Secondary battery self-discharge inspection method and self-discharge inspection device

Abstract

To appropriately determine good and defective secondary cells in a manufacturing process.SOLUTION: A secondary cell self-discharge inspection device performs self-discharge inspection in a process to successively manufacture a plurality of secondary cells, and comprises a charge-discharge device, a voltage measurement device, and a control device that includes a computing device. The control device individually acquires a voltage difference ΔV that represents a change of secondary cell voltage before and after an aging step, calculates the moving average MA of a plurality of secondary cells including the relevant secondary cell whose voltage difference ΔV has been acquired and the voltage difference ΔV of other secondary cells that have been manufactured adjacent to the relevant secondary cell, calculates a difference DA between the moving average MA and the voltage difference ΔV of the relevant secondary cell, and compares the difference DA with a preset threshold Ta to determine whether being defective or not.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method and apparatus for inspecting self-discharge of a secondary battery, and more particularly to a method and apparatus for inspecting self-discharge of a secondary battery that appropriately determines whether a secondary battery is good or bad during the manufacturing process. [Background technology]

[0002] 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 without 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 a high level of self-discharge and fails the inspection. It is then deemed a defective product and is generally discarded rather than shipped.

[0003] Figure 13 is a graph showing a method for determining the absolute value of the voltage difference ΔV [V] based on 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.

[0004] When inspecting such a voltage difference ΔV [V], there are many factors that cause variation in the self-discharge specific to the secondary battery, such as variation in the solid-solution elements of the positive electrode active material, variation in the additives in the electrolyte, and temperature variation within the battery.

[0005] Considering such variations, 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 bad) to prevent overlooking of defective products. As a result, some secondary batteries are judged as bad 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.

[0006] Therefore, the invention disclosed in Patent Document 1 is as follows: In this invention, 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 small internal short circuit, relative to the average value ΔVA of ΔV [V] that varies for each inspection lot.Then, an inspection method is disclosed in which batteries with ΔV [V] smaller than the value ΔVA - ΔVB are determined to be defective.

[0007] In this type of inspection method based on absolute values, which are based on 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 value. This can also result in a threshold value with a margin on the side of overdetection.

[0008] Figure 14(a) is a graph showing the voltage difference ΔV [V] between the secondary batteries consisting of material lot 1 and material lot 2 to be inspected. The vertical axis shows the voltage difference ΔV [V]. The horizontal axis shows 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 shows the probability, and the horizontal axis shows 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").

[0009] In the absolute value judgment described above, depending on the variations in the secondary batteries to be inspected, there is a possibility that the threshold value will have a margin on the side of overdetection. Here, it is assumed that the secondary batteries shown in Figure 14(a), both the secondary batteries contained in material lot 1 and the secondary batteries contained in material lot 2, are non-defective products with no self-discharge problems. Next, as shown in Figure 14(b), the average μ of these secondary batteries is calculated, and the variance σ 2is calculated and a probability distribution graph L1 is created. Here, an arbitrary threshold value (for example, +3σ) is set as the threshold value Tr, and secondary batteries with a standard deviation σ that exceeds the threshold value Tr are detected as abnormal values ​​and defective products. However, as described under the premise shown in FIG. 14(a), all products are non-defective products G and no defective products NG are included. However, regardless of how the threshold value Tr is set in FIG. 14(b), secondary batteries that exceed the threshold value Tr will be detected as defective products. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-190292 Summary of the Invention [Problem to be solved by the invention]

[0011] As described above, in the past, there was a possibility that secondary batteries that should have been judged as good products were over-detected as defective products, which resulted in the problem that good secondary batteries manufactured during the manufacturing process were judged as defective products.

[0012] The problem to be solved by the self-discharge inspection method and self-discharge inspection device for secondary batteries of the present invention is to appropriately determine whether a secondary battery is good or bad during the manufacturing process. [Means for solving the problem]

[0013] In order 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 of continuously manufacturing multiple secondary batteries, and includes a ΔV acquisition step of individually acquiring a voltage difference ΔV, which is the change in voltage of the secondary batteries before and after a discharge process under no load; 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 the voltage difference ΔV was acquired in the ΔV acquisition step; and a difference determination step of comparing the difference DA calculated in the difference calculation step with a predetermined threshold value Ta to determine whether the product is defective.

[0014] It is also desirable to further include a relative value calculation step of calculating the standard deviation of the voltage difference ΔV of each of the secondary batteries 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 batteries calculated in the relative value calculation step with a preset threshold value Tr to determine whether or not the secondary batteries are defective; and a final judgment step of determining as defective those secondary batteries that are determined to be defective in both the difference judgment step and the relative judgment step.

