BATTERY SYSTEM

Abstract

A battery system comprises multiple battery packs connected in parallel to an external system. The battery packs are connected to the external system via DC-DC converters. A control device discharges a first battery pack by activating the DC-DC converters to supply electricity from the first battery pack to other battery packs. After the first battery pack is discharged, the control device recharges it by operating the DC-DC converters to feed electricity from the other battery packs into the first battery pack. The control device calculates the full charge capacity of the first battery pack based on the amount of current that was charged into it.

Description

BACKGROUND OF THE INVENTION 1. Field of the invention

[0001] The present disclosure relates to a battery system. 2. Description of the related prior art

[0002] The unexamined Japanese patent application disclosure no. 2015-195653 (JP 2015-195653 A) describes a technology that estimates the battery capacity of a secondary battery in a battery system in which a plurality of secondary batteries are connected in parallel. In JP 2015-195653 A, one of the secondary batteries is designated as the priority battery, and only the priority battery is discharged or charged to estimate its battery capacity. BRIEF SUMMARY OF THE INVENTION

[0003] In JP 2015-195653 A, the parallel-connected secondary batteries are connected to a common charge and discharge circuit. Electricity is discharged from the priority battery to a load by operating the common charge and discharge circuit. When electricity is discharged from the priority battery to estimate its capacity, the electricity stored in the battery system decreases. This results in a situation where, after the battery capacity has been estimated, a discharge request to the battery system may not deliver the requested amount of electricity.

[0004] The purpose of the present disclosure is to avoid a decrease in the electricity stored in a battery system when a full charge capacity of a battery is measured.

[0005] A battery system within the meaning of the present disclosure is a battery system that performs charging and discharging between the battery system and an external system. The battery system comprises: a plurality of battery packs connected in parallel to the external system; a plurality of DC-DC converters configured to correspond to the battery packs and each arranged in a power line connecting the respective battery pack to the external system; and a control device that controls the DC-DC converters.The control device discharges a first battery pack, selected from among the available battery packs, until its state of charge (SOC) is equal to or lower than a first predetermined value. This is achieved by operating the DC-DC converters corresponding to the first battery pack and the other battery packs, respectively, to supply electricity from the first battery pack to the other battery packs. After this discharge, the control device recharges the first battery pack until its SOC is equal to or higher than a second predetermined value. This is achieved by operating the DC-DC converters to supply electricity from the other battery packs to the first battery pack. The control device then calculates the full charge capacity of the first battery pack based on the electricity supplied to it during this charging process.

[0006] In this configuration, the battery system comprises the battery packs, which are connected in parallel to the external system. The DC-DC converters are located in the power line connecting the battery packs to the external system. Each DC-DC converter is dedicated to one battery pack. The control device manages the DC-DC converters. The control device discharges a selected battery pack until its state of charge (SOC) is equal to or lower than the first predetermined value by operating the DC-DC converters to supply electricity from the first battery pack to the other battery packs.After the first battery pack is discharged, the control device charges it until its state of charge (SOC) is equal to or higher than the second predetermined value. This is achieved by operating the DC-DC converters to supply electricity from the other battery packs to the first battery pack. The control device calculates the full charge capacity of the first battery pack based on the amount of electricity charged into it as its SOC changed from the first predetermined value to the second. The full charge capacity of the first battery pack is reached through the exchange of electricity between the battery packs in the battery system to charge and discharge the first battery pack. This ensures that the measurement of the full charge capacity of a battery pack does not reflect a decrease in the total electricity stored in the battery system.

[0007] In the battery system of the present disclosure, the full charge capacity of a first battery pack can be calculated based on the electricity supplied during the discharge of the first battery pack. In this case, the control device charges a first battery pack selected from among the battery packs until the state of charge (SOC) of the first battery pack is equal to or greater than a third predetermined value by operating the DC-DC converters to supply electricity from the other battery packs to the first battery pack. After charging the first battery pack, the control device discharges the first battery pack until the state of charge (SOC) of the first battery pack is equal to or less than a fourth predetermined value by operating the DC-DC converters to supply electricity from the first battery pack to the other battery packs.The control device calculates the full charge capacity of the first battery pack based on the electricity discharged from the first battery pack while the SOC of the first battery pack changed from the third preset value to the fourth preset value.

[0008] In this configuration, the full charge capacity of the first battery pack is achieved through the exchange of electricity between the battery packs of the battery system. This prevents a decrease in the electricity stored in the battery system when measuring the full charge capacity of a battery pack.

