Redox flow battery and method for controlling redox flow battery

Abstract

To improve the charging or discharging efficiency of a redox flow battery.SOLUTION: A redox flow battery comprises: an electrolyte tank; a first battery cell that performs charging and discharging by using an electrolyte supplied from the electrolyte tank; a second battery cell that performs charging and discharging by using an electrolyte exhausted from the first battery cell; a pump for circulating the electrolyte; a first monitor cell that acquires a state of charge of the first battery cell; a second monitor cell that acquires a state of charge of the second battery cell; and a controller that controls at least one of a charging current and a discharging current in each of the first and second battery cells by using the state of charge of the first battery cell acquired by the first monitor cell and the state of charge of the second battery cell acquired by the second monitor cell.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a redox flow battery and a method for controlling a redox flow battery. [Background technology]

[0002] Redox flow batteries that use an oxidation-reduction reaction of ions to charge and discharge are known (see, for example, Patent Document 1). The redox flow battery system described in Patent Document 1 includes a monitor cell for measuring the state of charge of a battery cell that is charged and discharged. In this battery system, the state of charge measured by the monitor cell is used to control the flow rate of a pump for circulating the electrolyte within the system, thereby preventing overcharging or overdischarging of the electrolyte. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2015 / 122390 Summary of the Invention [Problem to be solved by the invention]

[0004] In a battery system that uses an electrolyte to charge and discharge (hereinafter also referred to as "charge and discharge") as in Patent Document 1, the voltage during charging and discharging cannot be made higher than the voltage at which water electrolysis occurs. For this reason, the battery system described in Patent Document 1 has a problem in that ions are present in the electrolyte that are not used for charging and discharging, resulting in poor charge and discharge efficiency. In addition, the battery system described in Patent Document 1 is required to be further improved in efficiency, such as by reducing the pump power. Thus, although the technology described in Patent Document 1 can suppress overcharging or overdischarging of the electrolyte, there is room for improvement in the efficiency of the entire system.

[0005] The present invention has been made to solve the above-mentioned problems, and has an object to realize efficient charging or discharging of a redox flow battery. [Means for solving the problem]

[0006] The present invention has been made to solve the above-mentioned problems, and can be realized in the following forms. A redox flow battery comprising: an electrolyte tank for storing an electrolyte; a first battery cell that is charged and discharged using an electrolyte supplied from the electrolyte tank; a second battery cell that is charged and discharged using an electrolyte discharged from the first battery cell; a pump that circulates an electrolyte between the first battery cell, the second battery cell, and the electrolyte tank; a first monitor cell that measures an electromotive force of the electrolyte supplied to the first battery cell to obtain a state of charge of the first battery cell; and a second monitor cell that measures an electromotive force of the electrolyte supplied to the second battery cell to obtain a state of charge of the second battery cell. and a control unit that controls at least one of a charge current and a discharge current in each of the first battery cell and the second battery cell using a state of charge of the first battery cell acquired by the first monitor cell and a state of charge of the second battery cell acquired by the second monitor cell, wherein the control unit controls at least one of a charge current and a discharge current in each of the first battery cell and the second battery cell such that a charge voltage or a discharge voltage of each of the first battery cell and the second battery cell, which is determined from the state of charge of the first battery cell and the state of charge of the second battery cell, is 1.55 V or less. The present invention can also be realized in the following forms.

[0007] (1) According to one aspect of the present invention, there is provided a redox flow battery comprising: an electrolyte tank that stores an electrolyte; a first battery cell that charges and discharges using the electrolyte supplied from the electrolyte tank; a second battery cell that charges and discharges using the electrolyte discharged from the first battery cell; a pump that circulates the electrolyte between the first battery cell, the second battery cell, and the electrolyte tank; a first monitor cell that acquires a state of charge of the first battery cell by measuring an electromotive force of the electrolyte supplied to the first battery cell; a second monitor cell that acquires a state of charge of the second battery cell by measuring an electromotive force of the electrolyte supplied to the second battery cell; and a control unit that controls at least one of a charging current and a discharging current in each of the first battery cell and the second battery cell by using the state of charge of the first battery cell acquired by the first monitor cell and the state of charge of the second battery cell acquired by the second monitor cell.

