ELECTRICITY STORAGE DEVICE AND METHOD FOR DETECTING ABNORMAL DISCHARGE IN ELECTRICITY STORAGE DEVICE

Abstract

To provide a power storage device and an abnormal discharge detection method for properly deciding a reference value for determining whether or not abnormal discharge occurs in a storage cell with the smallest balancer discharge electric quantity.SOLUTION: A power storage device comprises: a plurality of power storage cells; a balancer circuit which individually discharges each power storage cell; and a management unit. The management unit executes: reduction processing of discharging a power storage cell with relatively high voltage or residual electric quantity by the balancer circuit to reduce difference in voltage or difference in the residual electric quantity among the plurality of power storage cells; determination processing of comparing the difference between a balancer discharge electric quantity of a specific power storage cell with the smallest balancer discharge electric quantity discharged by the balancer circuit in a predetermined period and balancer discharge electric quantities of other power storage cells with a reference value to determine whether or not abnormal discharge occurs in the specific power storage cell; and decision processing of deciding the reference value on the basis of one of variation of the balancer discharge electric quantities of the plurality of power storage cells in the predetermined period and variation of self-discharge electric quantities of the plurality of power storage cells in the predetermined period.SELECTED DRAWING: Figure 7

Description

[Technical field]

[0001] The present invention relates to an electricity storage device and a method for detecting abnormal discharge in an electricity storage device. [Background technology]

[0002] In energy storage cells such as lithium-ion batteries, minute internal short circuits (an example of abnormal discharge) can occur due to metal contamination (the inclusion of foreign matter not used as a raw material during production, packaging, and transportation; hereafter abbreviated as metal contamination) inside the energy storage cell. In energy storage cells, a discharge path can be formed due to a failure (migration, dendrites, etc.) on the board of the management system that manages the energy storage cells, causing minute discharges (an example of abnormal discharge).

[0003] When an abnormal discharge occurs in a storage cell, the voltage or remaining amount of electricity decreases. When the voltage difference or remaining amount of electricity difference between the storage cells is reduced by discharging a storage cell with a relatively high voltage or a storage cell with a relatively high remaining amount of electricity, the storage cell where an abnormal discharge occurs has a low voltage or remaining amount of electricity, so there is less opportunity for it to be discharged by the balancer circuit. There is known a technique for determining whether an internal short circuit has occurred based on the difference in the amount of electricity discharged by a balancer circuit (hereinafter referred to as the balancer discharge amount of electricity) (see, for example, Patent Document 1). Specifically, the battery management system described in Patent Document 1 includes a cell balancing unit that balances a plurality of battery cells. The battery management system compares the difference between the maximum value (CB_max) of the cell balancing discharge capacity of each battery cell and the cell balancing discharge capacity (CB_n) of each battery cell with a reference value (REF), and determines that a battery cell among the plurality of battery cells whose difference is greater than the reference value is a short-circuited battery cell. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5117537 Summary of the Invention [Problem to be solved by the invention]

[0005] In the case of the battery management system described in the above-mentioned Patent Document 1, it is necessary to appropriately determine the reference value (REF) in order to accurately determine whether or not an internal short circuit has occurred. However, Patent Document 1 does not disclose how to determine the reference value, and there is room for improvement in determining an appropriate reference value.

[0006] This specification discloses a technique that can appropriately determine a reference value for determining whether or not abnormal discharge has occurred in a power storage cell having the smallest balancer discharge quantity of electricity. [Means for solving the problem]

[0007] The energy storage device comprises a plurality of energy storage cells, a balancer circuit that individually discharges each energy storage cell, and a management unit, wherein the management unit executes a reduction process that reduces the voltage difference or the difference in the remaining amount of electricity among the plurality of energy storage cells by discharging a storage cell having a relatively high voltage or remaining amount of electricity using the balancer circuit, a determination process that determines whether or not abnormal discharge is occurring in the specific energy storage cell by comparing the difference between the balancer discharge amount of electricity of a specific energy storage cell having the smallest balancer discharge amount of electricity discharged by the balancer circuit in a specified period and the balancer discharge amounts of electricity of other energy storage cells with a reference value, and a determination process that determines the reference value based on at least one of the variation in the balancer discharge amount of electricity of the plurality of energy storage cells in the specified period and the variation in the self-discharge amount of electricity of the plurality of energy storage cells in the specified period. Effect of the Invention

[0008] With the above configuration, it is possible to appropriately determine the reference value for determining whether or not abnormal discharge has occurred in the power storage cell having the smallest balancer discharge quantity of electricity. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic diagram of a vehicle equipped with a power storage device according to a first embodiment; [Diagram 2] Schematic diagram of a vehicle's power supply system [Diagram 3] Exploded perspective view of the power storage device [Figure 4A] Plan view of the storage element [Figure 4B] Cross-sectional view of line AA shown in FIG. 4A [Diagram 5] A block diagram showing an electrical configuration of a power storage device. [Figure 6] FIG. 1 is a schematic diagram for explaining the operation of a balancer circuit; [Figure 7] Flowchart of internal short circuit determination process [Figure 8] Schematic diagram to explain the plateau region [Figure 9] Schematic diagram to explain the variation in the amount of electricity in a storage cell DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] (Outline of this embodiment) (1) According to one aspect of the present invention, an energy storage device includes a plurality of energy storage cells, a balancer circuit that individually discharges each of the energy storage cells, and a management unit, and the management unit executes a reduction process in which a storage cell having a relatively high voltage or remaining amount of electricity is discharged by the balancer circuit to reduce a difference in voltage or a difference in remaining amount of electricity among the plurality of energy storage cells, a determination process in which a difference between a balancer discharge amount of electricity of a specific energy storage cell having the smallest balancer discharge amount of electricity discharged by the balancer circuit in a specified period and a balancer discharge amount of electricity of other energy storage cells is compared with a reference value to determine whether or not abnormal discharge has occurred in the specific energy storage cell, and a determination process in which a variation in the balancer discharge amount of electricity of the plurality of energy storage cells in the specified period and a variation in the self-discharge amount of electricity of the plurality of energy storage cells in the specified period are determined based on at least one of the variation in the balancer discharge amount of electricity of the plurality of energy storage cells in the specified period.

