JP7761510B2 - Secondary battery self-discharge inspection method and self-discharge inspection device - Google Patents

Description

This article relates to a method and device for testing self-discharge of secondary batteries, and more specifically to a method and device for testing self-discharge of secondary batteries that appropriately distinguishes between good and bad secondary batteries during the manufacturing process.

Traditionally, when manufacturing secondary batteries, finished products are inspected, including a self-discharge test during the aging process to determine whether self-discharge is within an acceptable range. The aging process involves charging the battery to a certain level and then maintaining it at a high temperature with no load. This process eliminates micro-short circuits within the secondary battery and stabilizes the electrodes. After the aging process is complete, the secondary battery's voltage difference ΔV [V] is measured. If the voltage difference ΔV [V] is greater than the set design value, the battery is deemed to have high self-discharge and fails the test. It is then deemed a defective product and is generally discarded rather than shipped.

Figure 13 is a graph showing a method for determining the absolute value of the voltage difference ΔV [V] from a common threshold T based on conventional design values. The vertical axis represents the voltage difference ΔV [V]. The horizontal axis represents the individual secondary batteries included in each lot.

When testing this type of voltage difference ΔV [V], there are many factors that cause variation in the self-discharge specific to the secondary battery, such as variations in the solid-solution elements in the positive electrode active material, variations in the additives in the electrolyte, and variations in temperature within the battery.

Taking this variability into account, the threshold value for determining whether a secondary battery is good or bad, the voltage difference ΔV [V], must be set with a margin toward overdetection (mistakenly detecting good products as defective) to prevent missing defective products. As a result, some secondary batteries are judged to be defective to be on the safe side, even when they are actually good. This has led to the problem of increased losses in the secondary battery manufacturing process.

The invention disclosed in Patent Document 1 therefore includes the following: Here, the terminal voltage during the aging process is set to the potential in a discharged state, and a reference value ΔVB is set as an absolute value, assuming the amount of terminal voltage drop of a defective battery with a minute internal short circuit, relative to the average value ΔVA of ΔV [V], which varies for each inspection lot. The inspection method then discloses a method of determining that a battery with a ΔV [V] smaller than the value ΔVA - ΔVB is defective.

When using an inspection method based on an absolute value, which is the average value of the entire lot, variations in the secondary batteries being inspected can cause the average value to fluctuate due to the influence of the voltage difference ΔV [V] between some secondary batteries, which can affect the threshold. This can also result in a threshold with a margin on the side of overdetection.

Figure 14(a) is a graph showing the voltage difference ΔV [V] between the secondary batteries to be inspected, consisting of material lot 1 and material lot 2. The vertical axis represents the voltage difference ΔV [V]. The horizontal axis represents the individual secondary batteries contained in each lot. Figure 14(b) is a graph showing a method for determining the voltage difference ΔV [V] as a relative value. The vertical axis represents the probability, and the horizontal axis represents the standard deviation σ of the voltage difference ΔV [V]. The threshold value Tr is set to an arbitrary standard deviation σ (for example, "+3σ").

In the absolute value determination described above, depending on the variation of the secondary batteries being inspected, the threshold value may have a margin toward overdetection. Here, for each secondary battery shown in FIG. 14(a), both the secondary batteries contained in material lot 1 and the secondary batteries contained in material lot 2 are assumed to be non-defective with no self-discharge issues. Next, as shown in FIG. 14(b), the mean μ and variance ^σ2 of these secondary batteries are calculated, and a probability distribution graph L1 is created. Here, an arbitrary threshold (e.g., +3σ) is set as threshold Tr, and secondary batteries with a standard deviation σ exceeding threshold Tr are detected as abnormal values and defective. However, as described under the premise shown in FIG. 14(a), all products are non-defective G, and no defective NG products are included. However, regardless of the setting of threshold Tr in FIG. 14(b), secondary batteries exceeding threshold Tr will be detected as defective.

JP 2013-190292 A

As mentioned above, in the past, there was a possibility that secondary batteries that should have been judged to be good products would be over-detected as defective. This caused the problem of good secondary batteries being judged as defective even when they were manufactured during the manufacturing process.

The problem that the secondary battery self-discharge inspection method and self-discharge inspection device of the present invention aim to solve is to properly determine whether a secondary battery is good or bad during the manufacturing process.

To solve the above problem, the self-discharge inspection method for secondary batteries of the present invention is a method for inspecting self-discharge of secondary batteries in a process for continuously manufacturing multiple secondary batteries, and includes: a ΔV acquisition step for individually acquiring a voltage difference ΔV, which is the change in voltage of the secondary batteries before and after a no-load discharge process; a moving average calculation step for calculating a moving average MA of the voltage difference ΔV between a target secondary battery, which is the secondary battery for which the voltage difference ΔV was acquired in the ΔV acquisition step, and a comparison secondary battery manufactured adjacent to the target secondary battery; a difference calculation step for calculating a difference DA between the moving average MA calculated in the moving average calculation step and the target secondary battery for which the voltage difference ΔV was acquired in the ΔV acquisition step; and a difference determination step for comparing the difference DA calculated in the difference calculation step with a preset threshold value Ta to determine whether the product is defective.

