The present invention relates to a method for operating a battery for a vehicle, in which parameters which describe an operating state of respective battery cells of the battery are determined, and a state of charge of at least one of the battery cells is reduced, if it is detected that the determined parameter or a combination of parameters of one of the battery cells exceeds a predetermined threshold. Furthermore, the present invention relates to a battery management system as well as to a battery arrangement. Moreover, the present invention relates to a computer program and a computer-readable medium.

Electrically driven vehicles comprise a battery or battery pack to provide electrical power to an electric drive motor. Such a battery usually comprises a plurality of battery cells, which are electrically connected to one another. The battery cells can be designed as lithium-ion cells. In addition, batteries or battery arrangements comprise a battery management system to control operation of the battery.

In an automotive lithium-ion battery pack, battery cells are oriented close together to improve the volumetric energy density of the entire system. Though packs are designed with software and hardware-based battery management systems (e.g. fuses, pressure relief valves, coolant loops), it is still possible for individual battery cells to enter a state in which self-heating occurs. During self-heating elevated temperatures set off exothermic chemical reactions within the cell and energy is dissipated as heat. An extreme case of self-heating is called thermal runaway where heat is generated faster than it can be dissipated to the surroundings, thereby leading to a rapid and uncontrollable rise in temperature. Thermal runaway can eventually lead to cell explosion due to a buildup of pressure from gaseous decomposition products. When the cell ruptures, flammable electrolyte materials catch fire and corrosive gases may be released. The transfer of heat from a cell in thermal runaway to the surrounding cells leads to propagation of thermal runaway events which is a great safety concern.

There are several ways in which a cell can be brought to a thermal runaway condition including, but not limited to, overcharging, elevated temperatures, and mechanical abuse or a combination of these. Regardless, it is critical to limit the impact of a thermal runaway event by minimizing propagation from one cell to the surrounding cells. Concepts to achieve this include the insertion of thermal gaps, ceramic buffers, or flame retardant insulation in the form of fibrous material or foams between cells. These solutions have been demonstrated to be effective, but they necessarily reduce the energy density of batteries by adding extra weight and volume.

In advanced battery packs with higher energies (enabling longer range driving), greater numbers of cells will be used and cell interconnections will be more parallelized and serialized. Battery management systems (BMS) will therefore be more complex, with greater numbers of sensors and a combination of active and passive cell balancing. Sensors allow the battery management system to detect parameters (e.g. temperature, pressure, voltage, impedance) outside of the normal range so that steps can be taken to limit cell damage or prevent the extreme case of thermal runaway.

One of the major factors that influences the onset of thermal runaway is cell state of charge (SoC). It has been shown in the literature that cells with lower SoC have a higher temperature threshold for thermal runaway than fully charged cells. This finding has been attributed to the thermal stability of the cathode (positive) electrode, where a more lithiated cathode material is more stable. The mechanism of thermal runaway is thought to be exothermic oxygen release from delithiated cathodes that occurs at approximately 200 to 300° C. Cell self-heating is known to begin at lower temperatures (approximately 100° C) due to decomposition of compounds at the anode interface, but heat flow from these reactions are typically an order of magnitude lower than decomposition reactions at the cathode and therefore do not directly induce thermal runaway.

Borner et al.:“Correlation of aging and thermal stability of commercial 18650-type lithium ion batteries", Journal of Power Sources 342 (2017): 382-392, demonstrates that cells with approximately 50% SoC exhibit thermal onset temperatures that are 40 to 50° C higher than those with 100% SoC. The lower stored energy of a lower SoC cell also means that if a thermal runaway event occurs, less heat will be released (leading to lower peak temperatures) and the duration of the event may be shorter. This property is the basis for shipping standards that mandate cells be transported in a low SoC state.

US 2013/0312947 A1 discloses a battery management system that includes a multiplicity of individual battery cells in a housing, a multiplicity of cooling passages in the housing within or between the multiplicity of individual battery cells and a multiplicity of sensors operably connected to the individual battery cells. The sensors are adapted to detect a thermal runaway event related to one or more of the multiplicity of individual battery cells. Furthermore, a management system is adapted to inject coolant into at least one of the multiplicity of cooling passages upon the detection of the thermal runaway event by the any one of the multiplicity of sensors, so that the thermal runaway event is rapidly quenched.

US 5 574 355 A describes a thermal runaway detection apparatus that is provided for use during charging of a battery. The thermal runaway detection apparatus includes circuitry for determining internal resistance or conductance of the battery under charge. A detection circuitry detects an increase in internal battery conductance or a decrease in internal battery resistance and provides an output. The output is indicative of a thermal runaway condition in the battery.

