Test method for self-discharge of batteries

Abstract

The application relates to a battery self-discharge test method, which comprises the following steps: charging a battery after capacity distribution processing until the battery capacity corresponding to the battery after the capacity distribution processing reaches a preset state of charge; performing first standing treatment on the charged battery at a preset temperature for a first preset time period; measuring a first open circuit voltage corresponding to the battery after the first standing treatment, and marking the corresponding time as a first preset time; performing second standing treatment on the battery after the first standing treatment at the preset temperature for a second preset time period, the length of the second preset time period being greater than that of the first preset time period; measuring a second open circuit voltage corresponding to the battery after the second standing treatment, and marking the corresponding time as a second preset time; and evaluating the self-discharge performance of the battery after the second standing treatment according to the first preset time, the second preset time, the first open circuit voltage and the second open circuit voltage, so that the test accuracy of the battery self-discharge is improved in general.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of battery testing, in particular to a battery self-discharge testing method. BACKGROUND

[0002] At present, lithium batteries for power storage have been widely concerned and applied. The difference and consistency of battery performance are important factors affecting its normal work. However, the difference in material, process and process capability of lithium battery manufacturers, and the consistency screening and determination of the same type of products are particularly important. The self-discharge performance is an important performance difference.

[0003] There are many reasons for the difference in self-discharge, mainly including internal micro-short circuit causing self-discharge and electrode liquid in positive and negative electrode oxidation and reduction capacity attenuation. The batteries with inconsistent self-discharge performance will have a large difference in SOC (State of Charge) after a period of storage, which will greatly affect its capacity and safety. Therefore, it is necessary to test the self-discharge performance of the battery to screen out abnormal batteries, and the detection accuracy of the self-discharge performance of the battery is particularly important. SUMMARY

[0004] In view of this, the present application provides a battery self-discharge testing method, which can greatly improve the detection accuracy of the self-discharge performance of the battery.

[0005] The testing method comprises:

[0006] charging the battery after capacity grading until the battery capacity corresponding to the battery after capacity grading reaches a preset state of charge;

[0007] carrying out first standing treatment on the charged battery at a preset temperature for a first preset time period;

[0008] measuring the first open circuit voltage corresponding to the battery after the first standing treatment, and marking the corresponding time as a first preset time;

[0009] carrying out second standing treatment on the battery after the first standing treatment at a preset temperature for a second preset time period, the length of the second preset time period being greater than the length of the first preset time period;

[0010] measuring the second open circuit voltage corresponding to the battery after the second standing treatment, and marking the corresponding time as a second preset time;

[0011] evaluating the self-discharge performance of the battery after the second standing treatment according to the first preset time, the second preset time, the first open circuit voltage and the second open circuit voltage.

[0012] In one embodiment, the current of the constant current charging is 0.1C-0.8C.

[0013] In one embodiment, the first preset time period is 0 to 5 days.

[0014] In one embodiment, the second preset time period is 1 to 5 days.

[0015] In one embodiment, the sum of the first preset time period and the second preset time period is less than or equal to 5 days.

[0016] In one embodiment, the first preset time period is 2 days, and the second preset time period is 3 days.

[0017] In one embodiment, the preset state of charge is 1-5% state of charge.

[0018] In one embodiment, the preset state of charge is 3%.

[0019] In one embodiment, the process of evaluating the self-discharge performance of the battery after the second static treatment according to the first preset time, the second preset time, the first open circuit voltage, and the second open circuit voltage includes:

[0020] Calculating a self-discharge rate of the battery after the second static treatment according to the first preset time, the second preset time, the first open circuit voltage, and the second open circuit voltage;

[0021] The self-discharge rate is compared with a preset self-discharge rate threshold to evaluate the self-discharge performance of the battery after the second rest treatment.

[0022] In one embodiment, in the step of calculating the self-discharge rate of the battery after the second static treatment, the formula used is as follows:

[0023] K=(OCV1-OCV2) / (T2-T1)

[0024] Wherein, K is the self-discharge rate of the battery after the second static treatment, OCV1 is the first open circuit voltage, OCV2 is the second open circuit voltage, T1 is the first preset time, and T2 is the second preset time.

