Battery system

Abstract

When measuring the full charge capacity of a battery, this suppresses the decrease in the battery system's stored energy. [Solution] The battery system 1 includes a plurality of battery packs 100a to 100d connected in parallel to an external system 2. Each battery pack 100a to 100d is connected in parallel to the external system 2 via a corresponding DC / DC converter 110a to 110d. The control device 200 operates the DC / DC converters 110a to 110d to supply power from battery pack 100a to battery packs 100b to 100d, thereby discharging battery pack 100a. After battery pack 100a is discharged, the control device 200 operates the DC / DC converters 110a to 110d to supply power from battery packs 100b to 100d, thereby charging battery pack 100a. The control device 200 calculates the full charge capacity of battery pack 100a based on the cumulative amount of current charged to battery pack 100a.

Description

Technical Field

[0001] The present disclosure relates to a battery system.

Background Art

[0002] Japanese Patent Application Laid-Open No. 2015-195653 (Patent Document 1) describes a technique for estimating the battery capacity of secondary batteries in a battery system in which a plurality of secondary batteries are connected in parallel. In this Patent Document 1, any one of the plurality of secondary batteries is set as a priority battery, and the battery capacity of the priority battery is estimated by discharging or charging only the priority battery.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In Patent Document 1, the secondary batteries connected in parallel are connected to a common charging circuit and a common discharging circuit. The common charging circuit and the common discharging circuit are operated to discharge from the priority battery to the load. When discharging from the priority battery to estimate the battery capacity of the priority battery, the power storage amount of the battery system decreases. Therefore, when there is a discharge request to the battery system after estimating the battery capacity, there is a concern that the required amount of power cannot be discharged.

[0005] An object of the present disclosure is to suppress a decrease in the power storage amount of a battery system when measuring the full charge capacity of a battery.

Means for Solving the Problems

[0006] The battery system of this disclosure is a battery system that charges and discharges with an external system. The battery system comprises a plurality of battery packs connected in parallel to each other with the external system, a plurality of DC / DC converters provided corresponding to the plurality of battery packs and each positioned on a power line connecting the corresponding battery pack to the external system, and a control device that controls the plurality of DC / DC converters. The control device operates the DC / DC converters corresponding to the first battery pack and each of the other battery packs to supply power from the first battery pack selected from the plurality of battery packs to the other battery packs, and discharges the first battery pack until its SOC (State of Charge) is less than or equal to a first predetermined value. After this discharge, the control device operates the DC / DC converters to supply power from the other battery packs to the first battery pack, and charges the first battery pack until its SOC is greater than or equal to a second predetermined value. In this charging, the control device calculates the full charge capacity of the first battery pack based on the power charged to the first battery pack.

[0007] In this configuration, the battery system comprises multiple battery packs connected in parallel to an external system. DC / DC converters are placed in the power lines connecting the battery packs to the external system. A DC / DC converter is provided for each battery pack. The control unit controls the multiple DC / DC converters. The control unit operates the DC / DC converters to supply power from a first battery pack selected from the multiple battery packs to the other battery packs, discharging the first battery pack until its State of Charge (SOC) falls below a first predetermined value. After the first battery pack is discharged, the control unit operates the DC / DC converters to supply power from the other battery packs to the first battery pack, charging the first battery pack until its SOC rises above a second predetermined value. The control unit calculates the full charge capacity of the first battery pack based on the power charged to the first battery pack until its SOC falls from the first predetermined value to the second predetermined value. Power is exchanged between multiple battery packs included in the battery system, charging and discharging the first battery pack to determine its full charge capacity. This suppresses the decrease in the battery system's stored energy when measuring the full charge capacity of the battery packs.

[0008] In the battery system of this disclosure, the full charge capacity of the first battery pack may be calculated based on the power of the first battery pack during discharge. In this case, the control device operates a DC / DC converter to supply power from other battery packs to the first battery pack selected from a plurality of battery packs, and charges the first battery pack until its State of Charge (SOC) is equal to or greater than a third predetermined value. After charging the first battery pack, the control device operates a DC / DC converter to supply power from the first battery pack to other battery packs, and discharges the first battery pack until its SOC is equal to or less than a fourth predetermined value. The control device calculates the full charge capacity of the first battery pack based on the power discharged from the first battery pack until its SOC falls from the third predetermined value to the fourth predetermined value.

