Cell balancing method, cell balancing device, and power storage system

Abstract

A cell balancing method according to the present invention comprises: a step for, when the voltage of any one of a plurality of single cells reaches a prescribed voltage, discharging each of the plurality of single cells until the voltage of that single cell reaches the prescribed voltage, and measuring the remaining capacity of that single cell from the product of the current flowing during the discharge and the flow time; and a step for, on the basis of the measured remaining capacity of each of the plurality of single cells, calculating a required balancing charge amount for each of the plurality of single cells, and thereafter, when a battery in a fully charged state is discharged, performing charging / discharging of a single cell, among the plurality of single cells, for which the absolute value of the required balancing charge amount is larger than a prescribed value, so as to compensate for the required balancing charge amount, thereby equalizing the remaining capacities of the plurality of single cells.

Description

Cell balancing method, cell balancing device, and power storage system

[0001] The present disclosure relates to a cell balancing method, a cell balancing device, and an electricity storage system for equalizing the remaining capacities of a plurality of cells.

[0002] Patent Document 1 discloses a technology in which the open circuit voltage (OCV) of each of a plurality of cells is measured, a remaining capacity correlated with the OCV is calculated for each of the plurality of cells, and the voltages of the plurality of cells are equalized based on the calculated remaining capacity of each of the plurality of cells.

[0003] International Publication No. 2021 / 192382

[0004] In the technology disclosed in Patent Document 1, when the internal resistance of a cell varies due to deterioration of the cell, the amount of voltage drop due to the internal resistance also varies, making it impossible to accurately measure the OCV. In other words, if the OCV cannot be accurately measured, the remaining capacity cannot be accurately calculated, and it becomes difficult to accurately align the remaining capacities of multiple cells.

[0005] A cell balancing method according to the present disclosure is a cell balancing method for equalizing the remaining capacities of a plurality of serially connected unit cells constituting a battery, the cell balancing method including: (a) a step of discharging the battery in a fully charged state, and when the voltage of any one of the plurality of unit cells reaches a predetermined voltage corresponding to a discharge end voltage, having a balancing circuit connected to each of the plurality of unit cells discharge the unit cell until the voltage of the unit cell reaches the predetermined voltage, and measuring the remaining capacity of the unit cell from the product of the current flowing through the balancing circuit and the time for which current is passed through the balancing circuit; and (b) calculating, for each of the plurality of unit cells, a balancing charge required for equalizing the remaining capacities of the plurality of unit cells based on the measured remaining capacities of each of the plurality of unit cells; and thereafter, when the fully charged battery is discharged, having the balancing circuit charge or discharge a unit cell of the plurality of unit cells whose absolute value of the balancing charge is greater than a predetermined value so as to compensate for the balancing charge, thereby equalizing the remaining capacities of the plurality of unit cells.

[0006] A cell balancing device according to the present disclosure is a balancing circuit for equalizing the remaining capacities of a plurality of serially connected unit cells constituting a battery, the balancing circuit comprising: a balancing circuit connected to each of the plurality of unit cells; and a control unit that controls the balancing circuit; when the voltage of any one of the plurality of unit cells reaches a predetermined voltage corresponding to a discharge end voltage, the control unit causes the balancing circuit to discharge each of the plurality of unit cells until the voltage of the unit cell reaches the predetermined voltage, measures the remaining capacity of the unit cell by multiplying the current flowing through the balancing circuit by the time the current is passed through the balancing circuit, and calculates, for each of the plurality of unit cells, a balancing charge required for equalizing the remaining capacities of the plurality of unit cells based on the measured remaining capacities of each of the plurality of unit cells; and thereafter, when the battery is discharged in a fully charged state, causes the balancing circuit to charge or discharge any unit cell of the plurality of unit cells whose balancing charge is greater than a predetermined value so as to compensate for the balancing charge, thereby equalizing the remaining capacities of the plurality of unit cells.

[0007] The energy storage system according to the present disclosure comprises a battery consisting of a plurality of cells connected in series, a balancing circuit for equalizing the remaining capacities of the plurality of cells, and a control unit for controlling the balancing circuit, wherein the balancing circuit comprises a DC-DC converter for charging and discharging each of the plurality of cells, and a current measuring unit for measuring the current flowing through the DC-DC converter, and the control unit causes the DC-DC converter to discharge each of the plurality of cells until the voltage of the cell reaches a predetermined voltage corresponding to the discharge end voltage, and measures the remaining capacity of each of the plurality of cells based on the product of the current flowing through the DC-DC converter and the time for which the current is flowing.

[0008] These comprehensive or specific aspects may be realized as a system, a method, an integrated circuit, a computer program, or a computer-readable recording medium such as a CD-ROM, or may be realized as any combination of a system, a method, an integrated circuit, a computer program, and a recording medium.

[0009] According to the cell balancing method and the like according to one aspect of the present disclosure, the remaining capacities of a plurality of cells can be accurately equalized.

[0010] FIG. 1 is a block diagram showing an example of a power storage system according to an embodiment. FIG. 2A is a diagram for explaining deterioration of a cell. FIG. 2B is a diagram for explaining variations in remaining capacity that occur when a cell is deteriorated. FIG. 3 is a flowchart showing an example of a cell balancing method according to an embodiment. FIG. 4 is a diagram for explaining a specific example of the cell balancing method according to an embodiment. FIG. 5 is a diagram for explaining a specific example of a process for measuring remaining capacity. FIG. 6 is a diagram for explaining a specific example of a process for aligning the remaining capacities of a plurality of cells.

[0011] Hereinafter, the embodiments will be specifically described with reference to the drawings.

[0012] The embodiments described below are all comprehensive or specific examples, and the numerical values, shapes, materials, components, arrangement and connection of the components, steps, and order of steps shown in the following embodiments are merely examples and are not intended to limit the present disclosure.

