A lithium ion battery capacity screening method

Abstract

The application discloses a lithium ion battery capacity grading and screening method, and relates to the field of lithium ion batteries, and comprises the following steps: S1, taking lithium batteries in the same capacity grading frame and standing; S2, discharging to 0% SOC at a constant current of 0.7C-1C, standing for more than 3 min, calculating the dispersion value sigma (standard deviation) and the median difference S, and excluding abnormal batteries; S3, charging the remaining batteries to 100% SOC at a constant current of 0.1C-0.7C, standing, and then calculating the sigma and S to exclude abnormalities; S4, saving the batteries meeting the standards. Then, the capacity can be measured by constant current and constant voltage charging, the internal resistance is tested after charging to 17% SOC, the batteries are stood for 3 days, the K value is calculated, and further screening is performed. The method utilizes the standing voltage rebound characteristics of full / empty batteries, accurately removes defective products such as tab abnormalities, and improves the screening efficiency and precision.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion batteries, and more specifically to a method for capacity grading and screening lithium-ion batteries. Background Technology

[0002] Lithium-ion batteries have become the most important rechargeable battery technology today due to their high energy density, long cycle life, and lightweight properties. Currently, the main technologies for defect screening of lithium-ion batteries during the capacity grading process include:

[0003] K-value (self-discharge rate) is a key parameter for measuring the self-discharge performance of lithium-ion batteries. It is usually expressed as the rate of voltage drop or capacity loss after the battery has been left to stand for a period of time. A K-value that is too high means that the battery is severely self-discharged. It can effectively eliminate defective products with excessive self-discharge, but it cannot eliminate 100% of them and its timeliness is too low.

[0004] Internal resistance selection involves screening out defective cells by testing the AC and DC internal resistance of lithium-ion batteries. Rigorous internal resistance testing and selection can effectively improve the consistency and reliability of battery packs. However, internal resistance can change and vary due to material instability, leading to a reduction in interception effect.

[0005] When grading lithium-ion batteries, the testing environment for all lithium batteries in the same grading frame should be consistent, and the voltage of lithium-ion batteries at different SOC (State of Charge) states should satisfy the charge-discharge curve relationship. Thus, it can be concluded that the theoretical voltage and the measured voltage have the greatest curvature change when fully charged and depleted. For defective products caused by structural abnormalities such as poor welding or tearing of the internal tabs of the cell, under the same testing environment, the voltage of good lithium-ion batteries should fluctuate within a certain range, while the voltage difference of defective lithium batteries should not be within the range. Summary of the Invention

[0006] Based on the above analysis, the present invention provides the following specific implementation scheme: a lithium-ion battery capacity assessment and screening method, characterized by comprising the following steps:

[0007] S1. Take a group of lithium batteries from the same capacity frame and let them stand.

[0008] S2. Discharge the lithium battery with a constant current, then let the lithium battery stand still, calculate the discrete value σ and the median difference S of the lithium battery after standing still, and exclude the lithium batteries whose cell voltage exceeds the discrete value σ and the median difference S.

[0009] S3. Charge the remaining lithium battery with a constant current, then let the remaining lithium battery stand still, let it stand still again, and then calculate the discrete value σ and the median difference S of the remaining lithium battery again, and exclude the lithium battery whose cell voltage exceeds the discrete value σ and the median difference S.

[0010] S4. Store the remaining compliant lithium batteries.

[0011] This invention utilizes the voltage rebound during capacity grading for comparison and screening. When fully charged, the voltage reaches the theoretical voltage of 3.65V and enters a resting state; the voltage will collectively decrease, but cells with abnormal tabs will still have a high voltage state. When fully discharged, the voltage reaches the theoretical voltage of 2.5V and enters a resting state; the voltage will collectively rise, and cells with abnormal tabs will show a significant rise in voltage, remaining in a high voltage state, thus screening out defective lithium batteries. This screening method involves taking a group of lithium batteries from the same capacity grading frame, subjecting them to resting, constant current discharge, calculating the difference between the discrete value σ and the median S to eliminate abnormal batteries, recalculating after charging and resting to eliminate more, and finally saving the batteries that meet the standards. This complete process significantly improves the screening efficiency and accuracy of the lithium-ion battery capacity grading process.

[0012] Preferably, the settling time is more than 3 minutes each time.

[0013] During the capacity grading process, after a lithium battery is discharged or charged, its voltage slowly stabilizes due to polarization and diffusion. If the settling time is insufficient, the voltage reading is prone to fluctuation, leading to errors in the calculation of the discrete value σ and the median difference S, resulting in the incorrect rejection of high-quality batteries or the retention of inferior ones. Setting a settling time of more than 3 minutes ensures that the open circuit voltage reaches a stable state, thereby obtaining accurate self-discharge characteristics and internal resistance information, and improving the reliability of the screening process.

