Secondary battery system, balance adjustment program, and balance adjustment method

Abstract

To provide a secondary battery system, a balance adjustment program, and a balance adjustment method.SOLUTION: A secondary battery system includes cell balance circuits 10 to 30 provided for each of battery strings STG1 to STG3 and a balance adjustment circuit 100 for switching whether or not to operate the cell balance circuit according to difference in an output current between the battery strings. The cell balance circuit includes: a plurality of cell coils L11 to L3n that are provided for each of the battery cells BC11 to BC3n and connected in parallel with the corresponding battery cell, a string coils L1 to L3 that are electromagnetically coupled to all of the cell coils provided corresponding to the plurality of battery cells included in one battery string and whose one end is connected to one of a positive or negative terminals of the corresponding battery string, and coil switches SW12, SW22, SW32 that switch whether the other end of the string coil is connected to or disconnected from the other of the positive terminal and the negative terminal of the corresponding battery string.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to, for example, a secondary battery system including a plurality of battery strings, and a balance adjustment program and a balance adjustment method used in the secondary battery system.

Background Art

[0002] In recent years, many vehicles using a motor as a power source such as electric vehicles have been proposed. In such vehicles, a battery pack in which a plurality of battery strings obtained by connecting a large number of battery cells in series are connected in parallel is used. In such a battery pack, it is required to balance the charging rates between the battery strings and the charging rates of the battery cells included in one battery string. By equalizing the charging rates between the battery strings and the charging rates of the battery cells included in one battery cell string, the risk of any of the battery strings or battery cells being in an over-discharged state can be reduced, and the utilization efficiency of the battery pack can be increased. Therefore, as an example of a technique for adjusting the balance of the charging rates of the battery strings or battery cells, Patent Documents 1 and 2 disclose such a technique.

[0003] The battery equalization device described in Patent Document 1 is a battery equalization device in a vehicle-mounted battery pack configured by connecting a plurality of battery stacks, each of which is configured by connecting a plurality of battery cells in series, in parallel. For each of the battery stacks, by calculating the internal resistance of the battery cells constituting the battery stack, a battery stack disconnection unit that disconnects a battery stack in which the difference between the maximum value and the minimum value of the internal resistance is equal to or greater than a threshold value from the power supply system, and a cell balance control unit that executes equalization control of the battery cells constituting the battery stack with respect to the disconnected battery stack. Further, the battery equalization device described in Patent Document 1 is a battery equalization device in a vehicle-mounted battery pack configured by connecting a plurality of battery stacks, each of which is configured by connecting a plurality of battery cells in series, in parallel. It includes a current measurement unit that measures the current flowing through each of the battery stacks, and a battery stack resistance control unit that controls the resistance value of a resistor connected in series to the output terminal of each of the battery stacks so that the current flowing through each of the battery stacks becomes equal according to the measurement result of the current flowing through each of the battery stacks.

[0004] The vehicle power supply system described in Patent Document 2 includes a power storage unit in which a plurality of cell strings, each of which has a plurality of cells connected in series, are connected in parallel, a voltage detection unit that detects the voltage of each of the plurality of cell strings, a plurality of switches inserted into each of the plurality of cell strings, and a management unit that executes an equalization process of turning on two or more of the plurality of switches to equalize the voltage / capacity between the plurality of cell strings in a non-driving state of the vehicle in which the present power supply system is mounted.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, in the technology described in Patent Document 1, in order to equalize the charging rates between battery cells, it is necessary to make the number of battery cells included in the battery stack redundant, which causes a problem of increasing the size of the battery pack. Also, in Patent Document 1, in order to equalize the charging rates of the battery stacks, an insertion resistor is required to be inserted in series with the battery stacks to absorb the difference in resistance values between the battery stacks, and there is a problem that the battery stacks cannot be used with low resistance and the battery utilization efficiency deteriorates. Further, in Patent Document 2, in order to equalize the charging rates of the cell strings, adjustment processing in a non-running state must be performed, and there is a problem that the charging rates between the cell strings cannot be adjusted in the draft. That is, the technologies described in Patent Documents 1 and 2 have a problem that the utilization efficiency of the battery pack cannot be sufficiently increased.