[0009] The state of charge (SOC) of the first battery pack can be estimated based on the voltage of the first battery pack.

[0010] In this configuration, the SOC of the first battery pack can be estimated based on the SOC-OCV (open circuit voltage) characteristics.

[0011] According to the present disclosure, a decrease in the electricity stored in a battery system can be avoided if the full charge capacity of a battery is measured. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Features, advantages and technical and industrial significance of embodiments of the invention are described below with reference to the accompanying drawings, in which the same signs denote the same elements and in which: Fig. Figure 1 is a schematic configuration diagram of a battery system according to an exemplary embodiment; Fig. Figure 2 is a flowchart showing an example of a process for calculating the full charge capacity, performed in a control device; Fig. Figure 3 is a diagram illustrating the SOC-OCV properties; Fig. Figure 4 is a flowchart showing an example of a process for calculating the full charge capacity, which is carried out in the control device in embodiment 2; and Fig. Figure 5 is a schematic configuration diagram of a battery system in a modified example. DETAILED DESCRIPTION OF THE EXECUTION EXAMPLES

[0013] Exemplary embodiments of the present disclosure are described in detail below with reference to the drawings. The same or equivalent parts in the drawings are designated by the same signs, and their descriptions are not repeated. Example 1

[0014] Fig. Figure 1 is a schematic configuration diagram of a battery system 1 according to the exemplary embodiment. As in Fig. As shown in Figure 1, the battery system 1 is connected to an external system 2 via a power line L. The battery system 1 can both receive electricity from the external system 2 and supply electricity to the external system 2. The battery system 1 comprises a plurality of battery packs 100. In the present embodiment, it comprises four battery packs 100a to 100d. The number of battery packs 100 is freely selectable and can be 10 or 20.

[0015] The Battery Pack 100 is a composite battery in which several individual batteries (battery cells) are connected, for example, in series. These battery cells can be 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 can also be nickel-metal hydride batteries. The Battery Pack 100 may be a battery pack (battery module) that was previously installed in a vehicle.

[0016] According to Fig. 1. The four battery packs 100a to 100d are connected in parallel to the external system 2. DC-DC converters 110 (110a to 110d) are provided in the power line L, which connects the battery packs 100a to 100d to the external system. The DC-DC converters 110a to 110d are bidirectional and are controlled by a control device 200. The DC-DC converters 110a to 110d control the charging and discharging of the respective battery packs 100a to 100d.

[0017] Each of the battery packs 100a to 100d is equipped with a monitoring module 120. The monitoring module 120 detects a voltage VB [V], a current IB [A], and a temperature TB of the corresponding battery pack 100a to 100d and outputs the detected values ​​to the control device 200. The current IB has positive and negative signs indicating the direction of flow. A current charging into the battery pack 100 (charging current) is recorded as a positive (+) value, while a current discharging from the battery pack 100 (discharging current) is recorded as a negative (-) value. The monitoring module 120 calculates a state of charge (SOC) of the corresponding battery pack 100a to 100d and outputs the calculated SOC to the control device 200. The SOC of the battery pack 100 can be calculated in the control device 200.The SOC is a state of charge of the battery pack 100, where a fully charged state is defined as SOC = 100 [%] and a fully discharged state as SOC = 0 [%].

[0018] The external system 2 comprises a power conditioning system (PCS) 10, a photovoltaic power generation device 20, a load 30, and a power grid PG. The battery packs 100a to 100d are connected to the PCS 10 in parallel via the respective DC-DC converters 110a to 110d.

[0019] The PCS 10 is a power conversion device capable of both AC-DC conversion (converting direct current to alternating current) and DC-AC conversion (converting direct current to alternating current). For example, the PCS 10 receives direct current from the photovoltaic power generation device 20. The PCS 10 supplies electricity to the load 30 in the form of alternating current. The load 30 includes electrical products used in households (e.g., air conditioners and lighting appliances). The PCS 10 exchanges alternating current between itself and the power grid PG.

[0020] The control device 200 comprises a processor and a memory and controls the battery system 1 by receiving commands from the PCS 10. In the present embodiment, the control device 200 calculates a full charge capacity of the battery pack 100 by controlling the DC-DC converters 110a to 110d.