[0011] In the case where there are a plurality of other storage cells, the above-mentioned "balancer discharge electricity amount of the other storage cells" may be the balancer discharge electricity amount of any one of the other storage cells, may be the average value of the balancer discharge electricity amounts of the other two or more storage cells, or may be the median value of the balancer discharge electricity amounts of the other two or more storage cells. In the case of any one of the other storage cells, the other one of the other storage cells may be the storage cell with the second smallest balancer discharge electricity amount after the storage cell with the smallest balancer discharge electricity amount in a predetermined period, or may be the storage cell with the largest balancer discharge electricity amount in a predetermined period.

[0012] Even if the storage cell is not connected to an electrical load, the voltage and remaining amount of electricity decrease due to self-discharge. The self-discharge amount of electricity [Ah] of the storage cell varies depending on the charging state and temperature of the storage cell, but the self-discharge amount of electricity may vary even if the charging state and temperature are the same. For example, consider a case where there are two storage cells and neither storage cell is experiencing abnormal discharge. In this case, the difference in the balancer discharge amount of electricity between the two storage cells in a specified period should ideally be 0, but in reality, the difference occurs due to the variation in the self-discharge amount of electricity. The control unit reduces the difference in the voltage or remaining amount of electricity between the two storage cells by the balancer circuit, so the difference in the balancer discharge amount of electricity between the two storage cells in a specified period corresponds to the true value of the variation in the self-discharge amount of electricity between the two storage cells in the specified period.

[0013] However, in addition to the above-mentioned variation in the self-discharge quantity of electricity, or instead of the variation in the self-discharge quantity of electricity, the variation in the balancer discharge quantity of electricity may cause a difference in the balancer discharge quantity of electricity. For example, in the case of a balancer circuit having a discharge resistor for each storage cell, the tolerance of the discharge resistor may cause variation in the balancer discharge quantity of electricity.

[0014] When neither of the storage cells is experiencing abnormal discharge, even if a difference occurs in the balancer discharge quantity of electricity between the two storage elements, the difference can be explained by the variation in the self-discharge quantity of electricity or the variation in the balancer discharge quantity of electricity. In contrast, when an abnormal discharge occurs in one of the storage cells, the storage cell experiencing abnormal discharge has fewer opportunities to be discharged by the balancer circuit, and a large difference occurs in the balancer discharge quantity of electricity between the storage cells that cannot be explained by the variation in the self-discharge quantity of electricity or the variation in the balancer discharge quantity of electricity.

[0015] According to the above-mentioned energy storage device, the reference value is determined based on at least one of the variation in the balancer discharge amount of electricity of the plurality of energy storage cells in a predetermined period and the variation in the self-discharge amount of electricity of the plurality of energy storage cells in a predetermined period. Therefore, the reference value for determining whether or not abnormal discharge is occurring in a specific energy storage cell can be appropriately determined in consideration of the variation in the balancer discharge amount of electricity and the variation in the self-discharge amount of electricity. In the above description, an example was given in which there are two storage cells, but the number of storage cells is not limited to two, and may be three or more.

[0016] (2) The management unit may determine the reference value based on a first parameter that may arise due to a frequency at which the specific storage cell and other storage cells are discharged by the balancer circuit during the specified period, and a second parameter that may arise due to a self-discharge amount of electricity of the specific storage cell and other storage cells during the specified period.

[0017] For example, if the difference between the balancer discharge quantity of a specific storage cell and the balancer discharge quantity of other storage cells in a predetermined period is equal to or less than the sum of the maximum difference in the balancer discharge quantity of electricity that may occur due to the variation in the balancer discharge quantity of electricity (one example of a first parameter) and the maximum difference in the balancer discharge quantity of electricity that may occur due to the variation in the self-discharge quantity of electricity (one example of a second parameter), the difference can be explained by the variation in the balancer discharge quantity of electricity and the variation in the self-discharge quantity of electricity. On the other hand, if the difference is greater than the above-mentioned sum, the difference cannot be explained by the variation in the balancer discharge quantity of electricity or the variation in the self-discharge quantity of electricity, so there is a high possibility that the specific storage cell is internally short-circuited. Therefore, if the above-mentioned sum is determined as the reference value, the reference value for determining whether the specific storage cell is internally short-circuited can be appropriately determined.

[0018] The reference value is not limited to the sum of the above-mentioned maximum values, and may be the square root of each maximum value. Instead of using the above-mentioned maximum values, the width of the variation may be determined based on 2σ of the variation (95% confidence interval in normal distribution of the balancer discharge electricity amount or self-discharge electricity amount), and the sum of the determined width or the sum of squares may be used as the reference value.

[0019] (3) As the balancer discharge electricity amount of the other storage cell, an average value of the balancer discharge electricity amounts of two or more other storage cells may be used.

[0020] According to the above-described power storage device, the reference value can be determined more appropriately than when the balancer discharge electricity amount of any one of the other power storage cells is used.

[0021] (4) The storage device may include a temperature measuring unit that measures a temperature of the storage cells, and the management unit may execute a recording process that acquires and records variation using a table in which the temperature and charge state of the storage cells correspond to the self-discharge amount of electricity of the storage cells over a certain period of time, and may obtain the variation in the self-discharge amount of electricity of the multiple storage cells by summing up the variation recorded by the recording process that is recorded for the specified period.

[0022] The inventors of the present application have found that the variation in the self-discharge amount of electricity of the storage cells varies depending on the state of charge (SOC) of the storage cells, the temperature of the storage cells, etc. The state of charge (SOC) can also be said to be the remaining amount of electricity. According to the above-described energy storage device, the variation in the self-discharge amount of electricity of the energy storage cell over a certain period of time can be obtained from a table in which the variation in the self-discharge amount of electricity of the energy storage cell over a certain period of time is associated with each combination of the temperature of the energy storage cell and the SOC of the energy storage cell, thereby making it possible to determine the variation according to the SOC and temperature of the energy storage cell.

[0023] (5) The plurality of storage cells may have a plateau region in which a change in voltage relative to a change in the state of charge is small.

[0024] A storage cell such as a lithium-ion secondary battery may be overcharged or overdischarged due to a malfunction of a peripheral device such as a charger or an electric load, or due to a variation in the amount of electricity between a plurality of storage cells. For this reason, when an abnormal state of a storage cell is detected in a storage device, the storage cell is generally protected by switching a current interruption device such as a relay or a semiconductor switch connected between the storage cell and the charger (or between the storage cell and the electric load) to an interruption state (open, open, off).