It is also desirable to further include a relative value calculation step of calculating the standard deviation of the voltage difference ΔV of each secondary battery included in the material lot based on the variance when the voltage difference ΔV of the secondary batteries included in the material lot is used as a population; a relative judgment step of comparing the standard deviation of the secondary battery calculated in the relative value calculation step with a preset threshold value Tr to determine whether the secondary battery is defective; and a final judgment step of determining as defective those secondary batteries determined to be defective in both the difference judgment step and the relative judgment step.

Furthermore, the comparison secondary battery manufactured adjacently in the moving average calculation step may be a secondary battery manufactured before or after the target secondary battery for which the voltage difference V was acquired in the ΔV acquisition step.

The step of calculating the moving average is preferably performed continuously across different material lots. The variance is preferably a population variance σ ² based on the total number of secondary batteries in the material lot. The no-load discharging step may be an aging step.

This can be suitably implemented when the secondary battery is a nickel-metal hydride battery.
In addition, the self-discharge inspection device for secondary batteries of the present invention is a self-discharge inspection device for secondary batteries that performs self-discharge inspection on secondary batteries in a process of continuously manufacturing a plurality of secondary batteries for each material lot, and is equipped with a voltage measurement device that measures the voltage of the secondary batteries, and a control device with an arithmetic unit, and the control device individually acquires a voltage difference ΔV, which is the change in voltage of the secondary batteries before and after the aging process, using the voltage measurement device, and calculates a moving average MA of a plurality of secondary batteries including a target secondary battery, which is the secondary battery from which the voltage difference ΔV was acquired, and a voltage difference ΔV of a comparison secondary battery manufactured adjacent to the target secondary battery, calculates a difference DA between the moving average MA and the voltage difference ΔV of the target secondary battery, and compares the difference DA with a predetermined threshold value Ta to determine whether the product is defective.

Furthermore, it is desirable that the control device further calculates the standard deviation σ of the voltage difference ΔV of each secondary battery included in the material lot based on the variance when the voltage difference ΔV of the secondary batteries included in the material lot is used as a population, compares the standard deviation σ with a preset threshold value Tr to determine whether the secondary battery is defective, and makes a final determination that the secondary battery is defective when compared with the threshold value Ta and when compared with the threshold value Tr.

The secondary battery self-discharge inspection method and self-discharge inspection device of the present invention make it possible to appropriately determine whether a secondary battery is good or bad during the manufacturing process.

1 is a block diagram showing a secondary battery manufacturing apparatus according to an embodiment of the present invention; 3 is a flowchart showing the procedure of a method for inspecting self-discharge of a secondary battery according to the present embodiment. FIG. 10 is a diagram showing the acquisition of a moving average MA of ΔV [V] in the difference determination. FIG. 10 is a diagram showing the acquisition of a moving average MA of ΔV [V] in the difference determination. 10 is a diagram showing the moving average MA of ΔV [V] of each secondary battery B1 to B11 obtained in the difference determination, and ΔV [V] of the secondary battery. FIG. 10 is a diagram showing the difference between the moving average MA of ΔV of each secondary battery B obtained in the difference determination and ΔV [V] of that secondary battery B. FIG. FIG. 10 is a diagram showing the necessity of cross-checking with relative judgment when there is a possibility of overdetection at a material lot switching section. This is the formula for calculating the population variance σ ² . FIG. 1 is a schematic diagram illustrating detection of defective products in a conventional self-discharge inspection. 10A and 10B are schematic diagrams illustrating defective product detection in a self-discharge inspection by difference judgment according to the present embodiment. 10 is a graph showing the ratio of overdetected non-defective products. 10 is a graph showing detection of defective products when the index for the conventional method is 100. 10 is a graph showing a method for determining the absolute value of a voltage difference ΔV [V] based on a common threshold T on the basis of conventional design values. Fig. 14(a) is a graph showing the voltage difference ΔV [V] between the secondary batteries to be inspected made from material lot 1 and material lot 2. Fig. 14(b) is a graph showing a method for determining the voltage difference ΔV [V] as a relative value.

The self-discharge inspection method and self-discharge inspection device for secondary batteries of the present invention will be described below with reference to Figures 1 to 11, focusing on an embodiment of a self-discharge inspection method and self-discharge inspection device for nickel-metal hydride batteries.

