A method for secondary selection of self-discharge battery

Abstract

The application provides a self-discharge battery secondary selection method, relates to the technical field of lithium ion batteries, and comprises the following steps: selecting a battery qualified in a constant-temperature full-charge chemical voltage; placing the selected battery under preset conditions; measuring the voltage V1 of the battery; measuring the voltage V2 of the battery after constant-temperature standing; selecting a battery with a voltage exceeding the preselected range of delta V1 as a poor voltage drop battery according to delta V1 = V1-V2; charging the poor voltage drop battery to a preset state of charge after multiple cycle charging and discharging under preset conditions; measuring the voltage V3 of the battery after standing; measuring the voltage V4 of the battery after constant-temperature standing; and selecting a battery in the preselected range of delta V2 as a good product according to delta V2 = V3-V4. Under the premise that the self-discharge settlement standard is tightened, the poor voltage drop battery, especially the battery close to the upper limit of the good product, is selected for secondary self-discharge, so that the selection accuracy of the primary good product battery is ensured, the risk of poor voltage drop in the later stage is reduced, and complaints are reduced.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and in particular to a method for secondary selection of self-discharging batteries. Background Art

[0002] Lithium-ion batteries are widely used due to their high energy density, long cycle life, and lack of memory effect. With increasing environmental protection requirements, lithium-ion batteries are increasingly used in new energy vehicles. In automotive modules, batteries are combined in series and parallel, placing high demands on battery consistency. The main criteria for determining consistency are internal resistance, capacity, voltage, and self-discharge rate. The so-called self-discharge rate, also commonly referred to as the K value, is the rate of voltage drop of a battery under certain storage conditions of temperature and time. A significant self-discharge voltage drop in a single battery within a battery pack can affect the performance of the entire pack, making it crucial to accurately identify batteries with poor self-discharge.

[0003] Existing battery self-discharge selection methods have some defects. For example, it is easy to select out batteries with large primary chemical self-discharge and slightly high voltage drop. These batteries are mainly caused by delayed polarization or SEI stabilization formation, and their performance is no different from that of good batteries. Good batteries are mistakenly downgraded to defective products, causing losses; or the batteries are subjected to multiple cycle discharge selection, wasting production capacity and resulting in waste of resources. Summary of the Invention

[0004] The purpose of the embodiments of the present application is to provide a method for secondary selection of self-discharging batteries, so as to select batteries with abnormal primary chemical self-discharge but normal secondary self-discharge, thereby increasing the yield.

[0005] In one aspect of an embodiment of the present application, a method for secondary selection of self-discharge batteries is provided, comprising selecting batteries with qualified formation voltage during full-charge formation at room temperature; allowing the selected batteries to stand under preset conditions; measuring the battery voltage V1; testing the battery voltage V2 after standing at room temperature; selecting batteries exceeding a preselected range of △V1 as batteries with poor voltage drop based on △V1=V1-V2; charging and discharging the batteries with poor voltage drop multiple times under preset conditions to charge the batteries to a preset state of charge; measuring the voltage V3 after allowing the batteries to stand; measuring the voltage V4 after allowing the batteries to stand at room temperature; and selecting batteries within the preselected range of △V2 as good products based on △V2=V3-V4.

[0006] Optionally, selecting a battery with a qualified formation voltage during full-charge formation at room temperature includes: fully charging the battery at room temperature, charging with a constant voltage cutoff current of 0.05C, and measuring the voltage as V0 after standing at room temperature for 8 hours, and selecting the battery with a qualified formation voltage; wherein C is the battery capacity.

[0007] Optionally, allowing the selected battery to stand under preset conditions includes: allowing the battery to stand at a preset temperature of 45° C. for 72 hours, and then allowing the battery to stand at room temperature for 8 hours.

[0008] Optionally, testing the voltage V2 of the battery after standing at room temperature includes: standing at room temperature for 14 days.

