WALUS SYLWIA (GB)
HALE CHRISTOPHER (GB)
US20050156575A1 | 2005-07-21 | |||
GB2537406A | 2016-10-19 | |||
US5686815A | 1997-11-11 |
Claims 1. A battery management system for a battery cell whose temperature decreases to a local minimum temperature during charging to a maximum capacity of the battery cell, the battery management system comprising: a charging module configured to charge the battery cell; a temperature sensor configured to measure a temperature indicative of the temperature of the battery cell during charging; and a controller configured to: control the charging module to charge the battery cell; receive the temperature measurements from the temperature sensor during charging of the battery cell; determine, from the received temperature measurements, at least one of: whether the temperature of the battery cell is approaching the local minimum temperature; whether the temperature of the battery cell is approximately at the local minimum temperature; and whether the temperature of the battery cell has passed through the local minimum temperature; and stop charging of the battery cell in response to determining that the temperature of the battery cell is approaching the local minimum temperature, in response to determining that the temperature of the battery cell is approximately at the local minimum temperature, and/or in response to determining that the temperature of the battery cell has passed through the local minimum temperature. 2. The battery management system of claim 1 , wherein determining at least one of: whether the temperature of the battery cell is approaching the local minimum temperature; whether the temperature of the battery cell is approximately at the local minimum temperature; and whether the temperature of the battery cell has passed through the local minimum temperature, comprises calculating a rate of change of the received temperature measurements with time. 3. The battery management system of claim 2, wherein determining whether the temperature of the battery cell is approaching the local minimum temperature comprises determining whether the calculated rate of change of the received temperature measurements has decreased below a threshold rate of change. 4. The battery management system of claim 3, wherein the controller is configured to determine a time at which the calculated rate of change of the measured temperature decreases below the threshold rate of change and stop charging of the battery cell when a predetermined time period has passed after the time at which the calculated rate of change of the measured temperature decreases below the threshold rate of change. 5. The battery management system of any of claims 2 to 4, wherein determining whether the temperature of the battery cell is approximately at the local minimum temperature comprises determining whether the calculated rate of change of the received temperature measurements changes from a negative rate of change to a positive rate of change. 6. The battery management system of any of claims 2 to 5, wherein determining whether the temperature of the battery cell has passed through the local minimum temperature comprises determining whether the calculated rate of change of the received temperature measurements is a positive rate of change. 7. The battery management system of any preceding claim, wherein determining whether the temperature of the battery cell is approaching the local minimum temperature comprises determining whether the temperature of the battery cell decreases below a first threshold temperature. 8. The battery management system of claim 7, wherein the first threshold temperature is greater than the local minimum temperature of the battery cell. 9. The battery management system of claim 7 or 8, wherein the first threshold temperature is a temperature of the battery cell which occurs at a state of charge of the battery cell which is less than a state of charge of the battery cell at which the local minimum temperature occurs during charging to a maximum capacity of the battery cell. 10. The battery management system of any of claims 7-9, wherein the first threshold temperature is a temperature of the battery cell which occurs at a state of charge of the battery cell, which is greater than 60%. 1 1. The battery management system of any of claims 7-10, wherein the controller is further configured to calculate the first threshold temperature during charging of the battery cell, wherein the first threshold temperature is calculated based at least in part on at least one temperature measurement taken during charging of the battery cell. 12. The battery management system of claim 11 , wherein the controller is further configured to: monitor the state of charge of the battery cell during charging of the battery cell; record a first measured temperature of the battery cell when a first predetermined state of charge is reached; and calculate the first threshold temperature based on the recorded first measured temperature. 13. The battery management system of claim 12, wherein the first threshold temperature is calculated by subtracting a predetermined temperature difference from the recorded first measured temperature. 14. The battery management system of claim 13, wherein the predetermined temperature difference is a proportion of a decrease in temperature of the battery cell which occurs when the battery cell is continuously charged between the first predetermined state of charge and a state of charge at which the temperature of the battery cell reaches the local minimum temperature. 15. The battery management system of any of claims 12-14, wherein the first predetermined state of charge is between about 50% and about 90%. 16. The battery management system of any preceding claim, wherein determining whether the temperature of the battery cell has passed through the local minimum temperature comprises determining whether the temperature of the battery cell is increasing with time. 17. The battery management system of claim 16, wherein the controller is further configured, during at least a part of charging of the battery cell, to stop charging of the battery cell in response to determining that the temperature of the cell is increasing. 18. The battery management system of claim 16 or 17, wherein determining that the temperature of the battery cell is increasing comprises detecting that the temperature of the battery cell is greater than a second threshold temperature. 19. The battery management system of claim 18, wherein the controller is further configured to calculate the second threshold temperature during charging of the battery cell, wherein the second threshold temperature is calculated based at least in part on at least one measurement of the temperature of the battery cell taken during charging of the battery cell. 20. The battery management system of claim 19, wherein the controller is further configured to: monitor the state of charge of the battery cell during charging of the battery cell; record a second measured temperature of the battery cell when a second predetermined state of charge is reached; and calculate the second threshold temperature based on the recorded second measured temperature. 21. The battery management system of claim 20, wherein the second threshold temperature is calculated by adding a temperature offset to the recorded second measured temperature. 22. The battery management system of any preceding claim, wherein the controller is further configured to: control the charging module to charge the battery cell to a state of charge which is the same as or greater than a state of charge at which the local minimum temperature occurs; determine, from the temperature measurements received from the temperature sensor during charging of the battery cell, a temperature of the battery cell at one or more states of charge. 23. The battery management system of claim 22, wherein the controller is configured to: determine from the temperature measurements received from the temperature sensor during charging of the battery cell, a first temperature of the battery cell at a first state of charge and a local minimum temperature of the battery cell reached during charging; and calculate a first temperature difference between the first temperature at the first state of charge and the local minimum temperature. 24. A battery management system for a battery cell whose temperature decreases to a local minimum temperature during charging to a maximum capacity of the battery cell, the battery management system comprising: a charging module configured to charge the battery cell; a temperature sensor configured measure a temperature indicative of the temperature of the battery cell during charging; and a controller configured to: control the charging module to charge the battery cell; receive the temperature measurements from the temperature sensor during charging of the battery cell; and stop charging of the battery cell in response to the temperature of the battery cell decreasing below a first threshold temperature. 25. The battery management system of claim 24, wherein the first threshold temperature is greater than the local minimum temperature of the battery cell. 26. The battery management system of claim 24 or 25, wherein the first threshold temperature is a temperature of the battery cell which occurs at a state of charge of the battery cell which is less than a state of charge of the battery cell at which the local minimum temperature occurs during charging to a maximum capacity of the battery cell. 27. The battery management system of any of claims 24-26, wherein the first threshold temperature is a temperature of the battery cell which occurs at a state of charge of the battery cell, which is greater than 60%. 28. The battery management system of any of claims 24-27, wherein the controller is further configured to calculate the first threshold temperature during charging of the battery cell, wherein the first threshold temperature is calculated based at least in part on at least one measurement of the temperature of the battery cell taken during charging of the battery cell. 29. The battery management system of claim 28, wherein the controller is further configured to: monitor the state of charge of the battery cell during charging of the battery cell; record a first measured temperature of the battery cell when a first predetermined state of charge is reached; and calculate the first threshold temperature based on the recorded first measured temperature. 30. The battery management system of claim 29, wherein the first threshold temperature is calculated by subtracting a predetermined temperature difference from the recorded first measured temperature. 31. The battery management system of claim 30, wherein the predetermined temperature difference is a proportion of a decrease in temperature of the battery cell which occurs when the battery cell is continuously charged between the first predetermined state of charge and a state of charge at which the temperature of the battery cell reaches the local minimum temperature. 32. The battery management system of any of claims 29-31 , wherein the first predetermined state of charge is between about 50% and about 90%. 33. The battery management system of any of claims 24-32, wherein the controller is further configured to determine whether the temperature of the battery cell is increasing with time. 34. The battery management system of claim 33, wherein the controller is further configured, during at least a part of charging of the battery cell, to stop charging of the battery cell in response to determining that the temperature of the cell is increasing. 35. The battery management system of claim 33 or 34, wherein determining that the temperature of the battery cell is increasing comprises detecting that the temperature of the battery cell is greater than a second threshold temperature. 36. The battery management system of claim 35, wherein the controller is further configured to calculate the second threshold temperature during charging of the battery cell, wherein the second threshold temperature is calculated based at least in part on at least one measurement of the temperature of the battery cell taken during charging of the battery cell. 37. The battery management system of claim 36, wherein the controller is further configured to: monitor the state of charge of the battery cell during charging of the battery cell; record a second measured temperature of the battery cell when a second predetermined state of charge is reached; and calculate the second threshold temperature based on the recorded second measured temperature. 38. The battery management system of claim 37, wherein the second threshold temperature is calculated by adding a temperature offset to the recorded second measured temperature. 39. The battery management system of any of claims 24-38, wherein the controller is further configured to: control the charging module to charge the battery cell to a state of charge which is the same as or greater than a state of charge at which the local minimum temperature occurs; determine, from the temperature measurements received from the temperature sensor during charging of the battery cell, a temperature of the battery cell at one or more states of charge. 40. The battery management system of claim 39, wherein the controller is configured to: determine from the temperature measurements received from the temperature sensor during charging of the battery cell, a first temperature of the battery cell at a first state of charge and a local minimum temperature of the battery cell reached during charging; and calculate a first temperature difference between the first temperature at the first state of charge and the local minimum temperature. 41. A battery management system for a battery cell whose temperature decreases to a local minimum temperature during charging to a maximum capacity of the battery cell, the battery management system comprising: a charging module configured to charge the battery cell; a temperature sensor configured to measure a temperature indicative of the temperature of the battery cell during charging; and a controller configured to: control the charging module to charge the battery cell; receive the temperature measurements from the temperature sensor during charging of the battery cell and determine whether the temperature of the battery cells is increasing with time; and stop charging of the battery cell in response to determining that the temperature of the battery cell is increasing with time. 42. The battery management system of claim 41 , wherein determining that the temperature of the cell is increasing comprises detecting that the temperature of the battery cell is greater than a second threshold temperature. 43. The battery management system of claim 42, wherein the controller is further configured to calculate the second threshold temperature during charging of the battery cell, wherein the second threshold temperature is calculated based at least in part on at least one measurement of the temperature of the battery cell taken during charging of the battery cell. 44. The battery management system of claim 43, wherein the controller is further configured to: monitor the state of charge of the battery cell during charging of the battery cell; record a second measured temperature of the battery cell when a second predetermined state of charge is reached; and calculate the second threshold temperature based on the recorded second measured temperature. 45. The battery management system of claim 44, wherein the second threshold temperature is calculated by adding a temperature offset to the recorded second measured temperature. 46. A battery management system for a battery cell whose temperature decreases to a local minimum temperature during charging to a maximum capacity of the battery cell, the battery management system comprising: a charging module configured to charge the battery cell; a temperature sensor configured to measure a temperature indicative of the temperature of the battery cell during charging; and a controller configured to: control the charging module to charge the battery cell; receive the temperature measurements from the temperature sensor during charging of the battery cell and calculate a rate of change of the measured temperature with time; and stop charging of the battery cell in response to the calculated rate of change meeting a predetermined criteria. 47. The battery management system of claim 46, wherein the predetermined criteria comprises the calculated rate of change of the measured temperature decreasing below a threshold rate of change. 48. The battery management system of claim 47, wherein the controller is configured to determine a time at which the calculated rate of change of the measured temperature decreases below the threshold rate of change and stop charging of the battery cell when a predetermined time period has passed after the time at which the calculated rate of change of the measured temperature decreases below the threshold rate of change. 49. The battery management system of any of claims 46 to 48, wherein the predetermined criteria comprises the calculated rate of change of temperature changing from a negative rate of change to a positive rate of change. 50. The battery management system of any of claims 46 to 49, wherein the predetermined criteria comprises the calculated rate of change being a positive rate of change. 51. A battery comprising a battery management system according to any preceding claim and at least one battery cell whose temperature decreases to a local minimum temperature during charging to a maximum state of charge of the battery cell. 52. A method of charging a battery cell whose temperature decreases to a local minimum temperature during charging to a maximum capacity of the battery cell, the method comprising: charging the battery cell; measuring a temperature indicative of the temperature of the battery cell during charging; determining, from the temperature measurements, at least one of: whether the temperature of the battery cell is approaching the local minimum temperature; whether the temperature of the battery cell is approximately at the local minimum temperature; and whether the temperature of the battery cell has passed through the local minimum temperature; and stopping charging of the battery cell in response to determining that the temperature of the battery cell is approaching the local minimum temperature, in response to determining that the temperature of the battery cell is approximately at the local minimum temperature, and/or in response to determining that the temperature of the battery cell has passed through the local minimum temperature. 53. A method of charging a battery cell whose temperature decreases to a local minimum temperature during charging to a maximum capacity of the battery cell, the method comprising: charging the battery cell; measuring a temperature indicative of the temperature of the battery cell during charging; and stopping charging of the battery cell in response to the temperature of the battery cell decreasing below a first threshold temperature. 54. A method of charging a battery cell whose temperature decreases to a local minimum temperature during charging to a maximum capacity of the battery cell, the method comprising: charging the battery cell; measuring a temperature indicative of the temperature of the battery cell during charging; determining whether the temperature of the battery cells is increasing with time; and stopping charging of the battery cell in response to determining that the temperature of the battery cell is increasing with time. 55. A method of charging a battery cell whose temperature decreases to a local minimum temperature during charging to a maximum capacity of the battery cell, the method comprising: charging the battery cell; measuring a temperature indicative of the temperature of the battery cell during charging; calculating a rate of change of the measured temperature with time; and stopping charging of the battery cell in response to the calculated rate of change meeting a predetermined criteria. |
FIELD OF THE INVENTION
[0001] This invention relates to battery management systems. This invention also relates to a battery comprising such a battery management system, as well as methods for charging of a battery cell.
BACKGROUND
[0002] In a typical lithium-sulphur battery, the positive electrode comprises a mixture of e.g. elemental sulphur and carbon supported on metal foil, while the negative electrode is a foil of lithium metal. During discharge, lithium at the negative electrode dissolves as lithium ions and the higher-order polysulphides at the positive electrode are reduced in successive steps to lower-order polysulphides until lithium sulphide is produced. During charging of the cell, lithium ions are reduced at the negative electrode made of lithium metal, and sulphide is re-oxidized to higher-order polysulphides at the positive electrode.
[0003] The higher-order polysulphides (which may, for example, include Ss 2 " , S6 2 ", and S4 2 ") generated at the positive electrode may be soluble in the electrolyte and can diffuse across the cell to the negative electrode where they are reduced to lower order polysulphides by reaction with metallic lithium. Depending on the state of charge of the cell, the concentration of the different polysulphide species in the cell can vary. The polysulphides are reduced at the lithium electrode then diffuse back to the positive electrode where they are re-oxidized again during charging. This shuttling of the polysulphides between the positive and negative electrode is a parasitic self-discharge process, widely referred to as the "polysulphide shuttle". In addition to self-discharge, the polysulphide shuttle also reduces the cycle life, decreases the charging efficiency, and lowers the power output of the lithium-sulphur cell.
