Battery management device, battery system, battery management method, and battery management program

Abstract

According to the present invention, a measurement circuit is connected by a voltage measurement line to each node of a plurality of cells connected in series and measures the voltage of each of the plurality of cells. A plurality of discharge circuits are connected in parallel with the plurality of cells, respectively, between the two adjacent voltage measurement lines. A control circuit executes equalization processing between the plurality of cells by controlling the plurality of discharge circuits on the basis of the voltages of the plurality of cells measured by the measurement circuit. The control circuit executes equalization processing when variations in voltages of the plurality of cells exceed a first threshold value, and determines that there is an error when the variations in the voltages of the plurality of cells exceed a second threshold value obtained by adding an offset value to the first threshold value.

Description

Battery management device, battery system, battery management method, and battery management program

[0001] The present disclosure relates to a battery management device, a battery system, a battery management method, and a battery management program that detect unsafe events in cells.

[0002] When lithium dentrite grows inside a lithium-ion battery cell and breaks through the separator, it short-circuits the positive and negative electrodes. If the lithium dentrite grows further and breaks through the separator to an even greater extent, the short-circuit between the positive and negative electrodes worsens. As the short-circuit worsens, the internal resistance decreases, increasing the risk of the lithium-ion battery cell catching fire or emitting smoke.

[0003] Regarding the detection of micro-short circuits in lithium-ion battery cells, Patent Document 1 discloses a method in which the average voltage of cells connected in series and the voltage of the cell being detected are calculated at a first time and a second time, respectively, and an abnormality is determined when the difference between the voltages at both times is equal to or greater than a threshold value.

[0004] WO 20 / 021889

[0005] Many battery systems are equipped with an equalization function to eliminate voltage variations between series-connected cells. Methods for detecting micro-short circuits based on the increase in voltage difference between cells must take into account the effect of the equalization process on the reduction in voltage difference between cells. The inventors have developed a method for detecting signs of internal cell short circuits more easily than conventional methods.

[0006] A battery management device according to an aspect of the present disclosure includes a measurement circuit connected to each node of a plurality of series-connected cells by voltage measurement lines and measuring the voltages of the plurality of cells, a plurality of discharge circuits connected in parallel to the plurality of cells between each of two adjacent voltage measurement lines, and a control circuit that performs an equalization process among the plurality of cells by controlling the plurality of discharge circuits based on the voltages of the plurality of cells measured by the measurement circuit. The control circuit performs the equalization process when a variation in the voltages of the plurality of cells exceeds a first threshold, and determines that an error has occurred when the variation in the voltages of the plurality of cells exceeds a second threshold obtained by adding an offset value to the first threshold.

[0007] Any combination of the above components, and conversion of the expression of the present disclosure into an apparatus, system, method, computer program, etc., are also valid aspects of the present disclosure.

[0008] According to the present disclosure, symptoms of an internal short circuit in series-connected cells can be easily detected.

[0009] Fig. 1 is a diagram for explaining the configuration of a battery system according to an embodiment. Fig. 2 is a diagram schematically showing the relationship between the rate of voltage drop and the rate of increase in the voltage difference between cells in the equalization process of the battery system according to the embodiment. Fig. 3 is a flowchart showing the flow of a battery management method performed by a battery management device according to the embodiment.

[0010] FIG. 1 is a diagram illustrating the configuration of a battery system 1 according to an embodiment. The battery system 1 includes a battery pack 20 and a battery management device 10. The battery pack 20 includes multiple cells E1-En connected in series. The number of cells connected in series is determined by the specifications of the load 2. The cells may be lithium-ion battery cells, nickel-metal hydride battery cells, lead battery cells, or the like. In the following description, an example using lithium-ion battery cells (nominal voltage: 3.6-3.7 V) is assumed.