[0015] The comparative secondary batteries manufactured adjacent to each other in the moving average calculation step may be secondary batteries manufactured before and after the target secondary battery for which the voltage difference V was acquired in the ΔV acquisition step.

[0016] It is desirable that the step of calculating the moving average is continuously performed across different material lots. The variance is a population variance σ based on the total number of secondary batteries in the material lot. 2 The no-load discharge step may be an aging step.

[0017] 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 multiple secondary batteries for each material lot, and is equipped with a voltage measuring 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 measuring device, and calculates a moving average MA of multiple 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.

[0018] Furthermore, it is desirable that the control device further calculates the standard deviation σ of the voltage difference ΔV of each of the secondary batteries 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 predetermined threshold value Tr to determine whether or not the battery is defective, and makes a final determination that the secondary battery is defective when determined to be defective by comparison with the threshold value Ta and when determined to be defective by comparison with the threshold value Tr. [Effects of the Invention]

[0019] According to the method and device for inspecting self-discharge of a secondary battery of the present invention, it is possible to appropriately determine whether a secondary battery is good or bad in the manufacturing process. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a block diagram showing a secondary battery manufacturing apparatus according to an embodiment of the present invention; [Figure 2] 3 is a flowchart showing the procedure of a method for inspecting self-discharge of a secondary battery according to the present embodiment. [Figure 3]FIG. 10 is a diagram showing the acquisition of the moving average MA of ΔV [V] in the difference determination. [Figure 4] FIG. 10 is a diagram showing the acquisition of the moving average MA of ΔV [V] in the difference determination. [Figure 5] FIG. 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. [Figure 6] 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. [Figure 7] 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. [Figure 8] This is the formula for calculating the population variance σ2. [Figure 9] FIG. 1 is a schematic diagram illustrating detection of defective products in a conventional self-discharge inspection. [Figure 10] 10A and 10B are schematic diagrams illustrating defective product detection in a self-discharge inspection by difference judgment according to the present embodiment. [Figure 11] 10 is a graph showing the ratio of overdetected non-defective products. [Figure 12] 10 is a graph showing detection of defective products when the index for the conventional method is set to 100. [Figure 13] 10 is a graph showing a method for determining the absolute value of a voltage difference ΔV [V] based on a common threshold T, based on conventional design values. [Figure 14] 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. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, a method and apparatus for inspecting self-discharge of a secondary battery according to the present invention will be described with reference to FIGS. 1 to 11 as an embodiment of a method and apparatus for inspecting self-discharge of a nickel-metal hydride storage battery.

[0022] <Configuration of the Secondary Battery of the Present 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 electrode plates 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.

[0023] Even after the battery elements are assembled, such nickel-metal hydride batteries are still incomplete and undergo initial charging and conditioning, followed by tests for capacity, voltage, internal resistance, etc. Furthermore, the battery is charged to a specified voltage and then aged at a high temperature without load. Aging eliminates short circuits and chemically stabilizes the electrodes. Self-discharge tests are generally conducted during this aging process.

[0024] 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-circuiting in such nickel-metal hydride batteries (see FIG. 10). For this reason, the method for inspecting self-discharge of secondary batteries according to this embodiment can be suitably applied to nickel-metal hydride batteries.

[0025] Although a nickel-metal hydride battery is exemplified as the secondary battery, it is not intended to be limited to a nickel-metal hydride battery, and it goes without saying that the present invention can be implemented with other secondary batteries such as a lithium-ion secondary battery as long as the present invention can be implemented.

[0026] <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 is used to continuously assemble a plurality of secondary batteries B for each material lot. A charging / discharging device 5 charges and discharges secondary batteries B that have been assembled in the assembly device 2 of the secondary battery manufacturing apparatus 1. The charging / discharging device 5 includes sensors such as a voltmeter and an ammeter (not shown). The charging / discharging device 5 is controlled by a control device 4. A self-discharge inspection device 3 according to this embodiment inspects the self-discharge of secondary batteries that have been controlled to a constant state of charge by the charging / discharging device 5. Since the aging step (FIG. 2: S1) according to this embodiment is intended to inspect for self-discharge, a relatively low charging rate (e.g., SOC 10% or less) is preferable because the voltage difference ΔV due to self-discharge increases.