[0025] The above-mentioned test method is to charge the battery after the capacity division treatment until the battery capacity corresponding to the battery after the capacity division treatment reaches a preset state of charge, and then perform a first static treatment on the charged battery at a preset temperature for a first preset time period, measure the first open circuit voltage corresponding to the battery after the first static treatment, and mark the corresponding time as the first preset time, further perform a second static treatment on the battery after the first static treatment for a second preset time period at a preset temperature, the length of the second preset time period is greater than the length of the first preset time period, measure the second open circuit voltage corresponding to the battery after the second static treatment, and mark the corresponding time as the second preset time, and evaluate the self-discharge performance of the battery after the second static treatment according to the first preset time, the second preset time, the first open circuit voltage and the second open circuit voltage, through two time periods. The static treatment of the first preset time period can take into account the characteristics of the rapid voltage drop of each battery in the initial stage. By setting the static treatment of the second preset time period, the disadvantage of relying solely on the static treatment of the first preset time period that each voltage drops rapidly and is not conducive to subsequent discharge performance judgment to screen abnormal batteries can be overcome. At this time, since the length of the second preset time period is greater than the length of the first preset time period, the voltage of the abnormal battery drops faster than that of the normal battery in the second preset time period, which can lay the foundation for the subsequent judgment of the self-discharge performance of the battery to screen out abnormal batteries, and generally improve the accuracy of the self-discharge test, simplify the charging and discharging process, save energy consumption, and quickly judge the self-discharge performance of each battery to screen out abnormal batteries, further saving the storage time and inventory pressure of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For those skilled in the art, other drawings can be obtained based on these drawings without creative work.

[0027] Figure 1 1 is a flow chart of a battery self-discharge testing method provided in an embodiment of the present application;

[0028] Figure 2 This is a schematic diagram of the open circuit voltages of three batteries after static treatment at different charging currents provided in an embodiment of the present application;

[0029] Figure 3 A schematic diagram showing the change in voltage drop over time of three batteries provided in an embodiment of the present application;

[0030] Figure 4A method flow diagram for evaluating the self-discharge performance of a second static treatment battery is provided for the embodiments of the present application.

[0031] Figure 5 A charging curve diagram of a battery is provided for the embodiments of the present application.

[0032] Figure 6 A differential capacity curve diagram of a battery is provided for the embodiments of the present application.

[0033] Figure 7 For Figure 6 A partial enlarged diagram of a differential capacity curve diagram of a battery. DETAILED DESCRIPTION

[0034] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings. Obviously, the described embodiments are only some of the embodiments of the present application, not all. Based on the embodiments in the present application, each of the following embodiments and its technical features can be combined with each other without conflict.

[0035] As Figure 1 A battery self-discharge test method is provided, which can greatly improve the detection accuracy of the battery self-discharge capacity. The test method comprises:

[0036] Step S110, charging the battery after the capacity distribution treatment until the battery capacity corresponding to the battery after the capacity distribution treatment reaches the preset state of charge.

[0037] After the formation treatment according to the normal process, the battery after the capacity distribution treatment needs to be charged first until the battery capacity corresponding to the battery after the capacity distribution treatment reaches the preset state of charge.

[0038] Step S120, performing first static treatment on the charged battery at a preset temperature for a first preset time period.

[0039] Wherein, at the preset temperature, the initial battery voltage drop of each battery after charging is relatively fast, which is not conducive to screening abnormal batteries by the speed of battery voltage drop at this time. Therefore, the first static treatment for the first preset time period is usually performed first.

[0040] Wherein, the preset temperature is usually 25℃-37℃, preferably 25℃.

[0041] Step S130, measuring the first open circuit voltage corresponding to the battery after the first static treatment, and marking the corresponding time as the first preset time.

[0042] Among them, after the first static treatment, the voltage drop of each battery has been greatly reduced. At this time, in order to measure the self-discharge performance of each battery, it is necessary to further measure the first open circuit voltage corresponding to the battery and mark the corresponding time as the first preset time.

[0043] Among them, batteries with qualified self-discharge performance are called normal batteries, and batteries with unqualified self-discharge performance are called abnormal batteries.

[0044] Step S140 , performing a second rest treatment on the battery after the first rest treatment at a preset temperature for a second preset time period, where the second preset time period is greater than the first preset time period.