[0009] With this configuration, power is exchanged between multiple battery packs included in the battery system, and the full charge capacity of the first battery pack is determined. Therefore, when measuring the full charge capacity of the battery packs, the decrease in the amount of stored energy in the battery system can be suppressed.

[0010] Preferably, the State of Charge (SOC) of the first battery pack may be estimated based on the voltage of the first battery pack.

[0011] With this configuration, the State of Charge (SOC) of the first battery pack can be estimated based on its SOC-OCV (Open Circuit Voltage) characteristics. [Effects of the Invention]

[0012] According to this disclosure, when measuring the full charge capacity of the battery, the decrease in the amount of charge stored in the battery system can be suppressed. [Brief explanation of the drawing]

[0013] [Figure 1] This is a schematic diagram of the battery system according to this embodiment. [Figure 2] This flowchart shows an example of the full charge capacity calculation process performed by the control unit. [Figure 3] This is a diagram illustrating the SOC-OCV characteristics. [Figure 4] This flowchart shows an example of the full charge capacity calculation process performed by the control device in Embodiment 2. [Figure 5] This is a schematic diagram of the battery system in a modified example. [Modes for carrying out the invention]

[0014] The embodiments of this disclosure will be described in detail below with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals, and their descriptions will not be repeated.

[0015] (Embodiment 1) Figure 1 is a schematic diagram of the battery system 1 according to this embodiment. As shown in Figure 1, the battery system 1 is connected to an external system 2 by a power line L. The battery system 1 can be powered by the external system 2 and can also discharge to the external system 2. The battery system 1 comprises a plurality of battery packs 100. In this embodiment, it comprises four battery packs 100a to 100d. The number of battery packs 100 is arbitrary and may be 10 or 20.

[0016] The battery pack 100 is a battery assembly formed by connecting multiple single cells in series, for example. The battery cells may be, for example, ternary lithium-ion batteries (hereinafter also referred to as "NMC batteries") or lithium iron phosphate batteries (hereinafter also referred to as "LFP batteries"). The battery cells may also be nickel-metal hydride batteries. The battery pack 100 may be a repurposed battery pack (battery module) that was installed in a vehicle.

[0017] Referring to Figure 1, the four battery packs 100a to 100d are connected in parallel to the external system 2. A DC / DC converter 110 (110a to 110d) is provided for each battery pack 100a to 100d on the power line L connecting the battery packs 100a to 100d to the external system. The DC / DC converters 110a to 110d are bidirectional DC / DC converters controlled by the control device 200. The DC / DC converters 110a to 110d control the charging and discharging of the corresponding battery packs 100a to 100d.

[0018] Each of the battery packs 100a to 100d is provided with a monitoring module 120. The monitoring module 120 detects the voltage VB [V], current IB [A], and temperature TB of the corresponding battery packs 100a to 100d and outputs them to the control device 200. The current IB has a positive or negative sign indicating the flow direction. The current (charging current) charged into the battery pack 100 is detected as a positive (+) value, and the current (discharging current) discharged from the battery pack 100 is detected as a negative (-) value. Further, the monitoring module 120 calculates the SOC of the corresponding battery packs 100a to 100c and outputs it to the control device 200. Note that the SOC of the battery pack 100 may be calculated by the control device 200. The SOC is the charge state of the battery pack 100, and is defined as SOC = 100 [%] for the fully charged state and SOC = 0 [%] for the fully discharged state.

[0019] The external system 2 includes a PCS (Power Conditioning System) 10, a solar power generation device 20, a load 30, and a power grid PG. Each of the battery packs 100a to 100d is connected in parallel to the PCS 10 via each of the DC / DC converters 110a to 110d.

[0020] The PCS 10 is a power conversion device capable of both AC / DC conversion (conversion from alternating current to direct current) and DC / AC conversion (conversion from direct current to alternating current). The PCS 10 receives DC power from, for example, the solar power generation device 20. The PCS 10 supplies AC power to the load 30. Note that the load 30 includes electrical products (such as air conditioners and lighting fixtures) used in homes. The PCS 10 exchanges AC power with the power grid 940.

[0021] The control device 200 includes a processor and a memory, receives commands from the PCS 10, and controls the battery system 1. Further, in the present embodiment, the DC / DC converters 110a to 110d are controlled to calculate the full charge capacity of the battery pack 100.

[0022] Figure 2 is a flowchart showing an example of the full charge capacity calculation process performed by the control device 200. This flowchart is executed when no power is being exchanged between the battery system 1 and the external system 2 (PCS10) (when no charging or discharging is occurring between the battery system 1 and the external system 2).

