IMPROVEMENT OF SAFETY OF BATTERIES CONTAINING PARALLEL ARRAYS OF RECHARGEABLE LITHIUM CELLS

TECHNICAL FIELD

The present invention relates to a device for, and methods of, monitoring a battery to provide an early warning of the failure of a cell of the battery. BACKGROUND ART

In the operation of a typical rechargeable lithium battery, lithium metal migrates from the anode to the cathode during the use of the battery. As the battery is recharged, lithium metal is plated back onto the anode. However, this plating process is not necessarily uniform, and repeated discharge/recharge cycles may result in the formation of lithium peaks or dendrites on the anode. Further use of a cell having substantially non-uniform lithium metal distribution on the anode will eventually result in the development of an internal short circuit in the single cell. In the development of this short circuit, a high resistance path is most likely formed in the initial stages. However, as a dendrite continues to grow and approaches the cathode, the path resistance will drop until low enough to allow the passage of a large current which will result in rapid localized heating. The formation of an internal short circuit or the rapid heating of a cell or group of cells results in the failure of these batteries.

In batteries containing a single series of cells, the user normally is aware of a failure in a single cell, because the entire battery stops working entirely. The user thus discards the battery as soon as a single cell fails. However, if a battery involves two or more cells connected in parallel, or two or more series strings of cells which are connected in parallel, then the user may not be aware of the failure of an individual cell and may continue to use the battery. This results because, although the failure of one of the

cells in one series stack may result in the failure of that stack, the remainder of the parallel stacks will continue to function normally. On continued use of the battery, the failed cell will become overheated and may burn, which can result in the rupture of the battery casing, an obviously undesirable consequence.

Previous attempts to protect against battery failures of this type have proven to be unreliable. One approach is to incorporate a thermal fuse to protect against the overheating of the battery. This approach, however, is only effective in the fortuitous instance where the failed cell is located adjacent the thermal fuse so that the heat generated can activate the fuse. Where this is not the case, the overheating in the area around the failed cell may be so localized as not to be detectable by a thermal fuse remote from the failed cell. In another approach, an electrical fuse is incorporated into a battery circuit to guard against the high currents indicative of a failing cell. This method is unreliable since a parallel-stack battery may be experiencing normal, design currents even though a cell therein has already failed. Thus, there still exists a need for a method and device which can reliably detect the impending failure of a cell in a parallel or series/parallel configured battery and protect against the consequences of an undetected failure. In particular, there are needs for methods and devices which can provide this protection at low cost. DISCLOSURE OF INVENTION One aspect of the present invention provides a circuit for monitoring a rechargeable battery including a plurality of stacks connected in parallel. Each of the plurality of parallel connected stacks have first and second ends and include either plural cells connected in series or one cell. The circuit is characterized by means for sensing a difference in an electrical condition between two of the plurality of parallel connected stacks in the battery to be

monitored, and means for providing a fault signal when the difference in the electrical condition deviates from a predetermined standard by more than a preselected threshold. The difference-sensing means may include voltage difference means for detecting a difference in voltage between a first point in one of the stacks and a second point in another stack parallel thereto, wherein the first and second points are intermediate the ends of the stacks. In this arrangement, the fault signal is provided when the difference in voltage deviates from a predetermined standard by more than a preselected threshold.

In another arrangement, the difference-sensing means may include current difference means for detecting a difference in current between one stack and another stack parallel thereto. The fault signal may be provided when the difference in current deviates from a predetermined standard by more than a preselected threshold.

Another aspect of the present invention provides an apparatus including a rechargeable lithium battery which includes a plurality of stacks connected in parallel. Each of the plurality of parallel connected stacks have a set of cells, each set of cells including either plural cells connected in series or one cell. The _. apparatus is characterized by means for detecting a difference in an electrical condition between the set of cells in one of the plurality of parallel connected stacks in the battery and the set of cells in another of the plurality of parallel connected stacks in the battery, and means for providing a fault signal when the difference in the electrical condition deviates from a predetermined standard by more than a preselected threshold.