[0013] (Embodiments) Hereinafter, a cell balancing method, a cell balancing device, and a power storage system according to embodiments will be described.

[0014] FIG. 1 is a block diagram illustrating an example of a power storage system 1 according to an embodiment.

[0015] The energy storage system 1 includes a cell balance device 10 and a battery 20. The cell balance device 10 is a device for equalizing the remaining capacities of the multiple unit cells connected in series that make up the battery 20. Note that, although the following description will be given using an example in which the multiple unit cells are five unit cells 21a-21e, the number of unit cells is not limited to five. For example, the battery 20 is a lithium-ion battery, and the multiple unit cells are often composed of 12 to 14 unit cells. Note that each unit cell may be composed of multiple batteries connected in parallel. For example, each of the five unit cells 21a-21e may be composed of multiple batteries connected in parallel, and the five unit cells 21a-21e may be connected in series. For example, the energy storage system 1 is mounted on a vehicle such as an electric vehicle. For example, the battery 20 is connected to auxiliary equipment mounted on the vehicle.

[0016] The cell balance device 10 includes a control unit 30 and a balancing circuit 40 .

[0017] The control unit 30 controls the balancing circuit 40. The balancing circuit 40 is, for example, an active cell balancing circuit (ACB circuit). The balancing circuit 40 is a circuit for equalizing the remaining capacities of the multiple unit cells connected in series that make up the battery 20 (i.e., performing cell balancing control), and is connected to each of the multiple unit cells. For example, the balancing circuit 40 includes a group of bidirectional switches, a DCDC converter 42, a current measurement unit 43, and a power supply 44.

[0018] The DCDC converter 42 is, for example, a bidirectional DCDC converter, and is controlled by the control unit 30 to supply power from a cell selected by a bidirectional switch group among the plurality of cells constituting the battery 20 to the power source 44, or to supply power from the power source 44 to a cell selected by a bidirectional switch group among the plurality of cells constituting the battery 20. In other words, the DCDC converter 42 charges and discharges each of the plurality of cells.

[0019] The current measurement unit 43 measures the current flowing in the DCDC converter 42. Specifically, the current measurement unit 43 is a current sensor that measures the current flowing from a cell selected by the bidirectional switch group from among the plurality of cells that make up the battery 20 to the power source 44 via the DCDC converter 42. The current measurement unit 43 may be provided in the DCDC converter 42, or may be realized by a coulomb counting function provided in the DCDC converter 42.

[0020] The power supply 44 is a power supply for temporarily storing the electric power of the plurality of cells in order to exchange electric power among the plurality of cells. Note that the type of the power supply 44 is not particularly limited, and may be a battery or a capacitor.

[0021] For example, the balancing circuit 40 includes a bidirectional switch group including six bidirectional switches 41a to 41f corresponding to the five battery cells 21a to 21e. The control unit 30 controls the balancing circuit 40 to exchange power among the battery cells, thereby equalizing the remaining capacities of the battery cells. For example, when transferring charge from the battery cell 21a to the battery cell 21b, the control unit 30 controls the bidirectional switches 41a and 41b to the ON state to select the battery cell 21a, and controls the DCDC converter 42 to supply power from the battery cell 21a to the power source 44, thereby discharging the battery cell 21a. Thereafter, the control unit 30 controls the bidirectional switches 41b and 41c to the ON state to select the battery cell 21b, and controls the DCDC converter 42 to supply power from the power source 44 to the battery cell 21b, thereby charging the battery cell 21b.

[0022] Although an example has been described in which the balancing circuit 40 is a cell selection type ACB circuit, the balancing circuit 40 is not limited to the cell selection type as long as it has a circuit configuration that allows power to be shared among a plurality of unit cells.

[0023] The control unit 30 is a computer including a processor (microprocessor) and a memory. The memory may be a read-only memory (ROM) or a random access memory (RAM), and can store programs to be executed by the processor. The control unit 30 is realized by the processor that executes the programs stored in the memory.

[0024] As deterioration of a cell progresses, the amount of energy that can be stored in the cell decreases. The degree of deterioration of each of the cells that make up the battery 20 varies depending on the usage conditions and installation environment of each cell, and therefore the degree of deterioration varies from cell to cell. This causes variations in remaining capacity. To address this, the balancing circuit 40 equalizes the remaining capacities of the multiple cells. Here, cell deterioration and variations in remaining capacity will be explained using Figures 2A and 2B.

[0025] FIG. 2A is a diagram illustrating the deterioration of a cell. FIG. 2B is a diagram illustrating the variation in remaining capacity that occurs when a cell is deteriorated. In FIGS. 2A and 2B, cells 1 to 5 are connected in series. Cells 1 to 5 shown in FIGS. 2A and 2B correspond to cells 21a to 21e, for example. In FIGS. 2A and 2B, the capacity of each cell is expressed as the volume of the container. Also, as an example, the OCV is shown to be 4.2 V when the SOC (State of Charge) of each cell is 100%, and 2.5 V when the SOC is 0%.

[0026] As shown in the upper part of FIG. 2A, when the battery 20 is new, the capacity of each cell is approximately the same. The fact that the capacity of each cell is approximately the same is expressed by the fact that the container volumes of each cell are the same. As shown in the lower part of FIG. 2A, as the deterioration of the battery 20 progresses, some cells are more deteriorated than others. Here, by changing the width of the containers of each cell, it is expressed that, of cells 1 to 5, cell 3 is the most deteriorated and cell 4 is the least deteriorated. As the deterioration of a cell progresses, the amount of energy that can be stored at an SOC of 100% decreases, but the amount of decrease varies for each cell depending on the degree of deterioration.