[0025] On the other hand, when a power storage device is mounted on a vehicle such as an automobile, if the current interruption device is set in an interrupted state to protect the power storage cells, the power supply to each electric load of the vehicle becomes unstable. For example, if the alternator (generator) of the vehicle breaks down and the power storage cells become overcharged, if the current interruption device is set in an interrupted state, the power supply to the electric loads will be maintained only by the broken alternator. This puts the vehicle in a situation where it is possible for the power supply to be lost at any time. In such a case, even if the alternator breaks down, it is preferable for the power storage device to maintain the power supply to the vehicle until the driver stops the vehicle in a safe place such as a roadside strip. That is, in the case of a power storage device mounted on a vehicle, it is preferable to continue to maintain the current interruption device in an energized state (closed, closed, off) for a certain period of time even if the power storage device is in an abnormal state.

[0026] As shown in Fig. 8, some storage cells have a plateau region in which the change in the open circuit voltage (OCV) of the storage cell relative to the change in SOC is small. Specifically, the plateau region is, for example, a region in which the change in OCV relative to the change in SOC is 2 [mV / %] or less. An example of a storage cell having a plateau region is an LFP / Gr-based (so-called iron-based) lithium-ion secondary battery in which LiFePO4 (lithium iron phosphate) is contained in the positive electrode active material and Gr (graphite) is contained in the negative electrode active material.

[0027] As shown in FIG. 9, when there is a variation in the amount of electricity in the storage cells having a plateau region, when the storage device is charged, the voltage of the storage cells whose SOC is in the plateau region is unlikely to increase even as charging progresses, but the voltage of the storage cells whose SOC is in a region higher than the plateau region (in other words, the storage cells with a large amount of remaining electricity) increases rapidly. This makes it more difficult to maintain the current interruption device in a conducting state for a certain period of time compared to when there is no variation in the amount of electricity. For this reason, in the case of a storage device having a plateau region, it is more desirable to reduce the difference in voltage or the difference in the amount of remaining electricity. However, when abnormal discharge occurs in the storage cells, variation occurs even if the difference is reduced by the balancer circuit.

[0028] If the driver is prompted to replace the power storage device via the vehicle when abnormal discharge occurs in the power storage cell, the driver can replace the power storage device with a normal power storage device, thereby suppressing variations in voltage and remaining charge caused by abnormal discharge. For this reason, in the case of a power storage device having a plateau region, it is particularly necessary to determine whether or not the power storage cell is abnormally discharged. According to the above-described power storage device, it is possible to determine whether or not the power storage cell is abnormally discharging, and therefore the present invention is particularly useful in the case of a power storage device having a plateau region.

[0029] (6) The power storage device may include a temperature measurement unit that measures a temperature of the power storage cell, and the management unit may determine, in the determination process, whether to base the reference value on the variation in the balancer discharge quantity of electricity or the variation in the self-discharge quantity of electricity, based on the temperature measured by the temperature measurement unit.

[0030] For example, when the temperature of the storage cell is low (low temperature), the variation in the self-discharge quantity of electricity of the storage cell is small enough to be ignored, so the variation in the self-discharge quantity of electricity is not used, and the reference value may be determined only from the variation in the balancer discharge quantity of electricity. When the temperature of the storage cell is high (high temperature), the influence of the self-discharge quantity of electricity is sufficiently dominant over the balancer discharge quantity of electricity, so the variation in the balancer discharge quantity of electricity is not used, and the reference value may be determined only from the variation in the self-discharge quantity of electricity. When the temperature is between low temperature and high temperature (normal temperature), the reference value may be determined based on both the variation in the self-discharge quantity of electricity and the variation in the balancer discharge quantity of electricity.

[0031] The invention disclosed in this specification can be realized in various forms, such as an apparatus, a method, a computer program for implementing the functions of these apparatus or methods, and a recording medium on which the computer program is recorded.

[0032] <Embodiment 1> The first embodiment will be described with reference to Figures 2 to 7. In the following description, the reference numerals of the drawings may be omitted for the same components, with some exceptions.

[0033] (1) Energy storage device An electricity storage device 1 according to a first embodiment will be described with reference to Fig. 1 and Fig. 2. As shown in Fig. 1, the electricity storage device 1 is mounted on a vehicle such as an automobile. As shown in Fig. 2, the electricity storage device 1 supplies power to an engine starting device 10 (a starter motor) and various auxiliary devices 12 (power steering, brakes, headlights, an air conditioner, a car navigation system, etc.) that are provided in the vehicle. The electricity storage device 1 is charged by a vehicle generator 13 (alternator).

[0034] (2) Configuration of the Energy Storage Device As shown in Fig. 3, the electricity storage device 1 includes a housing 71. The housing 71 includes a main body 73 and a lid 74 made of a synthetic resin material. The main body 73 is cylindrical with a bottom. The main body 73 includes a bottom portion 75 and four side portions 76. An upper opening 77 is formed at the upper end portion by the four side portions 76.

[0035] The container 71 contains the battery pack 30, which is made up of a plurality of power storage cells 30A, and a circuit board unit 72. The circuit board unit 72 is disposed on top of the battery pack 30. The lid 74 closes an upper opening 77 of the main body 73. An outer peripheral wall 78 is provided around the lid 74. The lid 74 has a protruding portion 79 that is generally T-shaped in plan view. A positive external terminal 80P is fixed to one corner of the front of the lid 74, and a negative external terminal 80N is fixed to the other corner.

[0036] The storage cell 30A is a secondary battery that can be repeatedly charged and discharged, specifically a lithium ion secondary battery. More specifically, the storage cell 30A is a lithium ion secondary battery having a plateau region in which the change in OCV relative to the change in SOC is small. An example of a lithium ion secondary battery having a plateau region is an iron-based lithium ion secondary battery in which iron is contained in the positive electrode active material. An example of an iron-based lithium ion secondary battery is an LFP / Gr-based lithium ion secondary battery in which LiFePO4 (lithium iron phosphate) is contained in the positive electrode active material and Gr (graphite) is contained in the negative electrode active material.

[0037] 4A and 4B, the power storage cell 30A contains an electrode assembly 83 together with a non-aqueous electrolyte in a rectangular parallelepiped case 82. The case 82 has a case body 84 and a lid 85 that closes the upper opening thereof. Although not shown in detail, the electrode body 83 is configured by disposing a separator made of a porous resin film between a negative electrode element in which a negative electrode active material is applied to a substrate made of copper foil and a positive electrode element in which a positive electrode active material is applied to a substrate made of aluminum foil. Both of these are strip-shaped, and are wound flatly so that they can be housed in the case body 84, with the negative electrode element and the positive electrode element being shifted to opposite sides in the width direction relative to the separator.