[0011] In the configuration of a battery system 1 according to the embodiment shown in FIG. 1 , a power line switch S1 that switches conduction / non-conduction between the battery pack 20 and the load 2 is inserted in a power line connecting the battery pack 20 and the load 2. In the example shown in FIG. 1 , the power line switch S1 is configured by a first transistor Q1 and a second transistor Q2, each configured as an N-channel metal-oxide-semiconductor field-effect transistor (MOSFET), connected in series in reverse orientation. In an N-channel MOSFET, a parasitic diode is formed in the source-to-drain direction. Connecting two N-channel MOSFETs in series in reverse orientation blocks current passing through the parasitic diode, forming a bidirectional switch. In the example shown in FIG. 1 , discharging current from the battery pack 20 can be cut off by turning off the first transistor Q1, and charging current to the battery pack 20 can be cut off by turning off the second transistor Q2.

[0012] When the battery system 1 according to the embodiment is used as a power storage system linked to a solar power generation system, the load 2 is a DC bus. A DC / DC converter for the solar cell and the DC side of an inverter connected to a commercial power system are connected to the DC bus. When the battery system 1 according to the embodiment is installed in a vehicle, the load 2 is an inverter and a motor, or a charger.

[0013] The battery management device 10 includes a measurement circuit 11, multiple discharge circuits connected in parallel to the multiple cells E1-En, and a control circuit 12. The measurement circuit 11 is configured as an AFE (Analog Front End) IC or an ASIC (Application Specific Integrated Circuit). The measurement circuit 11 is connected to each node of the multiple series-connected cells E1-En via multiple voltage measurement lines, and measures the voltage between each two adjacent voltage measurement lines to measure the voltage of each cell E1-En.

[0014] A discharge circuit is connected between each pair of adjacent voltage measurement lines. Specifically, a first discharge switch Sd1 and a first discharge resistor R1 are connected in series across the first cell E1, a second discharge switch Sd2 and a second discharge resistor R2 are connected in series across the second cell E2, a third discharge switch Sd3 and a third discharge resistor R3 are connected in series across the third cell E3, and so on, with an n-th discharge switch Sdn and an n-th discharge resistor Rn connected in series across the n-th cell En.

[0015] The measurement circuit 11 includes a multiplexer and an A / D converter. The multiplexer outputs the measurement voltages of the multiple cells E1-En to the A / D converter in a predetermined order. The A / D converter converts the analog measurement voltages input from the multiplexer into digital values. The measurement circuit 11 transmits the converted digital voltage values ​​of the cells E1-En to the control circuit 12 via the communication interface.

[0016] The measurement circuit 11 can measure the current flowing through the battery pack 20. A shunt resistor Rs is connected to the power line connecting the battery pack 20 and the load 2. As an example of a current measurement system, a differential amplifier amplifies the voltage across the shunt resistor Rs and outputs it to an A / D converter in the measurement circuit 11. The A / D converter converts the analog voltage indicating the current flowing through the battery pack 20, which is input from the differential amplifier, into a digital value. The measurement circuit 11 transmits the current value converted into a digital value to the control circuit 12 via the communication interface. Note that a Hall element may be used instead of the shunt resistor Rs.

[0017] At least one thermistor T1 is installed on the surface of the battery pack 20. The thermistor T1 is a temperature-sensing element whose resistance value changes depending on the temperature. A voltage divided by the thermistor T1 and a voltage-dividing resistor is output to an A / D converter in the measurement circuit 11. The A / D converter converts the input analog voltage indicating the surface temperature of the battery pack 20 into a digital value. The measurement circuit 11 transmits the temperature value converted into a digital value to the control circuit 12 via a communication interface. The thermistor T1 may be installed at multiple locations on the battery pack 20. Other temperature-sensing elements, such as a thermocouple or a platinum resistance thermometer, may be used instead of the thermistor T1.

[0018] The control circuit 12 controls the battery system 1 based on the voltage values ​​of the cells E1-En, the current values ​​flowing through the battery pack 20, and the temperature values ​​of the battery pack 20 received from the measurement circuit 11. The control circuit 12 is composed of a microcontroller, and the microcontroller executes a firmware program to perform the following processes.