[0027] 2B, when the battery 20 is discharged from a fully charged state in which each cell has a different amount of energy, the same amount of charge is discharged from each cell, so the SOC of cell 3, which is more deteriorated, i.e., has a smaller amount of stored energy, will reach 0% first. In a battery 20 in which cells are connected in series, discharging of the battery 20 must be stopped to prevent over-discharge of a cell whose SOC has reached 0%, so the SOC of each cell does not reach 0% simultaneously. In this way, variations occur in the remaining capacity of the multiple cells.

[0028] For example, as shown in FIGS. 2A and 2B , there is a correlation between OCV and SOC. Therefore, as in the technology disclosed in Patent Document 1, it is conceivable to measure the OCV, calculate the remaining capacity, which is correlated with the OCV, for each of a plurality of cells, and then align the remaining capacities of the plurality of cells based on the calculated remaining capacities of the plurality of cells. However, if the internal resistance of the cells varies due to deterioration of the cells, the amount of voltage drop due to the internal resistance also varies, making it impossible to accurately measure the OCV. In other words, if the OCV cannot be accurately measured, the remaining capacity cannot be accurately calculated, and it becomes difficult to accurately align the remaining capacities of the plurality of cells. Below, we will describe a cell balancing method that measures the remaining capacities of each of a plurality of cells without measuring the OCV of each of the cells, thereby accurately aligning the remaining capacities of the plurality of cells.

[0029] Fig. 3 is a flowchart showing an example of a cell balancing method according to an embodiment. Note that, because the cell balancing method is executed by the cell balancing device 10, Fig. 3 is also a flowchart showing an example of the operation of the cell balancing device 10 according to an embodiment. Fig. 3 also shows processes other than those of the cell balancing method. Specifically, steps S101 to S105, S109, and S114 are processes other than those of the cell balancing method. In other words, steps S106 to S108 and S110 to S113 are processes of the cell balancing method (i.e., processes of the control unit 30).

[0030] First, the remaining capacity of the battery 20 is confirmed (step S101). Step S101 and steps S102 to S105, S109, and S114, which will be described later, are executed by, for example, an ECU (Electronic Control Unit) that controls the battery 20. Note that these steps may also be executed by the control unit 30.

[0031] Next, it is determined whether the remaining capacity of the battery 20 is equal to or greater than a threshold value (step S102). The threshold value is the remaining dischargeable capacity of the battery 20, and is set appropriately depending on the performance of the battery 20, etc. For example, whether the remaining capacity of the battery 20 is equal to or greater than the threshold value is determined by measuring the OCV, since high accuracy is often not required. For example, whether the remaining capacity of the battery 20 is equal to or greater than a threshold value is determined by determining whether the voltage value of the battery 20 or the sum of the voltages of the multiple cells that make up the battery 20 is equal to or greater than a threshold voltage.

[0032] If it is determined that the remaining capacity of the battery 20 is less than the threshold value (No in step S102), discharging of the battery 20 is prohibited (step S103), and charging of the battery 20 is requested (step S104). For example, a user or the like is notified to charge the battery 20.

[0033] If it is determined that the remaining capacity of the battery 20 is equal to or greater than the threshold (Yes in step S102), discharging of the battery 20 is started (step S105). For example, it is assumed that the battery 20 in a fully charged state is to be discharged.

[0034] Next, the control unit 30 determines whether all of the capacity correction values ​​of the plurality of cells are equal to or less than a predetermined value (step S106). The capacity correction value is an example of a balancing charge amount required to equalize the remaining capacities of the plurality of cells, which is calculated for each of the plurality of cells in step S112 described below.

[0035] If the control unit 30 determines that all of the capacity correction values ​​of the plurality of cells are not equal to or less than the predetermined value, that is, that the capacity correction value of any of the plurality of cells is greater than the predetermined value (No in step S106), it performs cell balance charge / discharge (step S107). Cell balance charge / discharge is a process of equalizing the remaining capacities of the plurality of cells by having the balancing circuit 40 charge / discharge cells that have capacity correction values ​​greater than the predetermined value so as to compensate for the capacity correction values. A method for calculating the capacity correction value will be described later.

[0036] When the control unit 30 determines that all of the capacity correction values ​​of the plurality of cells are equal to or less than the predetermined value (Yes in step S106), it stops the operation of the balancing circuit 40 (step S108). When all of the capacity correction values ​​of the plurality of cells are small, the variation in the remaining capacities of the plurality of cells is small, so in this case, the control unit 30 does not perform the step of making the remaining capacities of the plurality of cells uniform.

[0037] Next, the discharge of the battery 20 is stopped (step S109). For example, the discharge of the battery 20 may be stopped by a user or may be automatically stopped when the voltage of one of the cells drops and reaches a predetermined voltage corresponding to the discharge cut-off voltage. The predetermined voltage may be the discharge cut-off voltage or a voltage somewhat higher than the discharge cut-off voltage (for example, the discharge cut-off voltage + 1 V).

[0038] The control unit 30 determines whether the voltages of the plurality of cells have dropped and whether the voltage of any of the plurality of cells has reached a predetermined voltage (step S110). For example, the cell balancing device 10 may have a function for measuring the voltage of each of the plurality of cells, and the control unit 30 may make this determination based on the measurement results. Alternatively, the control unit 30 may obtain the measurement results of the voltages of each of the plurality of cells from a BMS (battery management system) or the like, and make this determination based on the measurement results.

