Lithium ion battery grouping method

Abstract

The invention discloses a lithium ion battery matching method, and belongs to the technical field of lithium ion battery matching. The method comprises the following steps: in a constant-temperature environment, performing a series of specific charging, discharging and standing steps on the formed and aged lithium ion battery, including low-rate charging to a specific SOC, high-rate charging for a specific duration, low-rate discharging to a cut-off voltage, low-rate charging again, standing and the like, and collecting a plurality of key voltages V1 to V9 in the process of charging to a specific SOC, high-rate charging for a specific duration, low-rate discharging to a cut-off voltage, low-rate charging again, standing and the like. And by analyzing the voltage values or the difference values thereof, carrying out consistent gear division on the batteries, and finally grouping the batteries at the same gear into a battery pack. According to the method, the batteries with consistent capacity, internal resistance and self-discharge characteristics can be efficiently and quickly screened at low cost without complete full-charge and full-discharge capacity grading test and independent measurement of the internal resistance and the K value, and the overall consistency and the production efficiency of the battery pack are remarkably improved.

Description

Technical Field

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

[0002] Lithium-ion batteries, as a clean energy source, have been widely used in electric vehicles, energy storage systems, and other fields. Due to limitations in manufacturing processes, the capacity and voltage of a single lithium-ion battery are limited, and they typically need to be combined in series and parallel to form battery modules or battery packs to meet application requirements. When used in a group, the overall performance and lifespan of the battery pack are greatly affected by the consistency of the individual battery cells within it. If there are differences in parameters such as capacity, internal resistance, and self-discharge rate between battery cells, some batteries will be overcharged or over-discharged during charging and discharging, which not only reduces the effective capacity of the entire pack but also poses serious safety hazards.

[0003] Currently, the industry commonly uses a "capacity grading" method to screen for consistent batteries. This involves performing a complete charge-discharge cycle on each battery to accurately measure its capacity, and then grouping them according to the capacity value. While this method is effective, it has significant drawbacks: a complete charge-discharge test is time-consuming, consumes a large amount of electrical energy, and requires expensive capacity grading equipment, resulting in high production costs and low efficiency.

[0004] In addition, to ensure consistency, some solutions will also test the battery’s internal resistance (DCR, Direct Current Resistance) and self-discharge rate (K value), which further increases the complexity and time cost of the testing process. Summary of the Invention

[0005] The purpose of this invention is to provide a method and apparatus for matching lithium-ion batteries, so as to solve the problems of low efficiency and high cost of traditional capacity matching methods.

[0006] According to a lithium-ion battery packing method of the present invention, under constant temperature environment, for lithium-ion batteries after formation and aging, the following operations are performed: S1, charging the lithium-ion battery to a first SOC level at a first charging rate, and recording the charging start voltage V1 and end voltage V2; S2, letting the lithium-ion battery rest for a first duration, and then charging the lithium-ion battery for a second duration at a second charging rate, and recording the charging start voltage V3 and end voltage V4; S3, letting the lithium-ion battery rest for a third duration, and then discharging the lithium-ion battery to a specific cutoff voltage at a first discharging rate; S4, letting the lithium-ion battery rest for a fourth duration, recording the resting end voltage V5, and then charging the lithium-ion battery at a third charging rate. S5. Charge the lithium-ion battery to the second SOC level and record the charging end voltage V6; S6. Let the lithium-ion battery rest for five hours and record the rest end voltage V7. Then, let the lithium-ion battery that has completed the above charging and discharging steps rest and record the voltage V8 after the first day of rest and the voltage V9 after the seventh day of rest; S7. Based on at least one of the voltages V1 to V9, or the voltage difference calculated from any two of the voltages V1 to V9, classify the lithium-ion batteries into different grades and group the lithium-ion batteries classified into the same grade into a lithium-ion battery pack; wherein, the first charging rate, the third charging rate and the first discharging rate are all less than the second charging rate.

[0007] As a more specific implementation, the first charging rate in step S1 is 0.33C-0.5C.

[0008] As a preferred embodiment, the first charging rate in step S1, the first discharging rate in step S3, and the third charging rate in step S4 are the same.

[0009] As a preferred embodiment, the first SOC level is 20% to 50% SOC.

[0010] As a preferred embodiment, the second SOC level in step S4 is 20% SOC to 50% SOC.

[0011] As a preferred embodiment, the second charging rate in step S2 is 1C to 3C.

[0012] As a more specific embodiment, the second duration is 20 to 60 seconds, the first and fourth durations are 1 to 5 minutes, the third duration is 1 to 10 minutes, and the fifth duration is 3 to 10 minutes.

[0013] As a more specific embodiment, the specific cutoff voltage in step S3 is associated with the battery material type and ranges from 2.0V to 3.2V.

[0014] As a preferred embodiment, the temperature of the constant temperature environment is 20°C to 30°C.

[0015] As a more specific solution, the classification of lithium-ion batteries in step S6 further includes at least one of the following operations: classifying based on the difference between V2 and V1; classifying based on the difference between V4 and V3; classifying based on V5; classifying based on V6; and classifying based on the ratio of the difference between V8 and V9 to the resting time.