[0009] Optionally, selecting batteries that exceed a preselected range of △V1 as batteries with poor voltage drop based on △V1=V1-V2 includes: selecting batteries within the preselected range of △V1 of μ-3σ≤△V1<μ+3σ as good products, and performing secondary selection on defective products within the range of μ+3σ≤△V'1<μ+6σ; wherein △V'1=V1-V2, μ is the mean voltage drop, and σ is the standard deviation of the voltage drop.

[0010] Optionally, after charging and discharging the battery with poor voltage drop for multiple cycles under preset conditions, charging the battery to a preset state of charge includes: fully charging the battery with poor voltage drop at a constant current and constant voltage of 0.3C to 1C with a cut-off current of 0.05C, and fully discharging at 0.5C to 1C, for 3 to 6 cycles; charging at a constant current of 0.5C to 1C to a preset state of charge; where C is the battery capacity.

[0011] Optionally, the preset state of charge is at least 50% state of charge.

[0012] Optionally, measuring the voltage V3 after leaving the battery to rest includes: the resting time is 24 hours.

[0013] Optionally, measuring the voltage V4 after the battery is left at room temperature includes: leaving the battery at room temperature for 3 to 7 days.

[0014] Optionally, selecting batteries within a preselected range of △V2 as good products based on △V2=V3-V4 includes: batteries within the preselected range of △V2 being μ-3σ≤△V2<μ+3σ are good products; wherein μ is the mean value of the secondary voltage drop, and σ is the standard deviation of the secondary voltage drop.

[0015] The method for secondary selection of self-discharge batteries provided in the embodiment of the present application includes selecting batteries with qualified formation voltage during full-charge formation at room temperature; placing the selected batteries at rest under preset conditions; measuring the battery voltage V1; testing the battery voltage V2 after placing the batteries at rest at room temperature; selecting batteries that exceed a preselected range of △V1 as batteries with poor voltage drop based on △V1=V1-V2; charging and discharging the batteries with poor voltage drop multiple times under preset conditions to a preset state of charge; measuring the voltage V3 after placing the batteries at rest; measuring the voltage V4 after placing the batteries at rest at room temperature; and selecting batteries within the preselected range of △V2 as good products based on △V2=V3-V4. Under the premise of tightening the primary self-discharge settlement standards, secondary self-discharge selection is performed on batteries with poor voltage drop, especially those close to the upper limit of good products. This ensures the accuracy of the selection of primary good batteries, reduces the risk of poor voltage drop in later grouping, and reduces customer complaints. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments of the present application. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without creative work.

[0017] Figure 1 This is a flow chart of the method for secondary selection of self-discharging batteries provided in this embodiment;

[0018] Figure 2 It is a self-discharge scatter diagram of the method for secondary selection of self-discharging batteries provided in this embodiment. DETAILED DESCRIPTION

[0019] The technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the drawings in the embodiments of the present application.

[0020] In the description of this application, it should be noted that the terms "inner" and "outer" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, or the orientations or positional relationships in which the product of this application is typically placed when in use. These terms are intended solely to facilitate the description of this application and simplify the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, be constructed, or operate in a specific orientation. Therefore, they should not be construed as limitations on this application. Furthermore, the terms "first" and "second" and the like are used solely for distinction and should not be construed as indicating or implying relative importance.

[0021] It should also be noted that, unless otherwise expressly specified or limited, the terms "disposed" and "connected" should be understood broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to direct connections, indirect connections through an intermediate medium, or internal connections between two components. Those skilled in the art will understand the specific meanings of these terms in this application based on the specific circumstances.

[0022] Compared with the existing technology, the embodiment of the present application provides a secondary selection method for self-discharge batteries, which can not only avoid the waste of misjudgment of chemical self-discharge, but also effectively pick out physical self-discharge. It is simple to operate and feasible on the production line. This method is especially important in the mass production process. After being put into use, it greatly reduces customer complaints about poor pressure difference.

[0023] For details, please refer to Figure 1 As shown, the method for secondary selection of self-discharging batteries provided in the embodiment of the present application includes:

[0024] S100: Select batteries with qualified formation voltage during full-charge formation at room temperature.

[0025] Self-discharge calculation, the battery is fully charged at room temperature, the last step of constant voltage charging cutoff current is 0.05C, and the voltage is measured as V0 after standing at room temperature for 8 hours, and the battery with qualified formation voltage is selected; where C is the battery capacity.