WO 2015/149186 A1 discloses an apparatus for inhibiting thermal runaway of a battery cell. The apparatus uses a temperature sensor to measure a temperature of the cell and a discharge circuit, electrically coupled in series across terminals of the cell, to discharge the cell when its temperature exceeds a maximum normal operating temperature. When the temperature of the cell exceeds a maximum normal operating temperature, the state of charge (SoC) of the cell is decreased to a safe SoC.

It is an object of the present invention to provide a solution, how the impact of a thermal runaway in a battery of a vehicle can be reduced.

According to the invention this object is solved by a method, by a battery management system, by a battery arrangement, by a computer program as well as by a computer- readable medium having the features according to the respective independent claims. Advantageous embodiments of the invention are the subject matter of the dependent claims.

A method according to the invention serves for operating a battery for a vehicle.

Parameters which describe operating states of respective battery cells of the battery are determined. Furthermore, a state of charge of at least one of the battery cells, if it is detected that the determined parameter of one of the battery cells exceeds a

predetermined threshold, is reduced. It is provided that the state of charge of at least one battery cell is reduced, which is arranged in a predetermined surrounding area of the battery cell, of which the parameter exceeds the threshold value. The battery can be used in an at least partially electrically powered vehicle. The battery can comprise the plurality of battery cells, wherein the battery cells can be combined to form battery modules. The battery cells can be designed as lithium-ion-cells. The respective battery cells of a battery module or the battery can be electrically connected in series or parallel or a combination. The battery can be used to supply a drive motor of the vehicle and/or other electrical systems in the vehicle with electrical energy. In addition, the battery can be charged. This can take place during a charging process at a charging station and/or during recuperation.

During operation of the battery, the parameters of the respective battery cells can be detected. These parameters can be determined, for example, by means of a battery management system. The battery or a battery arrangement can comprise a plurality of sensors coupled to the respective battery cells. The battery management system is in particular designed to receive sensor data or measured values from the respective sensors and to determine the parameters therefrom. In particular, the parameters are determined such that self-heating of the battery cell can be derived on the basis of the parameters. The respective parameters can be compared to a predetermined threshold or thresholds. In other words, there exist behaviors of the battery cell in which a parameter or a combination of parameters is outside of a normal range or exceeds its predetermined threshold wherein these behaviors have previously been attributed to the onset of self heating. From this comparison it can then be deduced whether self-heating of the battery cell has already taken place or whether self-heating of the battery cell is imminent.

In addition, the battery management system can actively control energy flows on a cell- by-cell basis or a basis of several cells that is still smaller than a battery module. In particular, the battery management system is adapted to control the energy flows of each battery cell. The battery management system can reduce the state of charge of at least one of the battery cells. In addition, information can be stored in the battery management system describing the respective positions of the battery cells in the battery.

According to an essential aspect of the invention, the state of charge of at least one battery cell, which is arranged in a predetermined surrounding area of the battery cell, of which the parameter exceeds the threshold value, is reduced. In other words, the state of charge of at least one surrounding battery cell or of at least one battery cell that is located next to the battery cell at which the self-heating occurred is reduced. If the self-heating of one battery cell is detected, the battery management system can direct the surrounding battery cells to reduce their state of charge. This means that electrical energy is taken from the battery cells in the surrounding area. In this way, a“SoC reduction protocol” can be provided. By reducing the state of charge of a surrounding battery cell or cells in this way, the thermal runaway in the battery can be slowed down and/or the propagation can be prevented.

According to an embodiment, the battery cells are arranged side by side and the surrounding area is predetermined in such a way that it encloses at least the battery cell(s) which is arranged directly next to the battery cell, of which the parameter exceeds the threshold value. The surrounding area can include the battery cells which are located directly next to the battery cell with self-heating. Since the battery cells can be arranged in a row, the surrounding area can include the respective battery cell on the left side and the right side of the battery cell with self-heating. The surrounding area can also include the respective two battery cells on the left side and the right side of the battery cell with self heating. In other words, the surrounding area can include those battery cells in close contact or within a one to two to three (or greater) cell distance from the battery cell with self-heating. If the state of charge of the battery cells in the surrounding area is reduced, the propagation of the thermal runaway in the battery can be slowed or prevented.

According to a further embodiment, a temperature of the respective battery cells is detected as the parameter of the respective battery cells. The battery management system can have the ability to sense the temperature of the battery cells. For this purpose, the system can receive measured values from corresponding temperature sensors and/or other parameters which are correlated with temperature. In this case, the threshold can be set to a value at or below which self-heating typically initiates. Other behaviors that that could be indicative of an onset event include large fluctuations in voltage or current. Fluctuations in the voltage or current are indicative of an electrical short within the battery. Furthermore, an increase in cell pressure due to gassing from one of the components can be an indicator of self-heating. Alternatively or additionally, it can therefore be provided that the pressure, the voltage and/or the current are

determined as parameters. These parameters can also be detected with corresponding sensors or are already available to the system. On the basis of the parameters, the self heating of the battery cells can be detected in a simple and reliable manner.