[0016] The beneficial effects of this application are as follows: 1. This invention achieves battery consistency classification by collecting data on partial charge / discharge, rest, and voltages V1 to V9, skipping the traditional full charge / discharge capacity testing, significantly shortening testing time, reducing power consumption, and lowering equipment investment and maintenance costs.

[0017] 2. This invention uses a low charging rate of 0.33C–0.5C in the 20%–50% SOC range to reduce the impact of polarization and ensure that the voltages V1, V2, and V6 are true and reliable, providing a basis for accurate grading. The selection of this SOC range in this invention has sufficient voltage-SOC sensitivity to distinguish capacity differences, while avoiding the problems of large polarization and many side reactions in the low SOC range, thus balancing test accuracy and operational stability.

[0018] 3. This invention uses 1C–3C high-rate short-time charging combined with static storage to amplify and identify batteries with abnormal polarization internal resistance, replacing traditional internal resistance testers and simplifying the process.

[0019] 4. This invention sets the discharge cutoff voltage to 2.0V–3.2V, covering mainstream battery materials, without requiring adjustments to the process due to materials; at the same time, the rebound voltage V5 is used to evaluate the battery interface state and ohmic internal resistance.

[0020] 5. This invention selects a constant temperature of 20℃–30℃ for testing, controlling the voltage error caused by temperature to within 5mV, ensuring data consistency. At the same time, this temperature range is easy to achieve and suitable for mass production.

[0021] 6. This invention indirectly evaluates battery capacity by using the difference between V2 and V1 and V6, replacing the traditional capacity assessment method; it quickly screens polarization internal resistance by using the difference between V4 and V3; it evaluates ohmic internal resistance and interface state by using V5; and it evaluates self-discharge rate by using the ratio of the difference between V8 and V9 to the resting time, replacing the traditional K-value test; thus, it achieves comprehensive and rapid screening of capacity, internal resistance, and self-discharge, improving the consistency of the entire battery pack. Attached Figure Description

[0022] Figure 1 This is a flowchart of the lithium-ion battery packing method of the present invention.

[0023] Figure 2 This is a simplified structural diagram of the battery pack of the present invention.

[0024] Figure 3 This is a comparison chart of the discharge capacity of the entire battery pack by voltage group and capacity group.

[0025] Figure 4 This is a comparison chart of the DCR of the entire package for voltage and capacity groups.

[0026] Figure 5 This is a comparison chart of the voltage group and capacity group at the end of the full-pack charging process.

[0027] Figure 6 This is a comparison chart of the voltage group and capacity group at the end of the full-pack discharge. Detailed Implementation

[0028] This invention provides a method for grouping lithium-ion batteries, comprising the following steps: Under constant temperature conditions, for lithium-ion batteries after formation and aging: S1, charge the lithium-ion batteries to a first SOC level at a first charging rate, and record the charging start voltage V1 and end voltage V2; S2, allow the lithium-ion batteries to rest for a first duration, then charge them at a second charging rate for a second duration, recording the charging start voltage V3 and end voltage V4; S3, allow the lithium-ion batteries to rest for a third duration, then discharge them at a first discharging rate to a specific cutoff voltage; S4, allow the lithium-ion batteries to rest for a fourth duration, record the resting end voltage V5, and then charge them at a third charging rate... S5. Charge the lithium-ion battery to the second SOC level and record the charging end voltage V6; S6. Let the lithium-ion battery rest for five hours and record the rest end voltage V7. Then, let the lithium-ion battery that has completed the above charging and discharging steps rest and record the voltage V8 after the first day of rest and the voltage V9 after the seventh day of rest; S7. Based on at least one of the voltages V1 to V9, or the voltage difference calculated from any two of the voltages V1 to V9, classify the lithium-ion batteries into different grades and group the lithium-ion batteries classified into the same grade into a lithium-ion battery pack; wherein, the first charging rate, the third charging rate and the first discharging rate are all less than the second charging rate.

[0029] The method of this invention only includes partial charging, short-term high-rate charging, and low-rate discharging to the cutoff voltage, completely skipping the lengthy full charge and full discharge process required for accurate capacity measurement in traditional methods. This greatly shortens the testing and grouping time for a single battery, and because a complete charge-discharge cycle is avoided, the electrical energy consumed in the entire process is significantly reduced, directly lowering production costs. At the same time, it reduces the power, capacity, and energy consumption requirements of the testing equipment, shortens equipment occupancy time, and reduces the high investment and maintenance costs of capacity testing equipment.