Another aspect of the present invention provides a method of operating a rechargeable lithium battery including a plurality of stacks connected in

parallel. Each of the plurality of parallel connected stacks have a set of cells including either plural cells connected in series or one cell. The method is characterized by repeatedly charging and discharging the battery in a plurality of cycles each including charging and discharging portions while monitoring the battery so as to detect a difference in an electrical condition between the set of cells in one of the plurality of parallel connected stacks in the battery and the set of cells in another of the plurality of parallel connected stacks in the battery, and providing a fault signal when the difference in the electrical condition deviates from a predetermined standard by more than a preselected threshold. BRIEF DESCRIPTION OF THE FIGURES

FIGURE 1 is a block diagram showing a battery and a battery monitoring circuit according to one embodiment of the invention.

FIGURES 2 and 3 are block diagrams similar to Fig. 1 but depicting additional embodiments of the invention.

FIGURE 4 is a schematic view of typical components used in the circuit of Fig. 1.

FIGURE 5 is a graph depicting a typical difference in charging current and battery capacity on the repeated charging and discharging of a series/parallel configured battery. BEST MODE OF CARRYING OUT INVENTION

FIGURE 1 shows a series/parallel configured battery 5. When used in this application, the term "series/parallel configured battery* refers to a battery in which a plurality of cells are connected in series to form a stack, and where the stacks are then connected in parallel to one another. Thus, battery stacks 10 and 12 are connected in parallel and each stack consists of an equal number of series-connected cells. As shown in FIGURE 1, stack 10 consists of four identical series- connected cells identified as 14a, 14b, 14c and 14d.

Each cell 14 includes a lithium-bearing electrode or "anode" such as a mass of a lithium-containing metal alloy which may be in the form of a sheet or foil. Each cell further includes a counterelectrode or "cathode" adapted to take up lithium and an electrolyte including a lithium-containing substance dissolved in a non- aqueous solvent. Cells of this type are referred to herein as "lithium cells". During discharge of a lithium cell, lithium from the lithium-bearing electrode dissolves in the electrolyte and passes to the counterelectrode where the lithium is taken up with a corresponding release of electrical energy. When the cell configuration and the composition of the counterelectrode permit, the process can be reversed and the cell can be recharged by applying external electrical energy. During recharge, lithium leaves the counterelectrode and redeposits on the lithium-bearing electrode. Where the lithium-bearing electrode is a lithium-containing metal alloy, the redeposited lithium is plated onto the lithium-bearing electrode during recharge. Cells capable of repeated charging and discharging are referred to herein as "secondary" cells. Typically, each cell is enclosed in a sealed casing. Stack 12 is connected in parallel with stack 10 and consists of series-connected cells 16a, 16b, 16c and 16d, identical to cells 14.

The individual cells in stack 10 are electrically connected to one another by conductors such as wires, straps or electrically conductive components of the cell casings. Thus, conductor 15a is connected between the cathode of cell 14a and the anode of cell 14b. Similarly, conductor 15b is connected between the cathode of cell 14b and the anode of cell 14c, and conductor 15c is connected between the cathode of cell 14c and the anode of cell 14d. A further conductor 19 at a first or positive end of the stack connects the anode of cell 14a to the positive output conductor 30 of the battery at node 18. A conductor 21 at the second or

negative end of stack 10 connects the cathode of cell 14d to the negative battery output conductor 32 at node 22.

In like manner, cells 16a, 16b, 16c and 16d of stack 12 are connected in series by conductors 17a, 17b and 17c. Additionally, conductor 25 at the positive end of stack 12 connects the anode of cell 16a to positive battery output conductor 30 at node 24, and conductor 27 at the negative end of stack 12 connects the cathode of cell 16d to the negative battery output conductor 32 at node 28.