According to a further embodiment, at least one battery cell is discharged to reduce its state of charge, wherein the discharging is performed dependent on a temperature and/or a state of charge of at least one other battery cell. The maximum current rate at which the battery cells in the surrounding area can be discharged can be dependent on their original state of charge as well as their temperature, both of which could be varying dynamically. Rapidly discharging the battery cells surrounding the battery cell with the self-heating may effectively heat those cells. Therefore, close control over the surrounding cells by the battery management system can be necessary to reduce the likelihood of the“SoC reduction protocol” inducing self-heating or thermal runaway in the surrounding cells as well.

According to a further embodiment, for reducing the state of charge, energy is transmitted from at least one battery cell to a component of the vehicle that is not part of the powertrain. In this way it can be prevented that the drive train or the battery is heated by the dissipated energy. For example, the electrical energy from the at least one battery cell can be directed to energy dissipation systems. The use of energy dissipation systems could amount to an approximately 20 kW expenditure under certain conditions, which could effectively reduce the state of charge of four or more battery cells by 50% in less than 5 min.

According to a further embodiment, the energy is transmitted to a cooling device and/or a lighting device of the vehicle. The electrical energy can be directed into non-powertrain related systems in the vehicle, i.e. climate control and/or lighting. In this way, the energy taken from the battery cell for discharging can be used for other components of the vehicle. From this“SoC reduction protocol,” variants can be formulated such that cooling loops dedicated to individual cells or modules are activated by the energy expended by the cells surrounding the battery cell with self-heating. The manner in which the energy is expended by the cells surrounding the battery cell with self-heating is flexible as long as it does not contribute substantially to further cell or pack heating.

If it is detected that the parameter of a battery cell exceeds the threshold during charging of the battery, the charging operation is interrupted according to a further embodiment. The proposed“SoC reduction protocol” can also be useful for a malfunctioning battery undergoing a direct current fast charge event. In this case, the battery management system can first recognize an onset event in one battery cell, halt further charging, increase the load on the active battery pack cooling system to reduce the rate of cell temperature increase, and reduce the state of charge of the battery cells surrounding the battery cell with the self-heating. Thus, the charging of the battery and in particular a direct current fast charging can be securely performed. If it is detected in a further battery cell that the parameter exceeds the threshold value, the state of charge of at least one of the battery cells in the surrounding area of the further battery cell is reduced according to a further embodiment. Though this control scheme cannot eliminate the possibility of thermal runaway in a battery or battery pack, it can effectively slow and/or prevent propagation. Therefore, one could envision a scenario where a thermal event in a first battery cell induces thermal runaway in second battery cell which has already reached 50% SoC according to the“SoC reduction protocol.” The battery management system can recognize this event in the second cell and reduce the SoC of a third battery cell, adjacent to the second battery cell. Thus, the protocol would progressively slow the propagation of thermal runaway.

A battery management system according to the invention for a battery arrangement of a vehicle is configured to perform a method according to the invention. The current state of the art battery management systems do not include distributed sensors on a cell-by-cell level and they do not allow for cell-by-cell control of energy transfer for active state of charge balancing protocols. Current management systems can be implemented at the module level with a few sensors, i.e. a temperature sensor, and minimal control over individual cells. The proposed scheme for the“SoC reduction protocol” requires the implementation of systems with cell level control and distributed sensors. In particular, the battery management system according to the invention is adapted to control the state of charge of individual or small groups of battery cells.

A battery arrangement according to the invention comprises a battery management system according to the invention and a battery. The battery can comprise a plurality of battery cells. In addition, the battery arrangement can comprise a plurality of sensors for detecting parameters of the respective battery cells.

A vehicle according to the invention comprises a battery arrangement according to the invention. The vehicle may be an electric vehicle or a hybrid vehicle.

A further aspect of the invention relates to a computer program comprising instructions which, when the program is executed by a computing device of a battery management system, cause the computing device to carry out the method according to the invention.

A further aspect of the invention relates to a computer-readable medium comprising instructions which, when executed by a computing device of a battery management system, cause the computing device to carry out the method according to the invention. The preferred embodiments presented with respect to the method according to the invention and the advantages thereof correspondingly apply to the battery management system according to the invention, to the battery arrangement according to the invention, to the vehicle according to the invention, to the computer program according to the invention as well as to the computer-readable medium according to the invention.

Further features of the invention are apparent from the claims, the figures and the description of figures. The features and feature combinations mentioned above in the description as well as the features and feature combinations mentioned below in the description of figures and/or shown in the figures alone are usable not only in the respectively specified combination, but also in other combinations or alone without departing from the scope of the invention.