SUM MARY OF THE INVENTION
[0004] It has been found that a lithium sulphur cell may be charged in a manner that may reduce the risk of the polysulphide shuttle occurring. This can improve the efficiency of the charging process and prolong the cycle life of the cell. Without wishing to be bound by any theory, it has been found that, during the charge cycle of a lithium sulphur cell, the onset of shuttling or significant amounts of shuttling may be marked by the temperature of the cell dipping to a local minimum temperature during charging of the battery cell to a maximum state of charge. By stopping the charging cycle at or before this local minimum temperature is reached, the negative effects of polysulphide shuttling may be ameliorated. [0005] According to a first aspect of the invention there is provided a battery management system for a battery cell whose temperature decreases to a local minimum temperature during charging to a maximum capacity of the battery cell, the battery management system comprising: a charging module configured to charge the battery cell; a temperature sensor configured to measure a temperature indicative of the temperature of the battery cell during charging; and a controller configured to: control the charging module to charge the battery cell; receive the temperature measurements from the temperature sensor during charging of the battery cell; determine, from the received temperature measurements, at least one of: whether the temperature of the battery cell is approaching the local minimum temperature; whether the temperature of the battery cell is approximately at the local minimum temperature; and whether the temperature of the battery cell has passed through the local minimum temperature; and stop charging of the battery cell in response to determining that the temperature of the battery cell is approaching the local minimum temperature, in response to determining that the temperature of the battery cell is approximately at the local minimum temperature, and/or in response to determining that the temperature of the battery cell has passed through the local minimum temperature.
[0006] The battery cell is a battery cell whose temperature decreases to a local minimum temperature during charging to a maximum capacity of the battery cell. That is, during continuous charging of the battery cell to a maximum capacity of the battery cell, the temperature of the battery cell exhibits a behaviour of decreasing to a local minimum temperature. The temperature of the battery cell may subsequently increase to a temperature which is greater than the local minimum temperature. During charging of the battery cell to the maximum capacity, the rate of change of the temperature of the battery cell with respect to time is negative (i.e. the temperature decreases with time) shortly before the local minimum temperature is reached and may be positive (i.e. the temperature increases with time) shortly after the local minimum temperature is reached (e.g. in embodiments where the temperature rises again after having passed through the local minimum temperature). In such embodiments, the local minimum temperature is the temperature at which the rate of change of temperature with respect to time is substantially equal to zero.
[0007] In other embodiments, the temperature of the battery cell may, to a reasonable approximation, monotonically decrease with increasing state of charge while charging to a maximum capacity of the battery cell. In such embodiments, the local minimum temperature is the temperature of the battery cell at maximum capacity of the battery cell. The temperature of the battery cell does not therefore increase from the local minimum temperature during charging of the battery cell and may only increase from the local minimum temperature once discharging of the battery cell has begun.
[0008] The local minimum temperature generally occurs in the latter half of a complete charging cycle of the battery cell. That is, the local minimum temperature may occur at a state of charge of the battery cell which is greater than about 50%. The local minimum temperature may occur at a state of charge of the battery cell which is greater than about 70%. The local minimum temperature may occur at a state of charge of the battery cell which is less than about 95%. The local minimum temperature and/or the state of charge at which the local minimum temperature occurs may be dependent on one or more variable parameters. For example, the local minimum temperature and/or the state of charge at which the local minimum temperature occurs may vary as a function of ambient temperature and/or pressure conditions present during charging of a battery cell. Additionally or alternatively the local minimum temperature and/or the state of charge at which the local minimum temperature occurs may vary as a function of the age of the battery cell (i.e. the number of charge/discharge cycles that the cell has undergone) and/or may vary as a function of the rate of charge at which the cell is charged. The local minimum temperature of a given battery cell may therefore be different and/or may occur at a different state of charge during different charging cycles of the cell (which may, for example, take place under different conditions such as different ambient temperature and/or pressure conditions, and/or different rates of charge).
[0009] Whilst reference is made to a battery cell, whose temperature decreases to a local minimum temperature during charging to a maximum capacity of the battery cell, it will be appreciated that according to embodiments of the invention, charging of the battery cell is stopped prior to the temperature of the cell reaching the local minimum temperature. That is, during normal operation of a battery management system according to embodiments of the invention, the temperature of a battery cell may not reach the local minimum temperature (since charging of the battery cell is stopped prior to the local minimum temperature being reached). A battery cell is referred to as being a battery cell whose temperature decreases to a local minimum temperature during charging to a maximum capacity of the battery cell, to indicate an inherent property of the battery cell rather than to indicate behaviour of the battery cell during normal operation of a battery management system according to embodiments of the invention.
[0010] It will be appreciated that any battery cell may be manipulated such that its temperature exhibits a local minimum temperature during charging. For example, a battery cell could be artificially heated and/or cooled during charging and/or subjected to varying ambient temperature conditions which could cause the temperature of the cell to decrease to a local minimum temperature during charging. However, in such a scenario the local minimum temperature may be caused by specific manipulation of the battery cell rather than by an inherent temperature behaviour of the battery cell.
[0011] References herein to a battery cell whose temperature decreases to a local minimum temperature during charging to a maximum capacity of the battery cell are intended to refer to a battery cell which exhibits such temperature behaviour as an inherent property of the battery cell. For example, the temperature of a battery cell of the type contemplated herein may decrease to a local minimum temperature during a constant current charge cycle, where the temperature of the battery cell is at a steady state when charging begins and the battery cell is subjected to substantially constant ambient temperature conditions during charging. That is, when the temperature of the battery cell reaches a steady state equilibrium temperature with its surroundings before charging begins, the battery cell is charged at a constant rate and is subjected to a substantially constant ambient temperature, the temperature of the battery cell decreases to a local minimum temperature during charging to a maximum capacity. However, it will be appreciated that such conditions may not hold true during all charging cycles contemplated herein and will not hold true during all charging cycles during which the invention can be usefully applied. The above described conditions are used merely to explain that the temperature behaviour described herein is an inherent property of the battery cells in question and is not solely a function of temperature conditions to which the battery cell is subjected.
[0012] The battery cell is a rechargeable battery cell. The battery cell may, for example, be a lithium sulphur cell.
[0013] It is believed that, at least in some battery cells, the local minimum temperature occurs during charging of the battery cell approximately at a point at which a shuttle effect begins to occur in the battery cell. The shuttle effect may occur, for example, in a lithium sulphur cell when dissolved polysulphides, which are formed within the cell, shuttle between the anode and the cathode during charge cycling. The occurrence of the shuttle effect has been shown to degrade the capacity of the cell and/or the Coulombic efficiency of the cell over successive charge/discharge cycles. It is therefore desirable to prevent or reduce the occurrence of the shuttle effect during charging of the battery cell.
[0014] In some embodiments the controller may be configured to determine, whether the temperature of the battery cell is approaching the local minimum temperature, whether the temperature of the battery cell is approximately at the local minimum temperature, and whether the temperature of the battery cell has passed through the local minimum temperature. In other embodiments the controller may only be configured to determine whether one or two of these conditions has occurred. For example, the controller may be configured to determine whether the temperature of the battery cell is approaching the local minimum temperature but may not be configured to determine whether the temperature of the battery cell is approximately at the local minimum temperature, or whether the temperature of the battery cell has passed through the local minimum temperature.
[0015] In some embodiments the controller may be configured to stop charging of the battery cell: in response to determining that the temperature of the battery cell is approaching the local minimum temperature, in response to determining that the temperature of the battery cell is approximately at the local minimum temperature, and in response to determining that the temperature of the battery cell has passed through the local minimum temperature. In other embodiments, the controller may only be configured to stop charging of the battery cell in response to determining one or two of these conditions. For example, the controller may be configured to stop charging of the battery cell in response to determining that the temperature of the battery cell is approaching the local minimum temperature, but may not be configured to stop charging in response to determining that the temperature of the battery cell is approximately at the local minimum temperature, and may not be configured to stop charging of the battery cell in response to determining that the temperature of the battery cell has passed through the local minimum temperature.
[0016] Reference herein to the temperature of the battery cell approaching the local minimum temperature should be interpreted to mean that a rate of change of the temperature of the battery cell with respect to time (or equivalently with respect to state of charge during charging) has decreased such that the temperature is decreasing more rapidly towards the local minimum temperature than it previously was (e.g. at a previous time during charging, or equivalently at a lower state of charge). That is, the magnitude of a negative rate of change of temperature has increased. The temperature of the battery cell with respect to time (or equivalently state of charge) may pass through a knee point such that the rate of change begins decreasing (i.e. the magnitude of a negative rate of change begins increasing) as the temperature of the battery cell begins to approach the local minimum temperature. The temperature of the battery cell may be considered to be approaching the local minimum temperature after the knee point in the temperature has occurred and before the local minimum temperature is reached. The temperature of the battery cell with respect to time (or equivalently with respect to state of charge during charging) may additionally pass through an inflection point (at which the second derivative of temperature with respect to time changes from negative to positive). Occurrence of an inflection point may further indicate that the temperature is approaching the local minimum temperature. [0017] By monitoring the temperature of the battery cell during charging and using the temperature measurements to determine when to stop charging of the battery cell, may ensure that charging of the battery cell is stopped prior to significant onset of the shuttle effect. The extent to which the shuttle effect occurs during charging of the battery cell may therefore be advantageously reduced. Consequently, degradation of the capacity and/or the Coulombic efficiency of the cell over successive charge/discharge cycles is reduced, thereby extending the useful cycle life of the battery cell (i.e. the number of charge/discharge cycles for which the battery can be usefully used).
[0018] Measuring the temperature of a battery cell, and stopping charging of the cell based on the temperature measurements may provide advantages over controlling charging of a cell based on other properties of the cell (such as, for example, a state of charge, voltage etc.). The behaviour of one or more properties of a cell (e.g. state of charge, voltage etc.) as a function of time during charging of a cell may vary depending on the number of charging cycles which the cell has already undergone during the lifetime of the cell. For example, a cell may exhibit different behaviour during its first 10 or so charging cycles to the behaviour of an aged cell which has undergone, for example, in excess of 80 charging cycles. For this reason some characteristic behaviours of a cell which could be monitored in order to control charging of a cell may be different during different charging cycles. Such properties may therefore provide an unreliable indicator of when to stop charging of a cell in order to reduce the occurrence of the shuttle effect.
[0019] It has been found that in some battery cells the temperature of the cell reduces to a local minimum during charging, both early in the life of the cell (e.g. when the cell has only undergone a limited number of charging cycles, such as less than 10 charging cycles) and later in the life of the cell (e.g. when the cell has undergone a large number of charging cycles, such as in excess of 80 charging cycles). The temperature of the cell may therefore be advantageously used as a reliable indicator of the potential onset of the shuttle effect throughout the lifetime of the cell.
[0020] The charging module may be operable to generate a charging current to charge the cell. The charging module may, for example, be connectable to an electrical power supply but may not include a power supply itself. The charging module may be operable to adjust a potential difference which is held across the battery cell and/or adjust a charging rate at which the battery cell is charged. For example, under control of the controller the charging module may be operable to stop charging of the battery cell such as by cutting off a charging current delivered to the battery cell from a power supply. The charging module may comprise one or more switches controllable by the controller.
[0021] The temperature sensor may be any suitable apparatus for measuring temperature. The temperature sensor may, for example, include a plurality of different sensor modules arranged to measure temperature in a plurality of different positions on or in proximity to the battery cell. Suitable devices which may form all or part of a temperature sensor may include a negative temperature coefficient (NTC) and/or a positive temperature coefficient (PTC) chip thermistor, a thermocouple, an integrated circuit temperature sensor and/or any other suitable temperature sensing device familiar to those having ordinary skill in the art. In general the temperature sensor may have a suitable resolution to detect changes in temperature of a battery cell during charging of the battery cell.
[0022] In general, references to measuring the temperature of a battery cell should not be interpreted to be limited to direct temperature measurement of a single battery cell but should instead be interpreted to refer to measuring a temperature which is indicative of the temperature of one or more battery cells. For example, in some embodiments temperature may be measured on or proximate to a component which is thermally coupled to a battery cell (e.g. a thermally conductive material in contact with a battery cell). Such temperature measurements are indicative of the temperature of the battery cell even if they do not comprise direct measurement of the temperature of the battery cell itself. Such temperature measurements are therefore considered to be an example of measuring the temperature of the battery cell.
[0023] In some embodiments, a battery may comprise a plurality of battery cells connected in series and/or in parallel with each other. In such embodiments one or more temperature measurements may be taken which are considered to be indicative of the temperature of all of the plurality of battery cells. For example, the battery cells may be electrically connected to each other by a bus bar. The electrical connections between the battery cells and the bus bar may mean that the plurality of battery cells are thermally coupled to each other and to the bus bar. In such embodiments one or more temperature sensors may be positioned on or in proximity to the bus bar in order to measure a temperature which is indicative of the temperature of the bus bar. A measured temperature which is indicative of the temperature of the bus bar is also indicative of the temperature of one or more battery cells which are connected to the bus bar. Temperature measurements taken on or in proximity to a bus bar are therefore considered to be examples of measuring the temperature of a battery cell.
[0024] The controller may be any suitable apparatus for controlling the charging module based at least in part on measurements received from the temperature sensor. The controller may include control logic such as a processor (e.g. a micro-processor).
[0025] Determining at least one of: whether the temperature of the battery cell is approaching the local minimum temperature; whether the temperature of the battery cell is approximately at the local minimum temperature; and whether the temperature of the battery cell has passed through the local minimum temperature, may comprise calculating a rate of change of the received temperature measurements with time.
[0026] The absolute temperature of a battery cell may depend not only on the charging state of the battery cell but also on the ambient temperature conditions in which the battery cell is held. The absolute temperature of a battery cell may therefore vary between different charging cycles carried out under different ambient temperature conditions. The rate of change of temperature with time during a charging cycle may exhibit broadly similar behaviour during different charging cycles carried out under different ambient temperature conditions. That is, the general shape of a curve mapping battery cell temperature with time during charging may be relatively consistent under different ambient temperature conditions, whereas the absolute values of battery cell temperature may vary with different ambient temperature conditions.
[0027] The rate of change of temperature with respect to time (or equivalently with respect to state of charge during continuous charging) may therefore be advantageously used as a relatively consistent indicator which can be reliably used during different charging cycles carried out under different conditions.
[0028] Determining whether the temperature of the battery cell is approaching the local minimum temperature may comprise determining whether the calculated rate of change of the received temperature measurements has decreased below a threshold rate of change.
[0029] The threshold rate of change is a negative rate of change. If the calculated rate of change decreases below the threshold rate of change then the magnitude of the calculated rate of change (which is a negative rate of change) therefore increases above the magnitude of the threshold rate of change (which is a negative rate of change). Reference to the calculated rate of change decreasing below the threshold rate of change therefore, in general, indicates that the rate at which the temperature is decreasing, is itself increasing (i.e. a negative rate of change is becoming more negative). That is, the temperature is decreasing more rapidly (with respect to time).
[0030] The threshold rate of change may be a predetermined value. For example, the threshold rate of change may be determined by performing one or more calibration charging cycles and observing the rate of change of temperature of a battery cell during charging. Additionally or alternatively, the threshold rate of change may be determined by predicting the temperature behaviour of a battery cell based on knowledge of the chemistry of the battery cell. The threshold rate of change may be set to a rate of change whose magnitude is typically exceeded during charging of a battery cell only when the temperature of the battery cell is approaching the local minimum temperature. For example, the threshold rate of change may be set to a rate of change which is typically observed before the temperature of the battery cell approaches the local minimum temperature.
[0031] During a typical charging cycle of a battery cell the temperature of the battery cell (with respect to time) may pass through a knee point before decreasing to the local minimum temperature. The threshold rate of change may be set such that a measured rate of change decreasing below the threshold rate of change indicates that the knee point has been passed. The measured rate of change decreasing below the threshold rate of change may therefore indicate that the temperature of the battery cell is approaching the local minimum temperature.
[0032] The controller may be configured to determine a time at which the calculated rate of change of the measured temperature decreases below the threshold rate of change and stop charging of the battery cell when a predetermined time period has passed after the time at which the calculated rate of change of the measured temperature decreases below the threshold rate of change.