[0008] In order to prevent water decomposition of the electrolyte during charging / discharging of at least one of the first battery cell and the second battery cell, the voltages of the first battery cell and the second battery cell need to be controlled to a predetermined value or less. Therefore, some of the ions in the electrolyte supplied to the first battery cell are discharged without being used for charging / discharging. In this regard, according to the present configuration, the second battery cell can perform charging / discharging by using the ions in the electrolyte discharged from the first battery cell. This reduces the amount of ions recirculated by the pump among the ions not used for charging / discharging, thereby reducing the pump power. In other words, according to the redox flow battery of the present configuration, charging / discharging can be performed efficiently.

[0009] (2) In the redox flow battery of the above aspect, the control unit may control at least one of a charging current and a discharging current so that a charging voltage or a discharging voltage of each of the first battery cell and the second battery cell, which is determined from a charging state of the first battery cell and a charging state of the second battery cell, becomes 1.55 V or less. According to this configuration, the voltage of at least one of the first battery cell and the second battery cell during charging and discharging is controlled to a voltage that does not cause electrolysis of water, thereby making it possible to make charging and discharging more efficient.

[0010] (3) In the redox flow battery of the above aspect, the first monitor cell may detect a first voltage as the state of charge of the first battery cell, and the second monitor cell may detect a second voltage as the state of charge of the second battery cell, and the control unit may calculate a resistance value R of the first battery cell using a voltage difference obtained by subtracting the second voltage from the first voltage and a charging current I or a discharging current I of the first battery cell, and may use the calculated resistance value R to set a charging current I or a discharging current I that satisfies the following relational expression (1):

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[0011] The present invention can be realized in various forms, for example, in the form of a redox flow battery, a redox flow battery system, a control device for a redox flow battery, a control method for a redox flow battery and a computer program for executing these devices, a server device for distributing this computer program, a non-transitory storage medium on which a computer program is stored, etc. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic block diagram of a redox flow battery according to an embodiment of the present invention. [Diagram 2] FIG. 4 is an explanatory diagram of voltage changes during charging and discharging. [Diagram 3] FIG. 4 is an explanatory diagram of voltage changes during charging and discharging. [Figure 4] FIG. 2 is an explanatory diagram of the relationship between ions contained in an electrolyte and voltage. [Diagram 5] FIG. 1 is an explanatory diagram of the relationship between voltage and current with respect to changes in the molar concentration of an electrolyte. [Figure 6] 1 is a flowchart of a control method during charging of a redox flow battery. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] First Embodiment 1 is a schematic block diagram of a redox flow battery 100 according to an embodiment of the present invention. The redox flow battery 100 of this embodiment is a so-called vanadium redox flow battery in which a first battery cell 1 and a second battery cell 2 are charged and discharged (hereinafter also referred to as "charge and discharge") by changing the valence of vanadium ions in an electrolyte solution between the positive electrode and the negative electrode. In the redox flow battery 100 of this embodiment, the second battery cell 2 is charged or discharged using vanadium ions contained in the electrolyte solution that has passed through the first battery cell, thereby enabling efficient charging and discharging.

[0014] The redox flow battery 100 shown in FIG. 1 includes a first battery cell 1 and a second battery cell 2 that perform charging and discharging, a positive electrode electrolyte tank (electrolyte tank) 5 and a negative electrode electrolyte tank (electrolyte tank) 6 that store an electrolyte containing vanadium ions, a positive electrode pump (pump) 7 for circulating the electrolyte stored in the positive electrode electrolyte tank 5 to the first battery cell 1 and the second battery cell 2, a negative electrode pump (pump) 8 for circulating the electrolyte stored in the negative electrode electrolyte tank 6 to the second battery cell 2, circulation piping 9 that constitutes an electrolyte flow path circulated by the positive electrode pump 7 and the negative electrode pump 8, a first monitor cell 3, a second monitor cell 4, a DC / AC converter 20 (hereinafter simply referred to as “converter 20”) that converts DC current to AC current, and a control unit 10 that controls each unit. In the following, the first battery cell 1 and the second battery cell 2 will be collectively referred to as "battery cells 1, 2", the positive electrode electrolyte tank 5 and the negative electrode electrolyte tank 6 will be collectively referred to as "tanks 5, 6", the positive electrode pump 7 and the negative electrode pump 8 will be collectively referred to as "pumps 7, 8", and the first monitor cell 3 and the second monitor cell 4 will be collectively referred to as "monitor cells 3, 4".