[0026] S110: The selected battery is left to stand under preset conditions.

[0027] The batteries with qualified formation voltage were placed at a preset temperature of 45°C for 72 hours, and then placed at room temperature for 8 hours.

[0028] S120: Measure the battery voltage V1.

[0029] The battery with qualified formation voltage is kept at 45℃ for 72 hours and then kept at room temperature for 8 hours, and the voltage is measured as V1.

[0030] S130: After standing at room temperature, the battery voltage V2 is tested.

[0031] The time for standing at room temperature is 14 days, and the voltage V2 is tested after standing at room temperature for 14 days.

[0032] S140: According to ΔV1=V1-V2, the batteries that exceed the preselected range of ΔV1 are selected as batteries with poor voltage drop.

[0033] Good products are selected within the pre-selected range of △V1, μ-3σ≤△V1<μ+3σ, and defective products within the range of μ+3σ≤△V'1<μ+6σ are subject to secondary selection; wherein, △V'1=V1-V2, μ is the mean value of the voltage drop, and σ is the standard deviation of the voltage drop.

[0034] Specific as Figure 2 As shown, Figure 2 is a self-discharge scatter plot, μ is the mean voltage drop, and σ is the standard deviation of the voltage drop. The self-discharge good product range is adjusted from μ-3σ≤△V1<μ+3.8σ to μ-3σ≤△V1<μ+3σ. Experimental results show that within the tight range of μ+3σ≤△V1<μ+3.8σ, there are indeed batteries with abnormal voltage drop (battery 1# in Example 2), proving the necessity of tight control. Secondary settlement mainly selects defective products within the range of μ+3σ≤△V'1<μ+6σ, where △V'1=V1-V2. Finally, a portion of primary chemical self-discharge abnormal batteries are screened out, proving the feasibility of secondary settlement.

[0035] S150: The battery with poor voltage drop is charged and discharged for multiple cycles under preset conditions, so as to charge the battery to a preset state of charge.

[0036] When selecting batteries with poor voltage drop, a control group can be selected as a reference to demonstrate the feasibility of this method. For example, the poor voltage drop batteries and the control group can be cycled simultaneously, using a constant current and voltage of 0.3C to 1C for full charge, a cutoff current of 0.05C, and a full discharge of 0.5C to 1C for three to six cycles. The batteries can then be charged at a constant current of 0.5C to 1C to a preset state of charge (SOC), where C represents the battery capacity. The preset SOC is at least 50%.

[0037] S160: After the battery is left to rest, the voltage V3 is measured.

[0038] The standing time is 24 hours, and the voltage measured after standing for 24 hours after the cycle is V3.

[0039] S170: After the battery is left at room temperature, measure the voltage V4.

[0040] Specifically, it is stored at room temperature for 3 to 7 days. The voltage measured during the 3 to 7 days of storage at room temperature is recorded as V4.

[0041] S180: According to ΔV2=V3-V4, the batteries within the preselected range of ΔV2 are selected as good products.

[0042] Good products are within the pre-selected range of △V2: μ-3σ≤△V2<μ+3σ; where μ is the mean value of the secondary pressure drop and σ is the standard deviation of the secondary pressure drop.

[0043] Self-discharge calculation, select good batteries △V2=V3-V4 according to the secondary voltage drop mean μ and secondary voltage drop standard deviation σ.

[0044] When comparing with the control group, pick out the batteries with no abnormal secondary voltage drop, and compare them with the control group to see if there is no abnormal secondary voltage drop.

[0045] In summary, the method for secondary selection of self-discharge batteries provided in the embodiment of the present application is to fully charge the battery with poor voltage drop at a constant current and constant voltage of 0.3C-1C with a cut-off current of 0.05C, and fully discharge at 0.5C~1C, and cycle for 3 to 6 weeks. Then charge it with constant current and constant voltage to a certain SOC (state of charge), measure the voltage V3 after standing for 24 hours, and then measure the voltage V4 at room temperature for 3 to 7 days. According to △V2=V3-V4, select according to the secondary voltage drop mean μ and the secondary voltage drop standard deviation σ. At the same time, ten (the specific number is not limited) batteries with no abnormal primary voltage drop can be used as a control group, and finally batteries with abnormal primary chemical self-discharge and no abnormal secondary self-discharge are selected. These batteries have no difference from good products in various tests and can be used as good products, which increases the yield rate and increases the efficiency and profit.

