JP7597701B2 - Battery module inspection method - Google Patents

Description

This disclosure relates to a method for inspecting a battery module.

The lithium-ion battery module disclosed in JP 2021-044151 A (Patent Document 1) can be charged without requiring a circuit or the like for monitoring the charging state of each cell, and can prevent any single cell from becoming overcharged.

JP 2021-044151 A JP 2018-026210 A JP 2015-125091 A

Generally, after manufacturing a battery module, an inspection is performed to determine whether the battery module is a good product or a defective product. A battery module that meets a predetermined standard is determined to be a good product, while a battery module that does not meet the standard is determined to be a defective product. There is always a demand for improving the inspection accuracy in such battery module inspection.

This disclosure has been made to solve the above problems, and the purpose of this disclosure is to improve the inspection accuracy of battery modules.

(1) A method for inspecting a battery module according to an aspect of the present disclosure is a method for inspecting a plurality of battery modules connected in series. Each of the plurality of battery modules includes a plurality of cells connected in series. The inspection method includes a step of performing a charging process, a step of performing a determination process, and a step of performing the charging process and the determination process again. The step of performing the charging process includes a step of charging all cells included in the plurality of battery modules until the voltage of any one of the cells reaches a predetermined voltage, a step of calculating the capacity stored in each of all the charged cells, and a step of detecting the voltage of each of all the charged cells. The step of performing the determination process includes a step of determining, for each of all the cells charged by the charging process, whether the capacity of the cell is within a capacity tolerance range and the voltage of the cell is within a voltage tolerance range. The step of performing again includes a step of completely discharging each cell included in the reserved battery module when there is a reserved battery module among the plurality of battery modules that includes a cell whose capacity is not within the capacity tolerance range and whose voltage is not within the voltage tolerance range, and a step of performing the charging process and the determination process again for the reserved battery module in which each cell has been completely discharged.

(2) The determining step includes a step of determining whether the combination of the capacity and voltage of each of all cells charged by the charging process is located in any of the first to third regions defined on the capacity-voltage plane using a maximum capacity curve and a minimum capacity curve. The maximum capacity curve is a capacity-voltage curve passing through the intersection of the upper limit voltage of the voltage tolerance range and the maximum capacity of the capacity tolerance range. The minimum capacity curve is a capacity-voltage curve passing through the intersection of the upper limit voltage and the minimum capacity of the capacity tolerance range. The first region is a region sandwiched between the minimum capacity curve and the maximum capacity curve where the capacity is equal to or greater than the minimum capacity and the voltage is equal to or less than the upper limit voltage. The second region is a region sandwiched between the minimum capacity curve and the maximum capacity curve, other than the first region. The third region is a region other than the first region and the second region. The reserved battery module is a battery module that includes a cell located in the second region but does not include a cell located in the third region.

(3) Each of the multiple battery modules is a lithium ion battery having a bipolar structure.

As will be described in more detail later, there is a possibility that a reserved battery module (reserved item) may be erroneously determined to be defective when the voltage of a cell contained in the defective item reaches a predetermined voltage, even though the item is actually a good item. In the configurations (1) to (3) above, a complete discharge process is performed on the reserved battery module, and a charging process and a determination process are performed again. This makes it possible to improve the inspection accuracy of the reserved battery module.

This disclosure makes it possible to improve the inspection accuracy of battery modules.

1 is an overall configuration diagram of a battery inspection system according to an embodiment of the present disclosure; FIG. 2 is a schematic cross-sectional view of a battery unit. FIG. 2 is a schematic cross-sectional view of a battery module. 10 is a flowchart showing a processing procedure of an inspection process in a comparative example. FIG. 13 is a diagram for explaining a problem with an inspection process in the comparative example. 4A to 4C are diagrams illustrating an example of two types of curves according to the present embodiment. FIG. 2 is a diagram for explaining an area defined by a maximum capacity curve and a minimum capacity curve. 4 is a flowchart showing a processing procedure of an inspection process in the present embodiment.