[0015] The first battery cell 1 is supplied with electrolyte from each of the tanks 5 and 6. The battery cell 1 includes a positive electrode, a negative electrode, and an ion exchange membrane that separates the electrolyte on the positive electrode side from the electrolyte on the negative electrode side, which are not shown in FIG. 1. The first battery cell 1 is supplied with different electrolytes to the positive electrode side electrode and the negative electrode side electrode, respectively. The first battery cell 1 is connected to a converter 20 as shown in FIG. 1. The first battery cell 1 is charged and discharged by controlling the charge current or discharge current by the control unit 10. The second battery cell 2, like the first battery cell 1, includes a positive electrode, a negative electrode, and an ion exchange membrane that separates the electrolyte on the positive electrode side from the electrolyte on the negative electrode side. The second battery cell 2 is connected downstream of the first battery cell 1, and the electrolyte is supplied to the second battery cell 2 from the first battery cell 1.

[0016] The electrolyte stored in the positive electrode electrolyte tank 5 contains vanadium oxide ions (VO 2+ ) and vanadium dioxide ion (VO 2 + The electrolyte stored in the negative electrode electrolyte tank 6 contains divalent vanadium ions (V 2+ ) and trivalent vanadium ions (V 3+ ) and is sent to the negative side of the first battery cell 1 by the negative electrode pump 8.

[0017] The reaction shown in the following formula (2) occurs on the positive electrode sides of the battery cells 1 and 2. The reaction shown in the following formula (3) occurs on the negative electrode sides of the battery cells 1 and 2.

[0018]

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[0019] The circulation pipe 9 is a flow path that connects the first battery cell 1 and the second battery cell 2. The converter 20 converts the direct current generated or discharged by the first battery cell 1 and the second battery cell 2 into an alternating current.

[0020] The monitor cells 3 and 4 have a similar configuration to the battery cells 1 and 2, but as shown in Fig. 1, they are not connected to the converter 20 and are battery cells that do not contribute to charging or discharging. As shown in Fig. 1, the battery cells 1 and 2 and the monitor cells 3 and 4 are connected so that the electrolyte passes through them in the order of the first monitor cell 3, the first battery cell 1, the second monitor cell 4, and the second battery cell 2 from the upstream side of the pumps 7 and 8 through which the electrolyte is sent. Therefore, the second battery cell 2 charges and discharges using the electrolyte discharged from the first battery cell 1.

[0021] The first monitor cell 3 obtains the state of charge of the first battery cell 1 by measuring the electromotive force of the electrolyte supplied to the first battery cell 1. The second monitor cell 4 obtains the state of charge of the second battery cell 2 by measuring the electromotive force of the electrolyte supplied to the second battery cell 2. Specifically, the first monitor cell 3 detects the open circuit voltage of the first battery cell 1, and the second monitor cell 4 detects the open circuit voltage of the second battery cell 2.

[0022] The control unit 10 of this embodiment acquires the charging states of the battery cells 1, 2 acquired by the first monitor cell 3 and the second monitor cell 4, respectively, and the voltage of the converter 20. The control unit 10 controls the charging current or discharging current of each of the battery cells 1, 2 being charged or discharged, according to the acquired charging states of the battery cells 1, 2. Specifically, the control unit 10 controls the charging current and discharging current so that the voltage of each of the battery cells 1, 2 being charged or discharged is 1.55 V (volts) or less, at which electrolysis of water begins. In this embodiment, when the charging current or discharging current is the current I, the control unit 10 calculates the resistance R1 of the first battery cell 1 by dividing the voltage value obtained by subtracting the detected voltage (second voltage) of the second monitor cell 4 from the detected voltage (first voltage) of the first monitor cell 3 by the current I. The control unit 10 uses the calculated resistance R1 to control the current I so as to satisfy the following relational expression (4).

[0023]

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[0024] The control unit 10 controls the voltage V of the first battery cell 1 during charging of the battery cells 1 and 2. C1 reaches a preset voltage (open circuit voltage). In this embodiment, the control unit 10 controls the charging current so that the charging voltage of the second monitor cell 4 becomes 1.55 V or less, and when charging of the first battery cell 1 ends, charging of the second battery cell 2 also ends.