[0046] The method for secondary selection of self-discharge batteries provided in the embodiments of the present application, under the premise of tightening the primary self-discharge settlement standards, performs secondary self-discharge selection on batteries with poor voltage drop, especially batteries close to the upper limit of good products, so as to ensure the selection accuracy of primary good products, reduce the risk of poor voltage drop in later grouping, and reduce customer complaints; batteries with slightly high primary voltage drop are divided into two situations, one is caused by unstable chemical self-discharge caused by SEI film formation in the early stage, and the other is caused by slight physical self-discharge, which cannot be accurately selected by the primary self-discharge settlement. After secondary selection by the method for secondary selection of self-discharge batteries provided in the embodiments of the present application, not only is SEI formation stable, but physical self-discharge is also aggravated, making it easier to screen out; the voltage drop of the secondary selected good products is consistent with that of the control group, and there is no difference in performance test. The good batteries are disassembled in large quantities according to the voltage drop gradient without abnormal short circuit points, and the voltage drop after continued storage is consistent with that of the control group without abnormalities.

[0047] Therefore, the method for secondary selection of self-discharging batteries provided in the embodiments of this application is very effective and applicable for screening batteries with a voltage drop upper limit. After multiple experiments and disassembly, it is proven that batteries with a voltage drop upper limit do have two forms of self-discharge: high chemical self-discharge and slight physical self-discharge. If the selection criteria are relaxed at the outset, batteries with poor voltage drop are likely to appear later. If the criteria are tightened, batteries with high chemical self-discharge at the voltage drop upper limit will be wasted. Therefore, tightening the selection criteria at the outset and then using the method for secondary selection of self-discharging batteries provided in the embodiments of this application can greatly reduce customer complaints about poor voltage drop after shipment, while avoiding battery waste and effectively improving the competitiveness and efficiency of enterprises.

[0048] It should be noted that the method for secondary selection of self-discharging batteries provided in the embodiments of the present application is not limited to lithium-ion, sodium-ion and other plasma batteries; nor is it limited to cylindrical, aluminum-shell and soft-pack batteries.

[0049] The method for secondary selection of self-discharging batteries provided in the embodiments of the present application has the following self-discharging settlement standards: μ is the mean voltage drop, σ is the standard deviation of the voltage drop, and the range of qualified products is adjusted from μ-3σ≤△V1<μ+3.8σ to μ-3σ≤△V1<μ+3σ. The method for secondary selection of self-discharging batteries provided in the embodiments of the present application is mainly targeted at defective batteries with μ+3σ≤△V2<μ+6σ. The embodiments involved in the present application are all verified under this model system:

[0050] Example 1:

[0051] The batteries with poor self-discharge were charged to 4.2V at a constant current and constant voltage of 0.3C, with a cut-off current of 0.05C. They were then discharged to 2.75V at a constant current of 0.5C for 5 cycles. They were then charged at 0.5C for 60 minutes. After standing for 24 hours, the voltage was measured as V3. After the voltage test was completed, the batteries were stored at room temperature for 7 days and the voltage was measured again as V4. The mean and standard deviation were calculated using △V2=V3-V4 for settlement. A comparative test was conducted at the same time, with the same process. Batteries were selected and stored and disassembled according to the voltage drop gradient. The results are shown in Table 1.