[0039] When the control unit 30 determines that the voltage of any one of the plurality of cells has reached a predetermined voltage (Yes in step S110), it measures the remaining capacities of all of the plurality of cells (step S111). Specifically, the control unit 30 causes the balancing circuit 40 connected to each of the plurality of cells to discharge the cell until the voltage of the cell reaches the predetermined voltage, and measures the remaining capacity of the cell by multiplying the current flowing through the balancing circuit 40 by the time the current flows through the balancing circuit 40. More specifically, the control unit 30 causes the DCDC converter 42 to discharge the cell until the voltage of the cell reaches the predetermined voltage, and measures the remaining capacity of each of the plurality of cells by multiplying the current flowing through the DCDC converter 42 measured by the current measuring unit 43 by the time the current flows through the DCDC converter 42. In this way, by calculating the product of the current flowing from the cell and the time the current flows through the cell, it is possible to measure the remaining capacity of the cell without measuring the OCV of the cell.

[0040] The DC-DC converter 42 has a CCCV (Constant Current Constant Voltage) discharge function, and the control unit 30 may set the target voltage of each of the plurality of cells as a predetermined voltage and cause the DC-DC converter 42 to discharge each of the plurality of cells. If the voltage of a cell falls below the predetermined voltage, the cell may not be able to be charged. Therefore, by performing CCCV discharge, it is possible to prevent the voltage of the cell from falling below the predetermined voltage when measuring the remaining capacity of the cell.

[0041] Next, the control unit 30 calculates, for each of the plurality of cells, a capacity correction value (i.e., a charge amount required for balancing) required to equalize the remaining capacities of the plurality of cells based on the measured remaining capacities of each of the plurality of cells (step S112). For example, the control unit 30 calculates the difference between the remaining capacity of each of the plurality of cells and a reference capacity as the capacity correction value based on the measured remaining capacities of each of the plurality of cells. In this embodiment, the reference capacity is an average remaining capacity, which is the average value of the remaining capacities of the plurality of cells. The control unit 30 calculates the average remaining capacity of the plurality of cells, and calculates the difference between the remaining capacity of the cell and the average remaining capacity as the capacity correction value for each of the plurality of cells. In this embodiment, the difference obtained by subtracting the remaining capacity of the cell from the average remaining capacity, which is the reference capacity, is calculated as the capacity correction value. Thereafter, when discharging the battery 20, by charging and discharging the plurality of cells so that the remaining capacities of the plurality of cells equal the average remaining capacity in step S107, the amount of charge to be transferred is reduced, thereby shortening the time required for balancing. The control unit 30 includes a memory, and stores the calculated capacitance correction value in the memory so that it can be referenced when performing cell balancing charge / discharge in step S107. The memory is preferably a nonvolatile memory that retains the stored value even without power, in case the power source operating the cell balancing device 10 is turned off. If the memory already stores a capacitance correction value, the control unit 30 replaces the stored capacitance correction value with the newly calculated capacitance correction value in step S112 to update the capacitance correction value.

[0042] Next, the control unit 30 measures the remaining capacity of each of the plurality of cells and then uniformly distributes the charge discharged from each of the plurality of cells to the plurality of cells (step S113). For example, by distributing a charge according to the average remaining capacity to the plurality of cells, the charge discharged from each of the plurality of cells can be uniformly distributed to the plurality of cells. By uniformly distributing the total remaining capacity of the plurality of cells to each cell, even if a subsequent charging operation switches to discharging before the cells reach full charge, discharging can be started with the capacities of the cells balanced. Note that the flowchart in FIG. 3 depicts the control unit 30 executing step S113 after step S112. However, the control unit 30 may also uniformly distribute the charge discharged from each of the plurality of cells to the plurality of cells in step S113 after measuring the remaining capacity of all the cells in step S111 and before calculating the capacity correction value in step S112. That is, the control unit 30 may execute step S113 after executing step S111, regardless of step S112.

[0043] Thereafter, the battery 20 is charged, and in steps S106 and S107, cell balance charge / discharge is performed to compensate for the capacity correction value of a cell whose capacity correction value is greater than a predetermined value. In this embodiment, the capacity correction value is the difference obtained by subtracting the remaining capacity of the cell from the average remaining capacity, which is the reference capacity. In step S107, the control unit 30 balance charges / discharges the cells whose absolute value of the capacity correction value is greater than the predetermined value so that the cell is discharged less by the capacity correction value of the cell than other cells whose capacity correction values ​​(amount of charge required for balancing) are equal to or less than the predetermined value.

[0044] When the capacity correction values ​​are defined as above, the capacity correction value of cell 3, which is a more deteriorated cell, is a positive value, and the capacity correction value of cells 2 and 4, which are less deteriorated cells, is a negative value. For example, if the absolute values ​​of the capacity correction values ​​of cells 2, 3, and 4 are all greater than a predetermined value and the absolute values ​​of the capacity correction values ​​of cells 1 and 5 are all less than the predetermined value, the control unit 30 performs balanced charging and discharging of the cells in step S107 so that the cells (cells 2, 3, and 4) whose absolute values ​​of the capacity correction values ​​are greater than the predetermined value are discharged less by the capacity correction value of the cells than the other cells (cells 1 and 5) whose absolute values ​​of the capacity correction values ​​are equal to or less than the predetermined value. That is, in step S107, the control unit 30 performs balanced charging and discharging of the cells so that the cell (cell 3) whose capacity correction value is a positive value and whose absolute value is greater than the predetermined value is discharged less by the capacity correction value than the other cells (cells 1 and 5). On the other hand, in step S107, the cells (cells 2 and 4) whose capacity correction value is a negative value and whose absolute value is greater than a predetermined value are discharged less than the other cells (cells 1 and 5) by the capacity correction value, that is, the cells are balanced charged and discharged so that they are discharged more than the other cells (cells 1 and 5) by the absolute value of the capacity correction value.