[0038] A positive electrode terminal 87 is connected to the positive electrode element via a positive electrode collector 86, and a negative electrode terminal 89 is connected to the negative electrode element via a negative electrode collector 88. The positive electrode collector 86 and the negative electrode collector 88 each include a flat base 90 and a leg 91 extending from the base 90. A through hole is formed in the base 90. The leg 91 is connected to the positive electrode element or the negative electrode element. The positive electrode terminal 87 and the negative electrode terminal 89 each include a terminal body 92 and a shaft 93 protruding downward from the center of the lower surface of the terminal body 92. Of these, the terminal body 92 and the shaft 93 of the positive electrode terminal 87 are integrally formed from aluminum (a single material). In the negative electrode terminal 89, the terminal body 92 is made of aluminum, and the shaft 93 is made of copper, and these are assembled together. The terminal main bodies 92 of the positive electrode terminal 87 and the negative electrode terminal 89 are disposed on both ends of the lid 85 via gaskets 94 made of an insulating material, and are exposed to the outside from the gaskets 94 .

[0039] 4A, the lid 85 has a pressure relief valve 95. The pressure relief valve 95 is located between the positive terminal 87 and the negative terminal 89. The pressure relief valve 95 opens to reduce the internal pressure of the case 82 when the internal pressure of the case 82 exceeds a limit value.

[0040] (3) Electrical configuration of the power storage device 5, the power storage device 1 includes a battery pack 30, a BMU 31 (an example of a management device), and a communication connector 32. The battery pack 30 is connected to a positive external terminal 80P by a power line 34P, and is connected to a negative external terminal 80N by a power line 34N.

[0041] The battery pack 30 has 12 storage cells 30A connected in series and three in parallel. In Fig. 5, three storage cells 30A connected in parallel are represented by one battery symbol. The BMU 31 includes a current sensor 33 , a voltage measurement circuit 35 , a temperature sensor 36 (an example of a temperature measurement unit), a balancer circuit 38 , a current interruption device 39 , and a management unit 37 .

[0042] The current sensor 33 is located on the negative electrode side of the battery pack 30, and is provided on the negative power line 34N. The current sensor 33 measures the charge / discharge current [A] of the battery pack 30 and outputs the result to the management unit 37. The voltage measurement circuit 35 is connected to both ends of each storage cell 30A by a signal line. The voltage measurement circuit 35 measures the battery voltage [V] of each storage cell 30A and outputs the battery voltage [V] to the management unit 37. The total voltage [V] of the battery pack 30 is the sum of the voltages of the four storage cells 30A connected in series.

[0043] The temperature sensor 36 is of a contact type or a non-contact type, measures the temperature [°C] of the power storage cell 30A, and outputs the temperature to the management unit 37. Although omitted in Fig. 5, two or more temperature sensors 36 are provided. Each temperature sensor 36 measures the temperature of a different power storage cell 30A. The balancer circuit 38 is a passive balancer circuit 38 that reduces the voltage difference between the storage cells 30A by discharging the storage cell 30A having a relatively high voltage among the storage cells 30A. The balancer circuit 38 has a discharge resistor 38A and a switch element 38B for each storage cell 30A. The discharge resistor 38A and the switch element 38B are connected in series and are connected in parallel to the corresponding storage cell 30A. When the switch element 38B is turned on, the power of the corresponding storage cell 30A is discharged by the discharge resistor 38A.

[0044] The current interruption device 39 is provided on the power line 34P. A contact switch (mechanical type) such as a relay, or a semiconductor switch such as a FET (Field Effect Transistor) can be used as the current interruption device 39. The current interruption device 39 is switched by the management unit 37 between a conducting state (closed state, on state, closed state) and a blocking state (open state, off state, open state).

[0045] The management unit 37 includes a microcomputer 37A in which a CPU, a RAM, and the like are integrated into one chip, a storage unit 37B, and a communication unit 37C. The storage unit 37B is a rewritable storage medium in which various programs and data (including tables described later) are stored. The microcomputer 37A manages the power storage device 1 by executing the programs stored in the storage unit 37B. The communication unit 37C is a circuit that enables the BMU 31 to communicate with a vehicle ECU 14 (Engine Control Unit). The communication connector 32 is a connector to which a communication cable for communication between the BMU 31 and the vehicle ECU 14 is connected.

[0046] (4) Processing performed by the Management Department The following four processes executed by the management unit 37 will be described. -SOC estimation process Recording and processing of the balancer discharge quantity and the variation of the balancer discharge quantity Recording and processing of variations in the self-discharge amount of electricity in storage cells -Internal short circuit detection processing

[0047] (4-1) SOC estimation process Management unit 37 estimates the SOC of power storage device 1 by a current integration method. The current integration method is a method in which a current value of a charge / discharge current of power storage device 1 is measured at a predetermined time interval (e.g., 10 milliseconds) by current sensor 33, and the measured current value is added or subtracted from an initial value to estimate the SOC of power storage device 1. The current integration method is an example, and the method for estimating the SOC is not limited to this.

[0048] (4-2) Recording and processing of the balancer discharge quantity and the variation of the balancer discharge quantity As shown in Fig. 6, in the energy storage device 1, voltage variations may occur among the storage cells 30A. For convenience, in Fig. 6, the four storage cells 30A are labeled with the reference characters 1 to 4. When the voltage of any one of the storage cells 30A increases to a predetermined voltage, the management unit 37 controls the balancer circuit 38 to discharge the storage cell 30A so that the voltage of the storage cell 30A becomes substantially the same as the voltage of the storage cell 30A having the lowest voltage among the other storage cells 30A, thereby reducing the difference in voltage among the storage cells 30A (reduction process).

[0049] Instead of reducing the difference in voltage between the storage cells 30A, the difference in SOC (an example of the remaining amount of electricity) between the storage cells 30A may be reduced. Specifically, the management unit 37 may measure the SOC for each storage cell 30A, and when the SOC of any storage cell 30A increases to a predetermined SOC, the management unit 37 may control the balancer circuit 38 to discharge the storage cell 30A so that the SOC of the storage cell 30A becomes substantially the same as the SOC of the storage cell 30A having the lowest SOC among the other storage cells 30A.