[0023] In step 10 (hereinafter abbreviated as "S"), a battery pack 100 is selected to measure the full charge capacity. The selection of the battery pack 100 can be any method that allows for the sequential measurement of the full charge capacities of battery packs 100a to 100d, or, if only the full charge capacity of a specific battery pack 100 is to be measured, a battery pack 100 to measure the full charge capacity of may be selected. In this embodiment, first, battery pack 100a is selected as the battery pack to measure the full charge capacity of.

[0024] In S11, the selected battery pack 100 (in this process, battery pack 100a) is discharged. The control device 200 operates the DC / DC converters 110a to 110d and supplies the power stored in battery pack 100a to battery packs 100b to 100d, as shown by the dashed line in Figure 1. As a result, the power discharged from battery pack 100a charges battery packs 100b to 100d. In this process, battery pack 100a corresponds to the "first battery pack" of this disclosure, and battery packs 100b to 100d correspond to examples of the "other battery packs" of this disclosure.

[0025] In the subsequent S12, it is determined whether the SOC of the battery pack 100a is less than or equal to a predetermined value α. In this embodiment, the SOC of the battery pack 100 is calculated (estimated) based on the SOC-OCV characteristics (SOC-OCV curve). Figure 3 is a diagram illustrating the SOC-OCV characteristics. In Figure 3, the vertical axis is the OCV of the battery pack 100 (battery cell), and the horizontal axis is the SOC. In Figure 3, the solid line is the characteristics of the LPF battery, and the dashed line is the characteristics of the NMC battery. In an LPF battery, the relationship between OCV and SOC (hereinafter, this relationship will also be called the OCV curve) has a region where the change in the OCV curve is small (voltage flat region: plateau region). In an LPF battery, in the region where SOC is smaller than the plateau region (in Figure 3, the region where SOC is A or less), and in the region where SOC is larger than the plateau region (in Figure 3, the region where SOC is B or more), the OCV changes significantly in response to the change in SOC. These regions are also called non-plateau regions.

[0026] In LPF batteries, the accuracy of calculating SOC using SOC-OCV characteristics is low in the plateau region. In LPF batteries, the accuracy of calculating SOC using SOC-OCV characteristics is high in the non-plateau region. The predetermined value α is set to a value smaller than A in Figure 3, considering the case where the battery pack 100a is an LPF battery. The predetermined value α may be 0%. The predetermined value α may be set to the same value regardless of the battery type of the battery pack. The SOC of the battery pack 100a is calculated from the SOC-OCV characteristics in Figure 3, using the voltage VB of the battery pack 100a as a parameter.

[0027] If the State of Charge (SOC) of battery pack 100a is less than or equal to a predetermined value α, the result is positive and the process proceeds to S13. If the SOC of battery pack 100a is greater than the predetermined value α, the result is negative and the process returns to S11, where the battery pack 100a is discharged until the SOC becomes less than or equal to the predetermined value α.

[0028] In S13, the discharge of battery pack 100a is stopped, and charging of battery packs 100b to 100d is stopped. Charging and discharging are stopped by ceasing the operation of DC / DC converters 110a to 110d.

[0029] In S14, it is determined whether a predetermined time T1 has elapsed since the discharge of the battery pack 100a stopped. If the predetermined time T1 has elapsed, the determination is affirmative and the process proceeds to S15. If the predetermined time T1 has not elapsed, the determination in S14 is repeated. The predetermined time T1 is set, for example, to be a sufficient amount of time after the discharge of the battery pack 100a stops for the effects of concentration polarization, etc., to decrease.

[0030] In S15, the SOC calculated from the SOC-OCV characteristics using the voltage VB of the battery pack 100a as a parameter is stored as SOCs.

[0031] In the subsequent S16, the battery pack 100a is charged, and the current IB is integrated to calculate the integrated current amount ΣIB. The control device 200 operates the DC / DC converters 110a to 110d and supplies power stored in the other battery packs 100b to 100d to battery pack 100a, as shown by the dashed line in Figure 1. As a result, the power discharged from the other battery packs 100b to 100d is charged into battery pack 100a. The integrated current amount ΣIB [Ah] corresponds to the value obtained by integrating the current IB with respect to time, and is an example of the "power charged into the first battery pack" in this disclosure.