[0045] On the other hand, if it is determined that the voltage of none of the plurality of cells has reached the predetermined voltage (No in step S110), that is, if the discharging of the battery 20 has been stopped by a user or the like in step S109, it is determined whether the remaining capacity of the battery 20 is equal to or greater than a threshold (step S114). For example, whether the remaining capacity of the battery 20 is equal to or greater than a threshold is determined by measuring the OCV, since high accuracy is often not required. For example, whether the remaining capacity of the battery 20 is equal to or greater than a threshold is determined by determining whether the voltage value of the battery 20 or the sum of the voltages of the plurality of cells constituting the battery 20 is equal to or greater than a threshold voltage. Furthermore, because the discharging of the battery 20 has been stopped in step S109, it is easy to measure the OCV in step S114.

[0046] If it is determined that the remaining capacity of the battery 20 is less than the threshold value (No in step S114), charging of the battery 20 is requested (step S104).

[0047] If it is determined that the remaining capacity of the battery 20 is equal to or greater than the threshold value (Yes in step S114), the process ends. For example, if the vehicle is parked with the remaining capacity of the battery 20 sufficient, the result in step S114 is Yes.

[0048] In this way, the cell balancing method includes the steps of: (a) when the voltage of any one of the plurality of cells reaches a predetermined voltage corresponding to the discharge end voltage, having the balancing circuit 40 connected to each of the plurality of cells discharge the cell until the voltage of that cell reaches the predetermined voltage, and measuring the remaining capacity of that cell from the product of the current flowing through the balancing circuit 40 and the time for which the current flows; and (b) calculating, for each of the plurality of cells, a balancing charge required to equalize the remaining capacities of the plurality of cells based on the measured remaining capacities of each of the plurality of cells; and thereafter, when the fully charged battery 20 is discharged, having the balancing circuit 40 charge and discharge any cell among the plurality of cells whose balancing charge is greater than a predetermined value so as to compensate for the balancing charge, thereby equalizing the remaining capacities of the plurality of cells.

[0049] In addition, in the above (b), an average remaining capacity of the plurality of cells may be calculated based on the measured remaining capacity of each of the plurality of cells, and the difference between the remaining capacity of each of the plurality of cells and the average remaining capacity may be calculated as the amount of charge required for balancing.

[0050] In the above (a), after measuring the remaining capacity of each of the plurality of cells, the charge discharged from each of the plurality of cells may be distributed evenly to the plurality of cells.

[0051] Furthermore, if the remaining capacities of all of the plurality of cells measured in (a) are equal to or less than a predetermined allowable value, then (b) does not need to be performed. If the remaining capacities of all of the plurality of cells are equal to or less than a predetermined allowable value (a value slightly greater than a predetermined voltage), then the remaining capacities of the plurality of cells are already equal, and therefore the step of equalizing the remaining capacities of the plurality of cells does not need to be performed. In other words, if the deterioration of the cells progresses in the future and a cell with a remaining capacity greater than the predetermined allowable value is produced, then the step of equalizing the remaining capacities of the plurality of cells can be performed.

[0052] Furthermore, if the discharge of battery 20 is stopped before the voltage of any one of the plurality of cells reaches a predetermined voltage, the above steps (a) and (b) do not need to be performed. In this way, if the use of the device in which battery 20 is installed is stopped and the discharge of battery 20 is stopped before the voltage of any one of the plurality of cells reaches a predetermined voltage, the step of making the remaining capacities of the plurality of cells uniform does not need to be performed.

[0053] Next, a specific example of the cell balancing method will be described with reference to FIGS.

[0054] Fig. 4 is a diagram for explaining a specific example of the cell balancing method according to the embodiment. Fig. 4 shows the change over time in OCV of a plurality of cells. Note that Fig. 4 shows the OCV of 12 cells as the OCV of the plurality of cells that make up battery 20, but Figs. 5 and 6 will be explained using five cells as an example.

[0055] Fig. 5 is a diagram illustrating a specific example of a process for measuring remaining capacity. Fig. 6 is a diagram illustrating a specific example of a process for equalizing the remaining capacities of a plurality of cells. In Figs. 5 and 6, cells 1 to 5 are connected in series. In Figs. 5 and 6, only cell 3, which is the most deteriorated cell, is shown with a reduced volume. Although the other cells are also deteriorating, they are not shown.

[0056] As shown in "1. Fully Charged" in Figures 4 and 5, the fully charged battery 20 is discharged first. Cell 3 is the most deteriorated, and although the SOC of each cell is 100%, the amount of energy stored in cell 3 is less than that of the other cells.

[0057] As shown in "2. Discharge" in Figures 4 and 5, when battery 20 is discharged from a fully charged state in which the energy amount of each cell is different, the same amount of charge is discharged from each cell, resulting in variations in the remaining capacity of each cell.

[0058] As shown in "3. Discharge Stop" in Figures 4 and 5, the SOC of cell 3, which is the most deteriorated, i.e., the cell with the least amount of stored energy, reaches 0% first, and discharge is stopped. At this time, for example, cell 4 has the least deterioration, so the remaining capacity of cell 4 is the largest.

[0059] As shown in "4. Remaining Capacity Measurement" in Figures 4 and 5, the remaining capacity of each cell is measured. For example, the balance circuit 40 transfers the charge of cell 2 to the power supply 44, and the remaining capacity of cell 2 is measured by multiplying the current calculated when the charge of cell 2 is transferred by the current flow time. The charge of cell 2 transferred to the power supply 44 is then transferred to cell 3, which is already known to have no remaining capacity. The remaining capacity of other cells whose remaining capacity has not yet been measured is then measured. In this manner, the remaining capacity of each cell is measured. Note that for cell 3, which initially reaches a predetermined voltage corresponding to the discharge end voltage, the remaining capacity can be estimated to be zero, so the remaining capacity does not need to be measured again. Furthermore, if the power supply 44 has the ability to store a sufficient amount of charge, the remaining capacity does not need to be transferred to other cells.