The monitoring circuit includes a voltage difference detector 40 having a pair of input terminals 42 and 44 and an output terminal 46. Detector 40 is arranged to provide an output voltage at terminal 46 having a predetermined polarity and having a magnitude directly related to the absolute value of the difference between the voltages at input terminals 42 and 44. Detector 40 may incorporate a first amplifier 172 (FIG. 4) having an inverting input connected to input terminal 44, and a non-inverting input connected to input terminal 42. Amplifier 172 has a ground connection 173 connected to the negative output conductor 32 of the battery and a reference or power- supply terminal connected to a power supply bus 175 which in turn is connected to the positive output conductor 30 of the battery. Detector 40 also includes a second amplifier 174. Amplifier 174 is identical to amplifier 172 except that the inverting input of amplifier 174 is connected to input terminal 42 of the detector, whereas the non-inverting input of amplifier 174 is connected to input terminal 44 of the detector. The outputs of amplifiers 172 and 174 are connected through diodes 176 and 178 and voltage- dividing resistors 180 and 182 to ground, i.e., to the negative conductor 32 of the battery. The output terminal 46 of the detector is connected between

resistors 180 and 182. In operation, amplifier 174 provides an output voltage relative to ground proportional to (V ₄₄ - V ₄₂ ) where V ₄₄ is the voltage at input terminal 44 and V ₄₂ is the voltage at input terminal 42. Amplifier 172 provides a voltage relative to ground proportional to (V ₄₂ - V ₄₄ ) . Whenever V ₄₄ and V ₄₂ are unequal, one of these amplifier output voltages will be positive and one negative with respect to ground. The negative voltage will be blocked by one of diodes 176 and 178, whereas the positive voltage will be delivered via dividing resistors 180 and 182 to output terminal 46. Alternatively, detector 40 may incorporate a standard absolute value circuit.

Voltage difference detector 40 is connected between parallel stacks 10 and 12. Thus, conductor 36 is connected to conductor 15b at a node 20 between cells 14b and 14σ of stack 10, and to input terminal 42 of voltage difference detector 40. Similarly, conductor 34 is connected to conductor 17b at a node 26 between cells 16b and 16c of stack 12, and at the other end to input terminal 44 of voltage difference detector 40.

Positive battery output conductor 30 is connected through a control circuit 50 to positive battery terminal 60. Negative battery output conductor 32 is connected directly to negative battery terminal 62. The output terminal 46 of voltage difference detector 40 is connected to an input node 52 of control circuit 50 via conductive path 65, which has included thereon switch 67. Switch 67 is automatically actuated by current direction detector 68, which in turn is connected between positive battery terminal 60 and control circuit 50. Detector 68 is arranged to sense the direction of the current through battery terminal 60 and transmit this information to switch 67 via conductive path 69. Detector 68 and switch 67 are arranged to render conductive path 65 an open circuit during discharge and to render conductive path 65 a

completed circuit during charge. As discussed further herein below, it is desirable to monitor the condition of the battery during either the charging cycle or the discharging cycle, but not both. Thus, in an alternate embodiment switch 67 and detector 68 may be rearranged so that switch 67 is open during charge and closed during discharge.

Control circuit 50 includes a normally closed "one shot" switch or single-actuation switching device which will go open circuit upon receiving an input voltage from voltage difference detector 40 in excess of a predetermined actuation voltage. When switching circuit 50 goes to an open condition, it will electrically disable the battery. In the embodiment as shown in FIGURE 1, the total voltage between point 18 at one end of stack 1 and point 22 at the opposite end thereof will be essentially equal to the total voltage between the respective points 24 and 28 on stack 12. In a stack in which all of the cells are functioning normally, this total voltage will be evenly distributed across all of the individual cells. Thus, the intermediate voltage between points 18 and 20 in a properly functioning stack is expected to be of the same proportion to the total voltage as is the proportion of the number of cells between points 18 and 20 to the total number of cells in stack 10. For example, as shown in FIGURE 1, the intermediate voltage between points 18 and 20 will be essentially one half of the total voltage as point 20 is located at the midpoint of stack 10. Stated another way, the number of cells between point 20 and the negative end of stack 10 is equal to the number of cells between point 20 and the positive end of the stack. However, in a stack containing a cell which is about to fail, the total voltage will no longer be evenly distributed across all of the cells of the stack. Therefore, the intermediate voltage between points 18 and 20 may be either higher or lower than expected

depending on the location of the failing cell..