[0014] Preferably, the discharge uses a current rate of 0.7C to 1C to avoid reducing cycle life, and the charging uses a current rate of 0.1C to 0.7C to avoid lithium plating and thermal runaway caused by excessive temperature.

[0015] The first discharge accelerates the polarization effect with a current of 0.7C or higher, making the voltage dispersion more obvious. This makes it easier to accurately eliminate batteries with uneven performance by using the dispersion value σ and the median difference S.

[0016] Preferably, the discharge discharges the lithium battery cell to 0% SOC (state of charge), and the charging charges the lithium battery to 100% SOC.

[0017] By utilizing the rebound of cell voltage during capacity testing, the cell voltage can be fully tested by discharging to 0% SOC and charging to 100% SOC, thus improving the comprehensiveness and reliability of the screening method.

[0018] Preferably, the formula for calculating the difference between the discrete value σ and the median S is as follows:

[0019]

[0020]

[0021]

[0022]

[0023] Where Vmax represents the maximum voltage of the lithium battery group, and S is the median voltage difference of the lithium battery group. This indicates the median voltage of the lithium battery pack when the total number of cells is odd. When the total number of battery cells is even , This represents the standard deviation of the voltage of this group of lithium batteries. Here, is the average voltage of the lithium batteries in this group, and n is the total number of cells. This represents the number of battery cells.

[0024] The discrete value σ and median difference S of lithium battery cells are calculated by formula, with the two indicators complementing each other: σ captures group fluctuations, S targets high voltage anomalies, the median algorithm resists single-point contamination, and odd-even number processing ensures fairness.

[0025] Preferably, after step S3, the remaining lithium battery can be charged again using constant current and constant voltage to obtain a more accurate battery capacity.

[0026] This supplementary step enhances the screening depth, extending from voltage consistency to capacity accuracy. After initial screening, batteries may have inaccurate capacity claims. Constant current and constant voltage simulation of standard capacity testing eliminates polarization effects, yielding the approved capacity value.

[0027] Preferably, the constant current and constant voltage charging voltage limit is 3.65V.

[0028] 3.65V is a typical stable platform for lithium batteries, avoiding overvoltage lithium plating and ensuring accurate capacity readings.

[0029] Preferably, after the lithium battery is charged by constant current and constant voltage, it is allowed to stand for a period of time, then discharged, allowed to stand again, and then charged to 17% SOC for calculating the K value.

[0030] The cell exhibits a significant voltage curvature at 17% SOC. The voltage at 17% SOC is used to simulate storage scenarios, and the K-value quantifies the voltage decay rate.

[0031] Preferably, an internal resistance tester is used to test the internal resistance of the lithium battery after it has been charged to 17% SOC, and lithium batteries that do not meet the internal resistance standard are excluded.

[0032] This step targets dynamic impedance anomalies, with the 17% SOC internal resistance being the most sensitive. Lithium batteries that do not meet the internal resistance standard are eliminated using an internal resistance tester.

[0033] Preferably, the remaining lithium batteries after passing through the internal resistance tester are left to stand for more than 3 days, and the voltage change after standing is calculated to obtain the K value. Lithium batteries that do not meet the K value standard are excluded.

[0034] After internal resistance testing, the batteries were left to stand for more than 3 days. The voltage change was calculated to obtain the K value, and batteries that did not meet the standards were excluded. This long-term standing test verified the self-discharge stability and simulated a storage / transportation scenario. The 3-day standing exposure resulted in slow degradation.

[0035] The beneficial effects of this invention are as follows:

[0036] (1) By calculating the discrete value σ and the median difference S of the voltage of lithium batteries after standing in the same capacity frame as a threshold, batteries with abnormal voltage caused by poor electrode welding or tearing are effectively eliminated. This overcomes the limitations of K value not being able to eliminate 100% and internal resistance being easy to vary, thus improving the screening accuracy.

[0037] (2) Discharge to 0% SOC at a rate of 0.7C~1C, charge to 100% SOC at a rate of 0.1C~0.7C, and let stand for more than 3 minutes at the point of maximum curvature change to determine the dispersion. The detection cycle is short, avoiding the inefficiency of traditional K-value testing which requires standing for several days. It is suitable for large-scale capacity testing processes.

[0038] (3) Combining the subsequent constant current and constant voltage charging capacity verification, internal resistance test and 3-day K value calculation multiple screening, the remaining battery voltage is stable and highly consistent, significantly reducing the risk of self-discharge and abnormal internal resistance, and improving the overall performance of the battery pack. Attached Figure Description

[0039] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0040] Figure 1 This is a flowchart from the present invention;

[0041] Figure 2 This is a graph showing the relationship between voltage and SOC in this invention;

[0042] Figure 3 This is a graph showing the relationship between the voltage of the lithium battery when it is fully charged and the number of battery cells in this invention.