[0007] The present invention has been made in view of the above circumstances, and an object thereof is to improve the utilization efficiency of a battery pack.

Means for Solving the Problems

[0008] One aspect of the secondary battery system according to the present invention includes at least two battery strings in which a plurality of battery cells are connected in series, a plurality of cell balance circuits provided for each of the battery strings to reflux flyback current to the plurality of battery cells included in the corresponding battery string, and a balance adjustment circuit that switches whether to operate the cell balance circuit according to the difference in output current between the battery strings. The cell balance circuit includes a plurality of cell coils provided corresponding to each of the battery cells and connected in parallel with the corresponding battery cell, a string coil that is electromagnetically coupled to all of the cell coils provided corresponding to the plurality of battery cells included in one of the battery strings and has one end connected to one of the positive and negative terminals of the corresponding battery string, and a coil switch that switches between connecting and disconnecting the other end of the string coil to the other of the positive and negative terminals of the corresponding battery string. The coil switch is switched between a conductive state and a cutoff state by the balance adjustment circuit.

[0009] One aspect of the balance adjustment program according to the present invention is a balance adjustment program executed in an arithmetic device that controls a secondary battery system including at least two battery strings in which a plurality of battery cells are connected in series, and a plurality of cell balance circuits provided for each of the battery strings to reflux flyback current to the plurality of battery cells included in the corresponding battery string. The cell balance circuit includes a plurality of cell coils provided corresponding to each of the battery cells and connected in parallel with the corresponding battery cell, a string coil that is electromagnetically coupled to all of the cell coils provided corresponding to the plurality of battery cells included in one of the battery strings and has one end connected to one of the positive and negative terminals of the corresponding battery string, and a coil switch that switches between connecting and disconnecting the other end of the string coil to the other of the positive and negative terminals of the corresponding battery string. When the difference in output current between the battery strings becomes equal to or greater than a preset first threshold, the coil switch of the battery string with the larger output current is switched from the cutoff state to the conductive state.

[0010] One aspect of the balance adjustment method according to the present invention includes at least two battery strings in which a plurality of battery cells are connected in series, and a plurality of cell balance circuits provided for each of the battery strings to cause a flyback current to flow back to the plurality of battery cells included in the corresponding battery string. The cell balance circuit includes a plurality of cell coils provided corresponding to each of the battery cells and connected in parallel with the corresponding battery cell, and all of the cell coils provided corresponding to the plurality of battery cells included in one of the battery strings are electromagnetically coupled. A string coil having one end connected to one of the positive and negative terminals of the corresponding battery string, and a coil switch for switching to connect or disconnect the other end of the string coil to the other of the positive and negative terminals of the corresponding battery string. A balance adjustment method for controlling a secondary battery system having the above using an arithmetic device, wherein when the difference in output current between the battery strings becomes equal to or greater than a preset first threshold value, the coil switch of the battery string with the larger output current is switched from the off state to the on state.

Advantages of the Invention

[0011] According to the secondary battery system, balance adjustment program, and balance adjustment method of the present invention, the utilization efficiency of the battery pack can be improved.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0013] For the sake of clarity of explanation, the following descriptions and drawings have been appropriately omitted and simplified. In each drawing, the same reference numerals are assigned to the same elements, and duplicate explanations are omitted as necessary. Also, each element described in the drawings as a functional block that performs various processes can be composed of a CPU (Central Processing Unit), memory, and other circuits in terms of hardware, and can be realized by a program loaded into the memory or the like in terms of software. Therefore, it is understood by those skilled in the art that these functional blocks can be realized in various forms by only hardware, only software, or a combination thereof, and are not limited to any one of them. In each drawing, the same reference numerals are assigned to the same elements, and duplicate explanations are omitted as necessary.