The following describes in detail the embodiments of the present disclosure with reference to the drawings. Note that the same or corresponding parts in the drawings are given the same reference numerals and their description will not be repeated.

[Embodiment]
<Overall configuration of battery inspection system>
1 is an overall configuration diagram of a battery inspection system according to an embodiment of the present disclosure. The battery inspection system 100 includes connection terminals 101, 102. The connection terminals 101, 102 are configured to be capable of electrically connecting a battery unit 1. The battery unit 1 includes a plurality of battery modules 2 connected in series. Each of the plurality of battery modules 2 includes a plurality of cells. The battery inspection system 100 performs an inspection for each battery module 2 to determine whether it is a pass or fail product.

The configuration of the battery unit 1 will be described in more detail with reference to Figures 2 and 3. The battery unit 1 may be a newly manufactured new battery, or a used battery collected from a user.

The battery inspection system 100 further includes a DC power source 5, multiple voltage sensors 6, a current sensor 7, a temperature sensor 8, and a controller 9.

The DC power supply 5 is configured to charge and discharge the battery unit 1 according to control commands from the controller 9. The DC power supply 5 is, for example, an AC/DC converter. The DC power supply 5 converts AC power supplied from an external AC power supply (for example, a commercial power supply) 900 into DC power. The DC power supply 5 can also step up or step down the voltage of the DC power according to control commands from the controller 9.

Each of the multiple voltage sensors 6 is connected in parallel to a corresponding one of the multiple battery modules 2. Each voltage sensor 6 detects the voltage V of each cell in the corresponding battery module 2 and outputs the detected value to the controller 9.

The current sensor 7 is connected in series, for example, between the DC power supply 5 and the connection terminal 101. The current sensor 7 detects the current I supplied from the DC power supply 5 to the battery unit 1 and outputs the detected value to the controller 9.

The temperature sensor 8 detects the temperature T of the battery unit 1 (a representative battery module among the multiple battery modules 2) and outputs the detected value to the controller 9.

The controller 9 includes a processor 91 such as a CPU (Central Processing Unit), a memory 92 such as a ROM and a RAM, and an input/output interface (not shown). The controller 9 controls the DC power source 5 to completely discharge the battery module 2 or charge the battery module 2 (charging process). The controller 9 also performs a determination process to determine whether the battery module 2 is good or bad. The above-mentioned complete discharge process, charging process, and determination process will be described in detail later. The controller 9 may be configured by dividing it into two or more units for each function.

<Battery configuration>
2 is a schematic cross-sectional view of the battery unit 1. The battery unit 1 includes a module stack 31, a pair of restraining members 32, a pair of insulating members 33, a positive terminal 341, and a negative terminal 342.

The module stack 31 includes a plurality of battery modules 2 and a plurality of conductive plates 310. The battery modules 2 and the conductive plates 310 are arranged alternately in the stacking direction of the module stack 31 (the vertical direction in FIG. 2). The conductive plates 310 electrically connect adjacent battery modules 2 to each other. Each of the plurality of battery modules 2 is provided with a voltage detection wire 20 for detecting the voltage of each of the plurality of cells contained in the battery module (the voltage between two adjacent electrode layers 412 as described below). Note that although the number of battery modules 2 in FIG. 2 is three, the number of battery modules 2 is not particularly limited as long as it is two or more.

The pair of restraining members 32 restrain the module stack 31 from both sides in the stacking direction of the module stack 31.

One of the pair of insulating members 33 is disposed between one of the pair of restraining members 32 and one end of the module stack 31 in the stacking direction. The other of the pair of insulating members 33 is disposed between the other of the pair of restraining members 32 and the other end of the module stack 31 in the stacking direction.

The positive terminal 341 is electrically connected to the conductive plate 310 (lower end) located at one end of the stacking direction of the module stack 31. The positive terminal 341 is electrically connected to the connection terminal 101 (see FIG. 1). The negative terminal 342 is electrically connected to the conductive plate 310 located at the other end (upper end) of the stacking direction of the module stack 31. The negative terminal 342 is electrically connected to the connection terminal 102 (see FIG. 1).