<Configuration of Secondary Battery of This Embodiment>
The secondary battery B of this embodiment is exemplified by a nickel-metal hydride storage battery, which is an on-board battery for driving electric vehicles and hybrid vehicles, but is not limited thereto. In a nickel-metal hydride storage battery, for example, the positive electrode plate is formed by filling a positive electrode composite layer using nickel hydroxide or the like as a positive electrode active material on a porous nickel foam positive electrode substrate. Meanwhile, the negative electrode plate is formed by forming a negative electrode composite layer using a hydrogen storage alloy as a negative electrode active material on a negative electrode substrate made of a punched nickel plate. These positive and negative electrode plates are stacked with a separator interposed therebetween to form an electrode group. The electrode groups are housed in multiple battery cases provided in a battery case, connected in series, and external connection terminals are connected. An alkaline electrolyte such as a potassium hydroxide aqueous solution is poured into the battery case, and a lid is attached to the battery case to complete the assembly of the battery module.

Even after the battery elements have been assembled, such nickel-metal hydride batteries are still incomplete as batteries, and undergo initial charging and conditioning before being tested for capacity, voltage, internal resistance, etc. The battery is then charged to a specified voltage and aged at a high temperature without load. Aging eliminates short circuits and chemically stabilizes the electrodes. Self-discharge testing is generally performed using this aging process.

Nickel-metal hydride batteries are prone to variations in characteristics, such as self-decomposition of the positive electrode, due to variations in the characteristics of material lots. For this reason, it is difficult to distinguish between self-discharge due to variations in the characteristics of material lots and self-discharge due to minute short-term discharge in such nickel-metal hydride batteries (see Figure 10). For this reason, the self-discharge inspection method for secondary batteries according to this embodiment can be suitably applied to nickel-metal hydride batteries.

Note that while a nickel-metal hydride battery is used as an example of the secondary battery, it is not intended to be limited to nickel-metal hydride batteries, and it goes without saying that the present invention can be implemented with other secondary batteries, such as lithium-ion secondary batteries, as long as the present invention can be implemented.

<Secondary battery self-discharge inspection device>
FIG. 1 is a block diagram showing a secondary battery manufacturing apparatus according to this embodiment. In this secondary battery manufacturing apparatus 1, an assembly device 2 continuously assembles a plurality of secondary batteries B for each material lot. A charge/discharge device 5 charges and discharges secondary batteries B whose assembly is completed in the assembly device 2 of the secondary battery manufacturing apparatus 1. The charge/discharge device 5 includes sensors such as a voltmeter and an ammeter (not shown). The charge/discharge device 5 is controlled by a control device 4. The self-discharge inspection device 3 according to this embodiment inspects the self-discharge of secondary batteries controlled to a constant state of charge by the charge/discharge device 5. Since the aging process (FIG. 2: S1) according to this embodiment is intended to inspect for self-discharge, a relatively low charge rate (e.g., SOC 10% or less) is preferable because the voltage difference ΔV due to self-discharge increases.

The secondary battery self-discharge inspection device 3 of this embodiment includes a voltage measurement device 6 that measures the battery voltage of secondary battery B, and a control device 4 that includes a calculation device. This control device 4 is shared with the charge/discharge device 5.

Although not shown, the assembled secondary battery B is subjected to initial charging, conditioning, and various other tests, such as measuring the battery voltage and internal resistance.
Once the secondary battery is assembled, the charging/discharging device 5, controlled by the control device 4, begins the aging process (S1 in FIG. 2). In the aging process, the control device 4 charges the secondary battery B to a certain value using the charging/discharging device 5. Then, as a preparatory step, a heating device (not shown) heats the secondary battery B to a predetermined temperature. The aging process then begins, in which the secondary battery is left unloaded for a certain period of time. During this aging process, micro-short circuits caused by, for example, minute metal powder are eliminated. The battery is also chemically stabilized. In this embodiment, one of the purposes of the aging process is to inspect for self-discharge. Because the aging process is unloaded, no external power discharge occurs. However, micro-short circuits and discharge via the electrolyte can occur, resulting in a voltage drop due to "self-discharge." Large self-discharge results in rapid voltage drop even when the secondary battery is not in use, degrading battery performance. Therefore, once the secondary battery B is assembled, the charging/discharging device 5 first charges the secondary battery B to a set charge rate. Then, the self-discharge inspection device 3 performs the aging process (S1) as a self-discharge inspection.

In the control device 4, in the step of measuring the battery voltage V beforehand (S2) and the step of measuring the battery voltage V afterward (S3), the voltage measurement device 6 acquires and stores the voltages of the individual secondary batteries B1 to B9...

