Method for opportunistic balancing of charge between battery cells

Abstract

A method for balancing charge between battery cells, comprising: Operating a first battery cell at a first voltage and a second battery cell at a second voltage, wherein the first and second battery cells both operate in a first charge range; and Balancing the charge between the first battery cell and the second battery cell in a second charge range different from the first charge range, wherein the balancing of the charge is responsive to a voltage difference between the first voltage and the second voltage measured in the first charge range.

Description

TECHNICAL FIELD

[0001] This description relates to balancing charge between battery cells of a battery pack. In one example, the charge of battery cells is opportunistically balanced in response to a battery cell output voltage. BACKGROUND AND SUMMARY

[0002] Battery packs can be configured from multiple battery cells. Adding battery cells in series can increase the battery pack voltage, while adding battery cells in parallel can increase the ampere-hour rating of a battery pack. However, the amounts of charge stored in battery cells arranged in series can vary due to manufacturing variation. If the charge is allowed to vary from battery cell to battery cell, it is possible that less than the total battery pack capacity will be available to a battery pack load. For example, if one battery cell stores more charge than another, the charging and discharging of the battery cell may be dictated by the lower capacity battery cell, as it is more likely than a higher capacity battery cell to reach a battery cell voltage threshold during a battery cell charge or discharge.

[0003] US 2010 / 0 085 009 A1 discloses an apparatus and method for balancing cells using a voltage change pattern of a battery cell, wherein the voltage of each cell is measured, the OCV (Open Circuit Voltage) or SOC (State of Charge) of each cell is estimated using a voltage change pattern of each cell, including a current voltage and a past voltage, and a deviation in OCV or SOC between cells is eliminated by comparing the estimated OCV or SOC of each cell. In estimating the OCV of each cell, an output voltage error due to an IR drop is corrected. In this way, the SOC of each cell can be accurately estimated. Accurate estimation of the SOC can lead to a significant elimination of the SOC deviation of each cell.In addition, SOC estimation using an output voltage results in active cell balancing even during battery charging and discharging, minimizing SOC deviation of each cell.

[0004] US 2009 / 0096420 A1 discloses a monitoring circuit that monitors a cell voltage of multiple cells. The predetermined equalization time is set to a value proportional to the difference between the maximum voltage of a first cell and the minimum voltage of a second cell when the difference between the maximum and minimum voltage of the cells is greater than a predetermined threshold.

[0005] US 2010 / 0 194 339 A1 discloses a method for determining whether a maximum voltage among the voltages of a group of rechargeable battery cells is less than a reference voltage, where the reference voltage is between approximately 2 volts and 3.7 volts. The equalization time is calculated for one of the battery cells. It is determined whether a residual capacity difference or a voltage difference between two of the battery cells exceeds a reference value. The battery cells are equalized toward the end of discharge. The full charge capacities of the battery cells are calculated.

[0006] US 7,723,955 B2 discloses the electrical series connection of N lithium cells, where N is an integer greater than one. A balancing circuit is provided for each adjacent pair of lithium cells. This balances the charge stored in each cell so that the charge stored in one cell equals the charge stored in a second cell and the charge stored in each cell remains above a preset threshold. A voltage monitoring module uses a low-on-resistance differential multiplexer to select each cell and provide the associated voltage. A temperature monitoring module receives a signal indicative of a temperature associated with a battery pack.

[0007] Lithium-ion battery cells offer the ability to store relatively large amounts of charge in a small package. However, lithium-ion battery cells exhibit an open-circuit voltage profile that can make it difficult to determine when it is desirable to balance charge between battery cells. The open-circuit voltage profile of lithium-ion battery cells can further complicate determining how much charge is being removed from or added to a battery cell when the battery cell's open-circuit voltage is used to determine when battery cells are in a state of charge imbalance.

[0008] The present inventors have recognized the above problems and developed a strategy to overcome them.Specifically, the inventors have developed a method for balancing charge between battery cells, comprising: sensing an amount of charge stored in a first battery cell and a second battery cell at a first time; sinking or sourcing an amount of charge from the first battery cell and the second battery cell after sensing the amount of charge stored in the first battery cell and the second battery cell; and balancing the charge of a first battery cell or a second battery cell after drawing or sourcing the amount of charge from the first battery cell and the second battery cell in an amount of charge associated with a difference between the amount of charge stored in the first battery cell and the amount of charge stored in the second battery cell at the first time.

[0009] By determining whether a charge imbalance exists between battery cells at a first time point in time, where determining a battery cell's charge may be more reliable, and balancing the charge at a second time point in time, where determining a battery cell's charge may be less reliable, charge between battery cells can be balanced more accurately, allowing more of the battery's charge capacity to be used. For example, at lower battery voltages, it may be possible to more accurately determine a charge difference between battery cells because the battery voltage changes with respect to battery charge at a faster rate than at mid-range battery voltage ranges. However, it may be undesirable to balance charge between battery cells at lower voltages because some battery cells may be at lower voltage thresholds.However, after charging the battery cells, a charge difference determined at a lower battery charge value can be used at a higher battery charge value so that charge can be removed from or added to the battery cells without driving the battery cells to charge or discharge limits.

[0010] The above problems are solved by methods having the features of claims 1 and 11. Advantageous embodiments are the subject of the subclaims.

[0011] The present description can offer several advantages. Specifically, the strategy can improve battery cell charge balancing by providing improved estimates of the charge to be removed from or added to battery cells. Furthermore, the strategy provides accurate charge balancing without the cost of high-resolution voltage measurements.

[0012] The foregoing advantages and other advantages and features of the present description will be readily apparent from the following detailed description taken alone or in conjunction with the accompanying drawings.

[0013] It should be understood that the foregoing summary is provided to introduce, in a simplified form, a selection of concepts that are further described in the detailed description. It is not intended to identify key or essential features of the claimed subject matter, the scope of which is determined solely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that address any disadvantages noted above or in any part of this disclosure. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a schematic exploded view of a battery pack or battery assembly; Fig. 2 shows a schematic view of an exemplary battery module; Fig. 3 shows a schematic exploded view of an exemplary battery cell stack; Fig. 4 shows a schematic diagram of an exemplary battery charge equalization circuit; Fig. Figure 5A shows a prophetic example of battery cell charge curves for a new battery cell; Fig. Figure 5B shows a prophetic example of battery cell charge curves for an aged battery cell; Fig. Figure 6A shows a prophetic example of a charge difference between two battery cells at a lower battery voltage range; Fig. Figure 6B shows a prophetic example of a charge difference between two battery cells at a mid-battery voltage range; Fig. 7 and Fig. 8 shows a flow diagram of a method for balancing the charge of battery cells; and Fig. 9 shows a flowchart of a method for balancing battery cells based on a charge difference between battery cells. DETAILED DESCRIPTION

[0014] This description relates to balancing charge between battery cells of a battery pack. In one example, the battery cells may be in a Fig. 1 shown battery pack. Battery cells such as those shown in Fig. 2-3 shown, can be used as in Fig. 1. Charge differences between battery cells can be compensated by the voltages shown in the circuit diagram of Fig. 4 shown simplified circuit can be reduced or eliminated. Fig. 5A and Fig. 5B show battery cell voltage characteristics for new and aged battery cells. The method of Fig. 6 allows to detect when a resting voltage provides more information and to use the information during charge balancing between battery cells.