[0032] In S17, it is determined whether the State of Charge (SOC) of the battery pack 100a is greater than or equal to a predetermined value β. The predetermined value β is set to a value greater than B in Figure 3, taking into consideration the case where the battery pack 100a is an LPF battery. The predetermined value β may also be 100%. The predetermined value β may be set to the same value regardless of the battery type of the battery pack.

[0033] If the State of Charge (SOC) of battery pack 100a is greater than or equal to a predetermined value β, the result is positive and the process proceeds to S18. If the SOC of battery pack 100a is less than the predetermined value β, the result is negative and the process returns to S16, where charging of battery pack 100a is performed until the SOC becomes greater than or equal to the predetermined value β.

[0034] In S18, charging of battery pack 100a is stopped, and discharging from battery packs 100b to 100d is stopped. Charging and discharging are stopped by ceasing operation of DC / DC converters 110a to 110d. Also, the integration of current IB is terminated, and the integrated current amount ΣIB is stored.

[0035] In S19, it is determined whether a predetermined time T2 has elapsed since the charging of the battery pack 100a stopped. If the predetermined time T2 has elapsed, the determination is affirmative and the process proceeds to S20. If the predetermined time T2 has not elapsed, S19 is repeated. The predetermined time T2 is set, for example, to be a sufficient amount of time after the charging of the battery pack 100a stops for the effects of concentration polarization, etc., to decrease.

[0036] In S20, the SOC calculated from the SOC-OCV characteristics using the voltage VB of the battery pack 100a as a parameter is stored as SOCe.

[0037] In S21, the full charge capacity Fc [Ah] of the battery pack 100a is calculated from the following formula (1). Fc=ΣIB / ((SOCe-SOCs) / 100) (1) For example, when SOCe is 100% and SOCe is 0%, Fc is calculated as Fc = ΣIB.

[0038] Once S21 is processed, the current routine ends. In the next routine, in S10, a battery pack 100 whose full charge capacity has not been measured (for example, battery pack 100b) may be selected, and the same process may be performed, repeating the process until the full charge capacity of all battery packs 100 has been measured.

[0039] According to this embodiment, the control device 200 operates the DC / DC converters 110a to 110d to supply power from battery pack 100a selected from the plurality of battery packs 100a to 100d to battery packs 100b to 100d, and discharges battery pack 100a until its SOC is less than or equal to a predetermined value α. After discharging battery pack 100a, the control device 200 operates the DC / DC converters 110a to 110d to supply power from battery packs 100b to 100d to battery pack 100a, and charges battery pack 100a until its SOC is greater than or equal to a predetermined value β.

[0040] The control device 200 calculates the full charge capacity Fc of battery pack 100a based on the power (integrated current ΣIB) charged to battery pack 100a until the State of Charge (SOC) of battery pack 100a changes from a predetermined value α to a predetermined value β. Power is exchanged between multiple battery packs 100a to 100d included in the battery system 1, and battery pack 100a is charged and discharged to determine the full charge capacity Fc of battery pack 100a. Therefore, when measuring the full charge capacity Fc of battery pack 100a, the decrease in the amount of stored energy in the battery system 1 can be suppressed.

[0041] In this embodiment, the predetermined values ​​α and β are set as values ​​in the non-plateau region of the low SOC region and the high SOC region when the battery pack 100 is an LPF battery. Therefore, SOCs and SOCe can be calculated with accuracy, and the full charge capacity Fc can be measured with accuracy. In addition, since the difference between SOCs and SOCe can be made large, the full charge capacity Fc can be calculated with accuracy using the above equation (1).

[0042] In this embodiment, DC / DC converters 110a to 110d, provided for each battery pack 100a to 100d, are operated to charge and discharge the battery packs 100a to 100d. Therefore, power can be controlled for each battery pack 100a to 100d, resulting in good controllability. Furthermore, even if there is a voltage difference between the battery packs 100a to 100d, current recirculation can be suppressed.

[0043] (Embodiment 2) In the above embodiment, the full dielectric capacity was calculated using the battery's accumulated current ΣIB during charging of the selected battery pack 100. In embodiment 2, the full charge capacity is calculated using the current accumulated current ΣIB during discharging of the selected battery pack 100. Figure 4 is a flowchart showing an example of the full charge capacity calculation process executed by the control device 200 in embodiment 2. This flowchart is executed when no power is being exchanged between the battery system 1 and the external system 2 (PCS10) (when no charging or discharging is occurring between the battery system 1 and the external system 2).