The control device 4 comprises a computer equipped with a CPU, RAM, ROM, and storage means. The control device 4 controls the charge/discharge device 5 and the self-discharge inspection device 3. The control device 4 measures the battery voltage of each secondary battery B in a pre-battery voltage V measurement step (S2) and a post-battery voltage V measurement step (S3). The measurement results are stored in storage means, and the voltage difference ΔV [V] is also stored. The stored ΔV [V] is calculated using the procedure described below, and a differential judgment or relative judgment is performed to inspect the self-discharge of each secondary battery and determine whether the secondary battery is good or defective.

(Procedure of this embodiment)
As described above, conventional absolute value determination and relative value determination could result in overdetection as defective even when the battery was non-defective. This resulted in the problem of overdetection, in which non-defective secondary batteries manufactured during the manufacturing process were determined to be defective.

On the other hand, relative judgment alone can cause problems due to overdetection. Therefore, in this embodiment, we have decided to combine the concepts of absolute value judgment and relative judgment. However, simply combining absolute value judgment and relative judgment does not solve either problem.

Therefore, the present inventors have discovered a unique method to take advantage of the features of absolute value determination and relative determination.
(Secondary Battery Self-Discharge Inspection Method of the Present Embodiment)
2 is a flowchart showing the procedure of the method for inspecting self-discharge of a secondary battery according to this embodiment. The procedure of the method for inspecting self-discharge of a secondary battery according to this embodiment will be described below with reference to the flowchart.

<Acquisition of ΔV [V]>
In this method, first, the control device 4 starts the procedure of the aging step (S1) using the self-discharge inspection device 3. The aging step (S1) continues until the measurement of the battery voltage V of the secondary battery B is completed by the voltage measurement device 6 in the "post-battery voltage V measurement step (S3)." When the aging step (S1) starts, the secondary battery B, which has been charged to a predetermined charge rate (e.g., SOC 10%), is heated to a predetermined temperature. After the secondary battery B has been raised to the predetermined temperature, the temperature of the secondary battery B is maintained constant thereafter.

Once the temperature of secondary battery B has risen to a predetermined temperature, control device 4 measures the battery voltage V of secondary battery B using voltage measurement device 6 in the "Preliminary Battery Voltage V Measurement Step (S2)." Thereafter, a constant-temperature aging process is performed, and after a set time has elapsed, control device 4 measures the battery voltage V of secondary battery B using voltage measurement device 6 in the "Post-Battery Voltage V Measurement Step (S3)." Then, the procedure of "ΔV Acquisition Step (S4)" is executed. In the "ΔV Acquisition Step (S4)," a voltage difference ΔV [V] between the voltage measured in the "Preliminary Battery Voltage V Measurement Step (S2)" and the voltage measured in the "Post-Battery Voltage V Measurement Step (S3)" is acquired. The voltage difference ΔV is acquired for each secondary battery B1, B2, etc. In this embodiment, the secondary battery B to be inspected, for which the voltage difference ΔV [V] is acquired in this "ΔV Acquisition Step (S4)," is referred to as the "target secondary battery."

<Calculation of moving average MA>
In the subsequent "step of calculating moving average (S5)", a moving average MA of the voltage difference ΔV [V] between the "subject secondary battery" and a "comparison secondary battery" manufactured adjacent to each other for comparison is calculated.

In the secondary battery manufacturing process, secondary batteries B are produced consecutively on a production line. Here, "adjacent secondary batteries" includes at least the secondary batteries before or after the "target secondary battery." Furthermore, a moving average may be obtained for a total of five secondary batteries, two before and two after, or two before and two after.

Note that at the start of production, there will be no "previous adjacent secondary battery." In that case, the moving average can be calculated using only the data that can be obtained, or data that cannot be obtained can be substituted with other data. For example, the moving average MA is generally calculated using the "subject secondary battery" and two "comparison secondary batteries" adjacent to it in the front and rear, for a total of three secondary batteries. However, if the first secondary battery to be manufactured is the subject secondary battery, the moving average MA can be calculated using both the "subject secondary battery" and the adjacent "comparison secondary battery" behind it. Alternatively, the ΔV [V] of the "subject secondary battery" and the adjacent "comparison secondary battery" behind it can be doubled to calculate the moving average MA using the three ΔV [V].

This treatment can also be applied to cases where the final secondary battery produced becomes the "target secondary battery."
<Example of moving average MA>
3 is a diagram showing how the moving average MA of ΔV [V] is obtained in the difference determination. Here, the secondary battery B2 to be measured is the target secondary battery, and the preceding secondary battery B1 and the following secondary battery B3 are group G1, which are comparison secondary batteries. The moving average MA of group G1 consisting of these three secondary batteries B1 to B3 is calculated. This calculation is performed by simply averaging the sum of the three voltage differences ΔV [V] and dividing it by the number of secondary batteries B. The moving average MA calculated in this way is the moving average of secondary battery B2.