[0050] When discharging the storage cell 30A by the balancer circuit 38, the management unit 37 measures the amount of electricity discharged (balancer discharge amount of electricity [Ah]). Specifically, when discharging a certain storage cell 30A, the management unit 37 measures the voltage of the storage cell 30A by the voltage measurement circuit 35. The management unit 37 calculates the current value of the current discharged by the balancer circuit 38 every predetermined period from the voltage of the storage cell 30A and the resistance value of the discharge resistor 38A corresponding to the storage cell 30A according to Ohm's law. The management unit 37 measures the balancer discharge amount of electricity by integrating the current values ​​calculated every predetermined period.

[0051] Each time the management unit 37 discharges one storage cell 30A using the balancer circuit 38, it records in the memory unit 37B the measured balancer discharge quantity of electricity, the variation in the balancer discharge quantity of electricity described below, and the date and time of the discharge in association with the discharged storage cell 30A. The variation in the balancer discharge quantity of electricity will be explained. Since the balancer circuit 38 discharges the storage cell 30A through the discharge resistor 38A, the variation in the balancer discharge quantity of electricity occurs due to the tolerance of the discharge resistor 38A. For example, assume that the tolerance of the discharge resistor 38A used to discharge a certain storage cell 30A is ±1%, and the measured balancer discharge quantity of electricity is 10 mAh. In this case, the following formulas 1 and 2 show that the variation in the quantity of electricity actually discharged by the balancer circuit 38 with respect to the measured balancer discharge quantity of electricity is ±0.1 mAh. 10×(100 / 101)×(0.01)≒+0.1mAh ··· Formula 1 10×(100 / 99)×(-0.01)≒-0.1mAh ··· Formula 2

[0052] The variation in the balancer discharge electricity quantity can be expressed as the magnitude of the variation component contained in the measured balancer discharge electricity quantity, or the range of the magnitude of the variation contained in the balancer discharge electricity quantity.

[0053] (4-3) Recording and processing of variations in the self-discharge amount of electricity in storage cells Since it is difficult to actually measure the self-discharge amount of electricity of the storage cell 30A, the management unit 37 estimates the self-discharge amount of electricity of each storage cell 30A based on a combination of the temperature of the storage cell 30A and the SOC of the storage cell 30A. Specifically, as shown in Table 1 below, the memory unit 37B stores a table in which the standard self-discharge amount of electricity of the storage cell 30A over a certain period of time (e.g., one hour) and the variation in the self-discharge amount of electricity are associated with each other for each temperature of the storage cell 30A and for each SOC of the storage cell 30A. [Table 1]

[0054] In the example shown in Table 1, for example, when the temperature is 50°C and the SOC is 100%, the standard self-discharge quantity of electricity in a certain period of time is 1 mAh, and the variation in the self-discharge quantity of electricity at that time is ±0.1 mA. The management unit 37 obtains the variation in the self-discharge quantity of electricity corresponding to the SOC and temperature of the storage cell 30A from the table at regular intervals, and records it in the memory unit 37B in association with the date and time. For example, when the temperature is 50°C and the SOC is 100%, ±0.1 mA is recorded as the variation in the self-discharge quantity of electricity.

[0055] For convenience, in the first embodiment, the recording process is performed for only one of the storage cells 30A, and the variation in the self-discharge amount of electricity recorded for the one storage cell 30A is commonly used for the other storage cells 30A. Therefore, the variation in the self-discharge amount of electricity in a predetermined period will be the same for all storage cells 30A. The SOC may be estimated and the temperature measured for each storage cell 30A, and the recording process may be performed individually for each storage cell 30A.

[0056] The variation in the self-discharge quantity of electricity can also be expressed as the magnitude of the variation component contained in the estimated value of the self-discharge quantity of electricity, or the range of the magnitude of the variation contained in the estimated value of the self-discharge quantity of electricity.

[0057] (4-4) Internal short circuit detection process The management unit 37 determines whether or not the specific storage cell 30A has an internal short circuit (whether or not abnormal discharge has occurred in the specific storage cell 30A) by comparing the difference between the balancer discharge quantity of the storage cell 30A (specific storage cell 30A) having the smallest balancer discharge quantity of electricity in a specified period and the balancer discharge quantities of the other storage cells 30A in the specified period with a reference value.

[0058] In the first embodiment, the above-mentioned reference value is a value obtained by summing the maximum value of the difference in the balancer discharge amount of electricity that may occur during a specified period due to the frequency at which the specific storage cell 30A and the other storage cells 30A are discharged by the balancer circuit 38 (an example of a first parameter) and the maximum value of the difference in the balancer discharge amount of electricity that may occur during a specified period due to the self-discharge amount of electricity of the specific storage cell 30A and the other storage cells 30A (an example of a second parameter). In the following description, the predetermined period will be described as being the most recent month, but the predetermined period is not limited to the most recent month and can be determined as appropriate.

[0059] A flowchart of the process of determining whether or not there is an internal short circuit will be described with reference to Fig. 7. This process is executed, for example, when the engine of the vehicle is started. This process may be repeatedly executed at predetermined time intervals, such as every 10 minutes, after the engine of the vehicle is started.

[0060] In S101, the management unit 37 obtains the balancer discharge amount of electricity and the variation in the balancer discharge amount of electricity for each storage cell 30A in the most recent month. Specifically, the management unit 37 sums up the balancer discharge amount of electricity and the variation in the balancer discharge amount of electricity recorded in the most recent month among the balancer discharge amount of electricity and the variation in the balancer discharge amount of electricity recorded in the memory unit 37B for each storage cell 30A. In the following description, the sum of the variation in the balancer discharge amount of electricity recorded in the most recent month is simply referred to as the "variation in the balancer discharge amount of electricity for the most recent month."