[0019] The control circuit 12 estimates the SOC (State Of Charge) by combining the OCV (Open Circuit Voltage) method and the current integration method. The OCV method estimates the SOC based on the measured cell OCV and the cell's SOC-OCV curve. The cell's SOC-OCV curve is created in advance by the battery manufacturer based on characteristic tests and is registered in the control circuit 12 at the time of shipment.

[0020] The current integration method is a method for estimating the SOC based on the OCV at the start of cell charging and discharging and the integrated value of the measured current. With the current integration method, current measurement errors accumulate as the charging and discharging time increases. Therefore, it is preferable to use a weighted average of the SOC estimated by the current integration method and the SOC estimated by the OCV method.

[0021] The control circuit 12 converts the SOC of each cell E1-En into an actual capacity, combines these actual capacities to calculate the actual capacity of the battery pack 20, and can estimate the SOC of the battery pack 20 based on this actual capacity and the current full charge capacity (FCC: Full Charge Capacity) of the battery pack 20.

[0022] The control circuit 12 can estimate the current FCC of the battery pack 20 by dividing the current integrated value for the depth of discharge (DOD) by the DOD. The DOD is defined, for example, as the difference between the SOC at the start of charging and the SOC at the end of charging. The control circuit 12 can estimate the state of health (SOH) by dividing the current FCC by the initial FCC. A lower SOH value (closer to 0%) indicates more advanced deterioration.

[0023] When the control circuit 12 detects any of the abnormalities of overcharging, overdischarging, overcurrent, or overtemperature based on the voltage values, current values, and temperature values ​​of the multiple cells E1-En received from the measurement circuit 11, it sends a power line switch S1 shut-off signal to the measurement circuit 11, causing the measurement circuit 11 to turn off the power line switch S1.

[0024] The control circuit 12 controls the multiple discharge circuits based on the voltages of the multiple cells E1-En measured by the measurement circuit 11, thereby performing cell balancing among the multiple cells E1-En. In a typical passive cell balancing method, the other cells E1-En are discharged until the voltage of the cell with the lowest voltage (hereinafter referred to as the target value) is reached. The target value may be specified by the capacity, SOC, dischargeable amount, or chargeable amount.

[0025] The control circuit 12 sets the measured value of the cell with the lowest voltage among the multiple cells E1-En as a target value, and calculates the differences between the target value and the measured values ​​of the other multiple cells. The control circuit 12 calculates the discharge amounts of the other multiple cells based on the calculated differences. The control circuit 12 calculates the discharge times of the other multiple cells based on the calculated discharge amounts. The control circuit 12 generates a control signal for the equalization process including the discharge times of the multiple cells, and transmits it to the measurement circuit 11. The measurement circuit 11 controls the multiple discharge switches Qd1-Qdn to be on for the determined times based on the control signal received from the control circuit 12.

[0026] In this way, the equalization process applies current to the discharge resistors R1-Rn connected in parallel to each of the cells E1-En for a predetermined time, thereby lowering the voltage of all cells except the cell with the lowest voltage and equalizing the voltages of the cells E1-En. Hereinafter, the voltage drop rate during the equalization process is defined as α [mV / hour]. α is uniquely determined by the circuit constants (such as the resistance values ​​of the discharge resistors R1-Rn), the measurement circuit 11, and the control circuit 12.

[0027] On the other hand, if an internal micro-short circuit occurs due to metal deposition within a cell, the voltage of the cell where the micro-short circuit occurred gradually drops due to leakage current within the cell, disrupting the voltage balance between multiple cells E1-En. If the rate at which the voltage of the cell where the micro-short circuit occurred is β [mV / hour], the rate at which the voltage difference between cells E1-En increases can also be defined as β [mV / hour]. β gradually increases over time depending on the progression of the internal micro-short circuit.