Figure 4 shows how the moving average MA of ΔV [V] is obtained during difference determination. Here, secondary battery B3, the target secondary battery, is considered the target secondary battery, and the preceding secondary battery B2 and the following secondary battery B4 are group G2, which are comparison secondary batteries. The moving average MA of group G2, consisting of these three secondary batteries B2 to B4, is calculated. This calculation is performed by simply averaging the sum of the three voltage differences ΔV [V] and dividing it by the number of secondary batteries B. The moving average MA calculated in this way is the moving average of secondary battery B3.

<Calculation of difference DA>
5 is a diagram showing the moving average MA of ΔV [V] of each secondary battery B1 to B11 obtained in the difference determination, and the ΔV [V] of each secondary battery. Using the method shown in FIGS. 3 and 4, the graph shows the moving average MA of secondary batteries B1 to B11 as the target secondary batteries, with the batteries before and after them as the comparison secondary batteries. For secondary battery B1, only secondary battery B2 was used as the comparison secondary battery. For secondary battery B11, only secondary battery B10 was used as the comparison secondary battery.

In the "difference calculation step (S6)", the difference DA between the moving average MA calculated in the "moving average calculation step (S5)" and the "target secondary battery" whose voltage difference ΔV [V] was acquired in the "ΔV acquisition step" is calculated.

<Difference judgment>
FIG. 6 is a diagram showing the difference between the moving average MA of ΔV for each secondary battery B obtained in the difference determination and the ΔV [V] for that secondary battery B. The difference DA is expressed as the difference obtained by subtracting the moving average MA from the ΔV [V] for that secondary battery B. For example, as shown in FIG. 5, for secondary battery B1, ΔV [V] is greater than the moving average MA, so the difference is a positive number. Next, for secondary battery B2, as shown in FIG. 5, ΔV [V] is smaller than the moving average MA, so the difference is a negative number. For secondary battery B7, ΔV [V] is extremely large, so the difference DA is a large positive number. Similarly, for secondary battery B11, the difference DA is a large positive number.

It can be assumed that multiple adjacent secondary batteries (here, comparison secondary batteries relative to the target secondary battery) have roughly the same material and other characteristics. If there is a large difference between the ΔV [V] of the moving average MA and the ΔV [V] of the "target secondary battery," this indicates that the difference is not due to characteristics such as material variation within the material lot, but rather to differences in the inherent characteristics of the "target secondary battery." In this sense, this "difference DA" performs a relative evaluation within a narrow range. Conventional relative evaluations involve comparisons with the entire product or material lots. However, because the difference DA of this embodiment performs a relative evaluation within a narrow range, accuracy can be improved by comparing with multiple adjacent secondary batteries that can be assumed to have roughly the same material and other characteristics.

Here, the threshold value Ta is a reference value for determining the difference DA. In other words, a large difference DA means that ΔV [V] is extremely large relative to the moving average MA. In other words, this indicates that the target secondary battery has a large self-discharge compared to the comparison secondary battery, and therefore the target secondary battery can be determined to have a problem with self-discharge. If this threshold value Ta exceeds the range expected from the variation in the materials used in the manufactured secondary batteries, it can be determined that the large self-discharge is due to a problem with the "target secondary battery" itself. Because the variation in the materials used in manufactured secondary batteries is likely to be smaller within a narrower range, this threshold value Ta can also be set with greater precision.

Therefore, in the "difference determination step (S7)," the difference DA calculated in the "difference calculation step (S6)" is compared with a preset threshold value Ta to determine whether the product is good or defective. The determination result here is stored in the storage means of the control device 4.

<Moving average MA between different material lots>
FIG. 7 is a diagram showing the necessity of cross-checking with relative judgment when there is a possibility of overdetection at the material lot switching section.

In this embodiment, secondary batteries B from different material lots (production lots using different raw materials) are also manufactured continuously. In this case, it can be assumed that there is relatively little material variation within the same material lot. On the other hand, it can be assumed that there is relatively large material variation between different material lots. For this reason, when the moving average MA is calculated across different material lots, the variation in ΔV [V] of the comparison secondary battery is greater than the moving average MA within the same material lot. For this reason, the difference DA also tends to be larger. As a result, if the same threshold value Ta is used, there is a higher possibility of overdetection, in which a good product is judged to be defective.

Target secondary battery B5 is a secondary battery made using material lot 2. As shown in Figure 7, material lot 1 has a higher ΔV [V] overall than material lot 2, with an average of A1. On the other hand, material lot 2 has a lower ΔV [V] overall than material lot 1, with an average of A2. Note that secondary batteries B7 and B11 are defective products, so their influence is ignored. This results in an average difference ΔA between the average A1 of material lot 1 and the average A2 of material lot 2. As a result, the moving average MA is lower due to the influence of material lot 2, and the difference DA for the target secondary battery, secondary battery B4, becomes larger. For these reasons, the possibility of overdetection increases when moving average MA spans material lots.