The same holds true for the voltage across the cells of stack 12. Thus, if a cell in stack 12 is about to fail, the intermediate voltage between points 24 and 26 will be either higher or lower than expected depending on the location of the failing cell. Accordingly, when the cells of stacks 10 and 12 are functioning normally, the difference between the intermediate voltage appearing at points 20 and 26 will be substantially constant. As points 20 and 26 are at the midpoints of their respective stacks, as shown in FIGURE 1, the difference between the intermediate voltage at point 20 for stack 10 and the intermediate voltage at point 26 for stack 12 will be essentially zero. This is so regardless of variation in the actual values of the individual cell voltages, provided that all of the cells vary in substantially the same way.

This same result will occur even when points 20 and 26 are not at the midpoints of their respective stacks, but are located at another position on their respective stacks wherein that position is relatively the same from one stack to the next. In other words, if point 20 were located between cells 14a and 14b of stack 10 and point 26 were located between cell 16a and 16b of stack 12, the expected difference between the intermediate voltage at point 20 for stack 10 and the intermediate voltage at point 26 for stack 12 will be essentially zero. The expected difference between the intermediate voltage at point 20 for stack 10 and the intermediate voltage at point 26 for stack 12 will also be essentially zero when point 20 is located between cells 14c and 14d, and point 26 is located between cells 16c and 16d.

Voltage difference detector 40 will continuously monitor the difference between the intermediate voltage at point 20 for stack 10 and the intermediate voltage at point 26 for stack 12. The output voltage delivered at output 46 of the detector

and supplied to input node 52 of control circuit 50 will be directly related to the absolute value of the difference between the intermediate voltages. As long as the difference between the intermediate voltages at points 20 and 26 is within a specified threshold value from zero, the output voltage from voltage difference detector 40 will be below the predetermined activation voltage of circuit 50. However, if the voltage difference exceeds this preselected threshold, the output signal from voltage difference detector 40 will increase to above the actuation voltage of control circuit 50 which will then open to electrically disable the battery 5. Thus, the increased output voltage from detector 40 serves as a fault signal, and circuit 50 responds to this fault signal by disabling the battery.

FIGURE 2 shows another embodiment in accordance with the present invention. This embodiment is the same as that in FIGURE 1 except that conductors 34' and 36' are not connected at the midpoints of the respective stacks, but are at different positions on the stacks relative to one another. Thus, conductor 36' is connected between input terminal 42' on voltage difference detector 40' and node 20' between cells 14c' and 14d' of stack 10'. Conductor 34', on the other hand, is connected at one end to input terminal 44' of voltage difference detector 40' and at the opposite end to node 26' between cells 16a' and 16b' of stack 12'.

Accordingly, when the cells of stacks 10' and 12' are functioning normally, the difference between the stack 10' intermediate voltage at point 20' and the stack 12' intermediate voltage at point 26' will be substantially constant. Because points 20' and 26' are not in the same relative position on their respective stacks, this substantially constant difference between the stack 10' intermediate voltage at point 20' and the stack 12' intermediate voltage at point 26' will not be zero but instead will be substantially equal to a

predetermined non-zero standard value. The magnitude of this standard value will depend on the relative difference in position of points 20' and 26' on the respective stacks. Voltage difference detector 40' is similar to detector 40; however, detector 40' is modified to deliver a low output signal so long as the difference in the intermediate voltages is equal to the standard within the preselected threshold, and to deliver a high output voltage or fault signal whenever the difference in the intermediate voltages deviates from the standard by more than the preselected threshold.