[0033] As was explained above, if the rate of change of temperature decreases below the threshold rate of change, this may indicate that the temperature of the battery cell is approaching the local minimum temperature. This may indicate that charging of the battery cell should be stopped in order to prevent the local minimum temperature from being reached and in order to prevent significant onset of the shuttle effect. However, charging of the battery cell may still be continued as the temperature approaches the local minimum temperature without significant onset of the shuttle effect occurring. Charging may therefore continue after the measured rate of change has decreased below the threshold rate of change but may be stopped at a predetermined time period after the measured rate of change decreases below the threshold rate of change. The predetermined time period may be less than a typical time delay between the measured rate of change decreasing below the threshold rate of change and the local minimum temperature being reached, which typically occurs during charging of a battery cell. Charging is therefore advantageously stopped before the local minimum temperature is reached.
[0034] The predetermined time period may be determined based on one or more calibration charging cycles during which a typical time delay between the rate of change decreasing below a threshold rate of change and the local minimum temperature occurring is observed.
[0035] Determining whether the temperature of the battery cell is approximately at the local minimum temperature may comprise determining whether the calculated rate of change of the received temperature measurements changes from a negative rate of change to a positive rate of change. [0036] In the event that the calculated rate of change transitions from a negative rate of change to a positive rate of change, this typically indicates that the temperature has passed through a local minimum. When it is determined that the rate of change has transitioned from a negative rate of change to a positive rate of change then the temperature of the battery cell is therefore typically approximately at the local minimum temperature. Charging of the battery cell may be stopped in response to a determination that the temperature of the battery cell is approximately at the local minimum temperature, in order to prevent any further occurrence of the shuttle effect (which may begin approximately as the temperature reaches the local minimum temperature).
[0037] Stopping charging of the battery cell approximately as the local minimum temperature is reached may increase the proportion of the total capacity of the battery cell which is used during charging and discharging (when compared to, for example, stopping charging when the temperature is approaching the local minimum temperature).
[0038] Determining whether the temperature of the battery cell has passed through the local minimum temperature may comprise determining whether the calculated rate of change of the received temperature measurements is a positive rate of change.
[0039] In some battery cells the temperature of the battery cell increases with time during charging after the local minimum temperature has been reached. This may, for example, be caused by onset of the shuttle effect. If the calculated rate of change is a positive rate of change this may indicate that the local minimum temperature has been passed and may indicate occurrence of the shuttle effect.
[0040] In some embodiments, the controller may be configured to stop charging of the battery cell in the event that a positive rate of change is observed so as to provide a backup mechanism in case charging has not already been stopped before this point has been reached. For example, the controller may also be configured to stop charging if it is determined that the temperature of the battery cell is approaching the local minimum temperature and/or if it is determined that the temperature is substantially at the local minimum temperature. Typically a state of charge at which the local minimum temperature has been passed, will not therefore be observed, since charging will typically have been stopped before such a point has been reached. However, in the event that charging has continued until the local minimum temperature has been passed, charging may be stopped in order to prevent any further occurrence of the shuttle effect.
[0041] Determining whether the temperature of the battery cell is approaching the local minimum temperature may comprise determining whether the temperature of the battery cell decreases below a first threshold temperature.
[0042] As was explained above, the local minimum temperature of a given battery cell may be different and/or may occur at a different state of charge during different charging cycles of the cell (which may, for example, take place under different conditions such as different ambient temperature and/or pressure conditions, and/or different rates of charge). Consequently the first threshold temperature at which charging of the cell is stopped, may be different and/or may occur at a different states of charge during different charging cycles of the cell. That is, the first threshold temperature may not be a fixed property which remains the same during each charging cycle but may be an adaptive property which may be different during different charging cycles of a given battery cell.
[0043] By monitoring the temperature of the battery cell during charging and stopping the charging when the temperature of the cell decreases below the first threshold temperature, the extent to which the shuttle effect occurs during charging of the battery cell is advantageously reduced. Consequently degradation of the capacity and/or the Coulombic efficiency of the cell over successive charge/discharge cycles is reduced, thereby extending the useful cycle life of the battery cell (i.e. the number of charge/discharge cycles for which the battery can be usefully used).
[0044] The first threshold temperature may be a temperature which indicates that the state of charge of the battery cell is approaching a state of charge at which the local minimum temperature occurs. The first threshold temperature may be determined, at least in part, based on measurements taken during one or more calibration charging cycles of a battery cell. The one or more calibration cycles may, for example, be carried out on the same battery cell which is charged by the charging module and/or may be carried out on one or more reference cells (e.g. having similar properties to the battery cell which is charged by the charging module).
[0045] The first threshold temperature may be greater than the local minimum temperature of the battery cell.
[0046] During charging of the battery cell, the temperature of the battery cell may generally decrease. The threshold temperate may be a temperature of the battery cell which occurs during continuous charging prior to the temperature of the battery cell reaching the local minimum temperature. That is, charging of the battery cell is stopped before the temperature of the cell reaches the local minimum temperature (which would otherwise occur if charging of the battery cell were to be continued).
[0047] As was explained above, the local minimum temperature and/or the first threshold temperature may be different during different charging cycles of a battery cell. For example, the local minimum temperature of a battery cell may be a variable function of one or more of ambient temperature during charging, ambient pressure during charging, the age of the battery cell (i.e. the number of charging cycles which the battery cell has undergone to date), and the rate of charge during a charging cycle. The local minimum temperature may not therefore be a fixed property of the battery cell. The first threshold temperature may also be an adaptive property which may be different during different charging cycles.
[0048] Reference to the first threshold temperature being greater than the local minimum temperature should be interpreted to mean that during each charging cycle the first threshold temperature is greater than the local minimum temperature of the battery cell which would occur during a given charging cycle, if the charging were to be continued until the local minimum temperature is reached. It will be appreciated however, that charging of the battery cell may be stopped prior to the local minimum temperature being reached. The local minimum temperature during a given charging cycle therefore represents a local minimum temperature which would have been reached during the given charging cycle (i.e. under the ambient conditions and charging rate present during the charging cycle) had charging been continued to a state of charge at which the local minimum occurs.
[0049] Whilst the first threshold temperature may be greater than the local minimum temperature during each of a number of different charging cycles, the first threshold temperature and/or the local minimum temperature may be different during different charging cycles.
[0050] The first threshold temperature may be a temperature of the battery cell which occurs at a state of charge of the battery cell which is less than a state of charge of the battery cell at which the local minimum temperature occurs during charging to a maximum capacity of the battery cell.
[0051] Stopping charging of the battery cell in response to the temperature of the cell decreasing below the first threshold temperature may prevent the state of charge of the battery cell from reaching a state of charge at which the local minimum temperature would otherwise be reached. That is, charging of the battery cell is stopped before a state of charge of the battery cell reaches a state of charge at which the local minimum temperature would have occurred if charging of the battery cell were to be continued.
[0052] As was explained above, the state of charge at which the local minimum temperature occurs and/or the state of charge at which the first threshold temperature occurs may be different during different charging cycles of a battery cell. For example, the state of charge at which the local minimum temperature of a battery cell occurs may be a variable function of one or more of ambient temperature during charging, ambient pressure during charging, the age of the battery cell (i.e. the number of charging cycles which the battery cell has undergone to date), and the rate of charge during the charging cycle. The state of charge at which the local minimum temperature occurs may not therefore be a fixed property of the battery cell. The state of charge at which the first threshold temperature occurs may be an adaptive property which may be different during different charging cycles. [0053] Reference to the first threshold temperature being a temperature which occurs at a state of charge of the battery cell which is less than a state of charge of the battery cell at which the local minimum temperature occurs during charging to a maximum capacity of the battery cell, should be interpreted to mean that during each charging cycle the first threshold temperature is reached at a state of charge which is less than a state of charge at which the local minimum temperature of the battery cell would occur during a given charging cycle, if the charging were to be continued until the local minimum temperature is reached. It will be appreciated however, that charging of the battery cell may be stopped prior to state of charge at which the local minimum temperature occurs being reached. The state of charge at which the local minimum temperature occurs during a given charging cycle therefore represents a state of charge at which the local minimum temperature would have been reached during the given charging cycle (i.e. under the ambient conditions and charging rate present during the charging cycle) had charging been continued until the local minimum temperature occurs.
[0054] Whilst the state of charge at which the first threshold temperature occurs may be less than the state of charge at which the local minimum temperature occurs during each of a number of different charging cycles, the state of charge at which the first threshold temperature occurs and/or the state of charge at which the local minimum temperature occurs may be different during different charging cycles.
[0055] The first threshold temperature may be a temperature of the battery cell which occurs at a state of charge of the battery cell, which is greater than 60%.
[0056] The first threshold temperature may occur at a state of charge which is greater than about 70%, greater than about 80%, greater than about 85%, or even greater than about 90%. Typically the first threshold temperature may occur at a state of charge which is less than about 95%. As was explained above, the first threshold temperature and/or the state of charge at which the first threshold temperature occurs may be different during different charging cycles and may be a variable function of at least one of ambient temperature during charging, ambient pressure during charging, the age of the battery cell (i.e. the number of charging cycles which the battery cell has undergone to date), and the rate of charge during a charging cycle. The first threshold temperature may be set differently for different charging cycles according to one or more properties measured before or during a given charging cycle.
[0057] The controller may be further configured to calculate the first threshold temperature during charging of the battery cell, wherein the first threshold temperature is calculated based at least in part on at least one temperature measurement taken during charging of the battery cell. [0058] In some embodiments the first threshold temperature may be determined during charging of the battery cell. In some embodiments the first threshold temperature may be determined based upon a combination of predetermined data (e.g. collected during one or more calibration charging cycles) and temperature measurements made by the temperature sensor in real-time during charging of the battery cell. For example, a characteristic temperature decrease of a battery cell may be determined during one or more calibration charging cycles. The characteristic temperature decrease may be a temperature decrease which occurs whilst charging a cell between a predetermined state of charge (e.g. 80%) and a state of charge at which the local minimum temperature is observed (which may be greater than 80%).
[0059] During a charging cycle of the battery cell, the temperature of the cell at the predetermined state of charge (e.g. 80%) may be detected. The first threshold temperature may be determined based upon the detected cell temperature at the predetermined state of charge. For example, the first threshold temperature may be determined by subtracting a predetermined temperature difference from the detected cell temperature at the predetermined state of charge. The predetermined temperature difference may be based on a characteristic temperature decrease observed during one or more calibration charging cycles (as was described above). The predetermined temperature difference may be a proportion (e.g. 90%) of the characteristic temperature difference observed during one or more calibration charging cycles.
[0060] By calculating the first threshold temperature during charging of the battery cell and basing the calculation at least in part on at least one measurement of the temperature of the battery cell taken during charging of the battery cell, the first threshold temperature is set to a temperature which is relevant for the current charging cycle. As was explained above, the temperature behaviour of a battery cell may be different during different charging cycles and may, for example, depend on the ambient conditions, age of the cell and/or the rate of charge. Setting the first threshold temperature during a charging cycle to a first threshold temperature which is relevant for the current charging cycle advantageously means that the first threshold temperature is a reliable indicator of the potential onset of the shuttle effect across a variety of different conditions and charging cycles. The occurrence of the shuttle effect is therefore advantageously reduced and the useful lifetime of the battery cell is prolonged.
[0061] The controller may be further configured to: monitor the state of charge of the battery cell during charging of the battery cell; record a first measured temperature of the battery cell when a first predetermined state of charge is reached; and calculate the first threshold temperature based on the recorded first measured temperature. [0062] The first predetermined state of charge may be less than a state of charge at which the local minimum temperature occurs. The first predetermined state of charge may be set to be a state of charge which is less than the state of charge at which the local minimum temperature occurs during a vast majority of (or all) charging cycles under the vast majority of possible conditions (e.g. ambient temperature, pressure, charging rate etc.). The first predetermined state of charge may therefore be reached before the local minimum temperature occurs during a vast majority of possible different charging cycles carried out under a variety of different conditions. The first predetermined state of charge may, for example, be about 80%. Typically the first predetermined state of charge may be greater than about 50%. The first predetermined state of charge may be less than about 90%.
[0063] Whilst the local minimum temperature and/or the state of charge at which the local minimum temperature occurs may be different during different charging cycles. A change in temperature which occurs during charging from the first predetermined state of charge to a state of charge at which the local minimum temperature occurs may be relatively reliable. The temperature at the first predetermined state of charge therefore provides a reference point specific to the current charging cycle, which can be usefully used to calculate a first threshold temperature which is relevant to the current charging cycle.
[0064] The first threshold temperature may be calculated by subtracting a predetermined temperature difference from the recorded first measured temperature.
[0065] As was explained above, a change in temperature which occurs during charging from the first predetermined state of charge to a state of charge at which the local minimum temperature occurs may be relatively reliable. An expected temperature decrease of a cell during charging from the pre-determined state of charge to occurrence of the local minimum temperature may, therefore be used to set the predetermined temperature difference. This may allow the first threshold temperature to be set to a temperature which occurs prior to the local minimum temperature occurring, during a variety of different charging cycles. Stopping charging of the battery cell when the temperature reaches the first threshold temperature therefore advantageously prevents the battery cell from reaching a state of charge at which the local minimum temperature occurs.
[0066] The predetermined temperature difference may be a proportion of a decrease in temperature of the battery cell which occurs when the battery cell is continuously charged between the first predetermined state of charge and a state of charge at which the temperature of the battery cell reaches the local minimum temperature. [0067] The predetermined temperature difference may, for example, be about 90% or less of a decrease in temperature of the battery cell which occurs when the battery cell is continuously charged between the first predetermined state of charge and a state of charge at which the temperature of the battery cell reaches the local minimum temperature. In some embodiments the predetermined temperature difference may be less than about 85%, less than about 80%, less than about 75% or even less than about 70% of a decrease in temperature of the battery cell which occurs when the battery cell is continuously charged between the first predetermined state of charge and a state of charge at which the temperature of the battery cell reaches the local minimum temperature.
[0068] Setting the predetermined temperature difference as a proportion of the temperature difference between the first predetermined state of charge and the local minimum temperature improves the reliability with which charging is stopped prior to occurrence of the local minimum temperature. In particular, using a proportion of the temperature difference provides a margin of error such that if the temperature difference is less than expected, charging may still be stopped prior to the local minimum temperature being reached.
[0069] The first predetermined state of charge may be between about 50% and about 90%.
[0070] The first predetermined state of charge may be greater than about 60%. The first predetermined state of charge may be less than about 85%. The first predetermined state of charge may be about 80%.
[0071] Determining whether the temperature of the battery cell has passed through the local minimum temperature may comprise determining whether the temperature of the battery cell is increasing with time.
[0072] Whether or not the temperature of the battery cell is increasing with time, may be determined by measuring a first temperature of the battery cell at a first time and a second temperature of the battery cell at a second time, where the second time occurs after the first time. It may be determined that the temperature of the battery cell is increasing with time if it the second temperature measured at the second time is greater than the first temperature measured at the first time.
[0073] Determining whether the temperature of the battery cell is increasing with time may provide a further useful measure of the behaviour of the battery cell which may be used to stop charging of the cell at a point which reduces occurrence of the shuttle effect. For example, typically, during charging of a battery cell the temperature of the battery cell decreases until it reaches the local minimum temperature. If the temperature of the battery cell is increasing with time then this may indicate that the temperature has passed through the local minimum and the state of charge of the battery cell is greater than a state of charge at which the local minimum temperature occurs. If such an increase is detected then the controller may stop charging of the cell so as to reduce the occurrence of the shuttle effect.
[0074] The controller may be further configured, during at least a part of charging of the battery cell, to stop charging of the battery cell in response to determining that the temperature of the cell is increasing.
[0075] The controller may only stop charging of the battery cell in response to detecting that the temperature of the battery cell is increasing, during part of a process of charging a battery cell. For example, during a first part of charging of the battery cell, for example, up to a given state of charge (e.g. 70%) the controller may not stop charging of the battery cell if the temperature of the battery cell is increasing. However, once the state of charge of the battery cell is greater than the given state of charge (e.g. 70%) then the controller may stop charging of the battery cell (in response to detecting that the temperature is increasing). The given state of charge may in some embodiments be the same as the first predetermined state of charge described above.