[0027] The self-discharge inspection device 3 for a secondary battery of this embodiment includes a voltage measurement device 6 that measures the battery voltage of the secondary battery B, and a control device 4 that includes a calculation device. This control device 4 is shared with a charging / discharging device 5.

[0028] Although not shown in the figure, 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, under the control of the control device 4, begins the aging process (FIG. 2: S1). 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) raises 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, there is no external discharge of power. 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 a rapid voltage drop even when the secondary battery is not in use, degrading its performance. Therefore, once the secondary battery B is assembled, the charging / discharging device 5 first charges the secondary battery B to a set charging rate. Then, the self-discharge inspection device 3 performs the aging process (S1) as a self-discharge inspection.

[0029] In the control device 4, in a step (S2) of measuring the battery voltage V beforehand and a step (S3) of measuring the battery voltage V afterward, the voltage measurement device 6 acquires and stores the voltages of the individual secondary batteries B1 to B9 . . .

[0030] 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 step (S2) of measuring the pre-battery voltage V and a step (S3) of measuring the post-battery voltage V. 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 bad.

[0031] (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.

[0032] On the other hand, relative determination alone can cause problems due to overdetection. Therefore, in this embodiment, the concepts of absolute value determination and relative determination are combined. However, simply combining absolute value determination and relative determination does not necessarily solve either of the problems.

[0033] 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.

[0034] <Getting ΔV [V]> In this method, first, the control device 4 starts the procedure of step (S1) of the aging process using the self-discharge inspection device 3. Note that step (S1) of the aging process continues until measurement of the battery voltage V of the secondary battery B is completed by the voltage measurement device 6 in the "step (S3) of measuring the post-battery voltage V." When step (S1) of the aging process starts, first, the secondary battery B, which has been charged to a predetermined charge rate (e.g., SOC 10%), is heated and raised 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.

[0035] When 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 a "step of measuring battery voltage V beforehand (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 a "step of measuring battery voltage V afterward (S3)." Then, a procedure in a "step of acquiring ΔV (S4)" is executed. In the "step of acquiring ΔV (S4)," a voltage difference ΔV [V] between the voltage measured in the "step of measuring battery voltage V beforehand (S2)" and the voltage measured in the "step of measuring battery voltage V afterward (S3)" is acquired. The voltage difference ΔV is acquired for each of secondary batteries B1, B2, .... In this embodiment, the secondary battery B to be inspected, for which the voltage difference ΔV [V] is acquired in this "step of acquiring ΔV (S4)," is referred to as the "target secondary battery."

[0036] <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 it for comparison is calculated.

[0037] In the secondary battery manufacturing process, secondary batteries B are produced consecutively on a manufacturing line. Here, "adjacent secondary batteries" includes at least the secondary batteries before or after the "subject secondary battery." Furthermore, a moving average may be obtained for a total of five secondary batteries, two before and two after the subject secondary battery.

[0038] At the start of production, there is no "previous adjacent secondary battery." In that case, the moving average can be calculated using only the available data, or other data can be substituted for the unavailable 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 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 for all three ΔV [V].

[0039] This treatment can also be applied to cases where the final secondary battery produced becomes the "target secondary battery." <Example of moving average MA> FIG. 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 set as the target secondary battery, and the preceding secondary battery B1 and the following secondary battery B3 are set as comparison secondary batteries, forming a group G1. 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.

[0040] FIG. 4 is a diagram showing how the moving average MA of ΔV [V] is obtained in the difference determination. Here, the secondary battery B3 to be measured is set as the target secondary battery, and the preceding secondary battery B2 and the following secondary battery B4 are set as comparison secondary batteries, forming a group G2. 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.

[0041] <Calculating the difference DA> Fig. 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 those secondary batteries. Using the method shown in Figs. 3 and 4, the graph shows the moving average MA, with secondary batteries B1 to B11 as the target secondary batteries and 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.

[0042] In the "step of calculating the difference (S6)", the difference DA between the moving average MA calculated in the "step of calculating the moving average (S5)" and the "target secondary battery" whose voltage difference ΔV [V] was obtained in the "step of obtaining ΔV" is calculated.

[0043] <Difference judgment> FIG. 6 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. The difference DA is expressed as the difference obtained by subtracting the moving average MA from ΔV [V] of 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 it is a positive number. Next, for secondary battery B2, as shown in FIG. 5, ΔV [V] is smaller than the moving average MA, so it 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.