[0014] Also, the above-described program includes a set of instructions (or software code) for causing a computer to perform one or more functions described in the embodiments when loaded into the computer. The program may be stored in a non-transitory computer-readable medium or a tangible storage medium. By way of example and not limitation, the computer-readable medium or tangible storage medium includes random-access memory (RAM), read-only memory (ROM), flash memory, solid-state drive (SSD), or other memory technologies, CD-ROM, digital versatile disc (DVD), Blu-ray (registered trademark) disc, or other optical disc storage, magnetic cassette, magnetic tape, magnetic disk storage, or other magnetic storage devices. The program may also be transmitted on a transitory computer-readable medium or a communication medium. By way of example and not limitation, the transitory computer-readable medium or communication medium includes electrical, optical, acoustic, or other forms of propagated signals.

[0015] Embodiment 1 Fig. 1 shows a schematic diagram of the secondary battery system 1 according to Embodiment 1. In the example shown in Fig. 1, battery strings STG1 to STG3 among the battery packs included in the battery pack of the secondary battery system 1 are shown. The number of battery strings included in the secondary battery system 1 is at least one according to the specifications of the battery pack, but the balance adjustment method according to Embodiment 1 is more effective for a battery pack including two or more battery strings.

[0016] As shown in Fig. 1, the battery strings STG1 to STG3 are connected in parallel between the positive power line VH and the negative power line VL. And the secondary battery system 1 supplies power from the positive power line VH to a load circuit (for example, an external load) provided separately from the battery pack. Also, for the battery strings STG1 to STG3, the positive terminal is connected to the positive power line VH, and the negative terminal is connected to the negative power line VL.

[0017] The battery strings STG1 to STG3 have the same configuration. And in Fig. 1, the same components in the battery strings STG1 to STG3 are given corresponding reference numerals although they have different reference numerals. In the following description, the configuration of the battery string STG1 will be taken as an example to describe the configuration of the battery string.

[0018] As shown in Fig. 1, n battery cells (for example, battery cells BC11 to BC1n) are connected in series to the battery string STG1. In the secondary battery system 1 according to Embodiment 1, the number n of battery cells included in one battery string is a number that is neither excessive nor insufficient with respect to the voltage required by the external load. And the positive terminal of the battery cell BC11 closest to the positive power line VH is connected to the positive power line VH via the string switch SW11. Also, the negative terminal of the battery cell BC1n closest to the negative power line VL is connected to the negative power line VL. Here, in the secondary battery system 1, a current measurement unit AM1 is inserted into the wiring connecting the battery cell BC1n and the negative power line VL so as to be connected in series to the battery string STG1. The current measurement unit AM1 measures the output current of the battery string STG1 and notifies the balance adjustment circuit 100 described later of the current measurement value.

[0019] In FIG. 1, resistors representing the resistances with which battery cells BC11 to BC1n are connected in series to the battery are shown. This resistance schematically represents the internal resistance of the battery cell. This internal resistance has different resistance values individually due to manufacturing variations of the battery cell, deterioration of the battery, differences in charging rates, and the like.

[0020] Also, voltage measurement units VM11 to VM1n for measuring the output voltages of the individual battery cells are provided in the battery cells BC11 to BC1n. The voltage measurement units VM11 to VM1n notify the balance adjustment circuit 100, which will be described later, of the measured voltage values as the output voltages of the corresponding battery cells.