[0021] Fig. Figure 2 is a flowchart showing an example of a process for calculating the full charge capacity, which is executed in the control device 200. This flowchart is executed when no exchange of electricity takes place between battery system 1 and external system 2 (PCS 10) (when no charging and discharging takes place between battery system 1 and external system 2).

[0022] In step 10 (hereinafter abbreviated as "step"), a battery pack 100 is selected whose full charge capacity is to be measured. Any method can be used to select the battery pack 100, for example, one that can measure the full charge capacity of battery packs 100a to 100d successively. If the full charge capacity of only one specific battery pack 100 is to be measured, the battery pack 100 whose full charge capacity is to be measured can be selected. In the present embodiment, battery pack 100a is first selected as the battery pack whose full charge capacity is to be measured.

[0023] In S11, the discharge of the selected battery pack 100 (in the present method, battery pack 100a) is carried out. The control device 200 operates the DC-DC converters 110a to 110d to supply the electricity stored in battery pack 100a to battery packs 100b to 100d, as indicated by the long dashed short line in Fig. 1 indicated. The electricity taken from battery pack 100a is thus charged into battery packs 100b to 100d. In the present process, battery pack 100a corresponds to the “first battery pack” of the present disclosure, and battery packs 100b to 100d correspond to an example of “the other battery packs” of the present disclosure.

[0024] In the subsequent S12, it is determined whether the state of charge (SOC) of battery pack 100a is equal to or less than a predetermined value α. In the present embodiment, the SOC of battery pack 100 is calculated (estimated) based on the SOC-OCV characteristic curve. Fig. Figure 3 is a diagram illustrating the SOC-OCV characteristic curve. Fig. Figure 3 shows the ordinate axis representing the OCV value of battery pack 100 (battery cell) and the abscissa axis representing the SOC value. Fig. Figure 3: The solid line represents the characteristics of an LFP battery, and the dashed line represents the characteristics of an NMC battery. In the relationship between the OCV and the SOC (hereinafter also referred to as the "OCV curve") of the LFP battery, there is a region where the changes in the OCV curve are very small (flat voltage region: plateau region). In the LFP battery, the OCV changes in a region where the SOC is lower than in the plateau region (in Fig. 3. an area where the SOC is equal to or lower than A), and in an area where the SOC is higher than in the plateau area (in Fig. 3. An area where the SOC is equal to or higher than B), in response to changes in the SOC significantly. These regions are also referred to as non-plateau regions.

[0025] With the LFP battery, the accuracy of the SOC calculation using the SOC-OCV characteristics is low in the plateau region. With the LFP battery, the accuracy of the SOC calculation using the SOC-OCV characteristics is high in the non-plateau regions. The specified value α is in Fig. 3 is set to a value less than A to account for the case where battery pack 100a is an LFP battery. The predetermined value α can be 0 [%]. The predetermined value α can be set to the same value regardless of the battery type of the battery pack. Using the voltage VB of battery pack 100a as a parameter, the SOC of battery pack 100a is determined based on the SOC-OCV characteristics of Fig. 3 calculated.

[0026] If the state of charge (SOC) of battery pack 100a is equal to or lower than the predetermined value α, a positive determination is made and the process proceeds to S13. If the SOC of battery pack 100a is higher than the predetermined value α, a negative determination is made and the process returns to S11, where the discharge of battery pack 100a is carried out until the SOC is equal to or lower than the predetermined value α.

[0027] In S13, the discharge of battery pack 100a is stopped, and the charging of battery packs 100b to 100d is stopped. Charging and discharging are stopped because the operation of the DC-DC converters 110a to 110d is stopped.

[0028] In S14, it is determined whether a predetermined time T1 has elapsed since the discharge of battery pack 100a was stopped. If the predetermined time T1 has elapsed, this is confirmed, and the process proceeds to S15. If the predetermined time T1 has not yet expired, the determination in S14 is repeated. The predetermined time T1 is set, for example, so that the influence of concentration polarization, etc., becomes minimal after the discharge of battery pack 100a has ceased.

[0029] In S15, the SOC, which was calculated from the SOC-OCV characteristic curves using the voltage VB of the battery pack 100a as a parameter, is stored as SOCs.