[0015] Fig. 1 shows an exploded view of a battery assembly 1. The battery assembly may include a cover 10, coupling devices 12, a first cooling subsystem 14 (e.g., cold plate), a plurality of battery cell modules 16, a second cooling subsystem 18 (e.g., cold plate), and a tray 20. The cover may be attached to the tray using a suitable connecting device (e.g., bolts, adhesive, etc.) to form a housing surrounding the connecting devices, the cooling subsystems, and the battery modules in the assembled state.

[0016] The battery cell modules 16 may include multiple battery cells configured to store energy. Although multiple battery modules are shown, it should be understood that a single battery module may be used in other examples. The battery cell modules 16 may be placed between the first cooling subsystem 14 and the second cooling subsystem 18, with the battery modules positioned so that their electrical connections on a side 21 between the cooling subsystems face outward.

[0017] Each battery module may include a first side 23 and a second side 25. The first and second sides may be referred to as the upper and lower sides, respectively. The upper and lower sides may flank the electrical terminals, which may be described with reference to Fig. 2-3 are explained in more detail herein. In this example, the top side of each battery module in the battery assembly is positioned in a common plane. Similarly, the bottom side of each battery module in the battery assembly is positioned in another common plane. In other examples, however, only the top side or the bottom side of each battery module may be positioned in a common plane. In this way, the cooling subsystems may maintain direct contact with the top sides and bottom sides of the battery modules to increase heat transfer and improve cooling performance, as described in more detail herein, wherein the cooling subsystems and the battery modules may be in surface-sharing contact. Further details of an exemplary battery module are described herein with reference to Fig. 2-3. In other examples, only one of the cooling subsystems may be included in the battery assembly 1, such as an upper cooling subsystem (subsystem 14 in this example). Furthermore, the position, size, and geometry of the first and second cooling subsystems are exemplary in nature. Thus, based on various design parameters of the battery assembly, in other examples, the position, size, and / or geometry of the first and / or second cooling subsystems may be changed.

[0018] The battery assembly 1 may also include an electrical distribution module 33 (EDM), monitoring and balancing boards 35 (MBB), and a battery control module 37 (BCM). The voltage of battery cells in the battery cell modules 16 may be monitored and balanced by MBBs integrated on the battery cell modules 16. Battery cell balancing refers to equalizing electrical voltages between multiple battery cells in a battery cell stack. Furthermore, battery cell voltages may be balanced between battery cell stacks. MBBs may include multiple current, voltage, and other sensors. The EDM controls the distribution of energy from the battery pack to the battery load. In particular, the EDM includes contactors for connecting high-voltage battery energy to an external battery load, such as an inverter. The BCM enables supervisory control of battery pack systems.For example, the BCM can control additional modules in the battery pack, such as the EDM and the cell MBB. The BCM can consist of a microcontroller with memory, read-only memory, input ports, a real-time clock, output ports, and a Controller Area Network (CAN) port for communicating with systems outside the battery pack, as well as with MBBs and other battery pack modules.

[0019] Fig. 2 shows an exemplary battery module 200 that is used in the Fig. 1. The battery module 200 may include a battery cell stack with a plurality of stacked battery cells and output terminals 201. The stacking arrangement enables the battery cells to be tightly packed within the battery module.

[0020] Fig. 3 shows an exploded view of a portion of an exemplary battery cell stack 300. As shown, the battery cell stack is constructed sequentially from a housing heat sink 310, a battery cell 312, a compliant pad 314, a battery cell 316, etc. However, it should be understood that other arrangements are possible. For example, the battery cell stack may be constructed sequentially from a housing heat sink, a battery cell, a housing heat sink, etc. Furthermore, in some examples, the housing heat sink may be integrated with the battery cells.

[0021] The battery cell 312 includes a cathode 318 and an anode 320 for connection to a bus bar (not shown). The bus bar conducts charge from one battery cell to another. A battery module can be configured with battery cells connected in series and / or parallel. Bus bars connect battery cell terminals when the battery cells are combined in parallel. For example, the positive terminal of a first battery cell is connected to the positive terminal of a second battery cell to combine the battery cells in parallel. Bus bars also connect a positive and negative terminal of battery cell terminals when it is desired to increase the electrical voltage of a battery module. The battery cell 312 further includes a prismatic cell 324 containing electrolyte compounds. The prismatic cell 324 is in thermal communication with the cell heat sink 326.The cell heat sink 326 may be formed from a metal plate with the edges on one or more sides bent upwardly by 90 degrees to form a flanged edge. In the example of FIG. Fig. 3, two opposite sides include a flanged edge. However, other geometries are possible. Battery cell 312 is essentially identical to battery cell 316. Therefore, similar parts are referred to accordingly. Battery cells 312 and 316 are arranged so that their terminals are aligned and exposed. In the Fig. In the battery module 200 shown in Figure 2, the electrical connections are connected to allow energy to be drawn from each cell in the battery module. Back to Fig. 3, a compliant pad 314 is placed between the battery cell 312 and the battery cell 316. In other examples, however, the compliant pad need not be included in the battery cell stack.

[0022] The housing heat sink 310 may be formed from a metal plate having a base plate 328 with edges that are bent upwardly by 90 degrees on one or more sides to form a flanged edge. Fig. 3, a longitudinally oriented edge 330 and vertically oriented edges 332 are curved flanged edges. As illustrated, the housing heat sink is sized to accommodate one or more battery cells. In other words, one or more battery cells can be positioned within the base plate 328. Thus, the flanged edges of the battery cells can be in contact with the housing heat sink, and the bottom surface 329 of the battery cell 312 can be in contact with the bottom of the housing heat sink, facilitating heat transfer.

[0023] One of the longitudinally aligned edges 332 of the housing heat sink 310 may form part of the upper side 202 of the battery module 200, as shown in Fig. 2. Similarly, one of the longitudinally aligned edges 332 may form part of the bottom side of the battery module. Thus, the longitudinally aligned edges of the housing heat sink may contact the first and second cooling subsystems to enhance heat transfer. In this way, heat may be transferred from the battery cells to the exterior of the battery module.

[0024] The battery cells may be bound together by tie bands 204 and 205. The tie bands may be wrapped around the battery cell stack or may simply extend from the front of the battery cell stack to the back of the battery cell stack. In the latter example, the tie bands may be connected to a battery cover. In other examples, the tie bands may consist of threaded pins (e.g., metal threaded pins) screwed into the ends. Furthermore, various other strategies may be used to tie the cells together into the stack. For example, threaded rods connected to end plates may be used to provide the desired compression. In another example, the cells may be stacked in a rigid frame with a plate at one end that could slide back and forth against the cells to provide the desired compressive force.In still other examples, bars held in place by crank keys may be used to secure the battery cells. Thus, it is understood that various binding mechanisms may be used to hold the cell stack together, and the application is not limited to metal or plastic straps. A cover 206 provides protection for battery bus bars (not shown) that conduct charge from the multiple battery cells to output terminals of the battery module.