[0021] And in the secondary battery system 1, each battery string has a cell balance circuit. The cell balance circuit is provided for each battery string and causes a flyback current to flow back to the plurality of battery cells included in the corresponding battery string. In FIG. 1, cell balance circuit 10 is shown as the cell balance circuit corresponding to battery string STG1. Cell balance circuit 10 includes cell coils L11 to L1n, diodes D11 to D1n, string coils L1 to Ln, and coil switch SW12. The anode of diode D11 is connected to one end of cell coil L11, and the cathode is connected to the positive terminal of battery cell BC11. The other end of cell coil L11 is connected to the negative terminal of battery cell BC11. The anode of diode D12 is connected to one end of cell coil L12, and the cathode is connected to the positive terminal of battery cell BC12. The other end of cell coil L12 is connected to the negative terminal of battery cell BC12. The anode of diode D1n is connected to one end of cell coil L1n, and the cathode is connected to the positive terminal of battery cell BC1n. The other end of cell coil L1n is connected to the negative terminal of battery cell BC1n. That is, the diodes and cell coils connected in series have the same connection to the corresponding battery cell.

[0022] Also, the string coil L1 is electromagnetically coupled to all of the cell coils (for example, cell coils L11 to L1n) provided corresponding to a plurality of battery cells (for example, battery cells BC11 to BC1n) included in one battery string (for example, battery string STG1), and one end thereof is connected to one of the positive and negative terminals of the corresponding battery string STG1 (the negative terminal of the battery string STG1 in FIG. 1). Then, the coil switch SW12 switches whether to connect or disconnect the other end of the string coil L1 to the other of the positive and negative terminals of the corresponding battery string STG1 (the positive terminal of the battery string STG1 in FIG. 1).

[0023] And, the secondary battery system 1 according to Embodiment 1 has a balance adjustment circuit 100. The balance adjustment circuit 100 switches whether to operate the cell balance circuits 10, 20, 30 according to the difference in output current between the battery strings. More specifically, when the difference in output current between the battery strings becomes equal to or greater than a preset first threshold value, the balance adjustment circuit 100 switches the coil switch of the battery string on the side with the larger output current from the off state to the on state. Also, when the cell voltage difference between a plurality of battery cells included in one battery string becomes equal to or greater than a preset second threshold value, the balance adjustment circuit 100 switches the string switch of the battery string including the battery cells whose cell voltage difference has become equal to or greater than the second threshold value from the off state to the on state.

[0024] In the example shown in FIG. 1, in the example of the cell balance circuit 10, the OR circuit 11 is provided to control the coil switch SW12 from the off state to the on state regardless of whether the cell balance adjustment signal A1 or the string balance adjustment signal B1 is valid. Note that the cell balance adjustment signal A1 and the string balance adjustment signal B1 are based on the voltage measurement values (VM11 to VM1n, VM21 to VM2n, VM31 to VM3n input to the balance adjustment circuit 100 in FIG. 1) acquired from the battery cells included in the secondary battery system 1 and the current measurement values (AM1 to AM3 input to the balance adjustment circuit 100 in FIG. 1) acquired from the battery strings included in the secondary battery system 1, and the balance adjustment circuit 100 switches between the valid state and the invalid state.

[0025] In a battery pack, a battery string with a smaller combined internal resistance of battery cells tends to have a larger discharge amount. Also, within the same string, a battery cell with a smaller internal resistance has a larger discharge amount. Therefore, in the secondary battery system 1 according to Embodiment 1, the cell balance circuit and the balance adjustment circuit are used to reduce the difference in discharge amount between battery strings and the difference in discharge amount between battery cells.

[0026] Hereinafter, the balance adjustment method of the secondary battery system 1 performed using the cell balance circuits 10, 20, 30 and the balance adjustment circuit 100 will be described in detail. The balance adjustment method according to Embodiment 1 is assumed to be implemented by implementing the balance adjustment circuit 100 as dedicated hardware, or by implementing the balance adjustment circuit 100 by an arithmetic unit capable of executing a program and executing a balance adjustment program on the arithmetic unit.

[0027] FIG. 2 shows a flowchart for explaining the balance adjustment procedure in the secondary battery system 1 according to Embodiment 1. The balance adjustment procedure shown in FIG. 2 is implemented by the balance adjustment circuit 100 controlling the coil switches SW12, SW22, SW32. Also, in the secondary battery system 1 according to Embodiment 1, the balance adjustment process shown in FIG. 2 is periodically performed during the operation of the secondary battery system 1 such as during vehicle operation.