[0025] 2 and 3 are explanatory diagrams of voltage changes during charging and discharging. In FIG. 2, the voltage V C1 (solid line) and the detected voltage V of the first monitor cell 3 M1 (dotted line) and the detected voltage V of the second monitor cell 4 M2 2. FIG. 3 shows a graph enlarging an area AR1 in FIG. 2. During charging, the battery cells 1 and 2 are charged using vanadium ions in the electrolyte, so the higher the ion concentration of vanadium ions in the electrolyte, the smaller the resistance R of the battery cells 1 and 2. The ion concentration in the electrolyte flowing into the second monitor cell 4 located downstream of the first monitor cell 3 is lower than the ion concentration in the electrolyte flowing into the first monitor cell 3. Therefore, as shown in FIG. 2, the detected voltage V of the second monitor cell 4 M2 is the detected voltage V of the first monitor cell 3 M1 During charging, the first battery cell 1 has an increased voltage due to the resistance R for charging, so the voltage V C1 is the detected voltage V of monitor cells 3 and 4 M1 ,V M2 When the state of battery cells 1 and 2 changes from charging to discharging, the voltages V C1 ,V M1 ,V M2 The magnitude relationship is the opposite of the state of charge.

[0026] FIG. 4 is an explanatory diagram of the relationship between ions contained in the electrolyte and voltage. FIG. 4 shows the potential Ve (voltage) during charging, which changes according to the ratio (%) of tetravalent vanadium ions in the electrolyte supplied from the positive electrode electrolyte tank 5 to the first battery cell 1 and is calculated from the Nernst equation. As shown in FIG. 4, during charging, the higher the concentration of tetravalent vanadium ions, the higher the voltage of the first battery cell 1. In the redox flow battery 100 of this embodiment, for example, when the voltage of the second battery cell 2 is 1.42 V, about 50% of the vanadium ions capable of charging the second battery cell 2 remain in the electrolyte. When the voltage of the second battery cell 2 is 1.47 V, about 70% of the vanadium ions capable of charging the second battery cell 2 remain in the electrolyte. Therefore, in the second battery cell 2, the vanadium ions not used in charging the first battery cell 1 are used for charging.

[0027] 5 is a diagram illustrating the relationship between voltage and current with respect to changes in the molar concentration of the electrolyte. In FIG. 5, the current density (A / m) of the charging current controlled by the control unit 10 is plotted against the change in the molar concentration of the electrolyte when electrolytes of different molar concentrations are supplied to the first battery cell 1 during charging. 2 ) and the voltage (V) applied to the first battery cell 1. Specifically, when the molar concentration of vanadium ions is 500 mol / m 3 The voltage change is shown by black circles and solid lines, and the molar concentration is 850 mol / m 3 The voltage change is shown by the open triangle and the dashed line, and the molar concentration is 1600 mol / m 3 The voltage change is indicated by the white squares and the dashed line. The flow rate of the electrolyte, controlled by the pumps 7 and 8, is 6.3 ml / min.

[0028] 5, regardless of the molar concentration of the electrolyte, when the charging current by the control unit 10 increases, the voltage of the first battery cell 1 also increases. Also, the higher the molar concentration of the electrolyte, the lower the resistance during charging, and therefore the lower the voltage of the first battery cell 1 even with the same charging current. In this embodiment, the control unit 10 controls the charging current so that the voltage of each of the battery cells 1 and 2 does not exceed 1.55V.

[0029] 6 is a flowchart of a control method for charging the redox flow battery 100. As shown in FIG. 6, in the control flow for charging the redox flow battery 100, first, the control unit 10 operates the pumps 7 and 8 to start flowing a charging current to each of the battery cells 1 and 2, thereby starting charging (step S1). In this embodiment, by operating the redox flow battery 100 in advance, the initial charging current I 10 and the initial charging current I 20 The initial charging current I 10 ,I 20 is set to a current value within a range in which the voltage of each of the battery cells 1 and 2 does not exceed 1.55 V, using data previously obtained by operating the redox flow battery 100.

[0030] The control unit 10 performs a state acquisition step of detecting the voltage of the electrolyte flowing into each of the monitor cells 3 and 4 (step S2). M1 is the same as the voltage of the electrolyte flowing into the first battery cell 1. Similarly, the detected voltage V M2 is the same as the voltage of the second battery cell 2. After that, the monitor cells 3 and 4 continue to detect the voltage of the electrolyte. Note that the state acquisition step of step S2 corresponds to the first state acquisition step and the second state acquisition step.