[0061] An example of the balancer discharge amount of electricity and the variation in the balancer discharge amount of electricity for each storage cell 30A in the most recent month is shown in the following Table 2. For convenience, the four storage cells 30A are given the reference numbers 1 to 4 in Table 2. In the following explanation, the variation shown in Table 2 will be used as an example. [Table 2]

[0062] In S102, the management unit 37 identifies the storage cell 30A (here, storage cell 4) with the smallest balancer discharge amount of electricity in the most recent month from the balancer discharge amount of electricity of each storage cell 30A calculated in S101. Storage cell 4 is an example of a specified storage cell. In S103, the management unit 37 obtains a difference between the balancer discharge amount of electricity of the specific storage cell 30A (storage cell 4) in the most recent month and the balancer discharge amount of electricity of the other storage cells 30A (storage cells 1 to 3) in the most recent month. Specifically, the management unit 37 obtains a difference between the balancer discharge amount of electricity of the specific storage cell 30A in the most recent month and the average value of the balancer discharge amounts of electricity of all the other storage cells 30A in the most recent month by the following formula 3. |(105mAh+110mAh+85mAh) / 3-30mAh|=70mAh ··· Formula 3

[0063] In S104, the management unit 37 obtains a maximum value (an example of a first parameter) of the difference in the balancer discharge amount of electricity that may occur due to the frequency at which the specific storage cell 30A and the other storage cells 30A are discharged by the balancer circuit 38 in the most recent month. Specifically, the management unit 37 obtains the average value of the variations in the balancer discharge electricity amount of all the other storage cells 30A by the following formula 4. (|±5.25mAh|+|±5.5mAh|+|±4.25mAh|) / 3=5.0mAh ··· Equation 4

[0064] The management unit 37 calculates the maximum value of the difference in balancer discharge capacity that may occur due to the frequency at which the specific storage cell 30A and the other storage cells 30A are discharged by the balancer circuit 38 by summing the variation in the balancer discharge amount of electricity of the specific storage cell 30A and the average value of the variation in the balancer discharge amount of electricity of all other storage cells 30A using the following equation 5. |±1.5mAh|+5.0mAh=6.5mAh ··· Equation 5

[0065] From the above-mentioned equation 5, if the balancer discharge amount of electricity of a specific storage cell 30A varies downward by 1.5 mAh and the balancer discharge amount of electricity of all other storage cells 30A varies upward by 5.0 mAh, a difference of up to 6.5 mAh can occur between the balancer discharge amount of electricity of the specific storage cell 30A and the average value of the balancer discharge amount of electricity of all other storage cells 30A.

[0066] In S105, the management unit 37 calculates the total value of the variation in the self-discharge quantity of electricity of one storage cell 30A in the most recent month by summing up the variations in the self-discharge quantity of electricity recorded in the above-mentioned "recording process of the variation in the self-discharge quantity of electricity of the storage cell" recorded in the most recent month. For example, assume that the variations in the self-discharge quantity of electricity recorded in the most recent month are ±0.1mAh, ±0.05mAh, and ±0.01mAh. In this case, the total value of the variations in the self-discharge quantity of electricity recorded in the most recent month is ±0.16mAh (=±0.1mAh±0.05mAh±0.01mAh). In the following explanation, the total value of the variation in the self-discharge amount of electricity in the most recent month will be simply referred to as the "variation in the self-discharge amount of electricity in the most recent month."

[0067] In S106, the management unit 37 obtains the maximum value (an example of a second parameter) of the difference in the balancer discharge amount of electricity that may occur due to the self-discharge amount of electricity of the specific storage cell 30A and the other storage cells 30A in the most recent month. As described above, the management unit 37 executes the recording process of the variation in the self-discharge amount of electricity for only one of the storage cells 30A, and for the other storage cells 30A, the variation in the self-discharge amount of electricity recorded for that one storage cell is commonly used, so the variation in the self-discharge amount of electricity for each storage cell 30A in the most recent month is the same value. Therefore, the maximum value of the above-mentioned difference is the absolute value of the value obtained by multiplying the variation in the self-discharge amount of electricity for any one storage cell 30A in the most recent month by two. For example, if the total value of the variation in the self-discharge amount of electricity is ±0.16mAh, the maximum value of the above-mentioned difference is 0.32mAh.

[0068] For the sake of convenience, the following description will be given assuming that the variation in the self-discharge amount of electricity of each storage cell 30A in the most recent month is ±20mAh. In this case, if the self-discharge amount of electricity of the specific storage cell 30A varies downward by 20mAh and the self-discharge amount of electricity of all the other storage cells 30A varies upward by 20mAh, a difference of up to 40mAh can occur between the balancer discharge amount of electricity of the specific storage cell 30A and the average value of the balancer discharge amount of electricity of all the other storage cells 30A due to the variation in the self-discharge amount of electricity.

[0069] In S107, the management unit 37 determines a reference value by summing the maximum value of the difference in the balancer discharge quantity of electricity that may occur due to the frequency of discharge by the balancer circuit 38 obtained in S104 (an example of a first parameter) and the maximum value of the difference in the balancer discharge quantity of electricity that may occur due to the self-discharge quantity of electricity obtained in S106 (an example of a second parameter), as shown in the following equation 6. 6.5mAh+40mAh=46.5mAh ··· Formula 6

[0070] In S108, the management unit 37 compares the difference found in S103 with a reference value to determine whether the storage cell 30A with the smallest balancer discharge amount of electricity is internally short-circuited (an example of a determination process). In the above example, the difference found in S103 (70 mAh) is greater than the reference value (46.5 mAh), as shown in the following formula 7. In this case, the reason for the difference cannot be explained by the variation in the balancer discharge amount of electricity or the variation in the self-discharge amount of electricity, so the management unit 37 determines that the specific storage cell 30A is internally short-circuited. 46.5mAh<70mAh ··· Formula 7

[0071] On the other hand, if the difference calculated in S103 is equal to or smaller than the reference value, the management unit 37 determines that the energy storage cell 30A is not internally short-circuited. If the management unit 37 determines that the energy storage cell 30A is internally short-circuited, the process proceeds to S109, and if the management unit 37 determines that the energy storage cell 30A is not internally short-circuited, the process ends.

[0072] In S109, the management unit 37 notifies the vehicle ECU 14 that the power storage cell 30A has an internal short circuit. When the vehicle ECU 14 is notified of the internal short circuit, it prompts the driver to replace the power storage device 1 with a normal power storage device 1 by, for example, turning on a warning lamp related to the power storage device 1.

[0073] (5) Effects of the embodiment According to the energy storage device 1, the reference value is determined based on both the variation in the balancer discharge amount of electricity of the energy storage cells 30A during a predetermined period and the variation in the self-discharge amount of electricity of the energy storage cells 30A during a predetermined period. Therefore, the reference value for determining whether or not a specific energy storage cell 30A during a predetermined period has an internal short circuit can be appropriately determined in consideration of the variation in the balancer discharge amount of electricity and the variation in the self-discharge amount of electricity.