The battery module 2 is a battery having a bipolar structure (a so-called bipolar battery). In this embodiment, the battery module 2 is a lithium ion secondary battery.

<Battery module configuration>
3 is a schematic cross-sectional view of the battery module 2. The battery module 2 includes a laminate 21 and a sealing portion 22. The laminate 21 includes a plurality of bipolar electrodes 41, a plurality of separators 40, and an electrolyte.

The multiple bipolar electrodes 41 are stacked in a stacking direction (the up-down direction in FIG. 3). The bipolar electrode 41 includes an electrode plate 411 and an electrode layer 412.

The electrode plate 411 is, for example, a rectangular metal foil. The electrode plate 411 has a first main surface 411a and a second main surface 411b aligned in the stacking direction.

The electrode layer 412 is disposed on the inner periphery of the electrode plate 411. The electrode layer 412 is disposed on both sides of the electrode plate 411. The electrode layer 412 includes a positive electrode 43 and a negative electrode 44. The positive electrode 43 is disposed on the first main surface 411a of the electrode plate 411. The positive electrode 43 includes a positive electrode active material. The positive electrode 43 may contain a conductive material and a binder in addition to the positive electrode active material. The negative electrode 44 is disposed on the second main surface 411b of the electrode plate 411. The negative electrode 44 includes a negative electrode active material.

A negative terminal electrode is provided at one end of the laminate 21 in the stacking direction (the upper end in FIG. 3). No positive electrode 43 is provided on the first main surface 411a of the negative terminal electrode. A positive terminal electrode is provided at the other end (the lower end) of the laminate 21 in the stacking direction. No negative electrode 44 is provided on the second main surface 411b of the positive terminal electrode.

The separator 40 is made of insulating resin. The separator 40 is disposed between two adjacent electrode layers 412 (between the positive electrode 43 and the negative electrode 44). Specifically, the separator 40 is disposed between the negative electrode 44 of the bipolar electrode 41 located on one side of the stacking direction and the positive electrode 43 of the bipolar electrode 41 located on the other side of the stacking direction. By disposing the electrode layers 412 (positive electrode 43 and negative electrode 44) and the separator 40 as described above, a cell is formed between adjacent electrodes (including the bipolar electrodes and the terminal electrodes).

The sealing portion 22 is made of insulating resin (polypropylene, polyphenylene sulfide, etc.). The sealing portion 22 seals the periphery of the multiple bipolar electrodes 41 so that a space is formed between two adjacent bipolar electrodes 41. The sealing portion 22 seals the above-mentioned space on its inside. An electrolyte is contained in the space. The electrolyte is impregnated into the positive electrode 43, the negative electrode 44, and the separator 40.

The positive electrode active material, the negative electrode active material, the separator 40, and the electrolyte may be made of materials that are conventionally known as the positive electrode active material, the negative electrode active material, the separator, and the electrolyte of a lithium ion secondary battery. As an example, the positive electrode active material may be olivine-type lithium iron phosphate (LiFePO ₄ ). However, the positive electrode active material may be a ternary material in which a part of lithium cobalt oxide is replaced by nickel and/or manganese. The negative electrode active material may be graphite (carbon material). The separator may be polyolefin. The electrolyte includes an organic solvent (for example, a mixed solvent of DMC (dimethyl carbonate), EMC (ethyl methyl carbonate), and EC (ethylene carbonate)), a lithium salt (for example, LiPF ₆ ), and an additive (for example, LiBOB (lithium bis(oxalate)borate) or Li[PF ₂ (C ₂ O ₄ ) ₂ ]), etc.

Comparative Example
In order to facilitate understanding of the characteristics of the inspection process in this embodiment, a processing procedure of the inspection process in a comparative example will be described first. The configuration of the battery inspection system in the comparative example is similar to the configuration of the battery inspection system 100 in this embodiment (see FIGS. 1 to 3).