[0076] As was explained above, a determination that the temperature of the battery cell is increasing with time may indicate that the state of charge of the battery is greater than a state of charge at which the local minimum temperature occurs. Stopping charging of the battery in response to determining that the temperature of the battery cell is increasing prevents further charging of the battery in a situation in which the temperature of the cell was not detected to decrease below the first threshold temperature. For example, the first threshold temperature may have erroneously been set to a temperature which is less than the local minimum temperature. Consequently, charging of the battery cell may continue past a state of charge at which the local minimum temperature occurs. Monitoring whether the temperature of the battery cell increases with time therefore provides a backup mechanism by which charging of the battery cell is stopped before the battery cell is charged to full capacity. This advantageously reduces an occurrence of the shuttle effect during charging and thus prolongs the useful lifetime of the battery cell.
[0077] Determining that the temperature of the battery cell is increasing may comprise detecting that the temperature of the battery cell is greater than a second threshold temperature.
[0078] The controller may only stop charging of the battery cell in response to detecting that the temperature of the battery cell is greater than a second threshold temperature, during part of a process of charging a battery cell. For example, during a first part of charging of the battery cell, for example, up to a given state of charge (e.g. 70%) the controller may not stop charging of the battery cell if the temperature of the battery cell is greater than the second threshold. However, once the state of charge of the battery cell is greater than the given state of charge (e.g. 70%) then the controller may stop charging of the battery cell. The second temperature threshold may be a temperature which is greater than the temperature (or the expected temperature) of the battery cell at the given state of charge. The given state of charge may in some embodiments be the same as the first predetermined state of charge described above.
[0079] Typically, during charging of a battery cell the temperature of the battery cell decreases until it reaches the local minimum temperature. The second threshold temperature may be a temperature which is greater than the local minimum temperature. The temperature of the battery cell being greater than a second threshold temperature may indicate that the state of charge of the battery cell is greater than a state of charge at which the local minimum temperature occurs. As was explained in detail above it may be advantageous to prevent further charging of the battery cell once a state of charge is reached which is greater than a state of charge at which the local minimum temperature occurs.
[0080] Stopping charging of the battery in response to determining that the temperature of the battery cell increases above a second threshold temperature prevents further charging of the battery cell in a situation in which the temperature of the cell was not detected to decrease below the first threshold temperature. For example, the first threshold temperature may have erroneously been set to a temperature which is less than the local minimum temperature. Consequently, charging of the battery cell may continue past a state of charge at which the local minimum temperature occurs. Stopping charging of the battery cell if the temperature of the battery cell increases above the second threshold temperature therefore provides a backup mechanism by which charging of the battery cell is stopped before the battery cell is charged to full capacity. This advantageously reduces an occurrence of the shuttle effect during charging and thus prolongs the useful lifetime of the battery cell.
[0081] The controller may be further configured to calculate the second threshold temperature during charging of the battery cell, wherein the second threshold temperature is calculated based at least in part on at least one measurement of the temperature of the battery cell taken during charging of the battery cell.
[0082] In some embodiments the second threshold temperature may be determined during charging of the battery cell. In some embodiments the second threshold temperature may be determined based upon a measurement of the temperature of the battery cell at a second predetermined state of charge (e.g. 80%). The second threshold temperature may, for example, be set by adding a predetermined offset to a temperature of the battery cell which is measured at a second predetermined state of charge (e.g. 80%).
[0083] By calculating the second threshold temperature during charging of the battery cell and basing the calculation at least in part on at least one measurement of the temperature of the battery cell taken during charging of the battery cell, the second threshold temperature is set to a temperature which is relevant for the current charging cycle. As was explained above, the temperature behaviour of a battery cell may be different during different charging cycles and may, for example, depend on the ambient conditions, age of the cell and/or the rate of charge. Setting the second threshold temperature during a charging cycle to a threshold temperature which is relevant for the current charging cycle advantageously means that the second threshold temperature is a reliable indicator of the potential onset of the shuttle effect across a variety of different conditions and charging cycles. The occurrence of the shuttle effect is therefore advantageously reduced and the useful lifetime of the battery cell is prolonged.
[0084] The controller may be further configured to: monitor the state of charge of the battery cell during charging of the battery cell; record a second measured temperature of the battery cell when a second predetermined state of charge is reached; and calculate the second threshold temperature based on the recorded second measured temperature.
[0085] The second predetermined state of charge may be less than a state of charge at which the local minimum temperature occurs. The second predetermined state of charge may be set to be a state of charge which is less than the state of charge at which the local minimum temperature occurs during a vast majority of (or all) charging cycles under the vast majority of possible conditions (e.g. ambient temperature, pressure, charging rate etc.). The second predetermined state of charge may therefore be reached before the local minimum temperature occurs during a vast majority of possible different charging cycles carried out under a variety of different conditions. The second predetermined state of charge may, for example, be about 80%. Typically the second predetermined state of charge may be greater than about 50%. The second predetermined state of charge may be less than about 90%.
[0086] In some embodiments, the second predetermined state of charge is about the same or exactly the same as the first predetermined state of charge mentioned above. The second measured temperature may therefore be approximately the same as the first measured temperature mentioned above.
[0087] Whilst the local minimum temperature and/or the state of charge at which the local minimum temperature occurs may be different during different charging cycles. A change in temperature which occurs during charging from the second predetermined state of charge to a state of charge at which the local minimum temperature occurs may be relatively reliable. The temperature at the second predetermined state of charge therefore provides a reference point specific to the current charging cycle. For example, it can be relatively reliably ascertained that the temperature at the second predetermined state of charge is the maximum temperature of the battery cell during charging between the second predetermined state of charge and the local minimum temperature. The second measured temperature can therefore be used as a reference temperature which is not expected to be exceeded during charging until after charging as passed through the state of charge at which the local minimum temperature occurs.
[0088] The second threshold temperature may be calculated by adding a temperature offset to the recorded second measured temperature.
[0089] As was explained above, it may be reasonably assumed that the second measured temperature is the maximum temperature of the battery cell during charging between the second predetermined state of charge and the state of charge at which the local minimum temperature occurs. If, during charging beyond the second predetermined state of charge, the temperature of the battery cell is greater than the second measured temperature then this could therefore indicate that the state of charge of the battery cell is greater than the state of charge at which the local minimum temperature occurs. The second measured temperature is therefore a useful reference point to which the current temperature of the battery cell can be compared. However, during charging of the battery cell, relatively small fluctuations in the measured temperature of the battery cell may be observed. Such fluctuations may be referred to as noise. By adding a temperature offset to the recorded second measured temperature in order to set the second threshold temperature a circumstance in which noise in the temperature measurements causes the temperature to exceed the second threshold temperature before the temperature passes through the local minimum temperature. For example, shortly after the second threshold temperature is measured the temperature of the battery cell might briefly be measured to exceed the second measured temperature due to noise. If the second threshold temperature were to be set at the second measured temperature exactly, then charging of the battery cell might be stopped well before the local minimum temperature is reached (and before the temperature decreases below the first threshold temperature). The temperature offset may be greater than noise fluctuations which might be expected to occur in the measured temperature of the battery cell. Adding the temperature offset to the second measured temperature may therefore ensure charging of the battery cell is not stopped simply due to noise in the measured temperature.
[0090] The controller may be further configured to: control the charging module to charge the battery cell to a state of charge which is the same as or greater than a state of charge at which the local minimum temperature occurs; determine, from the temperature measurements received from the temperature sensor during charging of the battery cell, a temperature of the battery cell at one or more states of charge.
[0091] The battery cell may be charged to a state of charge, which is the same as or greater than a state of charge at which the local minimum temperature occurs in order to observe the behaviour of the cell at higher states of charge than would otherwise be reached during normal operation. For example, as was explained in detail above, according to embodiments of the invention charging may be advantageously stopped before the local minimum temperature is reached. However, during one or more charging cycles the battery cell may be deliberately charged to a state of charge beyond a state of charge where the local minimum temperature occurs (for example, the battery cell may be charged to capacity or at least close to capacity). Such a charging cycle may be referred to as a calibration charging cycle.
[0092] One or more calibration charging cycles may be used to determine one or more properties of the battery cell, which may be used to control subsequent charging of the battery cell. For example, one or more calibration charging cycles may be used to determine one or more of the first threshold temperature, the predetermined temperature difference and/or the second threshold temperature referred to above. The one or more properties may be determined from a single calibration charging cycle or may be determined by combining observations from a plurality of calibration charging cycles. For example, an average of observed quantities may be taken over a plurality of charging cycles.
[0093] The controller may be configured to: determine from the temperature measurements received from the temperature sensor during charging of the battery cell, a first temperature of the battery cell at a first state of charge and a local minimum temperature of the battery cell reached during charging; and calculate a first temperature difference between the first temperature at the first state of charge and the local minimum temperature.
[0094] The first state of charge at which the first measured temperature of the battery cell is determined may be less than a state of charge at which the local minimum temperature occurs. The first state of charge may be the same as the predetermined state of charge referred to above. The calculated first temperature difference may be used to set the predetermined temperature difference referred to above. For example, the predetermined temperature difference may be set as a proportion of the first calculated temperature difference. Alternatively a plurality of calibration charging cycles may be performed and the predetermined temperature difference may be based on a plurality of calculated first temperature differences. In some embodiments, the predetermined temperature difference may be set as a proportion (e.g. about 80%) of an average of a plurality (e.g. two) of calculated first temperature differences, determined during a plurality of calibration charging cycles.
[0095] According to a second aspect of the invention there is provided a battery management system for a battery cell whose temperature decreases to a local minimum temperature during charging to a maximum capacity of the battery cell, the battery management system comprising: a charging module configured to charge the battery cell; a temperature sensor configured measure a temperature indicative of the temperature of the battery cell during charging; and a controller configured to: control the charging module to charge the battery cell; receive the temperature measurements from the temperature sensor during charging of the battery cell; and stop charging of the battery cell in response to the temperature of the battery cell decreasing below a first threshold temperature.
[0096] The first threshold temperature may be greater than the local minimum temperature of the battery cell.
[0097] The first threshold temperature may be a temperature of the battery cell which occurs at a state of charge of the battery cell which is less than a state of charge of the battery cell at which the local minimum temperature occurs during charging to a maximum capacity of the battery cell.
[0098] The first threshold temperature may be a temperature of the battery cell which occurs at a state of charge of the battery cell, which is greater than 60%.
[0099] The controller may be further configured to calculate the first threshold temperature during charging of the battery cell, wherein the first threshold temperature is calculated based at least in part on at least one measurement of the temperature of the battery cell taken during charging of the battery cell.
[00100] The controller may be further configured to: monitor the state of charge of the battery cell during charging of the battery cell; record a first measured temperature of the battery cell when a first predetermined state of charge is reached; and calculate the first threshold temperature based on the recorded first measured temperature.
[00101] The first threshold temperature may be calculated by subtracting a predetermined temperature difference from the recorded first measured temperature.
[00102] The predetermined temperature difference may be a proportion of a decrease in temperature of the battery cell which occurs when the battery cell is continuously charged between the first predetermined state of charge and a state of charge at which the temperature of the battery cell reaches the local minimum temperature.
[00103] The first predetermined state of charge may be between about 50% and about 90%.
[00104] The controller may be further configured to determine whether the temperature of the battery cell is increasing with time. [00105] The controller may be further configured, during at least a part of charging of the battery cell, to stop charging of the battery cell in response to determining that the temperature of the cell is increasing.
[00106] Determining that the temperature of the battery cell is increasing may comprise detecting that the temperature of the battery cell is greater than a second threshold temperature.
[00107] The controller may be further configured to calculate the second threshold temperature during charging of the battery cell, wherein the second threshold temperature is calculated based at least in part on at least one measurement of the temperature of the battery cell taken during charging of the battery cell.
[00108] The controller may be further configured to: monitor the state of charge of the battery cell during charging of the battery cell; record a second measured temperature of the battery cell when a second predetermined state of charge is reached; and calculate the second threshold temperature based on the recorded second measured temperature.
[00109] The second threshold temperature may be calculated by adding a temperature offset to the recorded second measured temperature.
[00110] The controller may be further configured to: control the charging module to charge the battery cell to a state of charge which is the same as or greater than a state of charge at which the local minimum temperature occurs; determine, from the temperature measurements received from the temperature sensor during charging of the battery cell, a temperature of the battery cell at one or more states of charge.
[00111] The controller may be configured to: determine from the temperature measurements received from the temperature sensor during charging of the battery cell, a first temperature of the battery cell at a first state of charge and a local minimum temperature of the battery cell reached during charging; and calculate a first temperature difference between the first temperature at the first state of charge and the local minimum temperature.
[00112] According to a third aspect of the invention there is provided a battery management system for a battery cell whose temperature decreases to a local minimum temperature during charging to a maximum capacity of the battery cell, the battery management system comprising: a charging module configured to charge the battery cell; a temperature sensor configured to measure a temperature indicative of the temperature of the battery cell during charging; and a controller configured to: control the charging module to charge the battery cell; receive the temperature measurements from the temperature sensor during charging of the battery cell and determine whether the temperature of the battery cells is increasing with time; and stop charging of the battery cell in response to determining that the temperature of the battery cell is increasing with time.
[00113] Whether or not the temperature of the battery cell is increasing with time, may be determined by measuring a first temperature of the battery cell at a first time and a second temperature of the battery cell at a second time, where the second time occurs after the first time. It may be determined that the temperature of the battery cell is increasing with time if it the second temperature measured at the second time is greater than the first temperature measured at the first time.
[00114] Determining whether the temperature of the battery cell is increasing with time may provide a useful measure of the behaviour of the battery cell which may be used to stop charging of the cell at a point which reduces occurrence of the shuttle effect. For example, typically, during charging of a battery cell the temperature of the battery cell decreases until it reaches the local minimum temperature. If the temperature of the battery cell is increasing with time then this may indicate that the temperature has passed through the local minimum and the state of charge of the battery cell is greater than a state of charge at which the local minimum temperature occurs.
[00115] Stopping charging of the battery in response to determining that the temperature of the battery cell is increasing advantageously reduces an occurrence of the shuttle effect during charging and thus prolongs the useful lifetime of the battery cell.
[00116] Determining that the temperature of the cell is increasing may comprise detecting that the temperature of the battery cell is greater than a second threshold temperature.
[00117] The controller may only stop charging of the battery cell in response to detecting that the temperature of the battery cell is greater than a second threshold temperature, during part of a process of charging a battery cell. For example, during a first part of charging of the battery cell, for example, up to a given state of charge (e.g. 70%) the controller may not stop charging of the battery cell if the temperature of the battery cell is greater than the second threshold. However, once the state of charge of the battery cell is greater than the given state of charge (e.g. 70%) then the controller may stop charging of the battery cell. The second temperature threshold may be a temperature which is greater than the temperature (or the expected temperature) of the battery cell at the given state of charge.
[00118] Typically, during charging of a battery cell the temperature of the battery cell decreases until it reaches the local minimum temperature. The second threshold temperature may be a temperature which is greater than the local minimum temperature. The temperature of the battery cell being greater than a second threshold temperature may indicate that the state of charge of the battery cell is greater than a state of charge at which the local minimum temperature occurs. As was explained in detail above it may be advantageous to prevent further charging of the battery cell once a state of charge is reached which is greater than a state of charge at which the local minimum temperature occurs.
[00119] Stopping charging of the battery in response to determining that the temperature of the battery cell increases above a second threshold temperature prevents further charging of the battery cell. This advantageously reduces an occurrence of the shuttle effect during charging and thus prolongs the useful lifetime of the battery cell.
[00120] The controller may be further configured to calculate the second threshold temperature during charging of the battery cell, wherein the second threshold temperature is calculated based at least in part on at least one measurement of the temperature of the battery cell taken during charging of the battery cell.