<Relative Judging>
In such a case, the "step of calculating relative values (S8)" and the "step of determining relative values (S9)" are performed.

In the "step of calculating relative values (S8)," such overdetection is prevented by performing a relative judgment within the same material lot. The relative judgment calculates the population variance when the voltage difference ΔV [V] of secondary batteries B contained in one material lot is used as the population. Next, the population standard deviation σ of the voltage difference ΔV [V] of each secondary battery B contained in that material lot is calculated.

In the "step of relative determination (S9)", the calculated population standard deviation σ of each secondary battery B is compared with a preset threshold value Tr to determine whether the secondary battery is defective.
In this embodiment, ΔV [V] for all the units in each lot is totaled.

Fig. 8 shows the formula for calculating the population variance ^σ2 . Specifically, the population variance is equal to the average of the squares of the deviations from the mean value. The population variance " ^σ2 " of a population of data _x1 , _x2 , ..., xn, whose size is "n", can be expressed by the formula shown in Fig. 8 when the population mean value is represented by "μ" for " _x1 , _x2 , ..., _{xn "} . Here, " ^σ2 " is the population variance, and "σ" is the population standard deviation.

In addition, when the population of material lots is large, the "variance ^s2 " may be calculated based on a sample rather than by tallying all the target secondary batteries. When the "population standard deviation σ" is unknown, the "standard deviation s" may be used instead of the population standard deviation σ.

Also, when the "population standard deviation σ" is unknown, the unbiased deviation of the sample can be used as an estimate of the population standard deviation.
Note that, when used alone, this relative judgment cannot prevent overdetection, as described in the prior art. In other words, a secondary battery B that has been judged to be defective due to overdetection by the difference judgment of the difference DA based on the moving average MA across different material lots is judged to be a good product by the relative judgment, as this was an overdetection. The judgment results for each secondary battery B here are stored in the memory means of the control device 4.

<Final judgment>
In the "final judgment step (S10)," the secondary battery that was judged to be defective in both the difference judgment step (S7) and the relative judgment step (S9) is judged to be defective. In the final judgment step (S10), first, whether or not the secondary battery B to be inspected is a good product in the difference judgment step (S7) is read from the storage means of the control device 4. If it is judged to be a good product here, it is judged to be a good product (S11), and the process ends.

Meanwhile, in the step of determining the difference (S7), the controller 4 reads from its storage means whether the secondary battery B being inspected is a good product or not. If it is determined to be defective here, the controller 4 reads from its storage means whether it is a good product or not in the step of determining relative quality (S9). If it is determined to be a good product in the step of determining relative quality (S9), it is simply determined to be a good product (S11), and the process ends.

On the other hand, if the control device 4 reads the information about whether the product is good or bad from its storage means in the relative judgment step (S9) and determines that the product is defective, the product has already been determined to be defective in the difference judgment step (S7), so the product is finally determined to be defective (S12) and the process ends.

(Operation of this embodiment)
<Function of difference judgment>
FIG. 9 is a schematic diagram of defective product detection in a conventional self-discharge test. As shown in FIG. 9, self-discharge during the aging process includes self-discharge due to micro-short circuits specific to the secondary battery. On the other hand, there is also self-discharge due to self-decomposition of the positive electrode caused by variations in the raw materials of the material lot. These types of self-discharge cannot be distinguished by detecting ΔV [V] alone. Self-discharge due to self-decomposition of the positive electrode caused by variations in the raw materials of the material lot can be said to be a characteristic of the secondary battery, and cannot immediately be determined to be defective. However, in the past, because it was not possible to distinguish between these types of self-discharges by detecting ΔV [V] alone, there was a possibility that secondary batteries belonging to material lot 1, which were considered to be good products, were overdetected as defective.

Figure 10 is a schematic diagram of defective product detection in a self-discharge test using difference determination in this embodiment. In this embodiment, by using the moving average MA to cancel out ΔV [V] due to self-decomposition of the positive electrode caused by variations in raw materials common to the same material lot, it is possible to detect self-discharge caused by micro-short circuits or other factors specific to the secondary battery. In this way, in the difference determination step (S7) of this embodiment, secondary batteries experiencing self-discharge caused by micro-short circuits or other factors specific to the secondary battery can be more accurately detected using the detection threshold Ta.