[0030] In the subsequent S16, battery pack 100a is charged, with the current IB being integrated to calculate an integrated current ΣIB. The control device 200 controls the DC-DC converters 110a to 110d to feed the electricity stored in the other battery packs 100b to 100d into battery pack 100a, as shown in Fig. 1 is indicated by the long dashed line and the double short dashed line. The electricity drawn from the other battery packs 100b to 100d is thus charged into battery pack 100a. The integrated current ΣIB [Ah] corresponds to a value obtained by integrating the current IB over time and is an example of "the electricity charged into the first battery pack" within the meaning of the present disclosure.

[0031] In S17, it is determined whether the state of charge (SOC) of battery pack 100a is equal to or higher than a predetermined value β. The predetermined value β is set to a higher value than B in Fig. 3 is set to account for the case where battery pack 100a is an LFP battery. The predetermined value β can be 100 [%]. The predetermined value β can be set to the same value regardless of the battery type of the battery pack.

[0032] If the state of charge (SOC) of battery pack 100a is equal to or higher than the predetermined value β, a positive determination is made and the process proceeds to S18. If the SOC of battery pack 100a is lower than the predetermined value β, a negative determination is made and the process returns to S16, where charging of battery pack 100a is carried out until the SOC is equal to or higher than the predetermined value β.

[0033] In S18, the charging process of battery pack 100a and the discharging of battery packs 100b to 100d are stopped. Charging and discharging are halted because the operation of the DC-DC converters 110a to 110d is stopped. The integration of current IB is terminated, and the integrated current amount ΣIB is stored.

[0034] In S19, it is determined whether a predetermined time T2 has elapsed since the charging of battery pack 100a was stopped. If the predetermined time T2 has elapsed, this is confirmed, and the process proceeds to S20. If the predetermined time T2 has not yet expired, S19 is executed again. The predetermined time T2 is set, for example, so that the influence of concentration polarization, etc., becomes minimal after the charging process of battery pack 100a has stopped.

[0035] In S20, the SOC, which was calculated from the SOC-OCV characteristic curves using the voltage VB of the battery pack 100a as a parameter, is stored as SOCe.

[0036] In S21, a full charge capacity Fc [Ah] of the battery pack 100a is calculated using the following formula (1): Fc=∑IB / ((SOCe−SOCs) / 100)

[0037] For example, if the SOCe is 100 [%] and the SOCe is 0 [%], the calculation result is Fc= ΣIB.

[0038] Once S21 is completed, the current routine ends. In the next routine, S10, a battery pack 100 whose full charge capacity has not yet been measured (e.g., battery pack 100b) can be selected, and the same process can be performed. This process can be repeated until the full charge capacity of all battery packs 100 has been measured.

[0039] In the present embodiment, the control device 200 discharges battery pack 100a, selected from battery packs 100a to 100d, until the state of charge (SOC) of battery pack 100a is equal to or less than the predetermined value α, by operating the DC-DC converters 110a to 110d to supply electricity from battery pack 100a to battery packs 100b to 100d. After discharging battery pack 100a, the control device 200 charges battery pack 100a until its state of charge (SOC) is equal to or greater than the predetermined value β, by operating the DC-DC converters 110a to 110d to supply electricity from battery packs 100b to 100d to battery pack 100a.

[0040] The control device 200 calculates the full charge capacity Fc of battery pack 100a based on the electricity charged into battery pack 100a (integrated current ΣIB) while the state of charge (SOC) of battery pack 100a has changed from the predetermined value α to the predetermined value β. The full charge capacity Fc of battery pack 100a is obtained by exchanging electricity between battery packs 100a to 100d contained in battery system 1 to charge and discharge battery pack 100a. Thus, when measuring the full charge capacity Fc of battery pack 100a, a decrease in the electricity stored in battery system 1 can be avoided.

[0041] In the present embodiment, if battery pack 100 is an LFP battery, the predetermined values ​​α and β are set as values ​​in the non-plateau regions, which represent a low-SOC region and a high-SOC region, respectively. This allows the SOCs and SOCe to be calculated accurately, and thus the full charge capacity Fc can be measured precisely. Furthermore, a large difference between the SOC and SOCe can be ensured, allowing the full charge capacity Fc to be calculated accurately using formula (1) above.