[00121] In some embodiments the second threshold temperature may be determined during charging of the battery cell. In some embodiments the second threshold temperature may be determined based upon a measurement of the temperature of the battery cell at a predetermined state of charge (e.g. 80%). The second threshold temperature may, for example, be set by adding a predetermined offset to a temperature of the battery cell which is measured at a predetermined state of charge (e.g. 80%).
[00122] By calculating the second threshold temperature during charging of the battery cell and basing the calculation at least in part on at least one measurement of the temperature of the battery cell taken during charging of the battery cell, the second threshold temperature is set to a temperature which is relevant for the current charging cycle. As was explained above, the temperature behaviour of a battery cell may be different during different charging cycles and may, for example, depend on the ambient conditions, age of the cell and/or the rate of charge. Setting the second threshold temperature during a charging cycle to a threshold temperature which is relevant for the current charging cycle advantageously means that the second threshold temperature is a reliable indicator of the potential onset of the shuttle effect across a variety of different conditions and charging cycles. The occurrence of the shuttle effect is therefore advantageously reduced and the useful lifetime of the battery cell is prolonged.
[00123] The controller may be further configured to: monitor the state of charge of the battery cell during charging of the battery cell; record a second measured temperature of the battery cell when a second predetermined state of charge is reached; and calculate the second threshold temperature based on the recorded second measured temperature.
[00124] The second predetermined state of charge may be less than a state of charge at which the local minimum temperature occurs. The second predetermined state of charge may be set to be a state of charge which is less than the state of charge at which the local minimum temperature occurs during a vast majority of (or all) charging cycles under the vast majority of possible conditions (e.g. ambient temperature, pressure, charging rate etc.). The second predetermined state of charge may therefore be reached before the local minimum temperature occurs during a vast majority of possible different charging cycles carried out under a variety of different conditions. The second predetermined state of charge may, for example, be about 80%. Typically the second predetermined state of charge may be greater than about 50%. The second predetermined state of charge may be less than about 90%.
[00125] In some embodiments, the second predetermined state of charge is about the same or exactly the same as the first predetermined state of charge mentioned above. The second measured temperature may therefore be approximately the same as the first measured temperature mentioned above.
[00126] Whilst the local minimum temperature and/or the state of charge at which the local minimum temperature occurs may be different during different charging cycles. A change in temperature which occurs during charging from the second predetermined state of charge to a state of charge at which the local minimum temperature occurs may be relatively reliable. The temperature at the second predetermined state of charge therefore provides a reference point specific to the current charging cycle. For example, it can be relatively reliably ascertained that the temperature at the second predetermined state of charge is the maximum temperature of the battery cell during charging between the second predetermined state of charge and the local minimum temperature. The second measured temperature can therefore be used as a reference temperature which is not expected to be exceeded during charging until after charging has passed through the state of charge at which the local minimum temperature occurs.
[00127] The second threshold temperature may be calculated by adding a temperature offset to the recorded second measured temperature.
[00128] As was explained above, it may be reasonably assumed that the second measured temperature is the maximum temperature of the battery cell during charging between the second predetermined state of charge and the state of charge at which the local minimum temperature occurs. If, during charging beyond the second predetermined state of charge, the temperature of the battery cell is greater than the second measured temperature then this could therefore indicate that the state of charge of the battery cell is greater than the state of charge at which the local minimum temperature occurs. The second measured temperature is therefore a useful reference point to which the current temperature of the battery cell can be compared. However, during charging of the battery cell, relatively small fluctuations in the measured temperature of the battery cell may be observed. Such fluctuations may be referred to as noise. By adding a temperature offset to the recorded second measured temperature in order to set the second threshold temperature a circumstance in which noise in the temperature measurements causes the temperature to exceed the second threshold temperature before the temperature passes through the local minimum temperature. For example, shortly after the second threshold temperature is measured the temperature of the battery cell might briefly be measured to exceed the second measured temperature due to noise. If the second threshold temperature were to be set at the second measured temperature exactly, then charging of the battery cell might be stopped well before the local minimum temperature is reached (and before the temperature decreases below the first threshold temperature). The temperature offset may be greater than noise fluctuations which might be expected to occur in the measured temperature of the battery cell. Adding the temperature offset to the second measured temperature may therefore ensure charging of the battery cell is not stopped simply due to noise in the measured temperature.
[00129] According to a fourth aspect of the invention there is provided a battery management system for a battery cell whose temperature decreases to a local minimum temperature during charging to a maximum capacity of the battery cell, the battery management system comprising: a charging module configured to charge the battery cell; a temperature sensor configured to measure a temperature indicative of the temperature of the battery cell during charging; and a controller configured to: control the charging module to charge the battery cell; receive the temperature measurements from the temperature sensor during charging of the battery cell and calculate a rate of change of the measured temperature with time; and stop charging of the battery cell in response to the calculated rate of change meeting a predetermined criteria.
[00130] The predetermined criteria may comprise the calculated rate of change of the measured temperature decreasing below a threshold rate of change.
[00131] The controller may be configured to determine a time at which the calculated rate of change of the measured temperature decreases below the threshold rate of change and stop charging of the battery cell when a predetermined time period has passed after the time at which the calculated rate of change of the measured temperature decreases below the threshold rate of change.
[00132] The predetermined criteria comprises the calculated rate of change of temperature changing from a negative rate of change to a positive rate of change.
[00133] The predetermined criteria comprises the calculated rate of change being a positive rate of change.
[00134] According to a fifth aspect of the invention there is provided a battery comprising a battery management system according to any of the first, second, third of fourth aspects and at least one battery cell whose temperature decreases to a local minimum temperature during charging to a maximum state of charge of the battery cell. [00135] According to a sixth aspect of the invention there is provided a method of charging a battery cell whose temperature decreases to a local minimum temperature during charging to a maximum capacity of the battery cell, the method comprising: charging the battery cell; measuring a temperature indicative of the temperature of the battery cell during charging; determining, from the temperature measurements, at least one of: whether the temperature of the battery cell is approaching the local minimum temperature; whether the temperature of the battery cell is approximately at the local minimum temperature; and whether the temperature of the battery cell has passed through the local minimum temperature; and stopping charging of the battery cell in response to determining that the temperature of the battery cell is approaching the local minimum temperature, in response to determining that the temperature of the battery cell is approximately at the local minimum temperature, and/or in response to determining that the temperature of the battery cell has passed through the local minimum temperature.
[00136] According to a seventh aspect of the invention there is provided a method of charging a battery cell whose temperature decreases to a local minimum temperature during charging to a maximum capacity of the battery cell, the method comprising: charging the battery cell; measuring a temperature indicative of the temperature of the battery cell during charging; and stopping charging of the battery cell in response to the temperature of the battery cell decreasing below a first threshold temperature.
[00137] According to an eighth aspect of the invention there is provided a method of charging a battery cell whose temperature decreases to a local minimum temperature during charging to a maximum capacity of the battery cell, the method comprising: charging the battery cell; measuring a temperature indicative of the temperature of the battery cell during charging; determining whether the temperature of the battery cells is increasing with time; and stopping charging of the battery cell in response to determining that the temperature of the battery cell is increasing with time.
[00138] According to a ninth aspect of the invention there is provided a method of charging a battery cell whose temperature decreases to a local minimum temperature during charging to a maximum capacity of the battery cell, the method comprising: charging the battery cell; measuring a temperature indicative of the temperature of the battery cell during charging; calculating a rate of change of the measured temperature with time; and stopping charging of the battery cell in response to the calculated rate of change meeting a predetermined criteria.
[00139] Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.
BRIEF DESCRIPTION OF FIGURES
[00140] One or more embodiments of the invention are shown schematically, by way of example only, in the accompanying drawings, in which:
- Figure 1 is a schematic illustration of a battery according to an embodiment of the present invention;
Figure 2 is a schematic representation of the temperature and voltage of a battery cell during charging and discharging of the battery cell;
Figure 3 is a schematic representation of the temperature and voltage of a battery cell as a function of capacity during charge cycles carried out at three different temperatures;
Figure 4 is a schematic representation of the temperature of a battery cell as a function of state of charge during charging of the battery cell;
Figure 5 is a schematic representation of the discharge capacity and coulombic efficiency of a battery cell over a number of successive charge-discharge cycles;
Figure 6 is a schematic representation of the discharge capacity and coulombic efficiency of a battery cell over a number of successive charge-discharge cycles, when charging of the battery cell is controlled by a battery management system according to an embodiment of the invention;
- Figure 7 is a schematic representation of the discharge capacity and coulombic efficiency of a battery cell over a number of successive charge-discharge cycles, when charging of the battery cell is controlled by a battery management system according to an alternative embodiment of the invention;
Figure 8 is a schematic representation of the temperature of a battery cell as a function of time during charging; and
Figure 9 is a schematic representation of the temperature and voltage of an alternative battery cell during charging and discharging of the battery cell.
DETAILED DESCRIPTION
[00141] Before particular examples of the present invention are described, it is to be understood that the present disclosure is not limited to the particular battery management system, battery or method described herein. It is also to be understood that the terminology used herein is used for describing particular examples only and is not intended to limit the scope of the claims.
[00142] In describing and claiming the battery management systems, batteries and methods of the present invention, the following terminology will be used: the singular forms "a", "an", and "the" include plural forms unless the context clearly dictates otherwise. Thus, for example, reference to "a battery cell" includes reference to one or more of such elements.
[00143] Figure 1 is a schematic illustration of a battery 100 according to an embodiment of the present invention. The battery 100 comprises a battery cell 104 and a battery management system 102. The battery management system 102 includes the components shown within the dashed box 102 in Figure 1. Whilst the battery 100 which is shown in Figure 1 comprises a single battery cell 104, in other embodiments the battery 100 may comprise a plurality of battery cells 104. As will be explained in further detail below, the battery cell 104 is a battery cell 104 whose temperature decreases to a local minimum temperature during charging of the battery cell 104 to a maximum state of charge. The battery cell 104 is a rechargeable battery cell 104. The battery cell 104 may, for example, comprise a lithium sulphur cell. The battery cell 104 includes terminals 106a, 106b. Electrical connections may be established with the terminals 106a, 106b in order to charge and discharge the battery cell 104.
[00144] The battery management system 102 comprises a charging module 108, a temperature sensor 110 and a controller 112. The charging module 108 is operable to charge the battery cell 108. As is shown in Figure 1 , electrical connections may be established between the charging module and the terminals 106a, 106b so as to connect the charging module 108 across the battery cell 104. The charging module 108 may be operable to hold a potential difference across the battery cell 104 so as to charge the cell 104. The charging module 108 may, for example, be connectable to an electrical power supply (not shown) but may not include a power supply itself. The charging module 108 may be operable to adjust a potential difference which is held across the battery cell 104 and/or adjust a charging rate at which the battery cell 104 is charged.
[00145] The charging module 108 may comprise one or more switches and/or other electrical components arranged to control and/or condition electrical power which is provided to the battery cell 104. For example, the charging module 108 may be operable to connect a power supply across the battery cell 104 in order to charge the battery cell 104 and may be operable to disconnect the power supply from the battery cell 104 (e.g. by opening one or more switches) so as to stop charging of the battery cell 104.
[00146] The temperature sensor 110 is configured to measure the temperature of the battery cell 104. In particular, the temperature sensor 1 10 is configured to measure the temperature of the battery cell 104 during charging of the battery cell 104. The temperature sensor 1 10 may be in direct contact with one or more portions of the battery cell 104 or may be separated from the battery cell 104 but located close enough to the battery cell 104 that the temperature measurements made by the temperature sensor 1 10 are indicative of the temperature of the battery cell 104. In general, references to measuring the temperature of a battery cell should not be interpreted to be limited to direct temperature measurement of a single battery cell but should instead be interpreted to refer to measuring a temperature which is indicative of the temperature of one or more battery cells. For example, in some embodiments, temperature may be measured on or proximate to a component which is thermally coupled to a battery cell (e.g. a thermally conductive material in contact with a battery cell). Such temperature measurements are indicative of the temperature of the battery cell even if they do not comprise direct measurement of the temperature of the battery cell itself. Such temperature measurements are therefore considered to be an example of measuring the temperature of the battery cell.
[00147] In some embodiments, a battery may comprise a plurality of battery cells connected in series and/or in parallel with each other. In such embodiments one or more temperature measurements may be taken which are considered to be indicative of the temperature of all of the plurality of battery cells. For example, the battery cells may be electrically connected to each other by a bus bar. The electrical connections between the battery cells and the bus bar may mean that the plurality of battery cells are thermally coupled to each other and to the bus bar. In such embodiments one or more temperature sensors may be positioned on or in proximity to the bus bar in order to measure a temperature which is indicative of the temperature of the bus bar. A measured temperature which is indicative of the temperature of the bus bar is also indicative of the temperature of one or more battery cells which are connected to the bus bar. Temperature measurements taken on or in proximity to a bus bar are therefore considered to be examples of measuring the temperature of a battery cell.
[00148] The temperature sensor 1 10 may be any suitable apparatus for measuring temperature. In some embodiments, the temperature sensor 1 10 may include a plurality of different sensor modules arranged to measure temperature in a plurality of different positions on or in proximity to the battery cell 104. Suitable devices which may form all or part of a temperature sensor may include a negative temperature coefficient (NTC) and/or a positive temperature coefficient (PTC) chip thermistor, a thermocouple, an integrated circuit temperature sensor and/or any other suitable temperature sensing device familiar to those having ordinary skill in the art.
[00149] The controller 112 is configured to control the charging module 108 to charge the battery cell 104. For example, the controller 1 12 may generate control signals 1 14 for controlling the charging module 108 and output the control signals 114 to the charging module 108. The controller 1 12 is further configured to receive temperature measurements from the temperature sensor 1 10. In particular, the controller is configured to receive temperature measurements from the temperature sensor 1 10 during charging of the battery cell 104. The temperature measurements may, for example, be provided to the controller 1 12 in the form of temperature signals 1 16 generated at the temperature sensor 1 10. The temperature signals 1 16 are indicative of the temperature of the battery cell 104.
[00150] The controller 1 12 controls the charging module based, at least in part, on the temperature measurements 1 16 received from the temperature sensor 1 10. For example, the controller 1 12 may change the control signals 114 sent to the charging module 108 in dependence on the temperature signals 1 16 received from the temperature sensor 110. According to various embodiments of the invention, the controller 1 12 is configured to stop charging of the battery cell 104 in response to the temperature of the battery cell 104 decreasing below a first threshold temperature. Additionally or alternatively, the controller 1 12 may be configured to stop charging of the battery cell 104 in response to determining that the temperature of the battery cell 104 is increasing with time. For example, the controller 1 12 may be configured to stop charging of the battery cell 104 in response to the temperature of the battery cell 104 increasing above a second threshold temperature.
[00151] The controller 1 12 may stop charging of the battery cell 104 by generating a control signal 1 14 for causing the charging module 108 to stop charging of the battery cell 104. The controller 1 12 may determine that the temperature of the battery cell 104 has decreased below a first threshold temperature, is increasing with time and/or has increased above a second threshold temperature based on a temperature signal 116 received from the temperature sensor 1 10.
[00152] The controller 112 may be any suitable apparatus for controlling the charging module 108 based at least in part on measurements received from the temperature sensor 110. The controller 112 may include control logic such as a processor (e.g. a microprocessor).
[00153] As was mentioned above, the battery cell 104 is a battery cell 104 whose temperature decreases to a local minimum temperature during charging to a maximum capacity of the battery cell 104. Figure 2 is a schematic representation of the temperature and voltage of such a battery cell 104 during charging and discharging of the battery cell 104. The temperature of the battery cell 104 is shown in degrees Celsius (°C) in panel A of Figure 2. The voltage of the battery cell 104 is shown in Volts (V) in panel B of Figure 2. Both the temperature and the voltage are displayed as a function of continuous capacity in ampere hours (A h) during a charge-discharge cycle of the battery cell 104. [00154] In the example represented in Figure 2, an 11.5 A h long life battery cell is charged and discharged between 1.5 V and 2.45 V in a convection oven held at a temperature of 30°C.