[0025] The battery module may also include a front end cover 208 and a rear end cover 210 connected to the battery cell stack. The front and rear end covers include module openings 26. However, in other examples, the module openings may be included in a portion of the battery module containing the battery cells.

[0026] With reference now to Fig. Figure 4 shows a circuit diagram of an exemplary circuit for controlling battery cell balancing. The high potential sides of battery cells 1-3 are shown at 420-424. Since the battery cells are connected in series, the low potential side of one battery cell is connected to the higher potential of another battery cell. Battery cell balancing can be initiated by a microcontroller on the MBB. A simplified microcontroller output circuit for balancing battery cells is shown at 400-404.

[0027] The battery cell balancing circuit shown for battery cells 1-3 is essentially the same between battery balancing circuits, with the exception of the transistor bias resistors for the FET transistors that connect load resistors in parallel with the battery cells. The bias resistors are adjusted to compensate for the changing potential for each additional battery cell in the series of battery cells.

[0028] The charge of battery cell No. 2 is reduced when FET 412 is activated (e.g., closed), thereby coupling load resistor 414 in parallel with battery cell No. 2. FET 412 is activated when FET 410 conducts from a signal triggered by microcontroller signal input 402. When FET 410 conducts, one side of resistor 406 is referenced to ground, while the other side is coupled to resistor 408, which in turn is coupled to the high potential side of battery cell No. 2. Thus, the voltage developed between resistors 406 and 408, which is applied to the gate of FET 412, is related to the voltage of battery cell No. 1 with respect to ground and the ratio of resistors 406 and 408. FETs 426 and 428m, which balance battery cells Nos. 1 and 3, are the same as FET 412. Since the potential of battery cell 3 with respect to ground is different from that of battery cell No.2 increases, the bias resistors coupled to the gate of FET 426 are adjusted to ensure that FET 426 switches when the microcontroller circuit input 400 changes state. Similarly, the potential of battery cell 1 with respect to ground decreases from that of battery cell No. 2, and the bias resistors 416 and 418 coupled to the gate of FET 428 are adjusted to ensure that FET 428 switches when the microcontroller circuit input 404 changes state.

[0029] With reference now to Fig. Figure 5A shows a prophetic example of charge curves for a battery cell with full charge capacity. The curves shown can be used for the Fig. The system shown in 1-4 may be applicable.

[0030] The representation in Fig. Figure 5A has a Y-axis representing the battery cell open-circuit voltage, and the voltage increases in the direction of the Y-axis arrow. The X-axis represents the battery cell's percentage state of charge (SOC) (e.g., the percentage of the total rated charge the battery cell can store), and the percentage state of charge increases in the direction of the X-axis arrow. For example, a value of 10 along the X-axis represents 10% of the full charge a new battery cell is rated to store. A value of 100 indicates that the battery cell has the full charge capacity of a new battery.

[0031] Curves 502-522 are curves showing battery cell voltage versus battery cell state of charge percentage for various battery cell temperatures. Specifically, curve 502 is for a battery cell temperature of -15°C, curve 504 is for a battery cell temperature of -10°C, curve 506 is for a battery cell temperature of -5°C, curve 508 is for a battery cell temperature of 0°C, curve 510 is for a battery cell temperature of 5°C, curve 512 is for a battery cell temperature of 10°C, curve 514 is for a battery cell temperature of 15°C, curve 516 is for a battery cell temperature of 20°C, curve 518 is for a battery cell temperature of 25°C, curve 520 is for a battery cell temperature of 30°C, and curve 522 is for a battery cell temperature of 35°C.

[0032] Lines 550 and 552 denote boundaries of three battery cell charge ranges. The lower voltage charge range (e.g., range 1) lies from the left of line 550 to the Y-axis. The middle voltage charge range (e.g., range 2) lies between lines 550 and 552. The higher voltage charge range (e.g., range 3) lies to the right of line 552. The boundaries of the three battery cell charge ranges can vary depending on the measurement error when determining the battery cell resting voltage. For example, if the system has a voltage measurement error of 50 mV, a first set of battery cell charge ranges can be selected. If the system has a voltage measurement error of 100 mV, a second set of battery cell charge ranges, different from the first set of battery cell charge ranges, can be selected.It should be noted that voltage measurement errors during battery cell voltage measurement can come from one or more sources, such as ADC quantification errors and CPU rounding errors.

[0033] At the battery temperatures shown, it can be seen from curves 502-522 that the battery cell voltage increases with battery state of charge and battery temperature. Furthermore, in the mid-voltage range, the battery voltage changes little with increasing battery state of charge. For example, curve 508 changes little more than 0.1 volts between a 30 percent state of charge and a 95 percent state of charge. Thus, a small voltage change corresponds to a large percentage change in the charge stored in the battery cell. Therefore, in the mid-voltage range, an accurate and high-resolution voltage measurement is required to accurately determine a battery state of charge. In the lower voltage range, however, curve 508 changes by approximately 0.5 volts from 0 percent battery charge to 30 percent battery charge.Consequently, a lower-resolution battery cell resting voltage estimate allows for a more accurate estimate of a battery cell's state of charge. Similarly, in the higher voltage range, curve 508 changes by tenths of a volt from 95 to 100 percent of the battery's nominal charge value. Consequently, in the lower and higher battery cell voltage ranges, a more accurate battery state of charge can be estimated from the battery cell resting voltage.

[0034] In one example, a battery state of charge may be estimated in the lower or higher battery cell voltage range. As the battery is charged or discharged, it enters the mid-voltage range, where the battery state of charge may be revised in response to an integrated coulomb number estimated from battery cell current and charge or discharge time. Further, when battery cells are equalized in the mid-voltage range to extend the charge and discharge cycles of battery cells, the battery cells subjected to charge equalization are equalized according to the differences in the battery state of charge estimated while the battery cells were in the lower or high battery cell voltage ranges.For example, if a first battery cell has a voltage output of 3.05 volts, which corresponds to 10 percent of the battery cell's full charge capacity, and a second battery cell has a voltage output of 3.12 volts, which corresponds to 15 percent of the battery cell's full charge capacity, 5 percent of the charge capacity in the middle voltage range can be drawn from the second battery cell during a battery charging cycle, so that both battery cells reach 90 percent state of charge at the same time.

[0035] With reference now to Fig. Figure 5B shows a prophetic example of charge curves for an aged battery cell with less than full charge capacity. The curves shown can be used for the Fig. The system shown in 1-4 may be applicable.

[0036] The representation in Fig. 5B has an X and a Y axis that are the same as those in Fig. 5A. Furthermore, curves 502-522 represent the same battery operating conditions as in Fig. 5A. And line 550 denotes the same lower voltage limit as in Fig. 5A. Therefore, for the sake of brevity, the description of Fig. 5A for the similarly marked elements of Fig. 5B, and in the description of Fig. 5B, the description of these elements is omitted.