[0060] The average remaining capacity is calculated by averaging the measured remaining capacities of each cell. For example, if the remaining capacities of each cell are U1, U2, U3, U4, and U5, then (U1 + U2 + U3 + U4 + U5) / 5 is calculated. The difference between the remaining capacity of each cell and the average remaining capacity, which is the reference capacity, is then calculated as the required charge for balancing.

[0061] Thereafter, as shown in "5. Charging" in FIG. 4, the battery 20 is charged to a fully charged state.

[0062] As shown in "Balancing Operation During Discharge" in Figure 6, when the fully charged battery 20 is discharged, the balancing circuit 40 charges and discharges cells whose required charge for balancing is greater than a predetermined value to compensate for the required charge for balancing, thereby aligning the remaining capacities of the cells. For example, if the remaining capacity of cell 3 is 0 Ah and the average remaining capacity Uave is 100 Ah, the required charge for balancing Ucomp3 of cell 3 is Ucomp3 = -(U3 - Uave) = -(0 - 100) = 100 Ah. As a result, the battery 20 is discharged while 100 Ah of charge is charged to cell 3 from the other cells. For example, if the remaining capacity of cell 4 is 200 Ah and the average remaining capacity Uave is 100 Ah, the charge required for balancing cell 4, Ucomp4, is Ucomp4 = -(U4 - Uave) = -(200 - 100) = -100 Ah. As a result, 100 Ah of charge is discharged from cell 4 to the other cells, while the battery 20 is discharged. In this embodiment, if the battery 20 is not fully charged in "5. Charging" in FIG. 4 , the above-described balancing operation is not performed when the battery 20 is discharged. In other words, the above-described balancing operation is not performed when the battery 20 is discharged from a state that is not fully charged. However, if the battery 20 is fully charged in a subsequent charge, the above-described balancing operation may be performed when the battery 20 is subsequently discharged, even if the battery 20 is discharged from a state that is not fully charged.

[0063] In this way, when the battery 20 is discharged, the balancing circuit 40 charges and discharges the cells so that the remaining capacities of the cells are equalized, and as shown by "Discharge Stop" in Figure 6, the capacity of all cells can be used up and discharging can be stopped.

[0064] As described above, the remaining capacity of each of the plurality of cells can be measured by calculating the product of the current flowing through the balancing circuit 40 and the time the current is applied. Because the remaining capacity can be measured without measuring the OCV of each of the plurality of cells, the remaining capacity of each of the plurality of cells can be accurately measured even if the internal resistance of the cells varies due to cell deterioration. Therefore, the variation in the remaining capacities of the plurality of cells (in other words, the amount of charge required for balancing) can be accurately calculated, and the remaining capacities of the plurality of cells can be accurately matched.

[0065] The technology disclosed in Patent Document 1 requires the preparation of table data in which OCVs and remaining capacities are associated, which requires memory to store the table data and leads to increased costs. On the other hand, the cell balancing method disclosed herein eliminates the need to prepare such table data, enabling cost reductions.

[0066] Furthermore, in the technology disclosed in Patent Document 1, variations in remaining capacity (OCV variations) among a plurality of cells due to cell degradation occur immediately before the timing at which discharge is stopped, and cell balancing control must be performed in a short period of time immediately before the timing at which discharge is stopped, which may result in the cell balancing control not being able to be completed. On the other hand, according to the cell balancing method disclosed herein, the amount of charge required for balancing is calculated and stored in advance before the next discharge, and charging and discharging are performed from the fully charged state at the time of the next discharge to compensate for the amount of charge required for balancing, so that cell balancing control can be completed before discharge is stopped.

[0067] (Other Embodiments) As described above, the embodiments have been described as examples of the technology according to the present disclosure. However, the technology according to the present disclosure is not limited to these, and can be applied to embodiments in which modifications, substitutions, additions, omissions, etc. are made as appropriate. For example, the following modifications are also included in one embodiment of the present disclosure.

[0068] For example, in the above embodiment, an example has been described in which the average remaining capacity of the plurality of cells is calculated based on the measured remaining capacity of each of the plurality of cells, and the difference between the average remaining capacity of each of the plurality of cells is calculated as the amount of charge required for balancing, but this is not limiting. For example, the difference between the remaining capacity of a cell and an arbitrary value may be calculated as the amount of charge required for balancing.

[0069] For example, in the above embodiment, an example has been described in which the remaining capacity of each of the plurality of cells is measured and then the charge discharged from each of the plurality of cells is uniformly distributed to the plurality of cells, but this is not limiting. For example, the charge discharged from each of the plurality of cells does not have to be distributed to the plurality of cells. In other words, the charge discharged from each of the plurality of cells to measure the remaining capacity does not have to be returned to the plurality of cells.

[0070] For example, the present disclosure may be realized as a program for causing a computer (processor) to execute steps included in the cell balancing method. Furthermore, the present disclosure may be realized as a non-transitory computer-readable recording medium, such as a CD-ROM, on which the program is recorded.

[0071] For example, when the present disclosure is realized as a program (software), each step is performed by running the program using hardware resources such as a computer's CPU, memory, input / output circuits, etc. In other words, each step is performed by the CPU acquiring data from memory or input / output circuits, etc., performing calculations, and outputting the calculation results to memory or input / output circuits, etc.

[0072] In the above-described embodiment, each component included in the power storage system 1 or the cell balancing device 10 may be configured with dedicated hardware, or may be realized by executing a software program suitable for each component. Each component may be realized by a program execution unit such as a CPU or processor reading and executing a software program recorded on a recording medium such as a hard disk or semiconductor memory.

[0073] Some or all of the functions of the energy storage system 1 or the cell balancing device 10 according to the above-described embodiments are typically realized as an LSI, which is an integrated circuit. These may be individually implemented on a single chip, or some or all of them may be integrated on a single chip. Furthermore, the integrated circuit is not limited to an LSI, and may be realized using a dedicated circuit or a general-purpose processor. It is also possible to use an FPGA (Field Programmable Gate Array) that can be programmed after LSI manufacturing, or a reconfigurable processor that can reconfigure the connections and settings of circuit cells within an LSI.