Figure 4 is a flowchart showing the processing procedure of the inspection process in the comparative example. In S91, the controller 9 controls the DC power supply to discharge all cells included in the battery unit to a fully discharged state.

In S92, the controller 9 controls the DC power supply to start charging the battery unit 1. Since all the cells in the battery unit 1 are connected in series, all the cells are charged with the same current value.

In S93, the controller 9 determines whether the voltage of any of the cells in the battery unit has reached a predetermined upper limit voltage UL. The controller 9 returns the process to S92 and continues charging the battery unit until the voltage of any of the cells reaches the upper limit voltage UL (in other words, while the voltages of all the cells are below the upper limit voltage UL).

When the voltage of any cell reaches the upper limit voltage UL (YES in S93), the controller 9 ends charging of the battery unit (S94). Then, the controller 9 acquires the voltage of each of the multiple cells included in the battery unit (S95).

In S96, the controller 9 performs a pass/fail determination for each battery module based on the voltage acquired in S95. Specifically, the controller 9 determines for each battery module whether the voltages of all the cells included in the battery module are within a predetermined tolerance range. The tolerance range is the voltage range between the lower limit voltage LL and the upper limit voltage UL (LL≦V≦UL), and can be experimentally determined in advance according to the required specifications of the cells, etc.

If the voltages of all the cells included in the battery module are within the allowable range (YES in S96), the controller 9 determines that the battery module is a non-defective product (S97). On the other hand, if the voltage of at least one cell included in the battery module is outside the allowable range (NO in S96), the controller 9 determines that the battery module is defective (S98). This ends the series of processes.

Figure 5 is a diagram for explaining the problems with the inspection process in the comparative example. The horizontal axis represents the cell capacity (the amount of power stored in each cell). The vertical axis represents the cell voltage (the closed circuit voltage of each cell detected by the voltage sensor). The same applies to Figures 6 and 7 described below.

Here, we will use two of the multiple cells included in the battery unit as an example. These cells will be referred to as the "first cell" and the "second cell." The first cell and the second cell are included in different battery modules. It is assumed that when the same amount of power is charged, the voltage of the first cell is more likely to rise than the voltage of the second cell.

To explain in more detail, when forming the electrode layer 412 on the electrode plate 411 during the manufacture of the bipolar electrode 41, a weight defect may occur in which the weight per unit area of the electrode layer 412 does not reach a specified amount. The first cell is a cell composed of a bipolar electrode with a weight defect. The second cell is a cell composed of a normal bipolar electrode. In this case, the amount of electrode layer contained in the first cell is less than the amount of electrode layer contained in the second cell, which may result in the full charge capacity of the first cell being smaller than the full charge capacity of the second cell. As a result, when the same amount of power is charged, the voltage of the first cell is more likely to rise excessively compared to the voltage of the second cell.

Charging begins with both the first and second cells in a fully discharged state (cell capacity = 0). When the first and second cells are each charged with an amount of power ΔAh, the voltage of the first cell reaches the upper limit voltage UL (YES in S93 in FIG. 4). Then, charging of both the first and second cells ends (S94). The dashed line indicates that charging of the second cell does not exceed the amount of power ΔAh.

In this example, the first cell is actually defective (e.g., has a poor weight distribution) and the voltage of the first cell is likely to rise excessively. However, when charging is completed, the voltage of the first cell is equal to the upper limit voltage UL and is within the acceptable range. Therefore, the battery module including the first cell can be determined to be a good product (S97).

On the other hand, the second cell is actually a good product. If charging were to continue, the voltage of the second cell would rise to the acceptable range, as shown by the dashed line. Nevertheless, when charging is completed, the voltage of the second cell is below the lower limit voltage LL and is outside the acceptable range. Therefore, the battery module including the second cell is determined to be defective (S98).

In this way, in the comparative example, a battery module including a defective cell (first cell) may be erroneously determined to be good, while a battery module including a good cell (second cell) may be erroneously determined to be defective.