[0028] As shown in FIG. 2, when the secondary battery system 1 according to Embodiment 1 starts the balance adjustment process, it extracts the voltage of each battery cell from the acquired voltage measurement value for each battery string, and determines whether there is a battery cell whose voltage difference between battery cells within the same battery string is equal to or greater than a second threshold value (for example, the cell voltage difference threshold value) (step S1). The condition of this step S1 is hereinafter referred to as the cell voltage difference condition.

[0029] Next, in step S1, if there is no battery cell that satisfies the cell voltage difference condition (the NO branch of step S1), the process of step S3 is performed without performing the in-string balance control of step S2. On the other hand, when there is a battery cell that satisfies the cell voltage difference condition (the YES branch of step S1), the balance adjustment circuit 100 performs in-string balance control to switch the coil switch of the cell balance circuit of the battery string including the battery cell that satisfies the cell voltage difference condition from the off state to the on state (step S2).

[0030] Here, in the cell balance circuit of the secondary battery system 1 according to Embodiment 1, during the period when the coil switch is in the conductive state, since current flows into the battery cell from the positive electrode side through the cell coil corresponding to the battery cell whose voltage has dropped, that battery is in a charged state, and the other battery cells are in a discharged state. By such an operation, in the secondary battery system 1 according to Embodiment 1, the discharge amount of the battery cell with a low output voltage (that is, a low charge rate) is suppressed, and the discharge amount of the battery cell with a high output voltage (that is, a high charge rate) is increased, thereby suppressing the difference in charge rate between the battery cells.

[0031] Note that in the determination process of step S1, after performing the in-string balance control, it is preferable to change the cell voltage difference threshold value to a cell voltage difference release threshold value that is higher than the cell voltage difference threshold value and at which it can be determined that the cell voltage difference is sufficiently small. By performing such hysteresis control, the control can be stabilized by continuing the in-string balance control until the cell voltage difference becomes small.

[0032] Subsequently, in the secondary battery system 1 according to Embodiment 1, after the determination process in step S1 or the in-string balance control in step S2 is completed, it is determined whether there is a battery stack in which the inter-string current difference, which is the difference in the discharge amount between battery strings, calculated from the acquired current measurement values, is equal to or greater than a first threshold value (for example, the inter-string current difference threshold value) (step S3). Note that, as the inter-string current difference used for determination in step S3, either a cumulative value during the implementation periods of the previous balance adjustment process and the current balance adjustment process or a value calculated only from the current value at the timing of implementing step S3 may be used.

[0033] In step S3, if there is only a battery string in which the inter-string current difference is less than the inter-string current difference threshold value (the NO branch in step S3), the balance adjustment circuit 100 ends the balance adjustment process without performing step S4. On the other hand, in step S3, if there is a battery string in which the inter-string current difference is less than the inter-string current difference threshold value (the YES branch in step S3), the balance adjustment circuit 100 performs inter-string balance control to switch the coil switch of the cell balance circuit of the battery string on the side with the larger string current among the string sets exceeding the inter-string current difference threshold value from the off state to the on state (step S4), and returns the process to step S1 again.

[0034] Here, in the cell balance circuit of the secondary battery system 1 according to Embodiment 1, during the period when the coil switch is in the on state, since current flows into the battery string from the positive electrode side of the battery string through the string coil corresponding to the battery string with the decreased voltage, that battery string is in a charged state and the other battery strings are in a discharged state. By such an operation, in the secondary battery system 1 according to Embodiment 1, the discharge amount of the battery string with a low output voltage (that is, a low charge rate) and a large discharge amount is suppressed, and the discharge amount of the battery string with a high output voltage (that is, a high charge rate) is increased, thereby suppressing the difference in the charge rate between battery strings.