[0074] According to the energy storage device 1, the reference value is the sum of the maximum value of the difference in the balancer discharge amount of electricity caused by the variation in the balancer discharge amount of electricity between the specific energy storage cell 30A and the other energy storage cells 30A during a specified period and the maximum value of the difference in the balancer discharge amount of electricity caused by the variation in the self-discharge amount of electricity between the specific energy storage cell 30A and the other energy storage cells 30A during a specified period, so that the reference value for determining whether or not the specific energy storage cell 30A has an internal short circuit can be appropriately determined.

[0075] According to the energy storage device 1, the balancer discharge quantity of the other storage cell 30A is the average value of the balancer discharge quantities of two or more other storage cells 30A, so that the reference value can be determined more appropriately compared to the case where the balancer discharge quantity of any one of the other storage cells 30A is used.

[0076] According to the energy storage device 1, the variation in the self-discharge quantity of electricity of the energy storage cell 30A over a certain period of time can be obtained from a table in which the variation in the self-discharge quantity of electricity of the energy storage cell 30A over a certain period of time is associated with each combination of the temperature of the energy storage cell 30A and the SOC of the energy storage cell 30A, thereby making it possible to determine the variation in the self-discharge quantity of electricity according to the SOC and temperature of the energy storage cell 30A.

[0077] According to the power storage device 1, it is possible to determine whether or not the power storage cell 30A has an internal short circuit, and this is particularly useful in the case of a power storage device 1 having a plateau region.

[0078] <Other embodiments> The technology disclosed in this specification is not limited to the embodiments described above and in the drawings, and for example, the following embodiments are also included in the technical scope disclosed in this specification.

[0079] (1) In the above embodiment, an example has been described in which the reference value is the sum of the maximum value (first parameter) of the difference in the balancer discharge amount of electricity that may occur due to the frequency at which the specific storage cell 30A and the other storage cells 30A are discharged by the balancer circuit 38, and the maximum value (second parameter) of the difference in the balancer discharge amount of electricity that may occur due to the self-discharge amount of electricity of the specific storage cell 30A and the other storage cells 30A during a specified period.

[0080] In contrast to this, whether the reference value is to be determined based on the variation in the balancer discharge amount of electricity of each storage cell 30A or the variation in the self-discharge amount of electricity of each storage cell 30A may be determined based on the temperature measured by the temperature sensor 36. For example, when the temperature of the storage cell 30A is low (low temperature), self-discharge is difficult, so the variation in the self-discharge quantity of electricity of the storage cell 30A becomes negligibly small. Therefore, the variation in the self-discharge quantity of electricity is not used at low temperatures, and the reference value may be determined only from the variation in the balancer discharge quantity of electricity. When the temperature of the storage cell 30A is high (high temperature), the influence of the self-discharge quantity of electricity is sufficiently dominant over the balancer discharge quantity of electricity, so the variation in the balancer discharge quantity of electricity is not used, and the reference value may be determined only from the variation in the self-discharge quantity of electricity. When the temperature is between low temperature and high temperature (normal temperature), the reference value may be determined based on both the variation in the self-discharge quantity of electricity and the variation in the balancer discharge quantity of electricity.

[0081] Alternatively, if the tolerance of the discharge resistor 38A is very small, the variation in the balancer discharge quantity of electricity is negligible, so the reference value may be determined based only on the variation in the self-discharge quantity of electricity. Alternatively, depending on the balancer circuit, multiple storage cells 30A may be discharged by one discharge resistor 38A. In this case, there is no variation in the balancer discharge amount of electricity due to the tolerance of the discharge resistor 38A, so the reference value may be determined only from the variation in the self-discharge amount of electricity.

[0082] The reference value is not limited to the sum of the above-mentioned maximum values, and may be the square root of each maximum value. Instead of using the above-mentioned maximum values, for example, the width of the variation may be determined based on 2σ of the variation (95% confidence interval in normal distribution of the balancer discharge electricity amount or self-discharge electricity amount), and the sum of the determined width or the sum of squares may be used as the reference value.

[0083] (2) In the above embodiment, the balancer discharge amount of the other storage cells 30A is described by taking the average value of the balancer discharge amount of all the other storage cells 30A as an example. However, the balancer discharge amount of the other storage cells 30A is not limited to this. For example, the balancer discharge amount of the other storage cells 30A may be the median value of the balancer discharge amount of all the other storage cells 30A. Alternatively, it may be the balancer discharge amount of any one of the other storage cells 30A. The other one of the storage cells 30A may be the storage cell 30A having the second smallest balancer discharge amount of electricity after the specific storage cell 30A, or may be the storage cell 30A having the largest balancer discharge amount of electricity.

[0084] (3) In the above embodiment, the variation in the self-discharge quantity of electricity of the storage cell 30A is calculated by adding up the variations in the self-discharge quantity of electricity recorded in the memory unit 37B. However, the method of calculating the variation in the self-discharge quantity of electricity of the storage cell 30A is not limited to this. For example, the width of a 95% confidence interval in a normal distribution of the self-discharge quantity of electricity of the storage cell 30A in a predetermined period may be used as the variation in the self-discharge quantity of electricity. Alternatively, the variation in the self-discharge quantity of electricity of the storage cell 30A in a predetermined period may be a predetermined fixed value.

[0085] (4) In the above embodiment, a passive balancer circuit 38 has been described as an example of the balancer circuit 38. However, the balancer circuit 38 may be an active balancer circuit that reduces the difference by charging the lower-voltage storage cell 30A with the higher-voltage storage cell 30A.

[0086] (5) In the above embodiment, the balancer discharge quantity of electricity is calculated by calculating and integrating the current discharged by the balancer circuit 38 for each predetermined period according to Ohm's law from the voltage of the storage cell 30A and the resistance value of the discharge resistor 38A corresponding to the storage cell 30A, but the method of measuring the balancer discharge quantity of electricity is not limited to this. For example, the management unit 37 may measure the voltage of the storage cell 30A using the voltage measurement circuit 35, and when the voltage of the storage cell 30A drops to the same voltage as the lowest voltage storage cell 30A, convert the voltage difference between the voltage before and after discharge into the discharge quantity of electricity [Ah] using a predetermined calculation formula (or table). Alternatively, the current amount of electricity [Ah] of the storage cell 30A may be estimated from the voltage before discharge, and the current amount of electricity of the storage cell 30A may be estimated from the voltage after discharge, and the difference between them may be regarded as the balancer discharge amount of electricity.