[0044] It can be estimated that multiple adjacent secondary batteries (here, comparison secondary batteries relative to the target secondary battery) have approximately the same characteristics, such as materials. 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 in the material lot, but rather to differences in the inherent characteristics of the "target secondary battery." In this sense, this "difference DA" is a relative evaluation within a narrow range. Conventional relative evaluations involve comparisons with the entire product or material lots. However, the difference DA of this embodiment is a relative evaluation within a narrow range, and therefore accuracy can be improved by comparing with multiple adjacent secondary batteries that can be estimated to have approximately the same characteristics, such as materials.

[0045] 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 it can be determined that the target secondary battery has a problem with self-discharge. If this threshold value Ta exceeds the range expected from the variation in materials of the manufactured secondary batteries, it can be determined that the large self-discharge is due to a problem in the "target secondary battery" itself. Because the variation in materials of the manufactured secondary batteries is expected to be smaller within a narrow range, this threshold value Ta can also be set with higher precision.

[0046] Therefore, in the "step of determining difference (S7)", the difference DA calculated in the "step of calculating difference (S6)" is compared with a preset threshold value Ta to determine whether the product is good or bad. The determination result here is stored in the storage means of the control device 4.

[0047] <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.

[0048] In this embodiment, secondary batteries B from different material lots (production lots using different raw materials) are also continuously manufactured. 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 larger 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 high possibility of overdetection, in which a good product is judged to be defective.

[0049] 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, at an average of A1, than material lot 2. On the other hand, material lot 2 has a lower ΔV [V] overall, at an average of A2, than material lot 1. 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 lowered by the influence of material lot 2, and the difference DA for the target secondary battery, secondary battery B4, becomes larger. For these reasons, the moving average MA across material lots increases the possibility of overdetection.

[0050] <Relative Judgment> In such a case, the "step of calculating relative values ​​(S8)" and the "step of determining relative values ​​(S9)" are performed.

[0051] In the "step of calculating relative values ​​(S8)," such overdetection is suppressed 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 included 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 included in that material lot is calculated.

[0052] 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 or not the product is defective. In this embodiment, ΔV[V] for all the units in each lot is totaled.

[0053] Figure 8 shows the population variance σ 2 The population variance is specifically equal to the average of the squares of the deviations from the mean. For data x1, x2, ..., x whose size is "n", n The population variance of the population "σ 2 " is "x1,x2,...,x n When the population mean value is expressed as "μ", it can be expressed as shown in Figure 8. 2" is the population variance and "σ" is the population standard deviation.

[0054] In addition, when the population of material lots is large, it is better to use the "variance s" of the sample instead of totaling all the target secondary batteries. 2 If the "population standard deviation σ" is unknown, the "standard deviation s" can be used instead of the population standard deviation σ.

[0055] 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 as defective due to overdetection by the difference judgment of the difference DA based on the moving average MA across different material lots is judged as a non-defective 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.

[0056] <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.

[0057] Meanwhile, in the step of determining difference (S7), whether or not the secondary battery B to be inspected is a good product is read from the storage means of the control device 4. If it is determined to be a defective product here, whether or not it is a good product is determined in the step of relative determination (S9) is read from the storage means of the control device 4. If it is determined to be a good product in the step of relative determination (S9), it is simply determined to be a good product (S11) and the process ends.

[0058] On the other hand, if the product is judged to be defective in the relative judgment step (S9) by reading the information on whether it is a good product or not from the storage means of the control device 4, it has already been judged to be defective in the difference judgment step (S7), so the product is judged to be defective as a final judgment (S12) and the process ends.

[0059] (Action of this embodiment) <Function of difference judgment> Figure 9 is a schematic diagram of defective product detection using conventional self-discharge testing. As shown in Figure 9, self-discharge during the aging process can be caused by 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 it cannot be immediately determined to be defective. However, in the past, because it was not possible to distinguish between these types of self-discharge by detecting ΔV [V] alone, there was a possibility that secondary batteries belonging to material lot 1, which were considered to be good products, could be overdetected as defective.

[0060] 10 is a schematic diagram of defective product detection in a self-discharge test using the difference determination method of this embodiment. In this embodiment, the moving average MA is used 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, making it possible to detect self-discharge caused by micro-short circuits or other factors specific to the secondary battery. Thus, in the difference determination step (S7) of this embodiment, a secondary battery experiencing self-discharge caused by micro-short circuits or other factors specific to the secondary battery can be more accurately and appropriately detected using the detection threshold Ta.

[0061] <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 judgment, 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 made it possible to significantly reduce losses in the secondary battery manufacturing process.