[00155] The left-hand sides of panels A and B of Figure 2 display the temperature and voltage of the battery cell 104 during charging of the battery cell 104. That is, the battery cell 104 is charging up to the point indicated by the dashed line 202 in Figure 2. The battery cell 104 may be charged to a maximum capacity and the dashed line 202 in Figure 2 may indicate a point at which the battery cell 104 reaches a maximum capacity. In the example, shown in Figure 2 the battery cell 104 is charged at a rate of 0.1 C.
[00156] The right-hand side of panels A and B of Figure 2 display the temperature and voltage of the battery cell 104 during discharging of the battery cell 104. That is, the battery cell 104 is discharging after the point indicated by the dashed line 202 in Figure 2. In the example, shown in Figure 2, the battery cell 104 is discharged at a rate of 0.2 C.
[00157] During the charge-discharge cycle represented in Figure 2, it can be seen that during initial charging of the battery cell 104 the temperature generally decreases with increasing state of charge of the cell 104. Whilst some fluctuations in the temperature of the cell 104 can be seen during charging in Figure 2, there is a general trend of temperature decrease with increasing state of charge. Towards the end of the charging cycle, the rate of decrease of the temperature of the cell is seen to increase and the temperature of the battery cell decreases relatively rapidly to a local minimum temperature. The local minimum temperature is indicated with an arrow labelled 204 in panel A of Figure 2. As the battery cell 104 continues to be charged and the state of charge of the battery cell 104 further increases, the temperature of the battery cell increases again with increasing state of charge. The increase in temperature of the battery cell 104 which is seen after the local minimum temperature, is indicated with an arrow labelled 206 in panel A of Figure 2.
[00158] As can be seen from panel A of Figure 2, during continuous charging of the battery cell 104 to a maximum capacity of the battery cell, the temperature of the battery cell 104 exhibits a behaviour of decreasing to a local minimum temperature 204 before subsequently increasing to a temperature which is greater than the local minimum temperature 204. The battery cell 104 is therefore said to be a battery cell whose temperature decreases to a local minimum temperature 204 during charging to a maximum capacity of the battery cell 104. During charging of the battery cell 104 to the maximum capacity, the rate of change of the temperature of the battery cell 104 with respect to time is negative (i.e. the temperature decreases with time) shortly before the local minimum temperature 204 is reached and is positive (i.e. the temperature increases with time) shortly after the local minimum temperature 204 is reached. The local minimum temperature 204 is the temperature at which the rate of change of temperature with respect to time is substantially equal to zero. Typically the local minimum temperature 204 occurs at a state of charge of the battery cell 104 which is greater than about 70%. The local minimum temperature might occur at a state of charge which is less than about 95%.
[00159] In some instances the local minimum temperature 204 may also represent a global minimum temperature. That is, the local minimum temperature 204 may be the minimum temperature of the battery cell 104 during a charge-discharge cycle. However, it is possible that under certain conditions the battery cell 104 could reach a temperature which is less than the local minimum temperature 204, which is typically observed during charging and at a state of charge which is greater than about 70%, at another stage of a charge-discharge cycle. The local minimum temperature 204 is therefore referred to as a "local" minimum so as not to exclude the possibility of the temperature of the battery cell reaching a lower temperature at some other point in a charge-discharge cycle.
[00160] Without wishing to be bound by any theory, it is believed that the rise 206 in temperature of the battery cell 104 which occurs after the local minimum temperature 204 is caused by the onset of the shuttle effect. The shuttle effect may occur, for example, in a lithium sulphur battery cell when dissolved polysulphides, which are formed within the cell, shuttle between the anode and the cathode during charge cycling. The occurrence of the shuttle effect has been shown to degrade the capacity of the cell and/or the Coulombic efficiency of the cell over successive charge/discharge cycles. It is therefore desirable to prevent or reduce the occurrence of the shuttle effect during charging of the battery cell 104. For example, it may be desirable to stop charging of a battery cell 104 before significant onset of the shuttle effect occurs or at least at a point which reduces the occurrence of the shuttle effect (compared to charging the battery cell to full capacity). In some embodiments, charging of the battery cell 104 may be stopped at or before the local minimum temperature 204 occurs.
[00161] The behaviour of the temperature of a battery cell decreasing to a local minimum temperature 204 has been observed when charging a number of different battery cells of different capacities, under a variety of different conditions. For example, the temperature decreasing to a local minimum temperature during charging has been observed in battery cells having capacities in the range of about 3.4 A h to about 20 A h. The temperature decreasing to a local minimum temperature during charging has also been observed at a number of different ambient conditions (e.g. different ambient temperature conditions). For example, the temperature of battery cells has been observed to decrease to a local minimum temperature during charging when held in ambient temperature conditions of 10°C, 30°C and 50°C. The temperature of a battery cell has also been observed to decrease to a local minimum temperature during charging at a number of different charging rates (such as charging rates of 0.1 C and 0.15 C). The temperature of a battery cell decreasing to a local minimum temperature during charging has also been observed in cells of different ages (e.g. which have undergone a number of different charge-discharge cycles). For example, the temperature of a battery cell decreasing to a local minimum temperature has been observed in cells relatively early in their lifetime (e.g. after having undergone about 10 or less than 10 charge-discharge cycles) and later in their lifetime (e.g. having undergone over 80 charge-discharge cycles). The behaviour of decreasing to a local minimum temperature has also been observed in modules of battery cells comprising a plurality of battery cells connected in series and/or in parallel with each other.
[00162] Whilst the behaviour of the temperature of a battery cell decreasing to a local minimum temperature has been observed when charging a number of different battery cells under a number of different conditions, the local minimum temperature 204 and/or the state of charge at which the local minimum temperature occurs has been found to depend on one or more variable factors. For example, the local minimum temperature 204 and/or the state of charge at which the local minimum temperature occurs may depend on one or more of the ambient temperature and/or pressure conditions during charging, the rate of charging at which the battery cell is charged, the capacity of the battery cell 104 and/or the age of the battery cell (e.g. how many charge-discharge cycles the battery cell has undergone during its lifetime).
[00163] Figure 3 is a schematic representation of the temperature and voltage of a 10 A h cell as a function of capacity during charge cycles carried out under three different ambient temperature conditions. The data points labelled 301a represent the temperature of the battery cell during a charging cycle carried out under ambient temperature conditions of 30°C. The line labelled 301 b represents the voltage of the battery cell during the charging cycle carried out under ambient temperature conditions of 30°C. The data points labelled 302a represent the temperature of the battery cell during a charging cycle carried out under ambient temperature conditions of 50°C. The line labelled 302b represents the voltage of the battery cell during the charging cycle carried out under ambient temperature conditions of 50°C. The data points labelled 303a represent the temperature of the battery cell during a charging cycle carried out under ambient temperature conditions of 10°C. The line labelled 303b represents the voltage of the battery cell during the charging cycle carried out under ambient temperature conditions of 10°C.
[00164] As can be seen in Figure 3, the local minimum temperature 204 and the state of charge at which the local minimum temperature 204 occurs is dependent on the ambient temperature conditions under which a charging cycle is carried out. The local minimum temperature 204 and/or the state of charge at which the local minimum temperature 204 occurs have under some conditions also been observed to be a function of one or more of the rate of charging at which the battery cell is charged, the capacity of the battery cell 104 and/or the age of the battery cell.
[001 65] Figure 4 is a schematic representation of the temperature of a battery cell during charging as a function of state of charge of the battery cell. The temperature of the cell during charging which is shown in Figure 4 is similar to the representation shown in the top panel of Figure 2. According to embodiments of the invention, charging of a battery cell is stopped in response to detecting that the temperature of the battery cell has decreased below a first threshold temperature Ττι . The first threshold temperature Ττι may be a temperature which, when observed, indicates that the state of charge of the battery cell 104 is approaching a state of charge SOCmin at which a local minimum temperature T m in occurs. The first threshold temperature Ττι may be greater than the local minimum temperature Tmin and may therefore be reached, during charging of the battery cell 104 before the local minimum temperature Tmin is reached. That is, a state of charge at which the first threshold temperature Ττι occurs may be less than the state of charge at which the local minimum temperature Tmin occurs.
[001 66] The first threshold temperature Ττι may be a temperature of the battery cell which occurs at a state of charge which is greater than about 60%. In some embodiments, the first threshold temperature Ττι may occur at a state of charge which is greater than about 70%, greater than about 80%, greater than about 85% or even greater than about 90%. Typically the first threshold temperature occurs at a state of charge which is less than about 95%.
[001 67] The local minimum temperature Tmin may typically occur at a state of charge SOCmin which is greater than about 70%. The local minimum temperature Tmin may occur at a state of charge which is less than about 95%.
[001 68] The first threshold temperature Ττι may be determined based at least in part on one or more temperature measurements made during charging of the battery cell 104 and before the temperature of the battery cell 104 reaches the local minimum temperature Tmin. Additionally or alternatively, the first threshold temperature Ττι may be determined at least in part on one or more measurements taken during a one or more calibration charging cycles of a battery cell 104.
[001 69] During a calibration charging cycle of a battery cell 104, the battery cell 104 may be charged to a state of charge which is greater than the state of charge SOCmin at which the local minimum temperature Tmin occurs. The temperature response of the battery cell during charging may be measured and used to determine the first threshold temperature
[001 70] During a calibration charging cycle, the temperature of the battery cell may be recorded at a predetermined state of charge, which may be referred to as a first predetermined state of charge SOC1. The first predetermined state of charge SOC1 may be a state of charge which is less than a state of charge SOCmm at which the local minimum temperature T m in occurs during a vast majority of different charging cycles carried out under a number of different conditions. The first predetermined state of charge SOC1 may therefore be reached before the local minimum temperature 204 occurs during a vast majority of possible different charging cycles carried out under a variety of different conditions. The first predetermined state of charge might, for example, be about 80%. In general the first predetermined state of charge may be greater than about 50%. The first predetermined state of charge may be less than about 90%.
[00171] The temperature of the battery cell recorded at the first predetermined state of charge may be referred to as a calibration temperature and is denoted Tc. The temperature of the battery cell may be further monitored during a calibration charging cycle and the local minimum temperature Tmin may be recorded. The recorded local minimum temperature may be denoted Tmin. The temperature difference between the calibration temperature Tc and the recorded local minimum temperature Tmin is calculated. The calculated temperature difference may be referred to as a calibration temperature difference and is denoted AJc, where ATc=Tc-T m m.
[00172] In some embodiments the calibration temperature difference may be determined based on a plurality (e.g. two) of calibration charging cycles rather than just a single calibration charging cycle. For example, a plurality of calibration charging cycles may be performed during which a plurality of calibration temperatures Tc are measured (at the predetermined state of charge) and a plurality of local minimum temperatures Tmin recorded. In such embodiments, the calibration temperature difference AJc may be calculated as a difference between an average of the measured calibration temperatures Tc and an average of the recorded local minimum temperatures Tmin.
[00173] During at least some charging cycles, the calibration temperature difference AJc may be used to determine the first temperature threshold Ττι . In some embodiments, the first temperature threshold Ττι may be determined based upon the calibration temperature difference AJc and on one or more measurements of the temperature of the battery cell 104 taken during charging of the battery cell. For example, during charging of a battery cell 104, the temperature of the battery cell 104 at the first predetermined state of charge SOCi may be measured and referred to as a first measured temperature Ti (as shown in Figure 4). The first threshold temperature Ττι may be determined based on the first measured temperature Ti and the calibration temperature difference AJc- For example, the first threshold temperature Ττι may be determined by subtracting a proportion of the calibration temperature difference AJc from the first measured temperature. That is, the first threshold temperature Ττι may be determined according to Ττι=Τι - pATc, where p is between 0 and 1. In some embodiments the proportion p may be about 90% or less. The proportion of the calibration temperature difference pATc may be referred to as a predetermined temperature difference ΔΤ.
[00174] By setting the first threshold temperature based upon the calibration temperature difference AJc and the first measured temperature Ti , the first threshold temperature Ττι is set according to a knowledge of the typical temperature behaviour of a cell deduced during calibration (AJc) and a reference point which is relevant to the current charging cycle (Ti). Setting the temperature difference as a proportion of the temperature difference between the first predetermined state of charge and the local minimum temperature improves the reliability with which charging is stopped prior to occurrence of the local minimum temperature. Using a proportion p of the calibration temperature difference AJc provides a margin of error such that if the drop in temperature between the first measured temperature Ti and the local minimum temperature, is less than expected, charging may still be stopped prior to the local minimum temperature T m in being reached. The reliability with which charging is stopped prior to the local minimum temperature being reached is therefore improved.
[00175] Figure 5 is a schematic representation of the discharge capacity and coulombic efficiency of a battery cell over a number of successive charge-discharge cycles. In the particular example which is shown in Figure 5, the battery cell is an 11.5 Ah capacity lithium sulphur battery cell. The discharge capacity is depicted by solid dots labelled 401 and is shown in Ampere hours (Ah). The Coulombic efficiency is depicted by hollow dots labelled 402 and is shown as a percentage. Both the discharge capacity and the Coulombic efficiency are depicted as a function of the number of charge-discharge cycles which the battery cell has undergone. The data points shown in Figure 5 were obtained by performing 50 charge-discharge cycles of a battery cell where the battery cell was charged until a certain potential difference exists across the battery cell. In the specific example shown in Figure 5, the battery cell was charged until a potential difference of 2.45 V exists across the battery cell. The data points shown in Figure 5 therefore represent a prior art situation in which charging of the battery cell is not stopped in response to any temperature measurements of the battery cell.
[00176] It can be seen from Figure 5 that after the battery cell has undergone 50 charge- discharge cycles the discharge capacity of the battery cell has degraded to approximately 86% of the initial capacity (i.e. the capacity during the first charge-discharge cycle). In the example of Figure 5, the capacity of the battery cell decreases by an average of approximately 0.28% during each charge-discharge cycle. It can be further seen in Figure 5 that after the battery cell has undergone 50 charge-discharge cycles the Coulombic efficiency of the battery cell has decreased to approximately 76%. [00177] Figure 6 is a schematic representation of the discharge capacity and coulombic efficiency of a battery cell over a number of successive charge-discharge cycles, when charging of the battery cell is controlled by a battery management system according to an embodiment of the invention. Figure 6 is thus equivalent to the representation shown in Figure 5 except for the control of the charging of the battery cell which is used in order to generate the data points. In order to generate the data points shown in Figure 6 a 1 1.5 A h lithium sulphur battery was used, which is equivalent to the cell used to generate the data in Figure 5. The discharge capacity is depicted by solid dots labelled 501 and is shown in Ampere hours (Ah). The Coulombic efficiency is depicted by hollow dots labelled 502 and is shown as a percentage. Both the discharge capacity and the Coulombic efficiency are depicted as a function of the number of charge-discharge cycles which the battery cell has undergone.
[00178] The charge-discharge cycles represented in Figure 6 were carried out using a predetermined temperature difference set as a proportion of a calibration temperature difference, as was described above. The calibration temperature difference was determined by performing two calibration charge-discharge cycles and determining the temperature difference between a temperature recorded at a state of charge of 80% and the local minimum temperature. The calibration temperature difference was set as the mean of the temperature differences determined during the two calibration charge- discharge cycles. The predetermined threshold temperature was set at 90% of the calibration temperature difference. During subsequent charge-discharge cycles of the battery cell, the temperature of the battery cell was recorded when the state of charge of the battery cell reached 80% (the predetermined state of charge referred to in the described examples above). The temperature recorded at a state of charge of 80% is equivalent to the first measured temperature referred to above. The first temperature threshold was then set by subtracting the predetermined temperature difference from the first measured temperature. Charging of the battery cell was stopped, during each charging cycle, when the temperature of the battery cell was recorded as decreasing below the first temperature threshold.