[0044] In S30, a battery pack 100 is selected to measure the full charge capacity. The selection of the battery pack 100 may be the same as in S10, and in this embodiment, battery pack 100a is selected first.

[0045] In S31, battery pack 100a is charged. Similar to the charging in S16, the DC / DC converters 110a to 110d are activated to supply power stored in the other battery packs 100b to 100d to battery pack 100a, as shown by the dashed lines in Figure 1.

[0046] In S32, it is determined whether the SOC of the battery pack 100a is greater than or equal to a predetermined value b. The predetermined value b may be the same as the predetermined value β in S17. The predetermined value b may also be 100%. If the SOC of the battery pack 100a is greater than or equal to the predetermined value b, the determination is affirmative and the process proceeds to S33. If the SOC of the battery pack 100a is less than the predetermined value b, the determination is negative and the process returns to S31, and charging of the battery pack 100a is performed until the SOC becomes greater than or equal to the predetermined value b.

[0047] In S33, charging of battery pack 100a is stopped, and discharging from battery packs 100b to 100d is stopped, and the process proceeds to S34. In S34, it is determined whether a predetermined time T3 has elapsed since charging of battery pack 100a was stopped. If the predetermined time T3 has elapsed, the determination is affirmative and the process proceeds to S35. If the predetermined time T3 has not elapsed, S34 is repeated. The predetermined time T3 is the same value as the predetermined time T2 in S19.

[0048] In S35, the SOC calculated from the SOC-OCV characteristics using the voltage VB of the battery pack 100a as a parameter is stored as SOCs, and the process proceeds to S36.

[0049] In S36, the battery pack 100a is discharged, and the current IB is integrated to calculate the integrated current amount ΣIB. Discharging the battery pack 100a is performed in the same way as in S11, by operating the DC / DC converters 110a to 110d, and supplying the power stored in battery pack 100a to battery packs 100b to 100d as shown by the dashed line in Figure 1. The current IB detected by the monitoring module 120 is detected as a negative (-) value during discharge, so the integrated current amount ΣIB[Ah] is integrated as a negative value.

[0050] In S37, it is determined whether the State of Charge (SOC) of the battery pack 100a is less than or equal to a predetermined value a. The predetermined value a may be the same as the predetermined value α in S12. The predetermined value a may also be 0[%]. If the SOC of the battery pack 100a is less than or equal to the predetermined value α, the determination is affirmative and the process proceeds to S38. If the SOC is greater than the predetermined value a, the determination is negative and the process returns to S36, where the battery pack 100a is discharged until the SOC becomes less than or equal to the predetermined value a.

[0051] In S38, the discharge of battery pack 100a is stopped, and charging from battery packs 100b to 100d is stopped. The integration of current IB is completed, and the integrated current amount ΣIB is stored.

[0052] In S39, it is determined whether a predetermined time T4 has elapsed since charging of the battery pack 100a stopped. If the predetermined time T4 has elapsed, the determination is affirmative and the process proceeds to S40. If the predetermined time T4 has not elapsed, S39 is repeated. The predetermined time T4 may be the same value as the predetermined time T1 in S13.

[0053] In S40, the SOC calculated from the SOC-OCV characteristics using the voltage VB of the battery pack 100a as a parameter is stored as SOCe.

[0054] In S41, the full charge capacity Fc [Ah] of the battery pack 100a is calculated from equation (1) above. Note that the integrated current ΣIB is a negative value, and the value of (SOCe-SOCa) is also negative, so the full charge capacity Fc is a positive value.

[0055] Once S41 is processed, the routine ends. Then, as with the process in Figure 2, the process may be repeated until the full charge capacity of all battery packs 100 is measured.

[0056] According to this second embodiment, the control device 200 calculates the full charge capacity Fc of the battery pack 100a based on the power discharged from the battery pack 100a (integrated current ΣIB) until the State of Charge (SOC) of the battery pack 100a changes from a predetermined value b to a predetermined value a. Power is exchanged between the multiple battery packs 100a to 100d included in the battery system 1, and the battery pack 100a is charged and discharged to determine the full charge capacity Fc of the battery pack 100a. Therefore, when measuring the full charge capacity Fc of the battery pack 100a, the decrease in the amount of stored energy in the battery system 1 can be suppressed.