Therefore, in this embodiment, a configuration is adopted in which a pass/fail judgment is performed for each battery module taking into account the cell capacity in addition to the cell voltage. More specifically, two types of curves are prepared on the cell capacity-cell voltage plane, taking into account the allowable error (tolerance) of the basis weight of the electrode layer 412 (positive electrode 43 and negative electrode 44). Then, a pass/fail judgment is performed for each battery module (cell) according to the region defined by the two types of curves.

<Area defined by two types of curves>
FIG. 6 is a diagram showing an example of two types of curves in this embodiment. In this embodiment, in addition to the "voltage tolerance range" being defined by the upper limit voltage UL and the lower limit voltage LL for the cell voltage, a tolerance range is also defined for the cell capacity. This tolerance range is also called the "capacity tolerance range." The capacity tolerance range is a capacity range between the minimum capacity MIN and the maximum capacity MAX (MIN≦Ah≦MAX), and can be experimentally determined in advance according to the required specifications of the cell, etc.

More specifically, each of the multiple bipolar electrodes 41 (each cell) is configured so that the capacity of the positive electrode 43 is smaller than the capacity of the negative electrode 44. The maximum capacity MAX is the cell capacity when the basis weight of the positive electrode 43 is the maximum within the allowable error range. On the other hand, the minimum capacity MIN is the cell capacity when the basis weight of the positive electrode 43 is the minimum within the allowable error range.

The two types of curves are referred to as the "maximum capacity curve Lmax" and the "minimum capacity curve Lmin." The minimum capacity curve Lmin (see dashed line) passes through the intersection point a between the upper limit voltage UL of the voltage tolerance range and the minimum capacity MIN of the capacity tolerance range. The maximum capacity curve Lmax (see solid line) passes through the intersection point b between the upper limit voltage UL of the voltage tolerance range and the maximum capacity MAX of the capacity tolerance range. The intersection point between the maximum capacity curve Lmax and the lower limit voltage LL of the voltage tolerance range is referred to as c. Both curves pass through point d when cell capacity = 0.

Figure 7 is a diagram for explaining the area defined by the maximum capacity curve Lmax and the minimum capacity curve Lmin. In this embodiment, as shown in Figure 7, a "good area" and a "reserved area" are defined.

The good quality region is the region surrounded by the straight line ab, the curve bc, and the straight line ca. The curve bc is the curve between points b and c on the maximum capacity curve Lmax. The good quality region also includes points on the above three lines. The good quality region can also be said to be the region between the minimum capacity curve Lmin and the maximum capacity curve Lmax where the capacity is equal to or greater than the minimum capacity MIN and the voltage is equal to or less than the upper limit voltage UL.

The reserved area is the area surrounded by the straight line ac, the curve cd, and the curve ad. The curve cd is the curve between points c and d on the maximum capacity curve Lmax. The curve ad is the curve between points a and d on the minimum capacity curve Lmin. The reserved area includes points on the curve cd and the curve ad, but does not include points on the straight line ac. The reserved area can be said to be the area between the minimum capacity curve Lmin and the maximum capacity curve Lmax, other than the non-defective area.

Areas that are neither the good quality area nor the reserved area are referred to as the "defective area." In this embodiment, as described below, it is determined whether the combination of capacity and voltage of each cell (cell capacity, cell voltage) at the end of charging is within the good quality area, the reserved area, or the defective area.

The good product area corresponds to the "first area" in this disclosure. The reserved area corresponds to the "second area" in this disclosure. The defective product area corresponds to the "third area" in this disclosure. The upper limit voltage UL corresponds to the "predetermined voltage" in this disclosure.

<Processing flow>
8 is a flowchart showing the procedure of the inspection process in this embodiment. This flowchart is executed when a predetermined condition is met (for example, when an operator operates a start switch, not shown). Each step is realized by software processing by the controller 9 (processor 91), but may also be realized by hardware (electrical circuitry) arranged in the controller 9. Hereinafter, steps are abbreviated as S.