[0074] Furthermore, if an integrated circuit technology that can replace LSIs emerges due to advances in semiconductor technology or other derived technologies, that technology may naturally be used to integrate the components included in the energy storage system 1 or the cell balancing device 10 into integrated circuits.

[0075] In addition, this disclosure also includes forms obtained by making various modifications to the embodiments that a person skilled in the art would think of, and forms realized by arbitrarily combining the components and functions in each embodiment within the scope of the present disclosure.

[0076] (Additional Notes) The above description of the embodiments discloses the following techniques.

[0077] (Technology 1) A cell balancing method for equalizing the remaining capacities of a plurality of unit cells connected in series to form a battery, the cell balancing method comprising: (a) a step of, when the voltage of any one of the plurality of unit cells reaches a predetermined voltage corresponding to a discharge end voltage, causing a balancing circuit connected to each of the plurality of unit cells to discharge the unit cell until the voltage of the unit cell reaches the predetermined voltage, and measuring the remaining capacity of the unit cell from the product of the current flowing through the balancing circuit and the time for which current is passed through the balancing circuit; and (b) calculating, for each of the plurality of unit cells, a balancing required charge necessary to equalize the remaining capacities of the plurality of unit cells based on the measured remaining capacities of each of the plurality of unit cells; and thereafter, when the battery is discharged in a fully charged state, causing the balancing circuit to charge or discharge any unit cell of the plurality of unit cells whose absolute value of the balancing required charge is greater than a predetermined value so as to compensate for the balancing required charge, thereby equalizing the remaining capacities of the plurality of unit cells.

[0078] According to this method, the remaining capacity of each of the plurality of cells can be measured by calculating the product of the current flowing through the balancing circuit and the time the current is passed through it. Because the remaining capacity can be measured without measuring the OCV of each of the plurality of cells, the remaining capacity of each of the plurality of cells can be accurately measured even if the internal resistance of the cells varies due to deterioration of the cells. Therefore, the variation in the remaining capacities of the plurality of cells (in other words, the amount of charge required for balancing) can be accurately calculated, and the remaining capacities of the plurality of cells can be accurately matched.

[0079] (Technology 2) The cell balancing method according to Technology 1, wherein in (b), an average remaining capacity of the plurality of cells is calculated based on the measured remaining capacities of each of the plurality of cells, and for each of the plurality of cells, a difference between the remaining capacity of the cell and the average remaining capacity is calculated as the amount of charge required for balancing.

[0080] According to this, when discharging the battery, by charging and discharging the multiple cells so that the remaining capacity of the multiple cells becomes the average remaining capacity, the amount of charge transferred is reduced and the time required for balancing can be shortened.

[0081] (Technology 3) The cell balancing method according to Technology 1 or 2, wherein in (a), after measuring the remaining capacity of each of the plurality of single cells, the charge discharged from each of the plurality of single cells is uniformly distributed to the plurality of single cells.

[0082] According to this, by distributing the total remaining capacity of the multiple cells evenly to each cell, even if charging is subsequently performed and then switching to discharging before the cells are fully charged, discharging can be started with the capacities of each cell balanced.

[0083] (Technology 4) The cell balancing method according to any one of Technologies 1 to 3, wherein (b) is not performed if the remaining capacities of the plurality of single cells measured in (a) are all equal to or less than a predetermined allowable value.

[0084] According to this, if the remaining capacities of the plurality of cells are all equal to or less than a predetermined allowable value (a value slightly greater than a predetermined voltage), the remaining capacities of the plurality of cells are already equal, and therefore the step of equalizing the remaining capacities of the plurality of cells does not need to be carried out. In other words, if the deterioration of the cells progresses in the future and a cell with a remaining capacity greater than the predetermined allowable value is produced, the step of equalizing the remaining capacities of the plurality of cells can be carried out.

[0085] (Technology 5) A cell balancing method according to any one of Technologies 1 to 4, wherein, if discharging of the battery is stopped before the voltage of any one of the plurality of single cells reaches the predetermined voltage, (a) and (b) are not performed.

[0086] According to this, if the discharge of the battery is stopped before the voltage of any one of the multiple cells reaches a predetermined voltage, for example because the use of the device in which the battery is installed is stopped, there is no need to carry out a process to equalize the remaining capacity of the multiple cells.

[0087] (Technology 6) A balancing circuit for equalizing the remaining capacities of a plurality of serially connected unit cells constituting a battery, the balancing circuit comprising: a balancing circuit connected to each of the plurality of unit cells; and a control unit that controls the balancing circuit; when the voltage of any one of the plurality of unit cells reaches a predetermined voltage corresponding to a discharge end voltage, the control unit causes the balancing circuit to discharge for each of the plurality of unit cells until the voltage of the unit cell reaches the predetermined voltage, measures the remaining capacity of the unit cell by the product of the current flowing through the balancing circuit and the time for which the current is passed through the balancing circuit, and calculates, for each of the plurality of unit cells, a balancing required charge necessary to equalize the remaining capacities of the plurality of unit cells based on the measured remaining capacities of each of the plurality of unit cells; thereafter, when the battery is discharged in a fully charged state, the control unit causes the balancing circuit to charge or discharge any unit cell of the plurality of unit cells whose balancing required charge is greater than a predetermined value so as to compensate for the balancing required charge, thereby equalizing the remaining capacities of the plurality of unit cells.

[0088] This makes it possible to provide a cell balancing device that can accurately equalize the remaining capacities of a plurality of cells.