[0057] According to this second embodiment, as with the first embodiment, SOCs and SOCe can be calculated with high accuracy, and the difference between SOCs and SOCe can be made large, so the full charge capacity Fc can be calculated with high accuracy. Since DC / DC converters 110a to 110d are provided for each battery pack 100a to 100d, controllability is good, and even if there is a voltage difference between the battery packs 100a to 100d, current backflow can be suppressed.

[0058] (modified version) Figure 5 is a schematic diagram of the battery system S in a modified example. The battery system S comprises multiple sub-battery systems 1A to 1D. The sub-battery systems 1A to 1D are connected in parallel to the external system 2 (PCS10). (In Figure 5, the solar power generation device 20, the load 30, and the power grid PG are omitted.)

[0059] Sub-battery systems 1A to 1D have a configuration similar to battery system 1 in the embodiment, with multiple battery packs 100 and DC / DC converters 110 corresponding to each battery pack 100 connected in parallel. Each of the sub-battery systems 1A to 1D is equipped with corresponding relays R1 to R4 and is connected in parallel to the PCS 10 via relays R1 to R4. The control device 200A controls the operation of the DC / DC converters 110 and the opening and closing of relays R1 to R4.

[0060] In this modified example, when performing the full charge capacity calculation process shown in Figure 2 or Figure 4, relays R1 to R4 corresponding to the sub-battery systems 1A to 1D containing the battery pack 100 whose full charge capacity is to be measured are opened. For example, when calculating the full charge capacity of the battery pack 100 included in sub-battery system 1A, relay R1 is opened and relays R2 to R4 are closed. As a result, sub-battery system 1A is disconnected from PCS 10, enabling the full charge capacity calculation process shown in Figure 2 or Figure 4. Since sub-battery systems 1B to 1D are connected to PCS 10, power can be exchanged between the external system 2 and sub-battery systems 1B to 1D. The number of sub-battery systems 1A to 1D can be any number.

[0061] According to this modified example, when power is being exchanged between the battery system S and the external system 2, it becomes possible to measure the full charge capacity of the battery pack 100 by executing the full charge calculation process shown in Figure 2 or Figure 4.

[0062] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of this disclosure is indicated by the claims rather than by the description of the embodiments above, and all modifications within the meaning and scope equivalent to the claims are intended to be included. [Explanation of symbols]

[0063] 1, S battery system, 2 external systems, 10 PCS, 20 solar power generation devices, 30 loads, 100 battery packs, 110 DC / DC converters, 120 monitoring modules, 200, 200A control devices, PG power grid.

Claims

1. A battery system that performs charging and discharging in and out of an external system, The external system comprises a plurality of battery packs connected to each other in parallel, Multiple DC / DC converters are provided corresponding to the aforementioned multiple battery packs, and each is arranged in a power line connecting the corresponding battery pack to the external system, The system includes a control device for controlling the plurality of DC / DC converters, The control device is The DC / DC converters corresponding to the first battery pack and each of the other battery packs are operated to supply power from the first battery pack selected from the plurality of battery packs to the other battery packs, and the first battery pack is discharged until the State of Charge (SOC) of the first battery pack falls below a first predetermined value. After the discharge, the DC / DC converter is activated to supply power from the other battery pack to the first battery pack, and the first battery pack is charged until the State of Charge (SOC) of the first battery pack is equal to or greater than the second predetermined value. A battery system that, in the charging process, calculates the full charge capacity of the first battery pack based on the power charged to the first battery pack.

2. A battery system that performs charging and discharging in and out of an external system, The external system comprises a plurality of battery packs connected to each other in parallel, Multiple DC / DC converters are provided corresponding to the aforementioned multiple battery packs, and each is arranged in a power line connecting the corresponding battery pack to the external system, The system includes a control device for controlling the plurality of DC / DC converters, The control device is The DC / DC converters corresponding to the first battery pack and each of the other battery packs are operated to supply power from the other battery packs to the first battery pack selected from the plurality of battery packs, and the first battery pack is charged until the State of Charge (SOC) of the first battery pack is equal to or greater than the third predetermined value. After the charging, the DC / DC converter is operated to supply power from the first battery pack to the other battery pack, and the first battery pack is discharged until its State of Charge (SOC) falls below a fourth predetermined value. A battery system that, in the discharge described above, calculates the full charge capacity of the first battery pack based on the power discharged from the first battery pack.

3. The battery system according to claim 1 or 2, wherein the State of Cost (SOC) of the first battery pack is estimated based on the voltage of the first battery pack.

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