In S1, the controller 9 controls the DC power source 5 to discharge all cells contained in the battery unit 1 to a fully discharged state (full discharge process). If the battery unit 1 has never been charged (if charging is being performed for the first time after injection of electrolyte), the full discharge process in S1 can be omitted (skip). The controller 9 can determine whether or not a full discharge process is required depending on whether the voltages of the cells are uniform. If the voltages of the cells are not uniform (for example, if the voltage difference between the maximum voltage and the minimum voltage exceeds a predetermined value), the controller 9 determines that a full discharge process is required. On the other hand, if the voltages of the cells are uniform (for example, if the voltage difference is less than a predetermined value), the controller 9 can omit the full discharge process.

In S2, the controller 9 controls the DC power supply 5 to start charging the battery unit 1, for example, with a constant current. As described above, all cells in the battery unit 1 are connected in series, so all cells are charged with an equal current value. While the battery unit 1 is being charged, the controller 9 acquires the voltage of each cell, for example, at predetermined intervals.

In S3, the controller 9 determines whether the voltage of any of the cells in the battery unit 1 has reached the upper limit voltage UL. The controller 9 returns the process to S2 and continues charging the battery unit 1 until the voltage of any of the cells reaches the upper limit voltage UL. When the voltage of any of the cells reaches the upper limit voltage UL (YES in S3), the controller ends charging of the battery unit (S4).

In S5, the controller 9 calculates the capacity of each cell. More specifically, the controller 9 can calculate the capacity of each cell by multiplying the charging current I to the battery unit 1 by the charging time t (I x t). Note that the charging current I to the battery unit 1 is a constant current, which is an example. The controller 9 may calculate the capacity of each cell by integrating the charging current I(t), which changes over time, for each unit time Δt (Σ(I(t) x Δt)).

In S6, the controller 9 acquires the voltage of each cell (all cells included in the battery unit 1) detected by the voltage sensor 6. The processes in S2 to S6 are also referred to as the "charging process."

In S7 to S9, the controller 9 determines, for each battery module 2, in which region on the cell capacity-cell voltage plane each cell contained in that battery module 2 is located. Specifically, in S7, the controller 9 determines whether all cells contained in a certain battery module 2 are located in the non-defective region. If all cells are located in the non-defective region (YES in S7), the controller 9 determines that the battery module 2 is a non-defective product (S10).

If at least one cell is not in the non-defective area (NO in S7), the controller 9 determines whether any cell is in the reserved area (S8). If any cell is in the reserved area (YES in S8), the controller 9 determines whether any cell is in the defective area (S9). If no cell is in the defective area (NO in S9), i.e., if all cells outside the non-defective area are located in the reserved area, the controller 9 determines that the battery module 2 is a reserved item (S11).

If no cells are in the reserved area in S8 (NO in S8), i.e., if all cells outside the good area are located in the defective area, the controller 9 determines that the battery module 2 is defective (S12). Also, if the battery module 2 includes cells located in the reserved area but also includes cells located in the defective area, i.e., if there is a mixture of cells in the reserved area and cells in the defective area, the controller 9 determines that the battery module 2 is defective (S12). The processes of S7 to S12 are also referred to as "determination processes".

Battery modules 2 that are judged as good as a result of the judgment process are shipped as good. Battery modules 2 that are judged as defective are discarded or recycled without being shipped. In contrast, battery modules 2 that are judged as reserved are sent to an additional process, where a complete discharge process is performed, and the charging process and judgment process are performed again. In detail, the process is returned to S1 after excluding battery modules 2 that are judged as good or defective from the battery unit 1. As a result, the controller 9 charges all cells of the battery module 2 that is judged as reserved again (S2 to S4) after completely discharging all cells (S1). Furthermore, the controller 9 calculates the capacity of each cell and obtains the voltage of each cell (S5, S6). Then, the controller 9 judges in which region on the cell capacity-cell voltage plane each cell included in the battery module 2 that is judged as reserved is located (S7 to S12).