[0087] Alternatively, the resistance value of the discharge resistor 38A of the balancer circuit 38 may be stored in the management unit 37, and the discharged amount of electricity may be integrated by successively measuring the voltage change. Specifically, the balancer discharged amount of electricity may be calculated by the following formulas 8 to 10.

[0088] Balancer current I1 at time t1 = Cell voltage at time t1 / Discharge resistance value Equation 8 Balancer current I2 at time t2 = Cell voltage at time t2 / Discharge resistance value Equation 9 Amount of electricity discharged from the balancer between time t1 and time t2 = (I2-I1) × (t2-t1) Equation 10

[0089] Alternatively, the average value of the balancer current may be stored based on the normal voltage (e.g., 3.5 V) at which the balancer circuit 38 operates and the discharge resistor 38 A. Then, the management unit 37 may calculate the balancer discharge electricity amount by multiplying the balancer operation time by the average value of the balancer current.

[0090] (6) In the above embodiment, an example has been described in which it is determined whether or not the storage cell 30A has an internal short circuit. However, in addition to an internal short circuit, a small discharge may occur due to a failure of the BMU 31 (corresponding to the management system). Therefore, when the difference obtained in S103 is greater than the reference value, the management unit 37 may determine that at least one of an internal short circuit and a small discharge due to a failure of the BMU 31 has occurred, rather than determining that an internal short circuit has occurred.

[0091] (7) In the above embodiment, an example has been described in which, when the voltage of any one of the storage cells 30A rises to a predetermined voltage, the management unit 37 controls the balancer circuit 38 to discharge the storage cell 30A so that the voltage of that storage cell 30A becomes approximately the same as the voltage of the storage cell 30A having the lowest voltage among the other storage cells 30A. In contrast to this, the management unit 37 may use the storage cell 30A with the lowest voltage as a reference, and discharge all the other storage cells 30A so that the voltages of all the other storage cells 30A become substantially the same as the voltage of the reference storage cell 30A. Alternatively, the storage cell 30A with the lowest remaining amount of electricity may be used as a reference, and discharge all the other storage cells 30A so that the remaining amounts of electricity of all the other storage cells 30A become substantially the same as the remaining amount of electricity of the reference storage cell 30A.

[0092] The method of reducing the voltage difference (or the difference in the remaining amount of electricity) is not limited to this. For example, the amount of electricity to be discharged (or the discharge time) may be determined in advance according to the order, such as 18 mAh for the storage cell 30A with the highest voltage, 12 mAh for the storage cell 30A with the second highest voltage, and 6 mAh for the storage cell 30A with the third highest voltage.

[0093] (8) In the above embodiment, the power storage device 1 is described as being mounted on a vehicle such as an automobile. However, the power storage device 1 is not limited to being mounted on a vehicle and can be used for any purpose.

[0094] (9) In the above embodiment, a lithium ion secondary battery is used as the power storage cell 30A. However, the power storage cell 30A may be a capacitor that involves an electrochemical reaction. [Explanation of symbols]

[0095] 1...Electricity storage device 30A…Electric storage cell 36...Temperature sensor (an example of a temperature measuring part) 37…Management Department 38…Balancer circuit

Claims

1. A power storage device, A plurality of storage cells; A balancer circuit that individually discharges each storage cell; The management department and Equipped with The management unit a reduction process in which a storage cell having a relatively high voltage or remaining amount of electricity is discharged by the balancer circuit to reduce a difference in voltage or a difference in a remaining amount of electricity among the plurality of storage cells; a determination process for determining whether or not abnormal discharge has occurred in the specific storage cell by comparing a difference between a balancer discharge amount of a specific storage cell that has the smallest balancer discharge amount of electricity discharged by the balancer circuit during a predetermined period and the balancer discharge amounts of other storage cells with a reference value; A determination process of determining the reference value based on at least one of a variation in a balancer discharge amount of electricity of the plurality of storage cells during the predetermined period and a variation in a self-discharge amount of electricity of the plurality of storage cells during the predetermined period; A power storage device that performs the above.

2. The power storage device according to claim 1 , The management unit a first parameter that may be generated due to a frequency at which the specific storage cell and other storage cells are discharged by the balancer circuit during the predetermined period; A second parameter that may be generated due to the self-discharge amount of the specific storage cell and other storage cells during the predetermined period; The power storage device determines the reference value based on the

3. The power storage device according to claim 1 or 2, An electricity storage device, wherein an average value of the balancer discharge electricity amounts of two or more other electricity storage cells is used as the balancer discharge electricity amount of the other electricity storage cell.

4. The power storage device according to any one of claims 1 to 3, A temperature measuring unit that measures a temperature of the power storage cell, The management unit performing a recording process of acquiring and recording the variation using a table in which the temperature and the state of charge of the storage cell are associated with the self-discharge amount of the storage cell over a certain period of time; The variation recorded in the recording process during the predetermined period is summed up to determine the variation in the self-discharge amount of electricity of the plurality of storage cells.

5. The power storage device according to any one of claims 1 to 4, The plurality of storage cells have a plateau region in which a change in voltage relative to a change in a state of charge is small.

6. The power storage device according to any one of claims 1 to 5, A temperature measuring unit that measures a temperature of the power storage cell, In the determination process, the management unit determines whether to base the reference value on the variation in the balancer discharged amount of electricity or the variation in the self-discharged amount of electricity, based on the temperature measured by the temperature measurement unit.

7. A method for detecting abnormal discharge in a power storage device having a plurality of power storage cells and a balancer circuit that individually discharges each of the power storage cells, comprising: a reduction step of reducing a difference in voltage or a difference in a remaining amount of electricity among the plurality of storage cells by discharging a storage cell having a relatively high voltage or a relatively high remaining amount of electricity using the balancer circuit; a determination step of determining whether or not abnormal discharge has occurred in the specific storage cell by comparing a difference between a balancer discharge amount of a specific storage cell that has the smallest balancer discharge amount of electricity discharged by the balancer circuit during a predetermined period and the balancer discharge amounts of other storage cells with a reference value; determining the reference value based on at least one of a variation in a balancer discharge amount of electricity of the plurality of storage cells during the predetermined period and a variation in a self-discharge amount of electricity of the plurality of storage cells during the predetermined period; The abnormal discharge detection method includes:

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