<Suppression of overdetection>
FIG. 11 is a graph showing the ratio of overdetected non-defective products. Here, "overdetection" refers to the erroneous detection of excessively good products as defective. In a comparative example using conventional absolute value determination, the ratio of overdetected non-defective products was 0.21%, or approximately 2,100 per million. On the other hand, with the self-discharge inspection method of this embodiment, the ratio of overdetected non-defective products was dramatically reduced to 0.02%. This significantly reduced losses in the secondary battery manufacturing process.

<Preventing undetected defects>
12 is a graph showing the detection of defective products when the conventional example is set to an index of 100. In the comparative example, the detection of defective products by conventional absolute value judgment is set to an index of 100, while in the example using the self-discharge inspection method of this embodiment, the index was 101. This means that potential defective products that were judged to be non-defective or the like in the comparative example were correctly identified as defective products in the example.

According to the example of the self-discharge inspection method of this embodiment, there is an effect that good products and defective products can be correctly detected in this way.
(Effects of this embodiment)
(1) According to the self-discharge inspection method and self-discharge inspection device 3 for the secondary battery B of this embodiment, it is possible to appropriately determine whether the secondary battery is good or bad during the manufacturing process.

(2) In this embodiment, the voltage difference ΔV of secondary battery B is acquired in the ΔV acquisition step (S4). Then, in the moving average calculation step (S5), the moving average MA of the voltage difference ΔV between the target secondary battery Bo and the comparison secondary battery Bc manufactured adjacently is calculated. This allows for relative evaluation of secondary batteries with similar characteristics made from the same material lot.

(3) In the difference calculation step (S6), the difference DA is calculated by comparing the calculated moving average MA with the threshold value Ta. Then, in the difference determination step (S7), the difference DA is compared with the threshold value Ta, making it possible to easily and accurately detect defective products (NG) that exhibit self-discharge specific to the target secondary battery Bo.

(4) Similarly, by comparing the difference DA with the threshold value Ta, it is possible to easily and accurately detect whether a target secondary battery Bo that is a defective product NG has been overlooked as a non-defective product G.
(5) In addition, when the moving average MA is used across different material lots, a non-defective product G may be detected as a defective product NG in the difference determination step (S7). In this case, if there is no problem in the relative value calculation step (S8) and the relative determination step (S9) within the same material lot, the product is determined to be non-defective in the final determination step, thereby preventing overdetection.

(6) Therefore, secondary batteries B can be continuously manufactured using different material lots, thereby improving production efficiency.
(7) The relative judgment is performed using the mean value μ, population variance σ ² , and population standard deviation σ for all secondary batteries B belonging to the material lot as a population, and therefore an accurate judgment can be made.

(8) In this embodiment, step (S1) of the aging process is used as the no-load discharge process. This does not interfere with the normal production process of secondary batteries B, nor does it reduce production efficiency.

(9) In this embodiment, the secondary battery is exemplified as a nickel-metal hydride battery. Nickel-metal hydride batteries have a large change in ΔV due to self-discharge caused by material variations. Therefore, inspection using the self-discharge inspection method for secondary batteries of this embodiment has the advantage of enabling more appropriate inspection.

(10) The self-discharge inspection device 3 for secondary batteries of this embodiment can easily implement this embodiment by having the control device 4 execute the self-discharge inspection method for secondary batteries of this embodiment. Therefore, no special equipment is required and the method can be implemented using existing facilities.

(Another example)
In this embodiment, the secondary battery B is exemplified as a battery module of an in-vehicle nickel-metal hydride storage battery, but the target secondary battery can be a wide range of secondary batteries that require self-discharge inspection, such as lithium-ion secondary batteries, nickel-cadmium batteries, and all-solid-state batteries.

Furthermore, the use of the secondary battery B is not limited to vehicle use, but can also be used as a power source for computers, power storage facilities in homes and factories, and the like.
In this embodiment, continuous production across different material lots is assumed, but it is also possible to manufacture only a single material lot. In that case, the relative value calculation step (S8), the relative judgment step (S9), and the final judgment step (S10) can be omitted. In this case, the difference judgment step (S7) determines whether the product is good (G) or defective (NG).

In this embodiment, the moving average MA is calculated using three comparison secondary batteries Bc, which are adjacent to the target secondary battery Bo before and after the test target secondary battery Bo. However, as mentioned above, the comparison secondary battery Bc can also be the secondary battery adjacent to either the front or back of the target secondary battery Bo, and the moving average MA can be calculated using five comparison secondary batteries Bc, two of which are adjacent to the front and two of which are adjacent to the target secondary battery Bo.

Furthermore, although variations of the method for calculating the moving average at the start and end of production and for different material lots have been described above, those skilled in the art can also adopt other suitable methods.
In the relative value calculation step (S8), the mean value μ, population variance σ ² , etc. are calculated from all the secondary batteries B contained in the same material lot, which constitutes the population. However, as described above, this may also be done by extracting samples. The method of determination can be appropriately selected by those skilled in the art.