[0052] Table 1 Pressure drop data results

[0053] Battery number 1# 2# 3# 4# 5# 6# 7# 8# 9# 10# One-time selected voltage drop / mV 5.8 6.2 6.2 6.4 6.8 7.2 7.3 8.0 5.2 5.3 Secondary selection voltage drop / mV 4.0 4.8 6.2 4.9 7.3 7.5 4.4 7.8 4.8 4.3 Storage 30 days voltage drop / mV 7.1 7.3 9.5 7.3 10.2 8.8 7.5 11.1 7.2 7.5 Storage 60 days voltage drop / mV 7.6 7.8 13.9 7.9 15.7 13.9 7.8 17.6 7.8 8.1 Storage 120 days voltage drop / mV 8.2 8.3 15.8 8.5 18.2 16.8 8.2 21.5 8.2 8.3

[0054] Example 2:

[0055] The batteries that failed self-discharge were charged to 4.2V at a constant current and constant voltage of 0.5C, with a cut-off current of 0.05C. They were then discharged to 2.75V at a constant current of 1C for three cycles. They were then charged at 1C for 42 minutes. After standing for 24 hours, the voltage was measured as V3. After the voltage test was completed, the batteries were stored at room temperature for 5 days and the voltage was measured again as V4. The mean and standard deviation were calculated using △V2=V3-V4 for settlement. A comparative test was conducted over the same period, with the same process. Batteries were selected and stored and disassembled according to the voltage drop gradient. The results are shown in Table 2.

[0056] Table 2 Pressure drop data results

[0057] Battery number 1# 2# 3# 4# 5# 6# 7# 8# 9# 10# One-time selected voltage drop / mV 6.2 6.5 6.9 7.3 7.6 8.0 8.3 8.8 4.8 5.0 Secondary selection voltage drop / mV 8.3 4.9 5.0 4.8 8.9 4.9 9.2 9.0 4.8 4.9 Storage 30 days voltage drop / mV 15.5 7.2 7.1 7.1 14.9 6.9 16.2 16.7 7.4 7.3 Storage 60 days voltage drop / mV 20.9 7.6 7.4 7.7 21.8 7.2 24.3 25.9 7.8 7.8 Storage 120 days voltage drop / mV 25.8 8.0 7.9 8.2 29.6 7.8 30.5 326 8.0 8.1

[0058] Example 3:

[0059] The batteries with poor self-discharge were charged to 4.2V at 0.5C constant current and constant voltage, with a cut-off current of 0.05C. They were then discharged to 2.75V at 1C constant current for 6 cycles. They were then charged at 0.8C for 45 minutes. After standing for 24 hours, the voltage was measured as V3. After the voltage test was completed, the batteries were stored at room temperature for 3 days and the voltage was measured again as V4. The mean and standard deviation were calculated using △V2=V3-V4 for settlement. A comparative test was conducted at the same time, with the same process. Batteries were selected and stored and disassembled according to the voltage drop gradient. The results are shown in Table 3.

[0060] Table 3 Pressure drop data results

[0061] Battery number 1# 2# 3# 4# 5# 6# 7# 8# 9# 10# One-time selected voltage drop / mV 5.8 6.2 6.7 7.2 7.6 8.2 8.8 9.4 4.4 4.8 Secondary selection voltage drop / mV 5.1 4.9 8.7 5.0 4.8 14.8 4.8 12.5 4.8 4.7 Storage 30 days voltage drop / mV 6.6 6.5 13.5 6.8 6.9 20.5 7.1 19.6 7.2 6.9 Storage 60 days voltage drop / mV 7.2 7.0 20.7 7.5 7.5 29.6 7.7 25.8 7.9 7.8 Storage 120 days voltage drop / mV 8.0 7.8 28.8 8.2 8.0 40.1 8.1 36.5 8.2 8.3

[0062] In each of the above examples, only representative batteries were identified for number comparison and analysis. Batteries 9# and 10# in each example served as the control group for good products. The following conclusions can be drawn from Tables 1, 2, and 3:

[0063] a. After the second self-discharge settlement, some batteries with a voltage drop below the upper limit of good products will be converted to good products, while the other part will still be defective.