[00179] As can be seen in Figure 6, after performing 50 charge-discharge cycles, the capacity of the battery cell is approximately 95% of the initial capacity. The capacity of the battery cell decreases by an average of approximately 0.1 1 % during each charge- discharge cycle. It can be further seen in Figure 6 that after the battery cell has undergone 50 discharge-charge cycles the Coulombic efficiency of the battery cell remains at approximately 98%.
[00180] By comparing the data shown in Figure 6 with the data shown in Figure 5 it can be seen that by charging a battery cell using a battery management system according to an embodiment of the invention, the rate of decrease in capacity of the battery cell over successive charge-discharge cycles is decreased. Furthermore, the rate of decrease of the Coulombic efficiency of the battery cell over successive charge-discharge cycles is decreased. Without wishing to be bound by any theory it is believed that by using a battery management system according to an embodiment of the invention, the occurrence of the shuttle effect in the battery cell is reduced. Since the shuttle effect has been linked to degradation of the capacity and Coulombic efficiency of the battery cell, it is believed that reducing the occurrence of the shuttle effect leads to a slower rate of degradation of the capacity and Coulombic efficiency of the battery cell. The useful lifetime of the battery cell may therefore be extended.
[001 81 ] Embodiments have been described above in which charging of a battery cell is stopped in response to detecting that the temperature of the battery cell decreases below a first threshold temperature. In addition to, or as an alternative to such methods, charging of a battery cell may be stopped when it is detected that the temperature of the battery cell is increasing with time during charging. As was explained above, for example with reference to Figures 2 and 4, during continuous charging of a battery cell to full capacity, the temperature of the battery cell typically generally decreases with increasing state of charge until a local minimum temperature 204, T m in is reached. After the local minimum temperature 204, T m in is reached a substantial increase 206 in the temperature of the battery cell with increasing state of charge may be observed. In addition to, or as an alternative to, stopping charging of the battery in response to detecting a decrease in the temperature of the battery cell towards the local temperature minimum Tmin (as was described above), the charging of the battery cell may be stopped in response to determining that the temperature of the battery cell is increasing with time. That is, the controller 1 12 of the battery management system 102 may be further configured to determine whether the temperature of the battery cell 104 is increasing with time and may be configured to stop charging of the battery cell in response to detecting that the temperature of the battery cell is increasing with time.
[001 82] Whether or not the temperature of the battery cell is increasing with time may be determined by measuring a first temperature of the battery cell at a first time and a second temperature of the battery cell at a second time, where the second time occurs after the first time. If it is determined that the second temperature is greater than the first temperature then it may be determined that the temperature of the battery cell is increasing with time and thus charging of the battery cell may be stopped. It will be appreciated that if the temperature of the battery cell is increasing with time then this may indicate that the temperature has passed through the local minimum Tmin and the state of charge of the battery cell is greater than a state of charge SOCmin at which the local minimum temperature T m in occurs. Stopping charging in response to detecting that the temperature of the battery cell is increasing may therefore serve to reduce the occurrence of the shuttle effect in the battery cell.
[00183] Since it is known that a significant increase in temperature of a battery cell during charging is only expected after the battery cell has been charged up to a given state of charge, stopping charging of the battery cell in response to detecting that the temperature of the cell is increasing may only occur during a part of a charging cycle of a battery cell. For example, during charging of a battery cell up to a given state of charge (e.g. about 80%) the controller may not stop charging if an increase in the temperature of the battery cell is detected (since this is not likely to be caused by the onset of the shuttle effect). After the state of charge of the battery cell has increased to greater than the given state of charge then the controller may stop charging of the battery cell in response to detecting an increase in the temperature of the battery cell (since it is more likely that a temperature increase is being caused by the onset of the shuttle effect).
[00184] In some embodiments determining that the temperature of the battery cell is increasing may comprise detecting that the temperature of the battery cell is greater than a second threshold temperature ΤΪ2, an example of which is shown in Figure 4. The second threshold temperature ΤΪ2 is typically greater than the first threshold temperature Ττι which was described above, and is greater than the local minimum temperature T m in. The second threshold temperature Ττ2 may be set during charging of the battery cell and may, for example, be calculated based at least in part on one or more measurements of the temperature of the battery cell taken during charging of the battery cell. The second threshold temperature may typically occur at a stage of charge which is greater than about 70%. The second threshold temperature may occur at a state of charge which is less than about 99%.
[00185] In some embodiments, the temperature of the battery cell may be recorded when the state of charge of the battery cell reaches a predetermined state of charge. The predetermined state of charge may be referred to as a second predetermined state of charge SOC2 and the temperature measurement taken at the second predetermined state of charge SOC2 may be referred to as a second measured temperature T2. The second predetermined state of charge SOC2 is a state of charge which is less than a state of charge SOCmin at which the local minimum temperature Tmin occurs. Whilst not shown in Figure 4, in some embodiments, the second predetermined state of charge SOC2 may be about the same, or even exactly the same, as the first predetermined state of charge SOC1 which was described above. For example, the second predetermined state of charge SOC2 might be about 80%. [001 86] Since the second predetermined state of charge SOC2 is reached during charging prior to the state of charge SOCmin at which the local minimum temperature occurs, the second measured temperature T2 may provide a useful reference point which can be used to calculate the second threshold temperature ΤΪ2. Since the temperature of the battery cell is expected to decrease between the second predetermined state of charge SOC2 and the state of charge SOCmin at which the local minimum temperature occurs, it may be reasonably assumed that the second measured temperature T2 is the maximum temperature of the battery cell during charging between the second predetermined state of charge SOC2 and occurrence of the local minimum temperature T m in. If, during subsequent charging of the battery cell, the temperature of the battery cell is observed to be greater than the second measured temperature T2 then this may therefore indicate that the temperature of the battery cell has passed through the local minimum temperature T m in and is increasing.
[001 87] In some embodiments, the second threshold temperature ΤΪ2 is calculated by adding a temperature offset to the second measured temperatureT2. As was explained above, it might be reasonably assumed that the second measured temperature T2 is the maximum temperature of the battery cell during charging between the second predetermined state of charge and occurrence of the local minimum temperature Tmin. However, during charging of a battery cell, relatively small fluctuations in the measured temperature of the battery cell might be observed. Such fluctuations may be referred to as noise in the temperature measurements. Noise in the temperature measurements might cause the measured temperature of the battery cell to briefly rise above the second measured temperature T2 during charging of the cell between the second predetermined state of charge SOC2 and occurrence of the local minimum temperature Tmin. Calculating the second threshold temperature Ττ2 by adding a temperature offset to the second measured temperature may prevent charging of the battery from being stopped due to noise in the temperature measurements.
[001 88] For example, shortly after the second temperature measurement T2 is taken the measured temperature of the battery cell might briefly exceed the second measured temperature T2 due to noise in the temperature measurements. If the second threshold temperature Ττ2 were to be set at the second measured temperature T2 exactly, then charging of the battery cell might be stopped well before the local minimum temperature Tmin is reached (and before the temperature decreases below the first threshold temperature Ττι). The temperature offset may be greater than noise fluctuations which might be expected to occur in the measured temperature of the battery cell. Adding the temperature offset to the second measured temperature T2 may therefore ensure charging of the battery cell is not stopped simply due to noise in the measured temperature. [00189] In some embodiments, the controller 1 12 may be configured to determine whether the temperature of the battery cell 104 is increasing by determining a rate of change of the temperature of the battery cell 104 with time. For example, the controller 112 may calculate a rate of change of the temperature of the battery cell with time, based upon measurements received from the temperature sensor 1 10 at different times. The controller 1 12 may determine that the temperature of the battery cell 104 is increasing if the calculated rate of change of the temperature of the battery cell 104 is a positive rate of change.
[00190] In some embodiments, the controller 112 may determine that the temperature of the battery cell 104 is increasing if the calculated rate of change of the battery cell 104 is greater than a threshold rate of change. The threshold rate of change may be a positive rate of change which is greater than zero. For example, the threshold rate of change may be about 0.01 °C per minute or more. The threshold rate of change may be less than about 0.1 °C per minute.
[00191] The threshold rate of change may be sufficiently high to prevent noise in the temperature measurements from causing charging of the battery cell to stop. For example, noise in the temperature measurements may cause a calculated rate of temperature change to temporarily be positive. If the controller is configured to determine that the temperature is increasing and to stop charging of the cell merely when the calculated rate of change is positive, then noise may cause charging to be inadvertently stopped. By setting the threshold rate of change to be sufficiently high, inadvertent stopping of charging due to noise may be avoided.
[00192] As was described above, the controller 1 12 may be configured to stop charging of a battery cell 104 in response to determining that the temperature of the battery cell is increasing. This may be in addition to stopping charging of the battery cell if the temperature decreases below the first threshold temperature, or instead of stopping charging of the battery cell if the temperature decreases below the first threshold temperature.
[00193] For example, in some embodiments the controller 1 12 may be configured to stop charging of a battery cell 104 if it is determined that the temperature of the battery cell 104 is increasing and may be configured to stop charging of the battery cell 104 if the temperature decreases below the first threshold temperature Ττι . In such embodiments, stopping charging of the battery cell 104 in response to determining that the temperature of the battery cell is increasing prevents further charging of the battery cell 104 in a situation in which the temperature of the cell was not detected to decrease below the first threshold temperature Ττι . For example, the first threshold temperature Ττι may have erroneously been set to a temperature which is less than the local minimum temperature T m in. Consequently, charging of the battery cell 104 may continue past a state of charge SOCmin at which the local minimum temperature T m in occurs. Monitoring whether the temperature of the battery cell 104 increases with time therefore provides a backup mechanism by which charging of the battery cell 104 is stopped before the battery cell 104 is charged to full capacity.
[00194] In other embodiments, the controller 1 12 may be configured to stop charging of the battery cell 104 if it is determined that the temperature of the battery cell is increasing but may not be configured to stop charging of the battery cell 104 if the temperature decreases below a first threshold temperature Ττι. Figure 7 is a schematic representation of the discharge capacity and coulombic efficiency of a battery cell over a number of successive charge-discharge cycles, when charging of the battery cell is stopped when an increase in the temperature of the cell is detected. The data points shown in Figure 7 were generated using a 1 1.5 A h lithium sulphur cell, which is equivalent to the cells used to generate the data points shown in Figures 5 and 6. Similarly to Figures 5 and 6, the discharge capacity is depicted by solid dots labelled 601 and is shown in Ampere hours (A h). The Coulombic efficiency is depicted by hollow dots labelled 602 and is shown as a percentage. Both the discharge capacity and the Coulombic efficiency are depicted as a function of the number of charge-discharge cycles which the battery cell has undergone. During the charge-discharge cycles represented in Figure 7, charging of the battery cell was stopped when the rate of change of the temperature of the cell was greater than 0.034°C per minute.
[00195] As can be seen in Figure 7, after the battery cell has undergone 50 charge- discharge cycles the capacity of the battery cell is approximately 89% of the initial capacity of the battery cell. This represents a rate of decrease in the capacity of the battery cell of about 0.22% per charge-discharge cycle. After 50 charge-discharge cycles the Coulombic efficiency of the cell was about 95% of the initial Coulombic efficiency.
[00196] By comparing the data shown in Figure 7 with the data shown in Figure 5, it will be appreciated that stopping charging of the battery cell in response to detecting that the temperature of the battery cell is increasing (as shown in Figure 7) causes a significant improvement when compared to the prior art (as represented in Figure 5). Without wishing to be bound by any theory it is believed that by stopping charging in response to detecting an increase in temperature of the battery cell, the occurrence of the shuttle effect in the battery cell is decreased. Since the shuttle effect has been linked to degradation of the capacity and Coulombic efficiency of the battery cell, it is believed that reducing the occurrence of the shuttle effect leads to a slower rate of degradation of the capacity and Coulombic efficiency of the battery cell. The useful lifetime of the battery cell may therefore be extended. [00197] In some embodiments, charging of the battery cell may be stopped in response to determining a change in the rate of change of temperature of a battery cell with time. Figure 8 is a schematic representation of the temperature of a battery cell during continuous charging as a function of time. The temperature of the cell during charging which is shown in Figure 8 is similar to the representations shown in Figure 2 and 4 but with respect to time as opposed to capacity. In some embodiments the rate of change of temperature of the cell with respect to time may be determined during charging. The rate of change of temperature may be determined by measuring the temperature of the battery cell at a plurality of different times during charging. For example, as is shown in Figure 8, a first temperature measurement Ti of the battery cell may be measured at a first time ti and a second temperature measurement T2 of the battery cell may be measured at a second time t2. A first rate of change of temperature with respect to time between the first time ti and the second time t2 may be determined according to equation 1 below.
The first rate of change of the temperature of the battery cell between times and t ∑ is depicted in Figure 8 with a dashed line labelled 701.
[00198] The rate of change of the temperature of the battery cell may also be determined at other times. For example, as is shown in Figure 8, a third temperature measurement T3 of the battery cell is determined at a third time tz and a fourth temperature measurement T 4 of the battery cell is determined at a fourth time t 4 . A second rate of change of temperature with respect to time between the third time t3 and the fourth time t 4 may be determined according to equation 2 below and is depicted in Figure 8 with a dashed line labelled 702.
S _ _
dt t 4 -t 3 ^ '
[00199] For ease of illustration the first time ti , the second time t2, the third time t3 and the fourth time t 4 are displayed as having relatively large time separations between them. Consequently the rate of change is shown to change between the first time ti and the second time t2 and in particular the rate of change undergoes some relatively large changes between the third time t3 and the fourth time t 4 . The first and second rates of change 701 , 702 therefore appear only to represent an average rate of change over the time periods between the first and second times and between the third and fourth times (rather than representing an instantaneous rate of change given by a tangent to the line shown in Figure 8). However, it will be appreciated that the rate of change of temperature may be determined at a finer time resolution than is shown in Figure 8. That is, the time gaps between temperature measurements used to determine a rate of change may be smaller than the time gaps shown in Figure 8. [00200] In some embodiments temperature measurements which are used to determine a rate of change of temperature may be an average over a plurality of temperature measurements taken at different times. For example, each of the first ΤΊ , second T2, third T3 and fourth T 4 temperature measurements may be determined as an average over a plurality of temperature measurements taken in proximity to the first ti , second t2, third t3 and fourth t 4 times respectively. By taking an average over several temperature measurements the affect of any noise in the temperature measurements on a determined rate of change may be reduced.
[00201] It can be seen from Figure 8 that the first rate of change 701 of temperature between times ti and t2 is negative but with relatively small magnitude. The second rate of change 702 of temperature between times t3 and t 4 is also negative but has a larger magnitude than the first rate of change 701. The increase in magnitude of the gradient of temperature with respect to time indicates that the temperature has passed through a knee point 703 and is approaching the local minimum temperature 704. After passing through the knee point 703, the temperature pass through a point of inflection 705.
[00202] The knee point 703 may be thought of as a point at which the magnitude of the rate of change of temperature with respect to time begins increasing (with time). It will be appreciated that the rate of change of temperature with respect to time is negative at the knee point 703 and becomes more negative immediately after the knee point. Whilst the magnitude of the rate of change is increasing at the knee point 703, the rate of change itself is therefore decreasing. That is, a negative rate of change is becoming more negative.
[00203] Throughout this description the phrase "knee point" may be used generally to refer to the point at which the rate of change of temperature of a battery cell (with respect to time) begins decreasing such that the temperature of the battery cell is decreasing towards a local minimum temperature 704 (or equivalently the magnitude of a negative rate of change is increasing). References herein to the temperature of the battery approaching the local minimum temperature should be interpreted to mean that the temperature of the battery cell has passed the knee point 703 and is decreasing with time towards the local minimum temperature 704.