[0045] Among them, in order to measure the self-discharge performance of each battery to screen out abnormal batteries, it is necessary to further perform a second static treatment on the batteries after the first static treatment at a preset temperature for a second preset time period. The second time period is greater than the first preset time period. This is conducive to screening out abnormal batteries. The reason is that the second time period is greater than the first preset time period. At this time, the voltage of the abnormal battery drops faster than that of the normal battery in the second preset time period (that is, the voltage drop of the normal battery in the second preset time period is relatively small), which can lay the foundation for the subsequent determination of the battery self-discharge performance to screen out abnormal batteries. By setting the second time period to be greater than the first preset time period, it is conducive to laying the foundation for improving the test accuracy of the battery self-discharge performance.

[0046] Step S150 , measuring a second open circuit voltage corresponding to the battery after the second resting treatment, and marking the corresponding time as a second preset time.

[0047] Among them, in the second static treatment after the second preset time period, the voltage drop of the battery with qualified self-discharge performance in the first preset time period has basically dropped. At this time, during the second static treatment in the second preset time period, the voltage drop of the battery with qualified self-discharge performance in the second preset time period is smaller than that of the abnormal battery.

[0048] Step S160 , evaluating the self-discharge performance of the battery after the second resting treatment according to the first preset time, the second preset time, the first open circuit voltage, and the second open circuit voltage.

[0049] Among them, after obtaining the first preset time, the second preset time, the first open-circuit voltage and the second open-circuit voltage, the self-discharge performance of the battery after the second static treatment can be evaluated based on the speed at which the voltage drop of each battery decreases during the second static treatment in the second preset time period.

[0050] The above-mentioned test method uses static processing for two time periods, among which the static processing of the first preset time period can take into account the characteristic that the voltage drop of each battery is relatively fast in the initial stage. By setting the static processing of the second preset time period, it can overcome the disadvantage that each voltage drops relatively fast when relying solely on the static processing of the first preset time period, which is not conducive to subsequent discharge performance judgment to screen out abnormal batteries. At this time, since the length of the second preset time period is greater than the length of the first preset time period, the voltage of the abnormal battery drops faster than that of the normal battery in the second preset time period, which can lay the foundation for the subsequent judgment of the self-discharge performance of the battery to screen out abnormal batteries, and generally improve the accuracy of the self-discharge test, simplify the charging and discharging process, save energy consumption, and thus quickly judge the self-discharge performance of each battery to screen out abnormal batteries, further saving the storage time and inventory pressure of the battery.

[0051] In one embodiment, the charging is constant current charging, and the current of the constant current charging is 0.1C-0.8C.

[0052] Among them, if the constant current charging current is too large, although the charging time is short, it will cause battery polarization to have a greater impact on the battery self-discharge test performance; if the constant current charging current is too small, although the battery polarization has a smaller impact, it will cause the charging time to be too long. C is the nominal rated capacity of the battery.

[0053] By selecting a suitable constant current charging current of 0.1C-0.8C, the impact of the battery polarization process on the voltage drop of the battery at different times is reduced, while taking the charging time into account.

[0054] In one embodiment, Figure 2 As shown, the horizontal axis is the charging rate, and the vertical axis is the open circuit voltage after static treatment. Taking three batteries as an example, Cell-1, Cell-2 and Cell-3, as the charging rate of the constant current charging current increases, the open circuit voltage of each battery after static treatment after charging is basically the same, and the charging time is gradually shortened. Therefore, in order to reduce the impact of the charging process on the self-discharge performance test of each battery, and comprehensively considering the charging time, the constant current charging current can be further set to 0.1C-0.5C.

[0055] In one embodiment, the constant current charging current can be selected as 0.5C, while taking the charging time into consideration, to further reduce the impact of the charging process on the self-discharge performance test of each battery, that is, to reduce the impact of the battery polarization process on the voltage drop of the battery at different times.

[0056] In one embodiment, the first preset time period is 0.5 to 5 days.

[0057] The first preset time period should not be too long, otherwise it will be difficult to use the change in the voltage drop of the rechargeable battery to determine whether the self-discharge performance of each battery is qualified, which is not conducive to the subsequent process of screening abnormal batteries.

[0058] In one embodiment, the second preset time period is 1 to 5 days.