[0028] FIG. 2 is a diagram showing the relationship between the voltage drop rate α during equalization processing and the rate of increase β of the voltage difference ΔV between cells E1 and En. The voltage difference ΔV between cells E1 and En is a value indicating the voltage variation between cells E1 and En and is defined as the difference between the maximum and minimum voltages of cells E1 and En. The voltage difference ΔV between cells E1 and En may be defined as the difference between the average and minimum voltages of cells E1 and En, or as the difference between the median and minimum voltages of cells E1 and En. When calculating the average or median voltages of cells E1 and En, it is desirable to exclude the voltage of the cell with the lowest voltage.

[0029] At the time of shipping of the battery system 1 and when no internal micro-short circuit has occurred in cells E1-En, α>β. There is almost no difference in the internal resistance of cells E1-En, and the voltage difference ΔV is also small (the region in Figure 2 where no short circuit has occurred).

[0030] When a micro-short circuit occurs in some cells due to the inclusion of metal foreign matter, the displacement of the separator, the expansion of the positive and negative electrodes themselves, or lithium deposition due to use in harsh environments such as low temperatures, a difference in internal resistance occurs between the normal cell and the abnormal cell where the micro-short circuit has occurred, and this difference in internal resistance gradually increases. As a result, β gradually increases, and when α < β, the equalization process cannot keep up and the voltage difference ΔV increases (short circuit progression region in Figure 2).

[0031] As the lithium dendrites grow further and progress from a high-resistance short circuit to a low-resistance short circuit, the state changes to α<<β, causing a large short-circuit current to flow inside the cell, increasing the risk of fire and smoke (short-circuit growth region in Figure 2).

[0032] In this embodiment, a mechanism is introduced that detects signs of an increase in the voltage difference ΔV between cells E1-En and stops the use of battery system 1 in the short circuit progression region (a safe region with a low risk of fire or smoke) before reaching the short circuit growth region (a dangerous region with a high risk of fire or smoke).

[0033] 3 is a flowchart showing the flow of a battery management method by the battery management device 10 according to an embodiment. The measurement circuit 11 measures the voltages of the plurality of cells E1-En (S10). The control circuit 12 calculates the voltage difference ΔV between the measured plurality of cells E1-En (S11). The control circuit 12 compares the voltage difference ΔV with an equalization start threshold x (S12). The equalization start threshold x is set to, for example, approximately 10 mV to 30 mV.

[0034] If the voltage difference ΔV exceeds the equalization start threshold x (Y in S12), the control circuit 12 executes the equalization process (S13). If the voltage difference ΔV is less than the equalization start threshold x (N in S12), the control circuit 12 skips step S13 without executing the equalization process.

[0035] The control circuit 12 compares the voltage difference ΔV with the internal short-circuit detection threshold (x+y) (S14). The internal short-circuit detection threshold (x+y) is a value obtained by adding an offset value y to the equalization start threshold x. In this embodiment, the absolute value of the internal short-circuit detection threshold (x+y) is greater than the absolute value of the equalization start threshold x. The internal short-circuit detection threshold (x+y) is set within the range of the following (Equation 1):

[0036] (Maximum value of voltage difference ΔV that occurs naturally during normal use) < (x + y) < (voltage difference ΔV at the boundary with the area where cell safety is no longer ensured) ... (Equation 1) In other words, the internal short circuit detection threshold (x + y) is greater than the maximum value of voltage difference ΔV that occurs naturally during normal use, and smaller than the voltage difference ΔV at the boundary with the area where cell safety is no longer ensured.

[0037] The voltage difference ΔV at the boundary between the region where the safety of the cell is no longer ensured may be determined, for example, by determining the boundary between the short circuit progression region and the short circuit growth region of the battery pack 20 based on the results of experiments or simulations, and setting the voltage difference ΔV at that boundary.

[0038] The control circuit 12 may change the offset value y depending on the state or environment of the cell. The control circuit 12 may set the offset value y to be smaller as the state or environment of the cell deteriorates, thereby lowering the internal short-circuit detection threshold (x+y).

[0039] The control circuit 12 may change the offset value y by referring to at least one of the temperatures of the cells E1-En, the voltages of the cells E1-En, the SOCs of the cells E1-En, and the currents flowing through the cells E1-En. The median or average of the voltages or SOCs of the cells E1-En may be used as the voltages or SOCs of the cells E1-En.