[0037] Fig. 5B includes line 562, which redefines the boundary between the medium voltage charging range and the higher voltage charging range. In particular, Fig. 5B the middle voltage charging range between line 550 and line 562. The higher voltage charging range is to the right of line 562. During aging, a battery cell loses its capacity to store charge. This is shown in Fig. 5B is represented by charge curves 502-522, whose voltage increases before reaching the nominal voltage magnitude of 100% of the x-axis. In this example, the battery charge capacity is reduced to approximately 75 percent of the original nominal voltage storage capacity. In one example, the charge storage capacity of a battery cell may be reduced to a portion of the battery cell's nominal charge capacity if the battery voltage increases during charging to a value that is higher than expected based on Coulomb integration. Fig. 5B it is also evident that the voltage output from the battery cell does not change for the same amount of charge stored at lower charge levels compared to a battery with full charge storage capacity (e.g. Fig. 5A). Thus, the low voltage charging range remains the same for new battery cells and aged battery cells. Accordingly, battery state of charge estimates based on battery voltages in the lower voltage range for aged battery cells can be estimated using the same battery charging curves used to estimate the battery state of charge for new battery cells.

[0038] With reference now to Fig. Figure 6A shows a prophetic example of a charge difference between two battery cells at a lower battery voltage range. The representation in Fig. 6A has an X and a Y axis that are the same as those in Fig. 5A. Furthermore, the line 550, which indicates the lower voltage range, is the same as that shown in Fig. 6A. Therefore, for the sake of brevity, the description of Fig. 5A for the similarly marked elements of Fig. 6A, and in the description of Fig. 6A, the description of these elements is omitted.

[0039] Fig. 6A shows curves 604 and 606, which illustrate characteristic charging curves for two battery cells operating under substantially similar conditions (e.g., the same battery cell temperature). Curve 604 represents the charge storage capacity versus voltage of a first battery cell. Curve 606 represents the charge storage capacity versus voltage of a second battery cell. Curve 604 indicates that the first battery cell has a higher charge storage capacity than the second battery cell. It can be seen that curve 606 begins to rise with respect to voltage at a charge capacity approximately 8% lower than curve 604. As a result, the second battery cell has the capacity to store approximately 8% less charge than the first battery cell.

[0040] Marks 650 and 652 indicate charge levels at which corresponding amounts of charge are removed from the first battery cell and the second battery cell up to the point where the battery cells enter the lower battery charge range to the left of line 550. The charge remaining in the first battery cell is indicated by mark 652, while the charge remaining in the second battery cell is indicated by mark 650. Thus, it can be seen that when two battery cells with different charge capacities are discharged from a full charge level, there is an output voltage difference between the two battery cells. When the two battery cells are discharged sufficiently to enter the lower voltage range to the left of line 550, the voltage difference becomes more pronounced. In this example, a charge capacity difference of approximately 8% results in a voltage difference of a little more than 0.1 volts.The voltage difference is indicated by marker 602. As the battery cells are further discharged, the voltage difference increases even further. As a result, the difference between the charge capacities of the two battery cells becomes more apparent when the two battery cells are discharged to a lower voltage range of the battery charging curves 604 and 606.

[0041] With reference now to Fig. Figure 6B shows a prophetic example of a charge difference between two battery cells at a medium battery voltage range. The representation in Fig. 6B has an X and a Y axis that are the same as those in Fig. 5A and Fig. 6A. Furthermore, curves 604 and 606 as well as line 550, which indicates the lower voltage range, are the same as those shown in Fig. 6A. Therefore, for the sake of brevity, the description of Fig. 6A for the similarly marked elements of Fig. 6B, and in the description of Fig. 6B, the description of these elements is omitted.

[0042] In Fig. 6B, an equivalent amount of charge has been added to the first and second battery cells. The first battery cell is at a charge value indicated by the mark 660, while the second battery cell is at a charge value indicated by the mark 662. Even though a charge difference of approximately 8% remains between the first and second battery cells, the voltage value difference between the first and second battery cells is Fig. 6B at approximately 20 millivolts. This makes it more difficult to determine a charge difference between the first and second battery cells by simply charging the two battery cells. Since it is difficult to determine an amount of charge stored in each battery cell in the mid-voltage range, it may also be difficult to equalize a charge between two battery cells if the amount of charge added to or removed from the battery cell is based on the voltage of the battery cell while the battery cell is in the mid-voltage range. As with the method of Fig. 7 and Fig. 8, however, a charge difference can be determined in a lower voltage range, as in Fig. 6A, and then a battery cell in the middle voltage range Fig. 6B. It may be more desirable to extract or add voltage in the middle voltage range (e.g., while two battery cells are at charge levels 660 and 662), since the battery cells are often not allowed to enter the lower or higher voltage range due to charge or discharge constraints. Consequently, by knowing the charge value difference between battery cells, it is possible to perform cell balancing after predetermined charge / discharge cycles without having to remain in the low or high charge region of the battery cell charging curve.

[0043] With reference now to Fig. 7 and Fig. Figure 8 shows a flowchart for a method for balancing the charge of battery cells. The method of Fig. 7 and Fig. 8 is controlled by commands from the BCM control unit 37 or the MBB 35 of Fig. 1. The method 700 is also applicable to systems in which the charge is balanced by means of a passive device, as well as to systems in which the charge is shifted between battery cells.

[0044] At 702, method 700 determines the battery cell operating conditions. The battery cell operating conditions may include, but are not limited to, battery cell voltages, battery cell temperatures, and battery cell current flow. Method 700 proceeds to 704 after the battery cell operating conditions are determined.

[0045] At 704, method 700 assesses whether the battery cells are in an open-circuit voltage (OVC) state of charge estimation range (e.g., range 1 or 3 of Fig. 5A). In this example, the middle charge range (e.g. range 2 of Fig. 5A) No open-circuit voltage SOC estimation range, since the battery's open-circuit voltage changes little with the amount of battery charge. Battery charge estimation ranges can be empirically determined and stored in the memory of a battery pack controller, and a battery cell can be determined to be within a SOC estimation range based on the battery cell's open-circuit voltage. For example, if a battery cell has an open-circuit voltage of 2.9 volts, the battery cell can be determined to be in a lower voltage range where the battery's SOC can be estimated from the battery's open-circuit voltage. If the battery cell has an open-circuit voltage of 3.15 volts, the battery cell can be determined to be in a medium voltage range where the battery's SOC is not estimated based on the battery's current open-circuit voltage.Rather, the state of charge of a battery cell is estimated based on the amount of current the battery cell draws or delivers after leaving the lower or higher voltage range, and the battery charge, which is determined from the resting voltage of the battery cell in the resting voltage state of charge estimation range. If the battery cell has a resting voltage of 3.3 volts, the battery cell can be determined to be in the higher voltage range, where the battery state of charge is estimated based on the resting voltage of the battery. Thus, how the battery state of charge is estimated can be determined according to charge estimation ranges determined according to the battery voltage.