[0089] (Technology 7) An energy storage system comprising: a battery comprising a plurality of cells connected in series; a balancing circuit for equalizing the remaining capacities of the plurality of cells; and a control unit for controlling the balancing circuit, wherein the balancing circuit comprises a DCDC converter for charging and discharging each of the plurality of cells; and a current measuring unit for measuring a current flowing in the DCDC converter, wherein the control unit causes the DCDC converter to discharge each of the plurality of cells until the voltage of the cell reaches a predetermined voltage corresponding to a discharge end voltage, and measures the remaining capacity of each of the plurality of cells from the product of the current flowing in the DCDC converter and the time for which the current is flowing.

[0090] This allows the remaining capacity of each of the plurality of cells to be measured without measuring the OCV of each of the cells, so that even if the internal resistance of each cell varies due to deterioration of the cells, the remaining capacity of each of the plurality of cells can be accurately measured. Therefore, the variation in the remaining capacities of the plurality of cells (in other words, the amount of charge required for balancing) can be accurately calculated, and the remaining capacities of the plurality of cells can be accurately matched.

[0091] (Technology 8) The power storage system described in Technology 7, wherein the DC-DC converter has a CCCV discharge function, and the control unit sets the target voltage of each of the plurality of single cells as the predetermined voltage, and causes the DC-DC converter to discharge each of the plurality of single cells.

[0092] If the voltage of a cell falls below a predetermined voltage, the cell may not be able to be charged. Therefore, by performing CCCV discharge, the voltage of the cell can be prevented from falling below the predetermined voltage when measuring the remaining capacity of the cell.

[0093] The present disclosure can be applied to a balancing circuit for balancing the remaining capacities of a plurality of cells.

[0094] REFERENCE SIGNS LIST 1 Energy storage system 10 Cell balancing device 20 Battery 21a, 21b, 21c, 21d, 21e Single cell 30 Control unit 40 Balancing circuit 41a, 41b, 41c, 41d, 41e, 41f Bidirectional switch 42 DCDC converter 43 Current measurement unit 44 Power supply

Claims

1. A cell balancing method for equalizing the remaining capacities of a plurality of unit cells connected in series that constitute a battery, comprising: (a) a step of, when the voltage of any one of the plurality of unit cells reaches a predetermined voltage corresponding to a discharge end voltage, having a balancing circuit connected to each of the plurality of unit cells discharge the unit cell until the voltage of the unit cell reaches the predetermined voltage, and measuring the remaining capacity of the unit cell from the product of the current flowing through the balancing circuit and the time for which the current is passed through the balancing circuit; and (b) calculating, for each of the plurality of unit cells, a balancing required charge necessary to equalize the remaining capacities of the plurality of unit cells based on the measured remaining capacities of each of the plurality of unit cells, and thereafter, when the battery is discharged in a fully charged state, having the balancing circuit charge or discharge any unit cell of the plurality of unit cells whose absolute value of the balancing required charge is greater than a predetermined value so as to compensate for the balancing required charge, thereby equalizing the remaining capacities of the plurality of unit cells.

2. The cell balancing method according to claim 1, wherein in (b), an average remaining capacity of the plurality of cells is calculated based on the measured remaining capacity of each of the plurality of cells, and the difference between the remaining capacity of each of the plurality of cells and the average remaining capacity is calculated as the amount of charge required for balancing.

3. The cell balancing method according to claim 1 or 2, wherein in (a), after measuring the remaining capacity of each of the plurality of unit cells, the charge discharged from each of the plurality of unit cells is uniformly distributed to the plurality of unit cells.

4. The cell balancing method according to claim 1 or 2, wherein (b) is not performed if the remaining capacities of all of the plurality of single cells measured in (a) are equal to or less than a predetermined allowable value.

5. The cell balancing method according to claim 1 or 2, wherein (a) and (b) are not performed if discharging of the battery is stopped before the voltage of any one of the plurality of cells reaches the predetermined voltage.

6. A cell balancing device for balancing the remaining capacities of a plurality of serially connected unit cells constituting a battery, the cell balancing device comprising: balancing circuits connected to each of the plurality of unit cells; and a control unit that controls the balancing circuit, wherein when the voltage of any one of the plurality of unit cells reaches a predetermined voltage corresponding to a discharge end voltage, the control unit causes the balancing circuit to discharge each of the plurality of unit cells until the voltage of that unit cell reaches the predetermined voltage, and measures the remaining capacity of that unit cell by the product of the current flowing through the balancing circuit and the time for which it is passed; calculates, for each of the plurality of unit cells, a balancing required charge necessary to balance the remaining capacities of the plurality of unit cells based on the measured remaining capacities of each of the plurality of unit cells; and thereafter, when the battery is discharged in a fully charged state, causes the balancing circuit to charge or discharge any unit cell of the plurality of unit cells whose balancing required charge is greater than a predetermined value so as to compensate for the balancing required charge, thereby balancing the remaining capacities of the plurality of unit cells.

7. A power storage system comprising: a battery consisting of a plurality of cells connected in series; a balancing circuit for equalizing the remaining capacities of the plurality of cells; and a control unit for controlling the balancing circuit, wherein the balancing circuit comprises: a DCDC converter that charges and discharges each of the plurality of cells; and a current measuring unit that measures the current flowing in the DCDC converter, wherein the control unit causes the DCDC converter to discharge each of the plurality of cells until the voltage of that cell reaches a predetermined voltage corresponding to a discharge end voltage, and measures the remaining capacity of each of the plurality of cells from the product of the current flowing in the DCDC converter and the time for which it is flowing.

8. The power storage system according to claim 7, wherein the DC-DC converter has a CCCV (Constant Current Constant Voltage) discharge function, and the control unit sets target voltages of the plurality of single cells as the predetermined voltages, and causes the DC-DC converter to discharge each of the plurality of single cells.

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