[0040] The internal resistance of a cell tends to increase when the temperature is low. This increase in internal resistance is particularly large at low temperatures. The control circuit 12 can set a smaller offset value y as the temperature decreases. The designer may determine the relationship between the temperature and internal resistance of cells E1-En through experiments or simulations, and generate a map describing the relationship between temperature and offset value y so as to eliminate the effect of temperature on the internal resistance as much as possible. The control circuit 12 can determine the offset value y by referring to this map.

[0041] The internal resistance of a cell tends to increase as the cell voltage or SOC decreases. The control circuit 12 can set the offset value y to a smaller value as the cell voltage or SOC decreases. The designer may determine the relationship between the voltage or SOC and the internal resistance of cells E1-En through experiments or simulations, and generate a map describing the relationship between the voltage or SOC and the offset value y so as to minimize the effect of the voltage or SOC on the internal resistance. The control circuit 12 can determine the offset value y by referring to the map.

[0042] The greater the current flowing through the cell, the greater the potential risk of fire or smoke. The control circuit 12 can set the offset value y to be smaller the greater the current flowing through the cell. The designer may generate a two-dimensional or three-dimensional map describing the relationship between the offset value y and two or three of the three parameters: temperature, voltage (or SOC), and current. The control circuit 12 can determine the offset value y by referring to the two-dimensional or three-dimensional map.

[0043] For example, the control circuit 12 may set the offset value y to be smaller as the DOD increases. For example, when the battery system 1 according to the embodiment is used in a stationary energy storage system and the stationary energy storage system is used for backup purposes, the DOD becomes smaller, and when the stationary energy storage system is used for peak shift purposes, the DOD becomes larger.

[0044] For example, the control circuit 12 may set the offset value y to be smaller as the SOH of the cell decreases. The lower the SOH of a cell, the more damage has accumulated, and safety can be improved by lowering the internal short circuit detection threshold (x+y).

[0045] For example, the control circuit 12 may set the offset value y to be smaller as the rate of decrease in the SOH of the cell increases. The increase in the rate of decrease in the SOH is a phenomenon that is likely to occur at the end of the life of a cell.

[0046] For example, the control circuit 12 may set the offset value y to be smaller as the number of times overcharge protection, overdischarge protection, overcurrent protection, or overtemperature protection is activated increases. The more times these protections are activated, the higher the possibility that damage has accumulated in the cell, and the safety can be improved by lowering the internal short circuit detection threshold (x+y).

[0047] In step S14, if the voltage difference ΔV is less than the internal short-circuit detection threshold (x+y) (N in S14), the process proceeds to step S10, and the use of the battery pack 20 continues. If the voltage difference ΔV exceeds the internal short-circuit detection threshold (x+y) (Y in S14), the control circuit 12 determines that an unsafe event has occurred and notifies the upper device 51 (see FIG. 1) of the error. If charging or discharging is in progress, the control circuit 12 causes the measurement circuit 11 to turn off the power line switch S1 to stop charging or discharging (S15).

[0048] The higher-level device 51 displays the error on the user interface (UI) 52. The user contacts the manufacturer or retailer and calls a service technician. If the battery system 1 is a portable battery pack, the user may bring it to a service center. The service technician will disassemble the battery system 1 and repair or replace the assembled battery 20. If the voltage drop of a specific cell is due to an external short circuit, the problem may be fixed by replacing the part.

[0049] As described above, according to this embodiment, signs of an internal short circuit in a cell are detected by utilizing the relationship between the voltage drop rate α (voltage variation correction ability) in the equalization process and the rate of increase β of the voltage difference ΔV due to a micro-short circuit. When α<β, the voltage difference ΔV cannot be reduced by the equalization process.