[0059] The second preset time period is longer than the first preset time period. Since the second preset time period is longer than the first preset time period, the voltage of the abnormal battery drops faster in the second preset time period than that of the normal battery. At this time, by setting the static treatment during the second preset time period, it is possible to overcome the disadvantage that when relying solely on the static treatment during the first preset time period, each voltage drops too quickly, which is not conducive to subsequent discharge performance determination to screen abnormal batteries.

[0060] In one embodiment, the sum of the first preset time period and the second preset time period is less than or equal to 5 days.

[0061] like Figure 3 As shown, Figure 3 The diagram below shows the voltage drop of three battery groups (a, b and c) changing with time. Obviously, Figure 3 For cells with faster voltage drop in the early and middle stages, the voltage drop will still be faster in the later stages. For cells with slower voltage drop in the early stages, the voltage drop will also be slower in the later stages. From the trend point of view, the longer the cell is left standing, the smaller the error in the self-discharge test will be. However, the inventory pressure of the cell will increase in the later stages, which is not conducive to mass production. When the standing treatment time (i.e., the sum of the first preset time period and the second preset time period) is equal to 5 days, the voltage drop change of a normal battery tends to be stable (i.e., the voltage drop is close to zero). Therefore, in order to improve the overall efficiency, the sum of the first preset time period and the second preset time period can be further limited to not more than 5 days.

[0062] Among them, the unit d represents one day, Figure 3 The voltage drop on the vertical axis is calculated based on the voltage change between two consecutive days. Curve a represents the voltage drop change of the abnormal battery, and curves b and c represent the voltage drop changes of the normal battery pack, respectively. When the sum of the first preset time period and the second preset time period is equal to 5 days, the voltage drop of the abnormal battery is still large.

[0063] In one embodiment, Figure 4 As shown, step S160 includes:

[0064] Step S162 , calculating the self-discharge rate of the battery after the second rest treatment according to the first preset time, the second preset time, the first open circuit voltage, and the second open circuit voltage.

[0065] Step S164, comparing the self-discharge rate with a preset self-discharge rate threshold to evaluate the self-discharge performance of the battery after the second static treatment.

[0066] In one embodiment, the formula used in the step of calculating the self-discharge rate of the battery after the second static treatment in step S162 is as follows:

[0067] K = (OCV1-OCV2) / (T2-T1)

[0068] Wherein, K is the self-discharge rate of the battery after the second static treatment, OCV1 is the first open circuit voltage, OCV2 is the second open circuit voltage, T1 is the first preset time, and T2 is the second preset time.

[0069] In one embodiment, the preset state of charge is 1-5% state of charge.

[0070] Wherein, the greater the preset state of charge value, the longer the charging time, and the greater the influence of the battery voltage polarization process on the battery self-discharge performance detection; the smaller the preset state of charge value, the shorter the charging time, but too low state of charge, although the battery polarization is small, but will lead to the battery close to empty, and then adversely affect the battery performance storage.

[0071] In one embodiment, the battery capacity is 140 Ah, the battery self-discharge detection uses the method of steps S110 to S160, the preset temperature is 25℃, as shown in Figure 5 , Figure 5 is the battery charging curve, the horizontal coordinate is Cap, which represents the capacity of the battery, the unit is Ah, and the vertical coordinate is the battery voltage V0, according to Figure 5 further draw the DV / DQ curve (differential capacity curve) of the battery, as shown in Figure 6 , the horizontal coordinate is SOC (preset state of charge of the battery), and the vertical coordinate is the value of DV / DQ, in order to facilitate further analysis, further amplify the curve change of SOC from 0 to 7% in Figure 5 , as shown in Figure 7 Obviously, if the preset state of charge SOC value is too small, for example, 1% SOC is selected, it will lead to the battery close to empty, and then adversely affect the battery performance storage, if the preset state of charge SOC value is too large, for example, 7% SOC is selected, the charging time is too long, and the influence of the battery voltage polarization process on the battery self-discharge performance detection is greater. Combined with the actual situation, 3% SOC is selected, that is, the battery voltage is charged to 3.062V at this time, the voltage variation of the battery is small at this time, which can reduce the influence of the battery voltage polarization process on the battery self-discharge performance detection on the one hand, and on the other hand, it also takes into account the charging time and the influence on the battery performance, thereby improving the accuracy of the battery self-discharge detection as a whole.