[00204] Mathematically a point of inflection 705 is a point at which the second derivative of the temperature with respect to time is equal to zero. That is, the magnitude of the rate of change begins to decrease (or equivalently the negative rate of change at the inflection point 705 begins to increase before becoming zero at the local minimum temperature and becoming positive after the local minimum temperature).
[00205] When the temperature of the battery cell passes through a knee point 703, this indicates that the temperature is decreasing towards the local minimum temperature 704. When the temperature of the battery cell pass through the inflection point 705 this further indicates that the temperature is approaching the local minimum temperature 704.
[00206] It will be appreciated that by periodically determining the rate of change of temperature with time during charging, it may be determined whether or not the temperature is approaching the local minimum temperature 704 (or equivalently whether or not the knee point 703 and/or the point of inflection 705 has been passed). In some embodiments charging of a battery cell may be stopped in response to determining that a knee point 703 and/or a point of inflection 705 has been passed (and that the temperature of the cell is approaching the local minimum temperature). Such a determination may be made, for example, by comparing a determined rate of change of temperature with time to a threshold rate of change. For example, it may be determined that the temperature of the cell is approaching the local minimum temperature 704 if the rate of change of temperature (with respect to time) decreases below a threshold rate of change.
[00207] The threshold rate of change may be a predetermined value. For example, the threshold rate of change may be determined by performing one or more calibration charging cycles and observing the rate of change of temperature of a battery cell during charging. Additionally or alternatively, the threshold rate of change may be determined by predicting the temperature behaviour of a battery cell based on knowledge of the chemistry of the battery cell. The threshold rate of change may be set to a rate of change whose magnitude is typically exceeded during charging of a battery cell only when the temperature of the battery cell is approaching the local minimum temperature 704. For example, the threshold rate of change may be set to a rate of change which is typically observed prior to the knee point 703. For example, the threshold rate of change may be set approximately to the first rate of change 701 , which is observed prior to the knee point 703. If the determined rate of change of temperature of a battery cell decreases below the threshold rate of change, this may indicate that the temperature of the battery cell has passed through the knee point 703 and is decreasing towards the local minimum temperature 704.
[00208] It will be appreciated that the threshold rate of change is a negative rate of change. If the determined rate of change decreases below the threshold rate of change then the magnitude of the determined rate of change (which is a negative rate of change) therefore increases above the magnitude of the threshold rate of change (which is a negative rate of change). Reference to the determined rate of change decreasing below the threshold rate of change therefore, in general, indicates that the temperature is decreasing more rapidly (i.e. a negative rate of change is becoming more negative). That is, the temperature is decreasing at an increasing rate (with respect to time). [00209] As was explained above, if it is determined that the rate of change of temperature has decreased below the threshold rate of change this may indicate that the knee point 703 has been reached and/or has been passed. This may be referred to as detecting the knee point 703. However, it will be appreciated that such a method may not directly detect the knee point 703 but may merely determine an indication that the knee point 703 has been reached and/or passed.
[00210] In some embodiments, charging of a battery cell may be stopped in response to detecting that the rate of change of temperature of the battery cell has decreased below a threshold rate of change. For example, charging may be stopped immediately when it is determined that the rate of change of temperature has decreased below the threshold rate of change. Alternatively charging may be stopped at a time after it is determined that the rate of change of temperature has decreased below the threshold rate of change. For example, it may be determined that the rate of change has decreased below the threshold rate of change at a time W Charging of the battery may then be stopped at a time tthr+tdeiay. Where tdeiay is a predetermined time delay. The predetermined time delay tdeiay may be a time period which is less than a typical time period between a time when the knee point 703 is reached and a time when the local minimum temperature 704 is reached during charging. Stopping charging at a time tthr+tdeiay may therefore ensure that charging is stopped prior to the local minimum temperature being reached. The predetermined time delay tdeiay may, for example, be determined by carrying out one or more calibration charging cycles.
[00211] Additionally or alternatively, the determined rate of change of temperature may be used to detect occurrence of the inflection point 705. In some embodiments charging of the battery cell may be stopped in response to determining that the temperature has pass through the inflection point 705. For example, temperature measurements may be used to calculate the second derivative of temperature with respect to time. If the second derivative of temperature with respect to time passes through zero (i.e. the second derivate changes from a negative number to a positive number) then it may be determined that the inflection point 705 has been reached.
[00212] In some embodiments, charging of the battery cell may be stopped as soon as it is determined that the inflection point 705 has been reached. In other embodiments charging of the battery cell may be stopped at a predetermined time after it is determined that the inflection point 705 has been reached.
[00213] Determining that the knee point 703 has been reached and determining that the inflection point 705 has been reached may both be considered to be examples of determining that the temperature of the battery cell is approaching the local minimum temperature 704. Stopping charging of the battery cell in response to determining that the temperature of the battery cell is approaching the local minimum temperature may prevent the temperature reaching the local minimum temperature 704. As was explained in detail above, stopping charging of the battery cell before the local minimum temperature is reached advantageously stops charging before substantial onset of the shuttle effect begins. Consequently degradation of the performance of the battery cell (e.g. a reduced capacity and/or a reduced Coulombic efficiency) may be reduced and a useful lifetime of the battery cell may be prolonged.
[00214] In some embodiments, measurements of the rate of change of temperature of a battery cell may be used to detect occurrence of the local minimum temperature 704. For example, it may be determined that the local minimum temperature has been reached if it is detected that a measured rate of change of temperature shifts from a negative rate of change to a positive rate of change. In some embodiments charging of a battery cell is stopped in response to detecting occurrence of the local minimum temperature 704. As was explained in detail above continued charging of a better cell after the local minimum temperature is reached may result in significant occurrence of the shuttle effect. Stopping charging of a battery cell in response to detecting that the local minimum temperature has been reached may therefore reduce occurrence of the shuttle effect in the battery cell.
[00215] It will be appreciated that stopping charging of a battery cell in response to detecting occurrence of the local minimum temperature 704 may result in the battery cell being charged to a higher state of charge (before charging is stopped) than if charging is stopped in response to detecting a knee point 703 or a point of inflection 705. This may therefore increase the state of charge to which the battery is charged and may increase the portion of the battery cells available capacity which is used during a charge-discharge cycle. However, allowing the battery cell to reach the local minimum temperature during charging may result in some occurrence of the shuttle effect during charging. Consequently degradation of the performance of the battery cell (e.g. a reduction in the overall capacity and/or a reduction in the Coulombic efficiency) which occurs during each charging cycle may be increased when compared to embodiments in which charging of the battery cell is stopped prior to the local minimum temperature 704 being reached.
[00216] In some embodiments, a controller of a battery management system may be configured to stop charging of a battery cell in response to determining that the temperature is approaching the local minimum temperature (e.g. by detecting occurrence of the knee point 703 and/or the inflection point 705) and may be configured to stop charging of a battery cell in response to determining that a local minimum temperature 704 has occurred. That is, the controller may stop charging of the battery cell if it is detected that either the temperature is approaching the local minimum temperature 704 or that the local minimum temperature 704 has occurred. During typical operation, detection that the temperature is approaching the local minimum temperature 704 may occur prior to detection of the local minimum temperature 704. Stopping charging in response to detecting that the local minimum temperature 704 has occurred may therefore provide a backup mechanism by which charging is stopped in the event that detection of the temperature approaching the local minimum temperature (e.g. through detection of the knee point 703 and/or the inflection point 705) is missed.
[00217] Embodiments have been described above in which charging of a battery cell is stopped in response to one or more measurements of the rate of change of temperature of a battery cell. For example, charging may be stopped in response to determining that a measured rate of change indicates that a knee point 703 has occurred, in response to determining that a measure rate of change indicates that an inflection point 705 has occurred and/or in response to determining that a measured rate of change indicates that a local minimum temperature 704 has been occurred. Stopping charging of a battery cell in response to a detected behaviour of a rate of change of temperature may offer some advantages relative to stopping charging in response to a detected behaviour of an absolute measured temperature (for example, in response to determining that the temperature is less than a threshold temperature). For example, the behaviour of a rate of temperature change of a battery cell may be less dependent on ambient conditions in which the battery cell is held than the behaviour of the measured absolute temperature of the battery cell. As can be seen, for example, from the plots shown in the bottom half of Figure 3 the absolute temperature of a battery cell during charging can vary considerably depending on the ambient temperature conditions under in which the battery cell is held during charging. However, the rate of change of temperature with time during charging follows a broadly similar trend regardless of the ambient temperature conditions. The rate of change of temperature of a battery cell with time may therefore provide a reliable indicator of the behaviour of the battery cell under a range of different ambient temperature conditions.
[00218] Reference has been made above to performing one or more calibration charging cycles in order to determine one or more properties of a battery cell (e.g. a calibration temperature difference ΔΤΌ, a threshold rate of change etc.). One or more calibration charging cycles may be performed (for example, in order to determine a calibration temperature difference ΔΤΌ, a threshold rate of change etc.) on the same cell which will later be used in practice and whose charging will be controlled by a battery management system 102 according to an embodiment of the invention. The battery management system 102 may be further configured to carry out a calibration charging cycle. That is, the battery management system 102 may be configured to control the charging module to charge the battery cell 104 to a state of charge which is the same as or greater than a state of charge at which the local minimum temperature occurs. The controller 1 12 may be further configured to determine, from the temperature measurements received from the temperature sensor 110 during charging of the battery cell 104, a temperature of the battery cell 104 at one or more states of charge.
[00219] In particular, the controller 112 may be configured to determine from the temperature measurements received from the temperature sensor 1 10 during charging of the battery cell 104, a first temperature of the battery cell 104 at a first state of charge and a local minimum temperature of the battery cell 104 reached during charging. The controller may then calculate a calibration temperature difference between the first temperature at the first state of charge and the local minimum temperature. As was explained above, the calibration temperature difference may then be used to set a first threshold temperature during subsequent charge-discharge cycles of the same battery cell 104.
[00220] In some embodiments, one or more calibration charging cycles may be performed on a different battery cell to a cell whose charging is subsequently controlled by a battery management system according to an embodiment of the invention. For example, a reference battery cell may be used to perform one or more calibration charging cycles. Data determined during one or more calibration charging cycles of the reference battery cell may then be used in the control of charging of other similar battery cells. For example, a calibration temperature difference may be determined during one or more calibration charging cycles of a reference battery cell. The determined calibration temperature difference may subsequently be used to determine a first threshold temperature during charging of other similar cells.
[00221] Embodiments have been described above with reference to battery cells whose temperature exhibits the behaviour of decreasing to a local minimum temperature during charging and then subsequently increasing to temperatures greater than the local minimum temperature during further charging (as is shown, for example, in Figures 2, 4 and 8). However, the temperature of some battery cells may, to a reasonable approximation, substantially monotonically decrease during charging.
[00222] For example, some types of lithium sulphur battery cell may display the temperature behaviour of a monotonically decreasing temperature during charging. A particular example of a lithium sulphur cell whose temperature may monotonically decrease during charging may be a cell referred to as an ultra-light lithium sulphur cell. An ultra-light lithium sulphur cell may comprise multiple layers of anodes, porous polymeric separators and a plurality of cathodes stacked together alternately in a pouch (e.g. a pouch constructed from aluminium film laminated with polymer). The anodes may each comprise a metallic lithium foil (e.g. having a thickness of approximately 50 microns). Each cathode may comprise a mixture of sulphur, carbon and a binder material coated double-sided on an aluminium foil, which serves as a current collector. The layers which form the cell may be welded together and welded to current collector tabs, to which external connections may be made in order to charge and/or discharge the battery cell. The current collector tabs may, for example, comprise aluminium and nickel.
[00223] Figure 9 is a schematic representation of the temperature and voltage of such a battery cell during charging and discharging of the battery cell. The temperature of the battery cell is shown in degrees Celsius (°C) in panel A of Figure 9. The voltage of the battery cell is shown in Volts (V) in panel B of Figure 9. Both the temperature and the voltage are displayed as a function of continuous capacity in ampere hours (A h) during a charge-discharge cycle of the battery cell. In the example represented in Figure 9 a 17 A h battery cell is charged and discharged.
[00224] The left-hand sides of panels A and B of Figure 9 display the temperature and voltage of the battery cell during charging of the battery cell. That is, the battery cell is charging up to the point indicated by the dashed line 802 in Figure 9. The battery cell may be charged to a maximum capacity and the dashed line 802 in Figure 9 may indicate a point at which the battery cell reaches a maximum capacity. In the example, shown in Figure 9 the battery cell is charged at a rate of 0.2 C.
[00225] The right-hand side of panels A and B of Figure 9 display the temperature and voltage of the battery cell during discharging of the battery cell. That is, the battery cell is discharging after the point indicated by the dashed line 802 in Figure 9. In the example, shown in Figure 9, the battery cell is discharged at a rate of 0.2 C.
[00226] As can be seen in Figure 9, during charging of the battery cell, the temperature of the battery cell substantially monotonically decreases with increasing state of charge. At higher states of charge the rate of decrease of the temperature of the battery cell is shown to increase, as indicated by the arrow labelled 805. The temperature of the battery cell continues to decrease until reaching a minimum 804 at a maximum state of charge of the battery cell. Whilst the temperature of the battery cell does not increase with further charging after reaching the minimum temperature 804 (as is the case, for example, in Figure 2), the minimum temperature 804 is still considered to be an example of a local minimum temperature. That is, the battery cell whose charge-discharge behaviour is displayed in Figure 9 is considered to be an example of a battery cell whose temperature decreases to a local minimum temperature during charging to a maximum capacity of the battery cell.
[00227] During the charge-discharge cycle which is displayed in Figure 9, there may be little or no occurrence of the shuttle effect. Significant onset of the shuttle effect may in general cause an increase in the temperature of a battery cell. Since the temperature of the cell illustrated in Figure 9 monotonically decreases during charging and does not subsequently increase during charging (as was shown, for example, in Figure 2) this may indicate little or no occurrence of the shuttle effect during charging. However, it may still be possible for the shuttle effect to occur in the cell. For example, later in the lifetime of the cell (i.e. after the cell has undergone more charge-discharge cycles) onset of the shuttle effect may occur in the cell.
[00228] Despite there being little or no onset of the shuttle effect in the example, illustrated in Figure 9 it may still be advantageous to stop charging of the cell prior to the temperature reaching the local temperature minimum 804. For example, stopping charging of the cell prior to the temperature reaching the local temperature minimum 804 may reduce any reduction in battery cell capacity and/or may reduce any decrease in the Columbic efficiency of the battery cell after a given number of charge-discharge cycles. Stopping charging of the battery cell prior to the temperature of the battery cell reaching the local temperature minimum may therefore have similar benefits to those described above with reference to Figures 5 and 6, even if no significant onset of the shuttle effect occurs during charging to full capacity.
[00229] As was explained above, it will be appreciated that battery management systems according to embodiments of the invention may be advantageously used with the type of battery cell represented in Figure 9. For example, stopping charging of the battery cell when the temperature of the battery cell decreases below a first threshold temperature may increase a useful lifetime of the battery cell.
[00230] Reference is made throughout this specification to a state of charge of a battery cell. References to a state of charge, which may for example be given as a percentage, are intended to be relevant to an initial capacity of the battery cell. It will be appreciated that a total available capacity of a battery cell may decrease during the lifetime of the battery cell (i.e. after a number of charge-discharge cycles have been carried out). However, references to state of charge are intended to be relative to an initial capacity of the battery cell. That is, the state of charge is given relative to the available capacity of the battery cell before the capacity has decreased during use.
[00231] Whilst reference is made throughout this specification to charging a battery cell, such reference is intended to encompass apparatus and methods in which a plurality of battery cells are simultaneously charged. For example, a plurality of battery cells may be connected in series and/or in parallel with each other. The methods and apparatus disclosed herein may be used to charge and control charging of such a plurality of battery cells.
[00232] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.
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