○Furthermore, in this embodiment, the no-load discharge process has been described as an aging process, but it is not limited to an aging process as long as the voltage drop due to self-discharge can be properly measured.

The secondary battery manufacturing apparatus 1 of this embodiment shown in Figure 1 is a schematic block diagram showing the minimum required configuration, and the secondary battery manufacturing apparatus 1 and self-discharge inspection apparatus 3 are not limited to this configuration.

The numerical values, ranges, materials, etc. described in this specification are examples and can be optimized and implemented as appropriate by those skilled in the art.
The flowchart shown in FIG. 2 is an example of a procedure, and a person skilled in the art can add, delete, or change the procedure, or change the order of the procedure.

○The present invention is not limited to the embodiments, and those skilled in the art may add, delete, or modify the configuration without departing from the scope of the claims.

1... Secondary battery manufacturing device 2... Assembly device 3... Self-discharge inspection device 4... Control device (equipped with a calculation device) 5... Charging/discharging device 6... Voltage measurement device B (B1, B2...)... Secondary battery Bo... Target secondary battery Bc... Comparison secondary battery ΔV... Voltage difference MA... Moving average DA... Difference Ta... Threshold value μ... Population mean value σ ² ... Population variance σ... Population standard deviation Tr... Threshold value NG... Defective product G... Good product E... Overdetected secondary battery

Claims (7)

A method for inspecting self-discharge of secondary batteries in a process for continuously manufacturing a plurality of secondary batteries, comprising:
a ΔV acquisition step of individually acquiring a voltage difference ΔV, which is a change in voltage of the secondary battery before and after a no-load discharge process;
a moving average calculation step of calculating a moving average MA of the voltage difference ΔV between a target secondary battery, which is the secondary battery for which the voltage difference ΔV was acquired in the ΔV acquisition step, and a comparison secondary battery manufactured adjacent to the target secondary battery;
a difference calculation step of calculating a difference DA between the moving average MA calculated in the moving average calculation step and the target secondary battery for which a voltage difference ΔV has been acquired in the ΔV acquisition step;
a difference determination step of comparing the difference DA calculated in the difference calculation step with a preset threshold value Ta to determine whether the product is defective;
a relative value calculation step of calculating a standard deviation of the voltage difference ΔV of each of the secondary batteries included in the material lot based on a variance when the voltage difference ΔV of the secondary batteries included in the material lot is set as a population;
a relative determination step of comparing the standard deviation of the secondary battery calculated in the relative value calculation step with a preset threshold value Tr to determine whether the secondary battery is defective;
a final determination step of determining as defective a secondary battery that is determined to be defective in both the difference determination step and the relative determination step . 2. The method for inspecting self-discharge of a secondary battery according to claim 1, wherein the adjacently manufactured comparison secondary battery in the moving average calculation step is a secondary battery manufactured before or after the target secondary battery from which the voltage difference V was acquired in the ΔV acquisition step . 2. The method for inspecting self-discharge of a secondary battery according to claim 1 , wherein the step of calculating the moving average is performed continuously across different material lots. 4. The method for inspecting self-discharge of a secondary battery according to claim 1, wherein the variance is a population variance σ ² based on all secondary batteries in the material lot. 5. The method for inspecting self-discharge of a secondary battery according to claim 1, wherein the no-load discharge step is an aging step. 6. The method for inspecting self-discharge of a secondary battery according to claim 1, wherein the secondary battery is a nickel-metal hydride battery. A secondary battery self-discharge inspection device for inspecting self-discharge of secondary batteries in a process of continuously manufacturing a plurality of secondary batteries for each material lot, comprising:
a voltage measuring device for measuring the voltage of the secondary battery;
a control device having a computing device;
The control device
The voltage measuring device individually acquires a voltage difference ΔV, which is a change in voltage of the secondary battery before and after the aging process;
calculating a moving average MA of a plurality of secondary batteries including a target secondary battery, which is the secondary battery from which the voltage difference ΔV was acquired by the arithmetic device, and a comparison secondary battery manufactured adjacent to the target secondary battery;
calculating a difference DA between the moving average MA and the voltage difference ΔV of the target secondary battery;
The difference DA is compared with a preset threshold value Ta to determine whether the product is defective or not .
The control device
calculating a standard deviation σ of the voltage difference ΔV of each of the secondary batteries included in the material lot based on a variance when the voltage difference ΔV of the secondary batteries included in the material lot is set as a population;
The standard deviation σ is compared with a preset threshold value Tr to determine whether the product is defective or not.
A self-discharge inspection device for secondary batteries, characterized in that the secondary battery is finally judged to be defective when it is judged to be defective in comparison with the threshold value Ta and when it is judged to be defective in comparison with the threshold value Tr.