[0046] It should also be noted that the battery charge estimation ranges can be adjusted to account for battery aging and battery cell type. For example, a higher voltage charge range can be adjusted in response to battery cell aging. In particular, the higher voltage charge range limit can be moved from a higher percentage of the amount of charge that a battery cell can claim to store to a lower percentage of the amount of charge that the battery cell can claim to store. For example, the higher voltage charge range is Fig. 5A in response to battery aging to the higher voltage charging range of Fig. 5B. The higher voltage charging range will move to a lower percentage of the charge a battery cell is rated to store if a battery voltage increases before the battery stores its rated charge capacity. In one example, the amount of charge stored in a battery is estimated according to an amount of current a battery cell draws or sinks. For example, if a battery cell has a rated storage capacity of X amp-hours, and Y amps were applied to charge the battery cell for 0.5 hours, after the battery cell state of charge was estimated to be at 15 percent of the rated storage capacity while the battery cell is in the lower voltage range, then the battery state of charge can be estimated by 0.15*X+Y*0.5 amp-hours.

[0047] If method 700 determines that the battery cells are not within the open-circuit voltage charge estimation range, method 700 proceeds to 716. Otherwise, method 700 proceeds to 706.

[0048] At 706, method 700 judges whether one or more battery cells of a battery pack are in a lower voltage range. As explained above, a lower voltage range may be predetermined battery voltages stored in a memory of a battery pack controller. Furthermore, the lower voltage range may be adjusted with respect to battery cell temperature and battery cell type or chemistry. For example, Fig. 5A, to the left of line 550, a lower voltage range. If method 700 determines that the battery cell is in the lower voltage range, method 700 proceeds to 708. Otherwise, method 700 proceeds to 724 of Fig. 8, where the battery state of charge can be estimated based on battery cell resting voltages for battery cells that output higher voltages.

[0049] At 708, method 700 judges whether the battery cell with the lowest output voltage is at a lower charge threshold or whether all battery cells of the battery pack are in a lower voltage range. If so, method 700 proceeds to 710. Otherwise, method 700 proceeds to the end so that further charge can be removed from the battery cells so that the battery state of charge can be estimated from the battery resting voltage.

[0050] At 710, method 700 reads the battery cell voltage for each series battery cell and estimates the battery state of charge for battery cells that are in the predetermined lower voltage range. Battery cell voltages are read by sampling the battery cells. All battery cells of a battery pack or battery cell stack can be sampled substantially simultaneously by storing battery charge to capacitors and sampling the capacitors using an ADC. In one example, the battery state of charge is calculated from curves similar to those of Fig. 5A. The newly estimated state of charge for each battery cell in the lower voltage range becomes the basis for estimating the state of charge for each battery cell as the battery cells are charged and enter the mid-voltage range. Method 700 proceeds to 712 after the state of charge for each battery cell in the lower voltage range is estimated.

[0051] It should be noted that a single series battery cell can consist of multiple battery cells electrically connected in parallel. Thus, each of the parallel battery cells outputs an equivalent voltage.

[0052] At 712, method 700 estimates the state of charge for all series battery cells that do not enter the lower voltage range. The state of charge for series battery cells that do not enter the lower voltage range is determined according to the same procedure described below with reference to 716 and 718. After the state of charge for battery cells that are not in the lower voltage range is estimated, method 700 proceeds to 714.

[0053] At 714, method 700 determines a charge difference between series battery cells. In one example, the state of charge for the series battery cell outputting the lowest voltage is compared to the state of charge of the remaining series battery cells by subtracting the state of charge of the lowest series battery cell from the state of charge of the other series battery cells. In this way, the series battery cell with the lowest charge capacity can be compared to the charge capacity of other series battery cells, allowing it to be determined which battery cells are receiving charge or are being drained of charge during battery pack operation. After the charge difference between series battery cells is determined, method 700 proceeds to exit.

[0054] At 716, method 700 counts the charge drawn or delivered by series battery cells while operating in the mid-charge range, where it may be difficult to accurately determine a battery cell state of charge from the battery cell resting voltage alone. In one example, current flowing into or out of individual battery cell stacks may be monitored to determine an amount of current drawn or delivered by a series battery cell. In another example, the total amount of current drawn or delivered by a battery pack may be determined using a single current sensor. Then, the total amount of current may be divided between individual battery cell stacks and battery cells so that an amount of current drawn or delivered by an individual series battery cell may be determined. The current may be integrated over a charge or discharge cycle so that the total amount of current during the charge or discharge cycle may be determined.The integrated current amount can be converted to coulombs so that the amount of current flowing into and out of a battery cell can be determined. After determining a coulomb number that a battery draws or draws, method 700 proceeds to 718.

[0055] At 718, method 700 takes the counted number of coulombs for each series battery cell and adds it to a state of charge estimated for each battery cell from a resting battery voltage. Thus, the battery state of charge estimate in the mid-voltage range is based on a resting state of charge estimate based on battery voltage and a coulomb count estimate based on battery current. Each battery cell state of charge is updated as method 700 operates.

[0056] At 720, method 700 judges whether or not conditions for equalizing battery cell charge exist. In one example, battery cell charge equalization may be triggered once during a battery charge cycle. In other examples, battery cell charge equalization may be triggered once during a battery discharge cycle. In other examples, battery cell charge equalization may be triggered multiple times during battery charge and discharge cycles. For example, battery charge equalization may be triggered after a battery cell or battery pack draws a predetermined amount of current. On the other hand, battery charge equalization may be triggered after a battery cell or battery pack outputs a predetermined amount of current. In another example, battery charge equalization may be triggered when a predetermined charge or voltage difference is detected between battery cells.For example, an amount of charge stored in first and second battery cells may be estimated from the resting voltage of each battery cell while the battery cells were in a lower voltage range, as well as an amount of current the battery cells are drawing or sinking. If a charge difference greater than a predetermined amount is determined, battery cell charge balancing may be triggered. In other examples, battery cell charge balancing is triggered when one or more battery cells reach a predetermined voltage. Thus, battery cell charge balancing may be triggered when the battery cells are not in the resting voltage state of the charge estimation range and while preconditions are met.

[0057] If method 700 determines that conditions for battery cell charge balancing exist, method 700 proceeds to 722. Otherwise, method 700 proceeds to exit.

[0058] At 722, method 700 balances series battery cells in response to the difference between the charge differences between series battery cells as determined at 714 or 732. In some examples, the battery cells are configured such that each series battery cell of a battery cell stack draws or sources an amount of current equal to the amount of current drawn or sourced by other series battery cells of the battery cell stack. Therefore, a charge difference between series battery cells may be attributable to a difference in charge storage capacity between battery cells.