[0050] In this embodiment, by using an internal short-circuit detection threshold (x+y) obtained by adding an offset value y to the equalization start threshold x, it is possible to easily detect a state where α<β, in which the equalization process becomes ineffective, and to stop use of the battery system 1 before an internal short-circuit state occurs. Furthermore, by changing the offset value y according to the cell state and environment, it is possible to eliminate as much as possible the influence of the cell state and environment in determining whether an internal short circuit has occurred.

[0051] The present disclosure has been described above based on the embodiments. The embodiments are merely examples, and it will be understood by those skilled in the art that various modifications are possible in the combination of the respective components and processing steps, and that such modifications are also within the scope of the present disclosure.

[0052] For example, in the above-described embodiment, the battery system 1 is prohibited from use when the voltage difference ΔV exceeds the internal short-circuit detection threshold (x+y). However, the battery system 1 may be prohibited from use temporarily when the voltage difference ΔV exceeds the internal short-circuit detection threshold (x+y) for the first time, and may be prohibited from use permanently when the voltage difference ΔV exceeds the internal short-circuit detection threshold (x+y) for the second time.

[0053] The embodiment may be specified by the following items.

[0054] [Item 1] A battery management device (10) comprising: a measurement circuit (11) connected to each node of a plurality of cells (E1-En) connected in series by a voltage measurement line and measuring the voltage of each of the plurality of cells (E1-En); a plurality of discharge circuits (Sd1-Sdn, R1-Rn) connected in parallel with the plurality of cells (E1-En) between each of two adjacent voltage measurement lines; and a control circuit (12) that performs an equalization process among the plurality of cells (E1-En) by controlling the plurality of discharge circuits (Sd1-Sdn, R1-Rn) based on the voltages of the plurality of cells (E1-En) measured by the measurement circuit (11), wherein the control circuit (12) performs the equalization process when a variation in the voltages of the plurality of cells (E1-En) exceeds a first threshold, and determines that an error has occurred when the variation in the voltages of the plurality of cells (E1-En) exceeds a second threshold obtained by adding an offset value to the first threshold.

[0055] This allows easy detection of signs of an internal short circuit in the cells (E1-En).

[0056] [Item 2] The battery management device (10) according to Item 1, wherein the second threshold is set to a voltage difference lower than a region where the safety of the cells (E1-En) is not ensured.

[0057] This allows the use of the battery pack (20) to be stopped before it enters the dangerous area.

[0058] [Item 3] The battery management device (10) according to item 1, wherein the control circuit (12) changes the offset value by referring to at least one of the temperatures of the plurality of cells (E1-En), the voltages of the plurality of cells (E1-En), the SOC (State Of Charge) of the plurality of cells (E1-En), and the currents flowing through the plurality of cells (E1-En).

[0059] This makes it possible to detect signs of an internal short circuit while excluding the influence of temperature, voltage, SOC, and current.

[0060] [Item 4] The battery management device (10) according to Item 1, wherein the control circuit (12) sets the offset value to a smaller value as the SOH (State Of Health) of the cells (E1-En) decreases.

[0061] This allows for earlier detection of symptoms of an internal short circuit in a cell that is deteriorating, thereby improving safety.

[0062] [Item 5] The battery management device (10) according to item 1, wherein the variation in the voltages of the plurality of cells (E1-En) is defined by the difference between the maximum voltage and the minimum voltage of the plurality of cells (E1-En), the difference between the average voltage of the plurality of cells (E1-En) and the minimum voltage, or the difference between the median voltage of the plurality of cells (E1-En) and the minimum voltage.

[0063] This allows accurate determination of variations in the voltages of the multiple cells (E1-En).

[0064] [Item 6] A battery system (1) comprising: an assembled battery (20) including a plurality of cells (E1-En) connected in series; and the battery management device (10) according to any one of items 1 to 5.

[0065] This makes it possible to realize a battery system (1) that can easily detect signs of an internal short circuit in the cells (E1-En).