[0043] Figure 4 This is a graph showing the relationship between the voltage of the lithium battery in its empty state and the number of cells in this invention. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0045] like Figure 1 As shown, the present invention provides a method for capacity screening of lithium-ion batteries.

[0046] During capacity testing of lithium-ion batteries, the testing environment for all lithium batteries within the same capacity testing frame should be consistent, and as follows: Figure 2 As shown, the voltage of a lithium-ion battery under different SOC states satisfies the charge-discharge curve relationship, where 1 and 3 represent the measured voltage and 2 represents the theoretical voltage. Both the measured voltage and the theoretical voltage show the greatest curvature change under fully charged and empty states. Under the same test environment, the voltage of a good lithium-ion battery should fluctuate within a certain range, while the voltage of a defective battery should differ significantly.

[0047] S1. Take a group of 32 lithium batteries from the same capacity frame and let the group of lithium batteries stand still so that the normal cells can be restored to the theoretical voltage state. The standing time is ≥3 minutes.

[0048] This step selects a group of lithium batteries within the same capacity testing frame to ensure a consistent initial testing environment. These batteries are then left to stand for at least 3 minutes. After use or transportation, lithium batteries may exhibit surface polarization (concentration polarization, electrode polarization), causing the voltage to deviate from the theoretical value. Standing allows ion diffusion to reach equilibrium and polarization to subside, restoring the voltage of normal cells to their theoretical open-circuit voltage. Defective cells (such as those with electrolyte leakage or deteriorated active materials) cannot fully recover. By eliminating initial voltage drift interference, a benchmark is provided for subsequent discharge testing, ensuring the reliability of the screening data, avoiding misjudging normal batteries as defective products, and improving overall screening sensitivity.

[0049] S2. Perform constant current discharge on the lithium batteries within the same capacity frame at a rate of 0.7C or higher until the cells reach 0% SOC, at which point the voltage curvature change is greatest. Afterward, allow the lithium batteries to stand still and calculate the difference between the discrete value σ and the median value S of the lithium batteries after standing still. The specific formula is as follows:

[0050]

[0051]

[0052]

[0053]

[0054] Where Vmax represents the maximum voltage of the lithium battery group, and S is the median voltage difference of the lithium battery group. This indicates the median voltage of the lithium battery pack when the total number of cells is odd. When the total number of battery cells is even , This represents the standard deviation of the voltage of this group of lithium batteries. Here, is the average voltage of the lithium batteries in this group, and n is the total number of cells. This refers to the number of battery cells. For example... Figure 3 As shown, lithium batteries whose cell voltage exceeds the difference between the discrete value σ and the median value S are excluded.

[0055] At 0% SOC, the battery voltage plateau ends, entering a sharp drop region with the greatest voltage curvature, making it most sensitive to defects such as internal resistance and capacity unevenness. High-rate discharge amplifies defect manifestations (such as a sharp voltage drop in defective batteries). After resting, dynamic polarization is eliminated, accurately reflecting static dispersion. σ measures overall fluctuation, while the S value is more robust to extreme values, preventing a few abnormal batteries from dominating the judgment. By accurately identifying hidden defects at low SOC, the load on subsequent steps is reduced, the yield of capacity testing is improved, and energy waste is reduced.

[0056] S3. Let the remaining lithium batteries in the group rest for at least 3 minutes. Then charge them at a rate of 0.1~0.7C to 100% SOC. Let them rest again, and then calculate the discrete value σ and the median difference S of the remaining lithium batteries in the group again. Figure 4 As shown, lithium batteries whose cell voltage exceeds the difference between the discrete value σ and the median value S are excluded.

[0057] At 100% SOC, the voltage curve enters a plateau rising region, and the voltage change curvature is also at its maximum. High-rate charging amplifies cathode material defects (such as lithium dendrite formation or overcharge risk). Resting to recover OCV, combined with S2 low SOC data, forms a "dual-end" verification, avoiding omissions in single-sided testing (such as missing high SOC defects during discharge). Secondary calculation of discrete indicators further refines battery pack consistency. Through bidirectional screening, defective products are cumulatively eliminated, ensuring battery stability across the entire SOC range, significantly improving pack consistency and cycle life, and reducing later capacity decay complaints.

[0058] S4. Sort the remaining standard-compliant lithium batteries into different grades.

[0059] In one example, after the second calculation of the difference between the discrete value σ and the median S, the voltage is limited to 3.65V, and the remaining lithium battery cells are charged to 100% SOC. This step aims to obtain a more accurate representation of the lithium battery's capacity. Afterward, the remaining lithium battery is left to rest for at least 3 minutes, then subjected to constant current discharge. After resting, it is recharged to 17% SOC. Because the voltage change curvature of the cell is relatively large at 17%, the voltage at 17% SOC is used in the calculation of the K value.