[0042] In the present embodiment, the charging and discharging of the battery packs 100a to 100d is carried out by operating the DC-DC converters 110a to 110d, each of which is provided for the battery packs 100a to 100d. In this way, the electricity for each of the battery packs 100a to 100d can be controlled, which enables good controllability. Even in the case of a voltage difference between the battery packs 100a to 100d, current feedback can be avoided. Example 2

[0043] In the embodiment described above, the full charge capacity is calculated using the integrated current ΣIB during the charging of the selected battery pack 100. In embodiment 2, the full charge capacity is calculated using the integrated current ΣIB during the discharging of the selected battery pack 100. Fig. Figure 4 is a flowchart showing an example of a process for calculating the full charge capacity, which is carried out in the control device 200 in embodiment 2. This flowchart is executed when there is no exchange of electricity between battery system 1 and external system 2 (PCS 10) (when there is no charging and discharging between battery system 1 and external system 2).

[0044] In S30, a battery pack 100 is selected whose full charge capacity is to be measured. The selection of the battery pack 100 can be carried out as in S10, whereby in the present embodiment, battery pack 100a is selected first.

[0045] In S31, the charging of battery pack 100a is carried out. To charge battery pack 100a, as with the charging in S16, the DC-DC converters 110a to 110d are operated in such a way that the electricity stored in the other battery packs 100b to 100d is fed into battery pack 100a, as indicated by the long dashed double-short dashed line in Fig. 1 indicated.

[0046] In S32, it is determined whether the state of charge (SOC) of battery pack 100a is equal to or greater than a predetermined value b. The predetermined value b can be the same as the predetermined value β in S17. The predetermined value b can be 100 [%]. If the SOC of battery pack 100a is equal to or greater than the predetermined value b, a positive decision is made and the procedure proceeds to S33. If the SOC of battery pack 100a is less than the predetermined value b, a negative decision is made and the process returns to S31, where charging of battery pack 100a is carried out until the SOC is equal to or greater than the predetermined value b.

[0047] In S33, the charging of battery pack 100a and the discharging of battery packs 100b to 100d are stopped, and the process proceeds to S34. In S34, it is checked whether a predetermined time T3 has elapsed since the charging of battery pack 100a was stopped. If the predetermined time T3 has elapsed, this is confirmed, and the process proceeds to S35. If the predetermined time T3 has not yet elapsed, S34 is executed again. The predetermined time T3 is the same value as the predetermined time T2 in S19.

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

[0049] In S36, the discharge of battery pack 100a is carried out, integrating the current IB to calculate the integrated current ΣIB. To discharge battery pack 100a, as in S11, the DC-DC converters 110a to 110d are operated to supply the electricity stored in battery pack 100a to battery packs 100b to 100d, as shown by the long dashed line and short dashed line in S36. Fig. 1 indicated. The current IB recorded in the monitoring module 120 is recorded as a negative value (-) during discharge, and therefore the integrated current ΣIB [Ah] is integrated as a negative value.

[0050] In S37, it is determined whether the state of charge (SOC) of battery pack 100a is equal to or less than a predetermined value a. The predetermined value a can be the same as the predetermined value α in S12. The predetermined value a can be 0 [%]. If the SOC of battery pack 100a is equal to or less than the predetermined value a, a positive decision is made and the procedure proceeds to S38. If the SOC is higher than the predetermined value a, a negative decision is made and the process returns to S36, where the discharge of battery pack 100a is carried out until the SOC is equal to or less than the predetermined value a.

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

[0052] In S39, it is checked whether a predetermined time T4 has elapsed since the charging of battery pack 100a was interrupted. If the predetermined time T4 has elapsed, this is confirmed, and the process proceeds to S40. If the predetermined time T4 has not yet expired, S39 is executed again. The predetermined time T4 can be the same value as the predetermined time T1 in S13.

[0053] In S40, the SOC, which was calculated from the SOC-OCV characteristic curves 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 100a battery pack is calculated using the formula (1) above. The integrated current ΣIB is a negative value and the value of (SOCe - SOCs) is also negative, so the full charge capacity Fc is a positive value.

[0055] Once S41 has been processed, the current routine is terminated. This is similar to the procedure in Fig. 2. The procedure can be repeated until the full charge capacities of all battery packs 100 have been measured.

[0056] In embodiment 2, the control device 200 calculates the full charge capacity Fc of battery pack 100a based on the electricity discharged from battery pack 100a (integrated current ΣIB) while the state of charge (SOC) of battery pack 100a has changed from the predetermined value b to the predetermined value a. The full charge capacity Fc of battery pack 100a is obtained by exchanging electricity between battery packs 100a to 100d contained in battery system 1 to charge and discharge battery pack 100a. Thus, when measuring the full charge capacity Fc of battery pack 100a, a decrease in the electricity stored in battery system 1 can be avoided.