[0035] Note that in the determination process of step S3, after performing the string - to - string balance control in step S4, it is preferable to change the inter - string current difference threshold value to a value higher than the inter - string current difference threshold value and to an inter - string current difference release threshold value at which it can be determined that the inter - string current difference is sufficiently small. By performing such hysteresis control, the control can be stabilized by ensuring that the string - to - string balance control continues until the inter - string current difference becomes small.

[0036] Here, the change in the charging rate when the balance adjustment process is performed will be described. Therefore, FIG. 3 shows a graph for explaining the change in the charging rate in the secondary battery system 1 according to the first embodiment. The example shown in FIG. 3 is a case where the relative resistance values of the battery strings STG1 to STG3 are such that the battery string STG1 has a large resistance, the battery string STG2 has a medium resistance, and the battery string STG3 has a small resistance. In such a case of the internal resistance ratio, the current values of the respective battery strings are such that the battery string STG1 has a small current, the battery string STG2 has a medium current, and the battery string STG3 has a large current. Also, in FIG. 3, in order to show the effect of the balance adjustment process, a graph showing the time change of the charging rates of the three battery strings without the balance adjustment process is shown in the upper part, and a graph showing the time change of the charging rates of the three battery strings with the balance adjustment process is shown in the lower part, clearly showing the difference due to the presence or absence of the balance adjustment process.

[0037] As shown in FIG. 3, the charging rate of each battery string decreases earlier as the resistance value is smaller and the current value is larger. And by performing the balance adjustment process, the time change of the decrease in the charging rate of the battery string STG1 with a small resistance value and a large current value becomes smaller. On the other hand, for the battery string STG2 with a medium resistance value and a medium current value and the battery string STG3 with a large resistance value and a small current value, the time change of the decrease in the charging rate becomes larger in order to compensate for the output current decreased in the battery string STG1.

[0038] Moreover, when the balance adjustment process is performed, the time until the charging rate of the battery string STG1 reaches the lower limit of the charging rate becomes longer. The available state of the battery pack is until one string that reaches the lower limit of the charging rate occurs. Referring to FIG. 3, it can be seen that by performing the balance adjustment process, the time until the battery string STG1 with the fastest charging rate decrease reaches the lower limit of the charging rate is extended.

[0039] From the above description, in the secondary battery system 1 according to Embodiment 1, by performing balance control between strings using the balance adjustment circuit 100 and the cell balance circuit, the available time of the battery pack can be extended. Also, in the secondary battery system 1 according to Embodiment 1, by performing in-string balance control, the time until the battery cells reach the lower limit of the charging rate and the battery pack becomes unusable can be extended.

[0040] Further, in the secondary battery system 1 according to Embodiment 1, since the cell balance circuit can perform balance control of the charging rate between cells and between stacks even during the operation of the battery pack with current within the string, the effect of extending the usage period of the battery pack is higher than the balance control performed during the suspension of operation.

[0041] Also, in the secondary battery system 1 according to Embodiment 1, since only the comparison of the difference between the voltage value or current value and the threshold value is required without complex calculations for each individual battery cell or each individual battery string, the implementation is easy. In the secondary battery system 1 according to Embodiment 1, since there is no need for a component that reduces the efficiency such as a resistor connected in series with the battery string or battery cell, the battery string or battery cell can be utilized with maximum efficiency. The cell balance circuit of the secondary battery system 1 according to Embodiment 1 can perform two controls, balance control between battery cells and balance control between strings, with one circuit, so the circuit scale can be reduced.

[0042] Note that the present invention is not limited to the above-described embodiments, and can be appropriately modified without departing from the spirit thereof.