[0060] Then, an internal resistance tester is used to test the internal resistance. Lithium batteries that do not meet the requirements are discharged. The remaining lithium batteries are then left to stand for ≥3 days. The K value is calculated based on the voltage change. Lithium batteries that do not meet the K value requirements are discharged.

[0061] In a preferred embodiment of the invention, after step S3, the remaining battery is charged to 100% SOC with a voltage limit of 3.65V (to avoid overcharging damage) to calibrate the true capacity benchmark; after resting for ≥3 minutes, it is discharged to a low SOC with constant current, rested, and then charged to 17% SOC (voltage curvature peak region, sensitive to self-discharge and aging); the 17% SOC voltage is collected to calculate the K value (K=ΔV / Δt, self-discharge coefficient). Subsequently, the internal resistance is measured using an internal resistance tester (AC1kHz) to eliminate abnormalities; finally, after resting for ≥3 days, the voltage decay is monitored to accurately calculate the K value, and unqualified products are eliminated.

[0062] Voltage-limited charging ensures safe capacity calibration; a large 17% SOC curvature (high dV / dSOC) amplifies minute defects; long-term static storage simulates storage degradation, and the K-value quantifies the self-discharge rate (for defective batteries, K>th threshold is due to SEI film instability); internal resistance testing captures contact / material defects. Through comprehensive verification of capacity, internal resistance, and stability, the long-term reliability of the battery is significantly improved, making it suitable for high-end applications. Overall capacity rating efficiency is increased by 15%, resulting in significant economic benefits.

[0063] The prior description of this disclosure is provided to enable any person skilled in the art to make or use this disclosure. Various modifications to this disclosure will be apparent to those skilled in the art, and the general principles defined herein may be applied to other variations without departing from the spirit or scope of this disclosure. Therefore, this disclosure is not intended to be limited to the examples and designs described herein, but should be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0064] The embodiments of the present invention have been described in detail above. The description of the embodiments above is only for the purpose of helping to understand the method and core ideas of the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for capacity grading and screening lithium-ion batteries, characterized in that, Includes the following steps: S1. Take a group of lithium batteries from the same capacity frame and let them stand. S2. Discharge the lithium battery with a constant current, then let the lithium battery stand still, calculate the discrete value σ and the median difference S of the lithium battery after standing still, and exclude the lithium batteries whose cell voltage exceeds the discrete value σ and the median difference S. S3. Charge the remaining lithium battery with a constant current, then let the remaining lithium battery stand still, let it stand still again, and then calculate the discrete value σ and the median difference S of the remaining lithium battery again, and exclude the lithium battery whose cell voltage exceeds the discrete value σ and the median difference S. S4. Store the remaining compliant lithium batteries.

2. The screening method according to claim 1, characterized in that, Each settling time was more than 3 minutes.

3. The screening method according to claim 1, characterized in that, The discharge uses a current rate of 0.7C to 1C to avoid reducing cycle life, and the charging uses a current rate of 0.1C to 0.7C to avoid lithium plating and thermal runaway caused by excessive temperature.

4. The screening method according to claim 1, characterized in that, The discharge process discharges the lithium battery cell to 0% SOC (state of charge), and the charging process charges the lithium battery to 100% SOC.

5. The screening method according to claim 1, characterized in that, The formula for calculating the difference between the discrete value σ and the median S is as follows: ； Where Vmax represents the maximum voltage of the lithium battery group, and S is the median voltage difference of the lithium battery group. This indicates the median voltage of the lithium battery pack when the total number of cells is odd. When the total number of battery cells is even , This represents the standard deviation of the voltage of this group of lithium batteries. Here, is the average voltage of the lithium batteries in this group, and n is the total number of cells. This represents the number of battery cells.

6. The screening method according to claim 1, characterized in that, After step S3, the remaining lithium battery can be charged again using constant current and constant voltage to obtain a more accurate battery capacity.

7. The screening method according to claim 6, characterized in that, The constant current and constant voltage charging voltage limit is 3.65V.

8. The screening method according to claim 6, characterized in that, After constant current and constant voltage charging, the lithium battery is left to stand for discharge, then left to stand again, and then charged to 17% SOC for calculating the K value.

9. The screening method according to claim 8, characterized in that, The internal resistance of the lithium battery after charging to 17% SOC is tested using an internal resistance tester, and lithium batteries that do not meet the internal resistance standard are excluded.

10. The screening method according to claim 9, characterized in that, The remaining lithium batteries after passing through the internal resistance tester are left to stand for more than 3 days. The voltage change after standing is calculated to obtain the K value. Lithium batteries that do not meet the K value standard are excluded.

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