[0064] b. There is no obvious pattern in comparing the primary voltage drop gradient with the secondary settlement, that is, the primary voltage drop is lower than the defective product in the range of μ+3σ≤△V1<μ+3.8σ before tightening. There is also a certain probability of abnormality in the secondary. It can be seen that the necessity of primary tightening and secondary screening can be seen. However, the primary voltage drop of the battery is large, and the secondary voltage drop is likely to be large;

[0065] c. After the secondary settlement, a large number of batteries were disassembled according to the voltage drop gradient: all good batteries showed no abnormalities, while batteries with large voltage drops had obvious short circuits. This shows that this method is effective in selecting batteries for secondary self-discharge, effectively screening out chemical self-discharge batteries with poor primary voltage drops while amplifying batteries with slight physical self-discharge. The fact that there was no difference between the secondary selected good batteries and the primary good batteries proves the feasibility of this method, increases production yield, and improves enterprise efficiency.

[0066] The above description is merely an embodiment of the present application and is not intended to limit the scope of protection of the present application. For those skilled in the art, various modifications and variations of the present application are possible. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. A method for secondary selection of self-discharging batteries, characterized in that: include: Select batteries with qualified formation voltage during full charge formation at room temperature; placing the selected battery at rest under preset conditions; Measuring the voltage V1 of the battery; After standing at room temperature, the voltage V2 of the battery is tested; According to ΔV1=V1-V2, the battery that exceeds the preselected range of ΔV1 is selected as a battery with poor voltage drop; After charging and discharging the battery with poor voltage drop for multiple cycles under preset conditions, the battery is charged to a preset state of charge; After the battery is left to stand, the voltage V3 is measured; The voltage V4 is measured after the battery is left to stand at room temperature; According to ΔV2=V3-V4, the battery within the preselected range of ΔV2 is selected as a good product; The selecting the battery outside the preselected range of ΔV1 as a battery with poor voltage drop according to ΔV1=V1-V2 includes: Good products are selected within the preselected range of ΔV1, μ-3σ≤ΔV1<μ+3σ, and defective products within the range of μ+3σ≤ΔV'1<μ+6σ are selected again; wherein ΔV'1=V1-V2, μ is the mean value of the voltage drop, and σ is the standard deviation of the voltage drop; The step of charging and discharging the battery with poor voltage drop under preset conditions for multiple cycles to charge the battery to a preset state of charge comprises: The battery with poor voltage drop is fully charged at a constant current and constant voltage of 0.3C to 1C, with a cut-off current of 0.05C, and fully discharged at 0.5C to 1C, for 3 to 6 weeks; Charge at a constant current of 0.5C to 1C to the preset state of charge; where C is the battery capacity.

2. The method for secondary selection of self-discharging batteries according to claim 1, characterized in that: The battery selected in the battery room temperature full-charge formation process with a qualified formation voltage includes: The battery is fully charged at room temperature, the constant voltage charging cut-off current is 0.05C, and the voltage is measured to be V0 after standing at room temperature for 8 hours. The battery with qualified formation voltage is selected; where C is the battery capacity.

3. The method for secondary selection of self-discharging batteries according to claim 1, characterized in that: The step of placing the selected battery at rest under preset conditions comprises: After standing at the preset temperature of 45℃ for 72 hours, transfer to room temperature and stand for 8 hours.

4. The method for secondary selection of self-discharging batteries according to claim 1, characterized in that: The voltage V2 of the battery after standing at room temperature is tested as follows: The standing time at room temperature is 14 days.

5. The method for secondary selection of self-discharging batteries according to claim 1, characterized in that: The preset state of charge is at least 50% state of charge.

6. The method for secondary selection of self-discharging batteries according to claim 1, characterized in that: The measuring the voltage V3 after the battery is left at rest comprises: The standing time is 24 hours.

7. The method for secondary selection of self-discharging batteries according to claim 1, characterized in that: The voltage V4 measured after the battery is left at room temperature includes: The storage time at room temperature is 3 to 7 days.

8. The method for secondary selection of self-discharging batteries according to claim 1, characterized in that: The selecting of the battery within the preselected range of ΔV2 as a good product according to ΔV2=V3-V4 includes: Good products are within the preselected range of ΔV2, μ-3σ≤ΔV2<μ+3σ; wherein μ is the mean value of the secondary pressure drop, and σ is the standard deviation of the secondary pressure drop.

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