[0062] <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 good or the like in the comparative example were correctly identified as defective products in the example.

[0063] 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 in the manufacturing process.

[0064] (2) In this embodiment, the voltage difference ΔV of secondary battery B is acquired in a ΔV acquisition step (S4). Then, in a 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 adjacent to it is calculated. This allows for relative evaluation of secondary batteries with similar characteristics made from the same material lot.

[0065] (3) In the step (S6) of calculating the difference, the difference DA is calculated by comparing the obtained moving average MA with the threshold value Ta. Then, in the step (S7) of determining the difference, the difference DA is compared with the threshold value Ta, thereby easily and accurately detecting defective products NG that exhibit self-discharge specific to the target secondary battery Bo.

[0066] (4) Similarly, by comparing the difference DA with the threshold value Ta, it is possible to easily and accurately detect that 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 spans different material lots, a good product G may be detected as a defective product NG in the difference judgment step (S7). In this case, if there are no problems in the relative judgment within the same material lot in the relative value calculation step (S8) and the relative judgment step (S9), the product will be judged as a good product in the final judgment step, thereby preventing overdetection.

[0067] (6) Therefore, secondary batteries B can be continuously manufactured using different material lots, thereby improving production efficiency. (7) Relative judgment is performed by using all secondary batteries B belonging to the material lot as a population, with the mean value μ and population variance σ 2 , and the population standard deviation σ is used for the judgment, so that an accurate judgment can be made.

[0068] (8) In this embodiment, the step (S1) of the aging process is used as a no-load discharging process, which does not interfere with the normal production process of the secondary battery B and does not reduce production efficiency.

[0069] (9) In this embodiment, the secondary battery is a nickel-metal hydride battery. Nickel-metal hydride batteries have a large change in ΔV due to self-discharge caused by variations in materials. Therefore, the self-discharge inspection method for secondary batteries according to this embodiment has the advantage of enabling more appropriate inspection.

[0070] (10) The self-discharge inspection device 3 for secondary batteries of this embodiment can easily implement the self-discharge inspection method for secondary batteries of this embodiment by causing the control device 4 to execute the method. Therefore, no special equipment is required, and the method can be implemented using existing facilities.

[0071] (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.

[0072] 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. While this embodiment is intended for continuous production across different material lots, it is also possible to manufacture using only a single material lot. In this 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 or bad.

[0073] In this embodiment, the moving average MA is calculated using three secondary batteries adjacent to the target secondary battery Bo to be inspected, namely, the comparison secondary batteries Bc. However, as described above, the comparison secondary battery Bc may be the secondary battery adjacent to either the front or back of the target secondary battery Bo, and the moving average MA may be calculated using two comparison secondary batteries Bc. Furthermore, the moving average MA may be calculated using five comparison secondary batteries Bc, namely, two each of which are adjacent to the front and back of the target secondary battery Bo.

[0074] 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 and population variance σ are calculated from the total number of secondary batteries B contained in the same material lot, which is the population. 2 However, as mentioned above, the determination may be performed by extracting a sample. The determination method can be appropriately selected by those skilled in the art.

[0075] Although the no-load discharge process of this embodiment is described as an aging process, it is not limited to an aging process as long as the voltage drop due to self-discharge can be appropriately measured.

[0076] The secondary battery manufacturing apparatus 1 of this embodiment shown in FIG. 1 is a schematic block diagram showing the minimum required configuration, and the secondary battery manufacturing apparatus 1 and the self-discharge inspection apparatus 3 are not limited to this form.

[0077] 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.

[0078] The present invention is not limited to the embodiments, and those skilled in the art can add, delete, or modify the configuration without departing from the scope of the claims. [Explanation of symbols]

[0079] 1...Secondary battery manufacturing equipment 2...Assembly equipment 3...Self-discharge inspection device 4...Control device (with arithmetic unit) 5...Charging / discharging device 6...Voltage measuring device B(B1,B2…)…Secondary battery Bo: Target secondary battery Bc…Comparison secondary battery ΔV: voltage difference MA…moving average DA…Difference Ta...Threshold μ…Population mean value σ 2 ...mother variance σ…Population standard deviation Tr...Threshold NG…defective product G…Good product E: Over-detected secondary battery

Claims

1. 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 a secondary battery for which the voltage difference ΔV has been 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. 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.

3. 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 variance is a population variance σ based on the total number of secondary batteries in the material lot. ２ 4. The method for inspecting self-discharge of a secondary battery according to claim 1 or 3, wherein:

5. 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. 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.

7. 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.

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