In the following, the invention is explained in more detail based on preferred embodiments as well as with reference to the attached drawings. These show in:

Fig. 1 a schematic view of a vehicle that comprises a battery arrangement with a battery; and

Fig. 2 a schematic view of a battery arrangement, wherein the battery

arrangement comprises the battery and a battery management system.

In the figures, identical or functionally identical elements are provided with the same reference characters.

Fig. 1 shows a schematic representation of a vehicle 1 in a plan view. The vehicle 1 can be designed as an at least partially electrically driven vehicle, for example as an electric vehicle or as a hybrid vehicle. In addition, the vehicle 1 is designed as a passenger car. The vehicle 1 has a battery arrangement 2 that comprises a battery 3 and a cooling device 4 for cooling the battery 3. The battery 3 serves to supply an electrical drive motor 5 of the vehicle 1 with electrical energy.

Fig. 2 shows in a schematic representation the battery arrangement 2 with the battery 3 wherein the battery 3 comprises a plurality of battery cells C1 , C2, C3, C4, C5. In the present simplified embodiment, the battery 3 comprises five battery cells C1 to C5. The respective battery cells C1 to C5 comprise cell terminals 7, wherein in the present case only one terminal 7 is visible. The respective battery cells C1 to C5 can have a positive terminal and a negative terminal. The respective battery cells C1 to C5 are arranged in a row next to each other and in close contact. Between adjacent battery cells C1 to C5, a spacer 8 or gap pad is arranged.

In addition, the battery arrangement 2 comprises a battery management system 9, which is electrically connected via respective connecting lines 10, 1 1 to the terminals 7 of the battery cells C1 to C5. The battery management system 9 can be adapted to control an energy transfer to and from the individual battery cells C1 to C5. The electrical connections between the battery management system 9 and the battery cells C1 to C5 can enable individual battery cells C1 to C5 to transfer energy independently of the surrounding battery cells C1 to C5.

Furthermore, the battery arrangement 2 includes sensors 12, wherein in the present example one sensor 12 is associated with each battery cell C1 to C5. By means of the sensors 12 a parameter can be detected that describes an operating state of the respective battery cell C1 to C5. The parameter can be a pressure, a voltage and/or a current. The parameter can be the temperature of the battery cells C1 to C5. The battery management system 9 is connected with the sensors 12 for data transfer by means of respective connecting lines 6. In this way, the parameters of the respective battery cells C1 to C5 can be received by means of the battery management system 9 and compared with a predetermined threshold. Based on the comparison of the parameter with the threshold it can be detected by means of the battery management system 9, if a self heating occurred at the respective battery cells C1 to C5. For example, the onset of self heating can be detected if the temperature of a battery cell C1 to C5 exceeds a temperature of approximately 100° C.

In the present example, the battery management system 9 detects a parameter that is outside of the normal range of values in the battery cell C1 . In this case the battery management system 9 directs the battery cells C2 to C5 that are located in a

predetermined surrounding area 13 of battery cell C1 to reduce their state of charge (SoC). In the present embodiment, the surrounding area 13 includes those battery cells C2 to C5 that are within a distance of two battery cells to battery cell C1 with the self heating. In the present example, the battery cells C2 to C5 are located in the

predetermined surrounding area 13. By reducing the state of charge of the battery cells C2 to C5 in the surrounding area 13 a thermal runaway event in a battery 3 can be effectively slowed. To reduce the state of charge, the electrical energy of battery cells C2 to C5 can be directed to non-powertrain related systems in the vehicle 1. For example, the energy can be directed to a cooling system, i.e. the cooling device 4 of the battery arrangement 2, or to a lighting device 14 of the vehicle 1 .

The battery management system 9 comprises a computing device 15 on which a computer program or an algorithm is executed to provide this SoC reduction control scheme. In addition, the battery management system 9 comprises a storage device 16. On this storage device 16 the thresholds for the parameters can be stored. Furthermore, information that describes the positions of the individual battery cells C1 to C5 in the battery 3 can be stored on the storage device 16.

Though the SoC reduction control scheme cannot eliminate the possibility of a thermal runaway event in the battery 3, it can effectively slow and/or prevent propagation by recognizing the onset of a thermal event and responding preemptively. Thus, a protocol would enable the driver and passengers of an electric vehicle 1 with a battery 3 in thermal runaway more time to escape the vehicle 1 and for emergency crews to arrive on the scene of an event.

List of reference signs vehicle

battery arrangement battery

cooling device

drive motor

connecting line

terminal

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battery management system, 1 1 connecting line

sensor

surrounding area lighting device

computing device storage device

to C5 battery cell