[0059] The difference in battery cell charge capacity can be mitigated by adding or subtracting charge from individual series battery cells. For example, if it is determined that a first battery cell stores 100 more coulombs than a second battery cell, the charge of each battery cell, determined from the open-circuit voltages of each battery cell while the battery cells are in a lower voltage range, can be reduced by switching Fig. 4 For example, 100 coulombs can be removed from the first battery cell while the battery cells are in the middle voltage range. If both battery cells are supplied with the same amount of current, the cells can reach a voltage indicating a fully charged state essentially at the same time, so that all battery cells can be charged to a higher value. The amount of charge dissipated by the load resistor can be determined from the time and the amount of current flowing through the load resistor. Thus, even if the first battery cell has a higher charge capacity than the second battery cell, the battery cells will reach fully charged conditions at the same time. Similarly, if the two battery cells are supplying current, 100 coulombs can be removed from the higher capacity battery, so that both battery cells reach a low voltage threshold essentially at the same time.

[0060] In another example where charge may be shifted between battery cells (e.g., in U.S. patent application Ser. No. 12 / 820,411, which is hereby incorporated by reference in all respects), 100 coulombs of charge may be added to the second battery cell during a battery discharge cycle such that the first and second battery cells reach a lower voltage threshold substantially simultaneously. Similarly, 100 coulombs of charge may be removed from the first battery cell during a battery charge cycle such that the first and second battery cells reach a higher voltage threshold substantially simultaneously. In this manner, charge may be equalized between battery cells based on a battery state of charge estimated from a specified voltage range and a charge difference between battery cells.Furthermore, a charge between battery cells can be balanced based on a battery state of charge estimated from a specified voltage range and a charge estimated from current, as well as a charge difference between battery cells. The method of . Fig. 8 is a specific example of battery cell balancing that may be provided at 722. After balancing the charge between battery cells, method 700 proceeds to the end.

[0061] At 724, method 700 judges whether the battery cell with the highest output voltage is at a higher charge threshold or whether all battery cells of the battery pack are in a higher voltage range. If so, method 700 proceeds to 726. Otherwise, method 700 proceeds to the end so that further charge can be applied to the battery cells so that the battery state of charge can be estimated from the battery resting voltage.

[0062] At 726, method 700 reads the battery cell voltage for each series battery cell and estimates the battery state of charge for battery cells that are in the predetermined higher voltage range. In one example, the battery state of charge is calculated from curves similar to those of Fig. 5A. The newly estimated state of charge for each battery cell in the higher voltage range becomes the basis for estimating the state of charge for each battery cell as the battery cells are charged and enter the mid-voltage range. Method 700 proceeds to 728 after the state of charge for each battery cell in the higher voltage range is estimated.

[0063] At 728, method 700 estimates the state of charge for all series battery cells that do not enter the higher voltage range. The state of charge for series battery cells that do not enter the higher voltage range is determined according to the procedure described below with reference to 716 and 718. After the state of charge for battery cells that are not in the higher voltage range is estimated, method 700 proceeds to 730.

[0064] At 730, method 700 judges whether the counted charge per battery cell of a battery pack from 716-718 is less than a charge estimate from resting voltages in the higher voltage range. In other embodiments, method 700 judges whether the counted charge for each battery cell of a battery cell pack is less than charge estimates from resting voltages in the higher voltage range by a predetermined charge amount. For example, if method 700 determines from the resting voltage that a battery cell is at 95 percent of the battery cell's rated charge capacity, and if method 700 determines from battery cell current and a previous battery cell resting voltage (e.g., 716 and 718) that the battery cell is at 85 percent of the battery cell's rated charge capacity, method 700 may proceed to 734 if the 10 percent charge difference is greater than a predetermined threshold charge amount.However, if the predetermined threshold charge amount is a threshold of 20 percent, method 700 proceeds to 732.

[0065] If method 700 determines that a charge estimate based on counted coulombs is less than a rest voltage charge estimate, method 700 proceeds to 734. Otherwise, method 700 proceeds to 732.

[0066] At 734, the method 700 reduces the charge storage capacity of the battery cell due to aging. In one example, the charge storage capacity of the battery cell is reduced by an amount based on a number of coulombs added to or subtracted from a charge estimate based on open-circuit voltage. Further, the method 700 may reduce the total charge storage capacity from the nominal charge storage capacity of a battery cell such that the higher charge limit (e.g., 552 of Fig. 5A) corresponds to a lower charge value. Furthermore, reducing the total charge storage capacity of a battery cell from the nominal charge storage capacity can bring the charge estimate from the open-circuit voltage charge estimate and the charge estimated from a previous open-circuit voltage and integrated current amount into line. And reducing the total charge storage capacity of the battery from the nominal charge storage capacity can change when charge balancing occurs. For example, if charge balancing is scheduled when a battery cell is at 50 percent of charge storage capacity, charge balancing is started at a lower absolute charge value after the total charge storage capacity is reduced from the nominal charge storage capacity.Specifically, if the nominal charge capacity is 0.5 ampere-hours, charge equalization is initiated when the battery cell has a charge value of 0.25 ampere-hours. However, if the total charge capacity is reduced from the nominal charge capacity to a value of 0.4 ampere-hours, charge equalization is initiated when the battery cell has a charge value of 0.2 ampere-hours. After the total charge storage capacity of the battery cell is reduced due to aging, method 700 proceeds to 732.

[0067] At 732, method 700 determines a charge difference between series battery cells. In one example, the state of charge for the series battery cell outputting the highest voltage is compared to the state of charge of the remaining series battery cells by subtracting the state of charge of the highest series battery cell from the state of charge of the other series battery cells. In this way, the series battery cell with the highest charge capacity can be compared to the charge capacity of other series battery cells, allowing it to be determined which battery cells are receiving charge or are being drained of charge during battery pack operation. After the charge difference between series battery cells is determined, method 700 proceeds to exit.

[0068] Thus, the procedure of Fig. 7 and Fig. 8 discharges a charge from battery cells when the battery cells are at a first voltage and equalizes the charge of the battery cells based on or in response to a difference in charge stored by the battery cells after drawing or supplying charge to the battery cells. Furthermore, the method of Fig. 7 reduce the total charge storage capacity of a battery cell to account for aging, thereby allowing the battery cells to balance at different times in conjunction with the total charge storage capacity of the battery cells of the battery pack.

[0069] In this way, the procedure of Fig. 7 and Fig. 8 provides a method for balancing charge between battery cells, comprising: operating a first battery cell at a first voltage and a second battery cell at a second voltage, the first and second battery cells both operating in a first charge range; and balancing the charge between the first battery cell and the second battery cell in a second charge range different from the first charge range, wherein the balancing of the charge is responsive to a voltage difference between the first voltage and the second voltage. The method comprises wherein the first charge range is a lower charge range than the second charge range. The method comprises wherein the balancing of the charge comprises adding or removing charge from the first battery cell or the second battery cell.The method comprises wherein the second charge range occurs at a battery cell voltage at which a change in battery cell voltage of 0.1 volts produces a change in stored battery charge of less than 20 percent of the rated charge capacity of the first battery cell. The method comprises wherein the first charge range has a higher slope than the second charge range. The method comprises wherein an amount of charge added to or removed from the first or second battery cell is based on an amount of charge represented by the voltage difference.