[0072] In one embodiment, the preset state of charge is 3%, and the preset temperature is 25° C. For example, 16 batteries are used for testing, and a self-discharge test is performed on each battery according to the above steps S110 to S160 to calculate the self-discharge rate of each battery. The first horizontal row of Table 1 represents the first preset time period T1 and the second preset time period T2 (i.e., T1 and T2), with the unit being d (days). The first vertical column of Table 1 represents the self-discharge rate value (i.e., K value) of each battery. Ave represents the average K value, with the unit being mV. Obviously, considering the fluctuation stability of the standard deviation σ and the stability of the average K value Ave, the case where Ave-2.5σ is a negative value is first discarded; secondly, the case where the standard deviation σ is the smallest is sought. When σ=0.038, the case where the average K value Ave is smaller is further selected. In this case, the first preset time period T1 is 2 days, the second preset time period T2 is 3 days, and the average K value of each battery is 0.427.

[0073] Among them, the settings of the above-mentioned first preset time period and the second time period (the first preset time period T1 is 2 days, and the second preset time period T2 is 3 days) can fully ensure the release of the self-discharge voltage drop of each battery, so as to screen out abnormal self-discharge batteries, thereby improving the accuracy of the entire self-discharge test, and greatly reducing the storage space pressure of the entire battery during the self-discharge test.

[0074] Table 1

[0075]

[0076] That is, the above description is merely an embodiment of the present application and does not limit the patent scope of the present application. Any equivalent structural or equivalent process transformations made using the contents of the description and drawings of this application, such as the mutual combination of technical features between the various embodiments, or direct or indirect application in other related technical fields, are also included in the patent protection scope of the present application.

[0077] In addition, for structural elements with the same or similar characteristics, this application may use the same or different reference numerals to identify them. In addition, the terms "first" and "second" are used for descriptive purposes only and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more features. In the description of this application, the meaning of "multiple" is two or more, unless otherwise clearly and specifically defined.

[0078] In this application, the word "for example" is used to mean "serving as an example, instance, or illustration." Any embodiment described in this application as "for example" is not necessarily to be construed as preferred or advantageous over other embodiments. The above description is provided to enable any person skilled in the art to implement and use this application. In the above description, various details are listed for the purpose of explanation.

[0079] It should be understood that one of ordinary skill in the art will recognize that the present application can be practiced without using these specific details. In other embodiments, well-known structures and processes are not described in detail to avoid obscuring the description of the present application with unnecessary details. Therefore, the present application is not intended to be limited to the embodiments shown, but is intended to be consistent with the widest scope consistent with the principles and features disclosed in this application.

Claims

1. A method for testing battery self-discharge, characterized in that: include: Charging the battery after the capacity separation process until the battery capacity corresponding to the battery after the capacity separation process reaches a preset state of charge, wherein the preset state of charge is 1%-5% state of charge; performing a first resting treatment on the charged battery at a preset temperature for a first preset time period; measuring a first open circuit voltage corresponding to the battery after the first static treatment, and marking the corresponding time as a first preset time; performing a second rest treatment on the battery after the first rest treatment at a preset temperature for a second preset time period, wherein the second preset time period is longer than the first preset time period; measuring a second open circuit voltage corresponding to the battery after the second static treatment, and marking the corresponding time as a second preset time; Calculate the self-discharge rate of the battery after the second static treatment using the following formula based on the first preset time, the second preset time, the first open circuit voltage, and the second open circuit voltage; Wherein, K is the self-discharge rate of the battery after the second static treatment, OCV1 is the first open circuit voltage, OCV2 is the second open circuit voltage, T1 is the first preset time, and T2 is the second preset time; The self-discharge rate is compared with a preset self-discharge rate threshold to evaluate the self-discharge performance of the battery after the second rest treatment.

2. The testing method according to claim 1, wherein: The charging is constant current charging, the current of the constant current charging is 0.1C-0.8C, and C is the nominal rated capacity of the battery.

3. The testing method according to claim 1, wherein: The first preset time period is 0.5 to 5 days.

4. The testing method according to claim 3, wherein: The second preset time period is 1 to 5 days.

5. The testing method according to claim 4, characterized in that: The sum of the first preset time period and the second preset time period is less than or equal to 5 days.

6. The testing method according to claim 5, characterized in that: The first preset time period is 2 days, and the second preset time period is 3 days.

7. The testing method according to claim 1, wherein: The preset state of charge is 3%.

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