In the additional process, battery modules 2 determined to be defective have already been removed. Therefore, unlike the explanation in FIG. 5, battery modules 2 determined to be reserved do not include cells such as the first cell, whose voltage is likely to rise excessively. Battery modules 2 include cells such as the second cell that were in the reserved area when the initial charging was completed (in other words, they only advanced to the edge of the good area and did not reach the good area), but may be positioned in the good area if charging continues. Therefore, there is a possibility that the battery module 2 will be determined to be good by carrying out the additional process. If this happens, the battery module 2 will also be able to be shipped, and the yield can be improved.

As described above, in this embodiment, a category of "reserved items" is provided. Reserved items may be erroneously determined to be defective when the voltage of a cell contained in the defective item reaches the upper limit voltage UL, even though the item is actually a good item. Therefore, in addition to performing a full discharge process (S1) on reserved items, charging processes (S2 to S6) and determination processes (S7 to S12) are performed again. This makes it possible to improve the inspection accuracy of reserved items.

The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present disclosure is indicated by the claims rather than the description of the embodiments above, and is intended to include all modifications within the meaning and scope of the claims.

1 Battery unit, 2 Battery module, 20 Voltage detection line, 21 Laminate, 22 Sealing portion, 31 Module laminate, 310 Conductive plate, 32 Restraint member, 33 Insulating member, 341 Positive electrode terminal, 342 Negative electrode terminal, 40 Separator, 41 Bipolar electrode, 411 Electrode plate, 411a First main surface, 411b Second main surface, 412 Electrode layer, 43 Positive electrode, 44 Negative electrode, 5 DC power source, 6 Voltage sensor, 7 Current sensor, 8 Temperature sensor, 9 Controller, 91 Processor, 92 Memory, 100 Battery inspection system, 101, 102 Connection terminal.

Claims (2)

A method for testing a plurality of battery modules connected in series, comprising the steps of:
Each of the plurality of battery modules includes a plurality of cells connected in series;
The inspection method includes:
performing a charging process;
A step of executing a determination process;
and executing the charging process and the determination process again;
The step of performing the charging process includes:
charging all cells included in the plurality of battery modules until a voltage of any one of the cells reaches an upper limit voltage;
calculating the capacity stored in each of all the cells that have been charged;
and detecting the voltage of each of said charged cells;
the step of executing the determination process includes a step of determining, for each of all the cells charged by the charging process, whether the capacity of the cell is within a capacity allowable range between a minimum capacity and a maximum capacity , and whether the voltage of the cell is within a voltage allowable range between the upper limit voltage and the lower limit voltage ;
The minimum capacity is a cell capacity when the basis weight of the positive electrode of the cell is minimum within the range of an allowable error,
The maximum capacity is a cell capacity when the basis weight of the positive electrode of the cell is maximum within the range of allowable error,
The step of re-executing the battery module includes, when a reserved battery module is present among the plurality of battery modules,
fully discharging each cell in the reserve battery module;
and performing the charging process and the determining process again on the reserved battery module in which each cell has been fully discharged ;
the determining step includes a step of determining, for each of all the cells charged by the charging process, whether a combination of capacity and voltage of the cell is located in any of first to third regions defined by a maximum capacity curve and a minimum capacity curve on a capacity-voltage plane;
the maximum capacity curve is a charging curve showing a capacity-voltage characteristic passing through an intersection point between the upper limit voltage and the maximum capacity,
the minimum capacity curve is a charging curve showing a capacity-voltage characteristic passing through an intersection point between the upper limit voltage and the minimum capacity,
the first region is a region between the minimum capacity curve and the maximum capacity curve, in which the capacity is equal to or greater than the minimum capacity and the voltage is equal to or less than the upper limit voltage;
The second region is a region of the sandwiched region other than the first region,
the third region is a region other than the first region and the second region,
A method for inspecting a battery module , wherein the reserved battery module is a battery module that includes cells located in the second area but does not include cells located in the third area . 2. The battery module inspection method according to claim 1 , wherein each of the plurality of battery modules is a lithium ion battery having a bipolar structure.

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