[0070] The procedure of Fig. 7 and Fig. 8 also provides a method for balancing charge between battery cells, comprising: operating a first battery cell and a second battery cell in a first range of battery charge, the first and second battery cells operating at a charge imbalance indicated by a voltage difference between voltage of the first battery cell and voltage of the second battery cell; charging both the first and second battery cells to transition both the first and second battery cells to a second range of battery charge; and operating the first and second battery cells in the second range to reduce the charge imbalance, comprising adding or subtracting an amount of charge to the first or second battery cell in the second range of battery charge, the amount of charge based on the voltage difference in the first battery charge range.The method comprises wherein subtracting charge of the first or second battery cell equalizes charge between the first and second battery cells, and wherein the first battery charge range is a lower battery charge than the second battery charge range. The method comprises wherein the first battery charge range is a higher charge than the second battery charge range. The method comprises wherein a slope of battery voltage versus battery cell charge of the first range is higher than the slope of battery voltage versus battery cell charge of the second range. The method comprises wherein charge is added to the first or second battery cell using a transformer. The method comprises wherein charge is subtracted from the first or second battery cell using a resistor.The method further comprises basing a time of adding or subtracting charge from the first or second battery cell on an amount of charge stored in the first or second battery cell estimated from a current amount of draw and discharge of the first or second battery cell and an open-circuit voltage of the first or second battery cell in the second battery charge range. The method further comprises, after estimating the amount of charge stored in the first battery cell, integrating an amount of charge added to or subtracted from the first battery cell.

[0071] The procedure of Fig. 7 and Fig. 8 further provides a method for balancing charge between battery cells, comprising: estimating an amount of charge stored in a first battery cell in response to a resting voltage of the first battery cell, the resting voltage in a first voltage range of the battery cell output voltage; estimating an amount of charge stored in a second battery cell in response to a resting voltage of the second battery cell, the resting voltage in a first voltage range of the battery cell output voltage; reducing a total charge capacity of the first or second battery cell from a nominal charge storage capacity in response to a resting voltage of the first or second battery cell and an integrated charge capacity of the first or second battery cell; balancing charge of the first or second battery cell in a second range of battery cell output voltage in a charge amount,which is associated with a difference between the amount of charge stored in the first battery cell in the first voltage range of the battery cell output voltage and the amount of charge stored in the second battery cell in the first voltage range of the battery cell output voltage; and adjusting a time of equalizing the charge of the first or second battery cell in response to reducing the nominal charge storage capacity of the first or second battery cell. The method comprises, wherein the time of equalizing is associated with a voltage of the first or second battery cell. The method comprises, wherein equalizing the charge of the first or second battery cell comprises reducing the battery charge. The method comprises, wherein in the first voltage range of the battery output voltage, the battery voltage changes by at least a first voltage amount,when the battery charge changes by a first charge amount, and wherein in the second voltage range of the battery output voltage, the battery voltage changes by an amount less than the first voltage value when the battery charge changes by the first charge amount. The method comprises, wherein the balancing of the first battery cell is performed by means of a passive balancing circuit. The method comprises, wherein adjusting the timing of the charge balancing allows the charge balancing to occur at a lower battery voltage during battery cell charging.

[0072] With reference now to Fig. 9 a method for balancing battery cells based on a charge difference between battery cells. The method of Fig. 9 is controlled by commands from the BCM control unit 37 or the MBB 35 of Fig. 1. The method 900 is also applicable to systems in which the charge is balanced by means of a passive device, as well as to systems in which the charge is shifted between battery cells.

[0073] At 902, a SOC is determined for each battery cell of a battery cell stack or battery pack. In one example, the SOC of battery cells may be determined from the method of Fig. 7-8. In particular, the SOC of each battery cell may be determined when the battery cells are in a SOC range where the battery cell OVC changes by more than a threshold amount per predetermined SOC change. In other examples, the SOC of each battery cell may be determined when the battery voltage measurement error results in an SOC estimation error of less than a threshold amount. After the SOC of each battery cell is determined, method 900 proceeds to 904.

[0074] At 904, method 900 determines whether battery cell charge balancing has been requested. In one example, battery cell charge balancing is requested after a predetermined amount of charge has been added to or subtracted from the battery cells of a battery cell stack or battery pack. If battery cell charge balancing is requested, method 900 proceeds to 906. Otherwise, method 900 ends.

[0075] It should be noted that the battery cells may be charged or discharged during the course of battery pack operation between 902 and 904. In some examples, it may be desirable to balance the charge of battery cells in a battery charge range where there is little change in battery cell OCV compared to the change in battery cell SOC.

[0076] At 906, method 900 determines the battery cell or cells that had the lowest charge in the battery cell stack or battery pack while estimating battery SOC charge using battery OCV measurements. Method 900 also selects the battery cells that have a charge closest to the lowest charge battery cell. For example, if it is determined that out of five battery cells, one battery cell had a charge of 100 coulombs and a second battery cell had a charge of 150 coulombs, while the third through fifth battery cells had a charge of 200 coulombs during SOC estimation using OCV, method 900 selects the first battery cell as the lowest charge battery cell and the second battery cell as the battery cell closest in charge to the lowest charge battery cell.After selecting the battery cell closest to the lowest charge battery cell, method 900 proceeds to 908.

[0077] At 908, the method 900 determines an amount of charge to be discharged from the battery cells undergoing charge equalization. In one example, the method 900 may begin by deciding, at 902, to discharge the battery cell or cells that were closest in charge to the battery cell with the lowest charge by a fraction of the charge frequency between the lowest charge battery cell and the battery cell closest in charge to the lowest charge battery cell during the SOC estimation process. For example, if one battery cell had a charge of 100 coulombs and a second battery cell had a charge of 150 coulombs, while the third through fifth battery cells had a charge of 200 coulombs during SOC estimation using OCV, the method may determine to discharge the second battery cell by 25 coulombs during the battery cell charge equalization process.The battery cells that had a charge of 200 coulombs can be discharged by 125 coulombs, bringing the charge of the third through fifth battery cells close to the charge of the second battery cell. In another example, the charge of the third through fifth battery cells can be discharged by 25 coulombs plus a portion of the charge difference between the second battery cell and the third through fifth battery cells. For example, the third through fifth battery cells can be discharged by 25 coulombs plus 50 coulombs, for a total of 75 coulombs, during the charge equalization process. Thus, the third through fifth battery cells are removed by 25 coulombs plus 50% of the charge difference between the second battery cell and the third through fifth battery cells. In this way, the amount of charge removed from the battery cells with a higher charge during the OCV charge estimation process can be limited to a conservative amount.Consequently, there may be less chance that the charge of battery cells that are depleted of charge during the equalization process will fall below the charge of the battery cell with the lowest capacity.

[0078] In an alternative example, the charge of lower-capacity battery cells may be similarly increased to the value of the highest-capacity battery cells, thus reducing the possibility of charging battery cells to a value greater than that of the highest-capacity battery cell. After determining the amount of charge to be removed from each battery cell during the charge balancing process, method 900 proceeds to 910.