[0031] The control unit 10 detects the voltages V of the monitor cells 3 and 4. M1 ,V M2 and the charging current I 10 ,I 20 The control unit 10 calculates the resistance R1 of the first battery cell 1 and the resistance R2 of the second battery cell 2 using the resistances R1 and R2 of the first and second battery cells 1 and 2 and the detected voltages V of the monitor cells 3 and 4 (step S3). M1 ,V M2By substituting these values ​​into the equation (4), the control unit 10 calculates the charging currents I 10 ,I 20 to the calculated charging current (step S5). Note that the processes from step S3 to step S5 correspond to a control step.

[0032] The control unit 10 determines whether or not to terminate charging of the redox flow battery 100 (step S6). In this embodiment, the control unit 10 determines whether or not to terminate charging based on whether or not the voltage of the first battery cell 1 has reached a preset voltage. If the control unit 10 determines that the voltage of the first battery cell 1 has not reached the preset voltage (step S6: NO), it repeats the processes from step S3 onwards. If the control unit 10 determines that the voltage of the first battery cell 1 has reached the preset voltage, it terminates the control flow during charging of the redox flow battery 100.

[0033] As described above, in the redox flow battery 100 of this embodiment, the second battery cell 2 is charged and discharged using the electrolyte discharged from the first battery cell 1. The first monitor cell 3 acquires the state of charge of the first battery cell 1 by measuring the electromotive force of the electrolyte supplied to the first battery cell 1. The second monitor cell 4 acquires the state of charge of the second battery cell 2 by measuring the electromotive force of the electrolyte supplied to the second battery cell 2. The control unit 10 controls the charge current or discharge current of each of the battery cells 1 and 2 being charged or discharged according to the acquired states of charge of the battery cells 1 and 2. During charging and discharging of the first battery cell 1, in order to prevent water decomposition of the electrolyte, it is necessary to control the voltage of the first battery cell 1 to 1.55 V or less. Therefore, a part of the vanadium ions in the electrolyte supplied to the first battery cell 1 is discharged without being used for charging. In this embodiment, the second battery cell 2 can be charged using the vanadium ions in the electrolyte discharged from the first battery cell 1. As a result, the amount of vanadium ions not used for charging that are recirculated by the pumps 7 and 8 is suppressed, and the pump power can be reduced. That is, according to the redox flow battery 100 of the present embodiment, charging or discharging can be performed efficiently.

[0034] Furthermore, the control unit 10 of this embodiment controls the charging current and discharging current so that the voltage of each of the battery cells 1, 2 during charging or discharging is equal to or lower than 1.55 V (volts), at which water electrolysis begins. This prevents water electrolysis from occurring during charging or discharging of the battery cells 1, 2, making charging and discharging more efficient.

[0035] Furthermore, when the charging current or discharging current is the current I, the control unit 10 of this embodiment calculates the resistance R1 of the first battery cell 1 by dividing the voltage value obtained by subtracting the detected voltage (second voltage) of the second monitor cell 4 from the detected voltage (first voltage) of the first monitor cell 3 by the current I. The control unit 10 uses the calculated resistance R1 to control the current I so as to satisfy the above relational expression (4). In other words, the control unit 10 can use the calculated resistance R1 to determine the current I during charging and discharging within a range in which water electrolysis does not occur. As a result, water electrolysis does not occur during charging and discharging of the battery cells 1 and 2, thereby making charging and discharging more efficient.

[0036] <Modifications of the above embodiment> The present invention is not limited to the above-described embodiment, and can be embodied in various forms without departing from the spirit and scope of the invention. For example, the following modifications are also possible.

[0037] In the above embodiment, a vanadium redox flow battery is given as an example of the redox flow battery 100, but the redox flow battery may be a well-known redox flow battery that uses ions other than vanadium ions. The configuration of the redox flow battery and the control executed by it are not limited to the battery shown in the redox flow battery 100 having the configuration shown in Fig. 1 and can be modified. For example, the redox flow battery 100 may include three or more battery cells, and may include monitor cells corresponding to the number of battery cells.

[0038] In the present embodiment, the control unit 10 controls the currents of the battery cells 1 and 2 during charging and discharging so that the voltages of the battery cells 1 and 2 during charging and discharging are 1.55V or less, but may perform a different control. For example, the control unit 10 may control the current of only the first battery cell 1 during charging. In this case, the control unit 10 may set the charging current of the second battery cell 2 to be the same as the charging current of the first battery cell 1. The control unit 10 calculates the resistance value R1 by using the above relational expression (1) and then controls the current of the first battery cell 1, but may control the charging current of the first battery cell 1 by a different control. For example, the control unit 10 may have a correspondence map created in advance between the detected voltage of the first monitor cell 3 and the current at which the voltage of the first battery cell 1 becomes 1.55V or less, and may control the charging current using the correspondence map.