[0057] In embodiment 2, as in embodiment 1, the SOCs and the SOCe can be calculated precisely, and a large difference between the SOCs and the SOCe can be ensured, thus allowing the full charge capacity Fc to be calculated accurately. Since the DC-DC converters 110a to 110d are each provided for the battery packs 100a to 100d, controllability is good, and even with a voltage difference between the battery packs 100a to 100d, reverse current flow can be avoided. Modified example

[0058] Fig. Figure 5 is a schematic configuration diagram of a battery system S in a modified example. The battery system S comprises a plurality of sub-battery systems 1A to 1D. The sub-battery systems 1A to 1D are connected in parallel to each other to the external system 2 (PCS 10). Fig. 5 (photovoltaic device for power generation 20, load 30 and power grid PG not shown).

[0059] Sub-battery systems 1A to 1D are components similar to battery system 1 in the exemplary embodiments, each containing several battery packs 100 and the corresponding DC-DC converters 110 connected in parallel. Sub-battery systems 1A to 1D each include corresponding relays R1 to R4 and are connected to the PCS 10 in parallel via relays R1 to R4. A 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, the procedure for calculating the full charge capacity of Fig. 2 or Fig. 4. One of the relays R1 to R4, corresponding to one of the sub-battery systems 1A to 1D containing the battery pack 100 whose full charge capacity is to be measured, is opened. For example, when calculating the full charge capacity of the battery pack 100 contained in sub-battery system 1A, relay R1 is opened and relays R2 to R4 are closed. This interrupts the connection between sub-battery system 1A and PCS 10, thus preventing the calculation of the full charge capacity according to Fig. 2 or Fig. 4. Since the sub-battery systems 1B to 1D are connected to the PCS 10, electricity can be exchanged between the external system 2 and the sub-battery systems 1B to 1D. The number of sub-battery systems 1A to 1D is arbitrary and can be any number greater than one.

[0061] In this modified example, it is possible to measure the full charge capacity of the 100 battery pack by calculating the full charge capacity of Fig. 2 or Fig. 4 is carried out while electricity is exchanged between the battery system S and the external system 2.

[0062] The embodiments disclosed herein are in every respect illustrative and not to be understood as limiting. The scope of this disclosure is not defined by the description of the embodiments above, but by the claims, and is intended to encompass all modifications within the meaning and scope of the equivalents of the claims. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] JP 2015-195653

[0002] JP 2015-195653 A [0002, 0003]

Claims

[1] Battery system that performs charging and discharging between the battery system and an external system, wherein the battery system comprises: a large number of battery packs connected in parallel to each other to the external system; a multitude of DC-DC converters provided to match the battery packs, each arranged in a power line connecting the corresponding battery pack to the external system; and a control device that controls the DC-DC converters, wherein the control device a first battery pack, selected from the battery packs, is discharged until the charge level of the first battery pack becomes less than or equal to a first predetermined value by operating the DC-DC converters corresponding to the first battery pack and other battery packs, respectively, to supply electricity from the first battery pack to the other battery packs; after discharging, recharges the first battery pack until its state of charge is greater than or equal to a second predetermined value by operating the DC-DC converters to supply electricity from the other battery packs to the first battery pack; and a full charge capacity of the first battery pack calculated based on the electricity that was charged into the first battery pack during charging. [2] Battery system that performs charging and discharging between the battery system and an external system, wherein the battery system comprises: a large number of battery packs connected in parallel to each other to the external system; a multitude of DC-DC converters provided to match the battery packs, each arranged in a power line connecting the corresponding battery pack to the external system; and a control device that controls the DC-DC converters, wherein the control device charges a first battery pack, selected from the battery packs, until the charge level of the first battery pack is greater than or equal to a third predetermined value, by operating the DC-DC converters corresponding to the first battery pack and other battery packs respectively, to supply electricity from the other battery packs to the first battery pack; after charging, the first battery pack is discharged until the state of charge of the first battery pack becomes less than or equal to a fourth predetermined value, by operating the DC-DC converters to supply electricity from the first battery pack to the other battery packs; and a full charge capacity of the first battery pack calculated based on the electricity that was discharged from the first battery pack during discharge. [3] Battery system according to claim 1 or 2, wherein the state of charge of the first battery pack is estimated on the basis of a voltage of the first battery pack.

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