[0079] At 910, method 900 reduces the charge of battery cells that have a higher charge than the lowest charge battery cell. The charge of higher charge battery cells may be removed using a resistor or a charge storage device such as an inductor. For example, a resistor may be connected in parallel with a battery cell for a predetermined amount of time to remove a specified amount of charge from the battery cell. Each battery cell of a battery cell stack or battery pack that requires discharging may be discharged by an individually specified amount of charge.

[0080] In an alternative example, the charging of lower-charge battery cells may be increased using a charging device. For example, a charge stored in a coil may be transferred to a lower-charge battery cell to increase the amount of charge stored in the battery cell. After the charging of battery cells is adjusted, method 900 proceeds to 912.

[0081] At 912, method 900 determines the battery cell SOC using OCV at the next opportunity where SOC accuracy is higher than a predetermined value. In one example, the SOC is determined as in Fig. 7-8. After the battery cell SOC is determined, method 900 proceeds to 914.

[0082] At 914, method 900 judges whether the charge of a battery cell being discharged during charge equalization is greater than the charge of the lowest-charge battery cell determined during the OCV-SOC estimation process. If so, method 900 proceeds to 918. Otherwise, method 900 proceeds to 916.

[0083] Thus, method 900 assesses the state of charge of the battery cells to determine whether the amount of charge discharged from the battery cells brings the battery cells into balance with the lowest charge cell. In an alternative example, method 900 may judge whether the charge of a battery cell being charged during battery balancing is greater than that of the highest charge cell. If so, method 900 revises the charge capacity of battery cells with lower charge capacity. Otherwise, method 900 adjusts the amount of charge added to the battery cells during the charge balancing process.

[0084] At 916, method 900 revises the charge capacity estimate of the battery cells at a lower charge than the previous lowest charge battery cell, determined at 902. In one example, the charge capacity of the battery cell at a lower charge than the previous lowest charge battery cell may be revised by a predetermined amount or a portion of the amount of charge discharged during the charge balancing process. After updating the charge capacity of battery cells, method 900 proceeds to exit.

[0085] At 918, method 900 adjusts the charge reduction amount used during charge balancing. For example, if 25 coulombs were removed from a battery cell during the balancing process, then 30 coulombs may be removed from the battery cell during the next balancing process. The amount of charge removed from each battery cell may be updated after each balancing process so that, after a number of balancing sequences (e.g., iterations), all battery cells of the battery pack end up at substantially the same charge value after charge balancing. After adjusting the charge reduction amount of each battery cell in the battery cell stack or battery pack, method 900 proceeds to the end.

[0086] As can be understood by an average expert, the Fig.7-9 may be represented by instructions to a controller and may be represented by one or more of any number of processing strategies such as event-driven, interrupt-driven, multitasking, multithreading, and the like. Thus, various steps or functions may be performed in the sequence shown, in parallel, or in some cases skipped. Similarly, the order of processing is not required to achieve the objects, features, and advantages described herein, but is provided for ease of illustration and description. Although not specifically shown, one of ordinary skill in the art will recognize that one or more of the steps, functions, or methods shown may be performed repeatedly depending on the particular strategy used.

[0087] The following claims particularly point out certain combinations and subcombinations that are considered novel and non-obvious. These claims may refer to "a" element or "a first" element, or the equivalent thereof. Such claims are to be construed as encompassing the inclusion of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and subcombinations of the disclosed features, functions, elements, and / or properties may be claimed by amending the present claims or by presenting new claims in this or a related application. Such claims, whether broader or narrower, the same as, or different in scope than the original claims, are also considered to be included within the subject matter of the present disclosure.

Claims

[1] A method for balancing charge between battery cells, comprising: Operating a first battery cell at a first voltage and a second battery cell at a second voltage, wherein the first and second battery cells both operate in a first charge range; and Balancing the charge between the first battery cell and the second battery cell in a second charge range different from the first charge range, wherein the balancing of the charge is responsive to a voltage difference between the first voltage and the second voltage measured in the first charge range. [2] The method of claim 1, wherein the first charge range is a lower charge range than the second charge range, and wherein charge balancing is performed iteratively in response to a lowest capacity battery cell of a battery cell stack. [3] The method of claim 1, wherein the second charge range is associated with a battery cell voltage of a battery cell, wherein the state of charge of the first battery cell is estimated, and wherein a measurement error in the battery cell voltage results in a change in the state of charge estimate of the first battery cell by more than a threshold value. [4] The method of claim 1, wherein in the first charge range the change in voltage produces a higher change in stored battery charge than in the second charge range. [5] The method of claim 1, wherein the first battery charge range is a higher charge than the second battery charge range. [6] The method of claim 1, wherein a slope of battery voltage versus battery cell charge of the first region is higher than the slope of battery voltage versus battery cell charge of the second region. [7] The method of claim 1, wherein charge is added to the first or second battery cell by means of a transformer. [8] The method of claim 1, wherein charge is subtracted from the first or second battery cell by means of a resistor. [9] The method of claim 1, further comprising: basing a timing of adding or subtracting charge from the first or second battery cell on an amount of charge stored in the first or second battery cell estimated from an amount of current drawn and discharged by the first or second battery cell and an open circuit voltage of the first or second battery cell in the second battery charge range. [10] The method of claim 1, further comprising, after estimating the amount of charge stored in the first battery cell, integrating an amount of charge added to or subtracted from the first battery cell. [11] A method for balancing charge between battery cells, comprising: Estimating an amount of charge stored in a first battery cell in response to a resting voltage of the first battery cell, the resting voltage in a first voltage range of the battery cell output voltage; Estimating an amount of charge stored in a second battery cell in response to a resting voltage of the second battery cell, the resting voltage in a first voltage range of the battery cell output voltage; Reducing a total charge capacity of the first or second battery cell from a nominal charge storage capacity in response to an open circuit voltage of the first or second battery cell and an integrated charge capacity of the first or second battery cell; Balancing charge of the first or second battery cell in a second range of battery cell output voltage in an amount of charge associated with a difference between the amount of charge stored in the first battery cell in the first voltage range of battery cell output voltage and the amount of charge stored in the second battery cell in the first voltage range of battery cell output voltage; and Adjusting a timing of equalizing the charge of the first or second battery cell in response to reducing the rated charge storage capacity of the first or second battery cell. [12] The method of claim 11, wherein the timing of the equalization is associated with a voltage of the first or second battery cell. [13] The method of claim 11, wherein balancing the charge of the first or second battery cell comprises reducing the battery charge. [14] The method of claim 11, wherein in the first voltage range of the battery output voltage, the battery voltage changes by at least a first voltage amount when the battery charge changes by a first charge amount, and wherein in the second voltage range of the battery output voltage, the battery voltage changes by an amount less than the first voltage value when the battery charge changes by the first charge amount. [15] The method of claim 11, wherein the balancing of the first battery cell is performed by means of a passive balancing circuit. [16] The method of claim 11, wherein adjusting the timing of equalizing charge allows equalizing charge to occur at a lower battery voltage during battery cell charging.

Images