Explanation of Reference Numerals

[0043] 1 Secondary battery system 10, 20, 30 Cell balance circuit 11, 21, 31 OR circuit 100 Balance adjustment circuit D11~D1n, D21~D2n, D31~D3n Diode L1, L2, L3 String coil L11~L1n, L21~L2n, L31~L3n Cell coil AM1, AM2, AM3 Current measurement unit VM11~VM1n, VM21~VM2n, VM31~VM3n Voltage measurement unit BC11~BC1n, BC21~BC2n, BC31~BC3n Battery cell SW11, SW21, SW31 String switch SW12, SW22, SW32 Coil switch STG1, STG2, STG3 Battery string

Claims

1. At least two battery strings in which a plurality of battery cells are connected in series, A plurality of cell balance circuits provided for each of the battery strings and causing a flyback current to flow back to the plurality of battery cells included in the corresponding battery string, A balance adjustment circuit that switches whether to operate the cell balance circuit according to the difference in output current between the battery strings, The cell balance circuit is, A plurality of cell coils provided corresponding to each of the battery cells and connected in parallel with the corresponding battery cell, All of the cell coils provided corresponding to the plurality of battery cells included in one of the battery strings are electromagnetically coupled, and one end is connected to one of the positive and negative terminals of the corresponding battery string, A coil switch that switches between connecting and disconnecting the other end of the string coil to the other of the positive and negative terminals of the corresponding battery string, The coil switch is a secondary battery system in which the conduction state and the cutoff state are switched by the balance adjustment circuit.

2. The balance adjustment circuit switches the coil switch of the battery string on the side with the larger output current from the cutoff state to the conduction state when the difference in output current between the battery strings becomes equal to or greater than a preset first threshold value. The secondary battery system according to claim 1.

3. The balance adjustment circuit switches the coil switch of the battery string including the battery cell having a cell voltage difference equal to or greater than the preset second threshold value from the cutoff state to the conduction state when the cell voltage difference between the plurality of battery cells included in one of the battery strings becomes equal to or greater than the preset second threshold value. The secondary battery system according to claim 2.

4. The secondary battery system according to claim 1, further comprising a plurality of diodes provided in each of the plurality of cell coils, having a cathode connected to the positive terminal of the corresponding battery cell and an anode connected to one end of the corresponding cell coil.

5. At least two battery strings in which a plurality of battery cells are connected in series, A plurality of cell balance circuits provided for each of the battery strings and causing a flyback current to flow back to the plurality of battery cells included in the corresponding battery string, The cell balance circuit is, A plurality of cell coils provided corresponding to each of the battery cells and connected in parallel with the corresponding battery cell; A string coil that is electromagnetically coupled to all of the cell coils provided corresponding to the plurality of battery cells included in one of the battery strings, and one end of which is connected to one of the positive and negative terminals of the corresponding battery string; A balance adjustment program executed in an arithmetic unit that controls a secondary battery system having a coil switch that switches whether to connect or disconnect the other end of the string coil to the other of the positive and negative terminals of the corresponding battery string, The balance adjustment program that switches the coil switch of the battery string on the side with the larger output current from the off state to the on state when the difference in output current between the battery strings becomes equal to or greater than a preset first threshold.

6. At least two battery strings in which a plurality of battery cells are connected in series; A plurality of cell balance circuits provided for each of the battery strings and configured to reflux flyback current to the plurality of battery cells included in the corresponding battery string, The cell balance circuit is A plurality of cell coils provided corresponding to each of the battery cells and connected in parallel with the corresponding battery cell; A string coil that is electromagnetically coupled to all of the cell coils provided corresponding to the plurality of battery cells included in one of the battery strings, and one end of which is connected to one of the positive and negative terminals of the corresponding battery string; A balance adjustment method for controlling a secondary battery system having a coil switch that switches whether to connect or disconnect the other end of the string coil to the other of the positive and negative terminals of the corresponding battery string, using an arithmetic unit, The balance adjustment method that switches the coil switch of the battery string on the side with the larger output current from the off state to the on state when the difference in output current between the battery strings becomes equal to or greater than a preset first threshold.

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