Again, voltage difference detector 40' continuously monitors the difference between the intermediate voltage at point 20' for stack 10' and the intermediate voltage at point 26' for stack 12', and the output voltage delivered at output terminal 46' and supplied to input node 52' of control circuit 50' will be directly related to the absolute value of this difference. As long as the difference between the intermediate voltages at points 20' and 26' is within a specified threshold value from the predetermined non-zero standard, the output voltage from the voltage difference detector 40' will be below the predetermined activation voltage of circuit 50'. However, if the voltage difference exceeds this preselected threshold, the output signal from voltage difference detector 40' will increase to above the activation voltage of control circuit 50'and serve as a fault signal. If this occurs during a portion of the charge/discharge cycle when switch 67' is closed, the fault signal will be transmitted to control circuit 50'. Control circuit 50' will respond to this fault signal by opening to electrically disable the battery 5'. FIGURE 3 shows yet another embodiment in accordance with the present invention as applied to the series/parallel configured battery 5' ' which is the same as batteries 5 and 5' described above.

The individual cells in a stack are connected in series to one another by conductors, as described more fully above. However, in this embodiment, the anode of cell 14a'' of stack 10" is not connected directly to positive battery output conductor 30'', but has included therebetween current detector 70. Current detector 70 is a simple device which converts the current through stack 10" into a corresponding voltage by utilizing a conductor 71 with low but known resistance and a difference amplifier 73 having inverting and non-inverting inputs connected to the opposite ends of low-resistance conductor 71. Conductor 76 is connected between the anode of cell 14a'' and terminal 72 at one end of low-resistance conductor 71. Conductor 78 is connected between terminal 74 at the other end of low-resistance conductor 71 and the positive battery output conductor 30" at node 18".

Similarly, positioned between the anode of cell 16a'' of stack 12" and the positive battery output conductor 30'' is current detector 80 which is constructed similarly to current detector 70 to convert the current through stack 12" to a corresponding voltage. Thus, current detector 80 includes conductor 81 which has a low but known resistance and a difference amplifier 83 having inverting and non-inverting inputs connected to the opposite ends of low resistance conductor 81. Conductor 86 is connected between the anode of cell 16a' ' and terminal 82 at one end of low resistance conductor 81, and conductor 88 is connected between terminal 84 at the other end of low resistance conductor 81 and the positive battery output conductor 30" at node 24".

The monitoring circuit includes voltage difference detector 40" which is connected between the respective outputs of difference amplifiers 73 and 83 which carry the voltages corresponding to the respective currents through stacks 10" and 12". Therefore,

output terminal 79 of difference amplifier 73 is connected to input terminal 42'' of voltage difference detector 40" by conductor 90, and output terminal 89 of difference amplifier 83 is connected to input terminal 44" of voltage difference detector 40'' by conductor 92.

Again, positive battery output conductor 30'' is connected through a control circuit 50'' to positive battery terminal 60'', and negative battery output conductor 32" is connected directly to negative battery terminal 62". The output terminal 46" of voltage difference detector 40'' is connected to input node 52" of control circuit 50'' via conductive path 65".

In the embodiment as shown in FIGURE 3, the current through stack 10" as detected by current detector 70 will be essentially equal to the current in stack 12" as detected by current detector 80 when all of the cells in both stacks are functioning normally. When this is the case, the difference between the current through stack 10'' and the current through stack 12'', as well as the difference between the corresponding voltages generated at terminals 42" and 44'', will be essentially zero. However, when one of the cells in either stack is about to fail, the resistance of that stack will increase resulting in a proportional decrease in current through the stack and a subsequent and proportional decrease in the corresponding voltage generated at the respective input terminal of the voltage difference detector 40''. Thus, once the current in stacks 10" and 12" are converted to corresponding voltages which are input at terminals 42" and 44" of voltage difference detector 40'', this embodiment works the same as that previously described with reference to FIGURE 1. Current detectors 70 and 80 will continuously monitor the currents through stacks 10" and 12" and convert them into corresponding voltages to be input into voltage difference detector 40" at input terminals 42''