[0066] [Item 7] A battery management method comprising: a step of measuring the voltage of each of a plurality of cells (E1-En) connected in series; and a step of, when a variation in the voltages of the plurality of cells (E1-En) exceeds a first threshold, performing an equalization process among the plurality of cells (E1-En) by controlling a plurality of discharge circuits (Sd1-Sdn, R1-Rn) connected in parallel to the plurality of cells (E1-En) based on the voltages of the plurality of cells (E1-En); and a step of determining that an error has occurred when the variation in the voltages of the plurality of cells (E1-En) exceeds a second threshold obtained by adding an offset value to the first threshold.

[0067] This allows easy detection of signs of an internal short circuit in the cells (E1-En).

[0068] [Item 8] A battery management program that causes a computer to execute the following steps: when a variation in voltage of a plurality of cells (E1-En) connected in series exceeds a first threshold, a process of performing an equalization process among the plurality of cells (E1-En) by controlling a plurality of discharge circuits (Sd1-Sdn, R1-Rn) that are connected in parallel to the plurality of cells (E1-En) based on the voltages of the plurality of cells (E1-En), and a process of determining that an error has occurred when the variation in voltage of the plurality of cells (E1-En) exceeds a second threshold obtained by adding an offset value to the first threshold.

[0069] This allows easy detection of signs of an internal short circuit in the cells (E1-En).

[0070] REFERENCE SIGNS LIST 1 Battery system 2 Load 20 Assembled battery E1-En Cell 10 Battery management device 11 Measurement circuit 12 Control circuit Rs Shunt resistor T1 Thermistor Q1-Q2 Transistor S1 Power line switch R1-Rn Discharge resistor Sd1-Sdn Discharge switch

Claims

1. A battery management device comprising: a measurement circuit connected to each node of a plurality of cells connected in series by a voltage measurement line and measuring the voltage of each of the plurality of cells; a plurality of discharge circuits connected in parallel to the plurality of cells between each of two adjacent voltage measurement lines; and a control circuit that performs an equalization process among the plurality of cells by controlling the plurality of discharge circuits based on the voltages of the plurality of cells measured by the measurement circuit, wherein the control circuit performs the equalization process when a variation in voltages of the plurality of cells exceeds a first threshold, and determines that an error has occurred when the variation in voltages of the plurality of cells exceeds a second threshold obtained by adding an offset value to the first threshold.

2. A battery management device according to claim 1, characterized in that the second threshold is set to a voltage difference lower than the region where the safety of the cell is no longer ensured.

3. The battery management device of claim 1, characterized in that the control circuit changes the offset value by referring to at least one of the temperatures of the multiple cells, the voltages of the multiple cells, the SOC (State Of Charge) of the multiple cells, and the currents flowing through the multiple cells.

4. The battery management device according to claim 1, wherein the control circuit sets the offset value to be smaller as the SOH (State Of Health) of the cell decreases.

5. The battery management device of claim 1, characterized in that the variation in voltage of the multiple cells is defined as the difference between the maximum voltage and the minimum voltage of the multiple cells, the difference between the average voltage and the minimum voltage of the multiple cells, or the difference between the median voltage and the minimum voltage of the multiple cells.

6. A battery system comprising: a battery pack including a plurality of cells connected in series; and the battery management device according to any one of claims 1 to 5.

7. A battery management method comprising the steps of: measuring the voltage of each of a plurality of cells connected in series; when a variation in the voltages of the plurality of cells exceeds a first threshold, controlling a plurality of discharge circuits connected in parallel to the plurality of cells based on the voltages of the plurality of cells to perform an equalization process among the plurality of cells; and when the variation in the voltages of the plurality of cells exceeds a second threshold obtained by adding an offset value to the first threshold, determining that an error has occurred.

8. A battery management program that causes a computer to execute the following steps: when a variation in voltage of multiple cells connected in series exceeds a first threshold, an equalization process is performed between the multiple cells by controlling multiple discharge circuits that are connected in parallel to the multiple cells based on the voltages of the multiple cells; and when the variation in voltage of the multiple cells exceeds a second threshold obtained by adding an offset value to the first threshold, a process that determines that an error has occurred.

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