[0039] The redox flow battery 100 in the above embodiment includes the monitor cells 3 and 4 as a component for acquiring the state of charge of each of the battery cells 1 and 2, but the state of charge may be acquired by other components. The state of charge of the battery cells 1 and 2 may be acquired from the potential of the electrolyte measured by a voltmeter, or may be acquired from the hue, transparency, and absorbance of the electrolyte measured by a spectrophotometer.

[0040] Although the present aspect has been described above based on the embodiment and modified examples, the above-mentioned embodiment of the aspect is intended to facilitate understanding of the present aspect and does not limit the present aspect. The present aspect may be modified or improved without departing from the spirit and scope of the claims, and equivalents are included in the present aspect. Furthermore, if a technical feature is not described as essential in this specification, it may be deleted as appropriate. [Explanation of symbols]

[0041] 1…1st battery cell 2…Second battery cell 3…1st monitor cell 4…Second monitor cell 5…Positive electrode electrolyte tank (electrolyte tank) 6...Anode electrolyte tank (electrolyte tank) 7…Positive electrode pump (pump) 8…Negative electrode pump (pump) 9…Circulation piping 10...Control section 20...DC / AC converter 100...Redox flow battery AR1…area I…Current I 10 …Initial charging current of the first battery cell I 20 …Initial charging current of the second battery cell R1: Resistance of the first battery cell R2: Resistance of the second battery cell V C1 …DC / AC converter voltage V M1 …Detection voltage of the first monitor cell V M2 …Detection voltage of the second monitor cell

Claims

1. A redox flow battery, an electrolyte tank storing an electrolyte; a first battery cell that is charged and discharged using an electrolyte supplied from an electrolyte tank; a second battery cell that is charged and discharged using the electrolyte discharged from the first battery cell; a pump that circulates an electrolyte between the first battery cell, the second battery cell, and the electrolyte tank; a first monitor cell that measures an electromotive force of an electrolyte supplied to the first battery cell to obtain a state of charge of the first battery cell; a second monitor cell that measures an electromotive force of an electrolyte supplied to the second battery cell to obtain a state of charge of the second battery cell; a control unit that controls at least one of a charging current and a discharging current in each of the first battery cell and the second battery cell, using a charging state of the first battery cell acquired by the first monitor cell and a charging state of the second battery cell acquired by the second monitor cell; Equipped with the control unit controls at least one of a charging current and a discharging current so that a charging voltage or a discharging voltage of each of the first battery cell and the second battery cell, which is determined from a charging state of the first battery cell and a charging state of the second battery cell, is 1.55 V or less.

2. The redox flow battery according to claim 1, the first monitor cell detects a first voltage as the state of charge of the first battery cell; the second monitor cell detects a second voltage as the state of charge of the second battery cell; The control unit is calculating a resistance value R of the first battery cell using a voltage difference obtained by subtracting the second voltage from the first voltage and a charging current I or a discharging current I of the first battery cell; The calculated resistance value R is used to set a charging current I or a discharging current I that satisfies the following relational formula (1). [0010]

3. A method for controlling a redox flow battery, comprising: a first state acquisition step of acquiring a state of charge of a first battery cell, the first battery cell being charged and discharged using an electrolyte supplied from an electrolyte tank, by measuring an electromotive force of the electrolyte supplied to the first battery cell; a second state acquisition step of acquiring a state of charge of a second battery cell, the second battery cell being charged and discharged using the electrolyte discharged from the first battery cell, by measuring an electromotive force of the electrolyte supplied to the second battery cell; a control step of controlling at least one of a charging current and a discharging current in each of the first battery cell and the second battery cell, using the charging state of the first battery cell acquired in the first state acquisition step and the charging state of the second battery cell acquired in the second state acquisition step; Equipped with The control step controls at least one of a charging current and a discharging current so that the charging voltage or the discharging voltage of each of the first battery cell and the second battery cell, which is calculated from the charging state of the first battery cell and the charging state of the second battery cell, is 1.55 V or less.

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