and 44", respectively. The output voltage delivered at output terminal 46'' of the detector and supplied to input node 52'' of control circuit 50'' will be directly related to the absolute value of the difference between the corresponding voltages generated at input terminals 42" and 44''. As long as the difference between the input voltages at terminals 42" and 44" is within a specified threshold value from zero, the output voltage from voltage difference detector 40'' will be below the predetermined actuation voltage of circuit 50''. However, if the voltage difference exceeds this preselected threshold, the output signal from the voltage difference detector 40'' will increase to above the actuation voltage of control circuit 50'', thereby serving as a fault signal. During charge, when switch 67" is closed, the fault signal will be transmitted to control circuit 50", and control circuit 50" will respond by opening to electrically disable battery 5 ' ' . The threshold value which must be surpassed in order to actuate control circuits 50, 50' and 50'' must be carefully chosen to reliably disable the battery upon sensing the impending failure of a cell or group of cells, but not under normal operating conditions. Thus, if the threshold value is set too low, the battery will be disabled upon experiencing normal fluctuations in either charging or discharging voltages or currents. However, if the threshold value is set too high, the battery may fail and rupture before being disabled. FIGURE 5 shows the effect that a failing cell will typically have on the difference in current through parallel stacks of cells when monitoring is performed during the charging cycle. Thus, the current difference normally stays at a relatively low but fluctuating level through a substantial number of charge/discharge cycles as the capacity of the battery slowly lessens. As a cell or group of cells in one of the parallel stacks is about to fail there typically is a dramatic increase in

the charging current difference, yet the battery still has a substantial capacity so that the impending failure of the cell will be unknown to the user. Continued charging and discharging of the battery beyond this point will cause the failing cell to become overheated which may cause it to burn, possibly resulting in the rupture of the battery casing.

By selection of an appropriate threshold value, the battery will continue to operate during small fluctuations in charging current difference, but will be disabled when a dramatic increase in charging current difference is detected, well before the consequences of a failed cell are realized. Thus, as shown in FIGURE 5, the selection of the threshold value as shown will disable the battery after about 30 charge/discharge cycles, even though the battery may have continued to function for well over 40 charge/discharge cycles. Therefore, the proper selection of the threshold value provides a substantial margin of protection before the cell fails completely. Typical threshold values are approximately 0.1-0.3 Amps difference in stack current or approximately 1.5 Volts difference in intermediate voltage when monitoring is done during the charging cycle, and 0.6-0.8 Amps difference in stack current or approximately 2.0 Volts difference in intermediate voltage when monitoring is done during the discharge cycle. Because the charging and discharging cycles require different threshold values, it is desirable to monitor the condition of the battery during only one of these cycles. In the embodiments discussed above, switches 67, 67' and 67" and the associated current direction detectors effectively disable the monitoring system and the battery disabling control circuit 50, 50' or 50'' during the discharge portion of each cycle. These components can be rearranged to disable the monitoring and control system during charge and leave it in operation during discharge. Alternatively, however, the monitoring circuit may be arranged to respond to a

signal from the current direction sensing device and automatically adjust the threshold to the appropriate value for that portion of the charge/discharge cycle which the battery is experiencing. While the figures in this Application show each stack to consist of four series-connected cells, a stack may consist of any practical number of cells either less than or more than the number shown. For the embodiments shown in FIGURES 1 and 2, each stack must consist of at least two cells connected in series so that an intermediate voltage between the cells may be monitored. However, any number of cells greater than two may also be incorporated into a stack and the intermediate voltage monitored at a point between any of the cells. In the embodiment shown in FIGURE 3, only one cell is necessary in each stack in order to determine the current through the stack, but any number of cells greater than one may also be monitored.

Additionally, while the foregoing descriptions of the preferred embodiments refer to only two stacks of cells in parallel, it is apparent that any number of parallel stacks may be monitored in accordance with the present invention, wherein the battery will be disabled when a cell or group of cells in any one of the parallel stacks is about to fail. Further, it should be noted that it is not necessary that each stack consist of an equal number of series-connected cells so long as the total voltage from one end of the stack to the opposite end is the same for all the stacks in parallel connection.

INDUSTRIAL APPLICABILITY

The circuit and method of the present invention provide for monitoring a rechargeable battery to provide an early warning of the failure of a cell of the battery.