[0030] As those skilled in the art know, the relationship between voltage and SOC of lithium-ion batteries is not linear, but rather has plateau and non-plateau regions. In the high SOC range (e.g., 50%-90% SOC), most batteries (e.g., lithium iron phosphate, ternary lithium) are in a voltage plateau region, where voltage changes slowly with SOC. For example, for lithium iron phosphate batteries around 3.2V, a 20% change in SOC may only correspond to a 0.1V voltage change. At this point, voltage is not sensitive to capacity, making it difficult to distinguish capacity deviations through voltage differences. However, in the low SOC range (e.g., ≤10% SOC), the battery has left the plateau region and entered a phase of rapid voltage decline. The slope of voltage change with SOC increases significantly. For example, for lithium iron phosphate batteries, a 5% change in SOC from 10% to 5% may result in a voltage drop from 2.8V to 2.0V, a change of 0.8V. At this point, even a small difference of 1%-2% in SOC can lead to a significant voltage difference of tens or even hundreds of mV. In other words, capacity differences are amplified by voltage differences, which is the meaning of "high sensitivity."

[0031] However, while the low SOC range (below 10%) offers higher sensitivity, it presents two critical issues: Firstly, in this range, the battery's internal resistance increases with decreasing charge, and the polarization voltage accounts for a higher proportion during charging and discharging, leading to poor voltage data stability. Secondly, near 0% SOC, side reactions such as lithium plating and electrolyte decomposition may occur, affecting test repeatability and accelerating battery aging under long-term cycling. The 20%-50% SOC range, on the other hand, represents the battery's chemically stable range, characterized by low internal resistance and minimal polarization. Its voltage changes during charging and discharging more closely approximate the actual open-circuit voltage, resulting in high data repeatability and making it suitable as a benchmark range for batch screening.

[0032] Furthermore, the 20%-50% SOC range falls within the non-voltage plateau region of lithium-ion batteries. While the slope of voltage change with SOC is not as steep as in the range below 10% SOC, it is still sufficiently significant. For example, the voltage change rate of lithium iron phosphate in this range is approximately 0.01-0.02V / %SOC, meaning a 1% change in SOC corresponds to a 10-20mV voltage change. This sensitivity is sufficient to distinguish the required capacity consistency for battery packing; a 1% capacity difference can be converted into a 10-20mV voltage difference, which can be accurately identified using conventional voltage acquisition equipment (accuracy ≥1mV), meeting the screening requirements. Therefore, this invention selects the 20%-50% SOC range to measure voltage and determine capacity. This is an optimization based on the stability and operability requirements of the battery packing scenario. This range provides sufficient sensitivity to distinguish capacity differences while ensuring the stability and safety of the testing process, making it the optimal choice for balancing accuracy and efficiency.

[0033] In an embodiment of the present invention, the first charging rate in step S1 is 0.33C-0.5C, and the first SOC level is 20% SOC to 50% SOC. The second SOC level in step S4 is also 20% SOC to 50% SOC. This is because using a low charging rate of 0.33C-0.5C within the voltage-sensitive range of 20%-50% SOC minimizes the impact of battery polarization on voltage sampling accuracy, ensuring the authenticity and reliability of the collected voltage data V1, V2, and V6. This lays a solid foundation for subsequent accurate grading. Simultaneously, controlling the SOC within a moderate range avoids full charging or deep discharging, significantly shortening testing time and saving energy.

[0034] In embodiments of the present invention, the first charging rate in step S1, the first discharging rate in step S3, and the third charging rate in step S4 are the same. This is a simplified solution that simplifies the process flow and equipment control logic, uses the same standard charging rate for key steps, reduces the complexity requirements on testing equipment, and makes the entire testing process more stable, easier to control and implement, thus reducing equipment costs and maintenance difficulty. It should be noted that this is not the optimal solution, as some batteries are not suitable for higher charging rates. Therefore, the optimal solution needs to be verified on the production line based on the product.

[0035] In an embodiment of the present invention, the second charging rate in step S2 is 1C to 3C; the second charging duration is 20 seconds to 60 seconds, the first and fourth charging durations are 1 minute to 5 minutes, the third charging duration is 1 minute to 10 minutes, and the fifth charging duration is 3 minutes to 10 minutes. The reason is that high-rate charging and discharging of the battery can screen out batteries with abnormal internal resistance. Batteries with high polarization internal resistance differ from normal batteries during constant current charging, manifested as a difference in voltage difference before and after charging. Therefore, batteries with high polarization internal resistance can be identified through high-current charging. High-rate charging of 1C-3C can effectively amplify and expose batteries with abnormal polarization internal resistance within tens of seconds, replacing the traditional cumbersome internal resistance tester.

[0036] In an embodiment of the present invention, the specific cutoff voltage in step S3 is 2.0V to 3.2V. This range covers the low SOC range voltage of mainstream battery materials, such as approximately 2.0V for lithium iron phosphate, approximately 3.0V for ternary lithium, and approximately 3.2V for lithium cobalt oxide, without the need for individual adjustments for each material, thus enhancing the versatility of the solution. 2.0V-3.2V corresponds to an SOC of less than 10%, at which point battery polarization is essentially eliminated, and the battery electrode surface returns to equilibrium. For the battery interior, batteries with high polarization internal resistance are usually due to poor internal interfaces, affecting the insertion and extraction of lithium ions between the positive and negative electrodes during charging and discharging. Therefore, batteries with high internal resistance can be identified by the rebound voltage during the resting process, so V5 can accurately reflect the internal interface state.

[0037] In embodiments of the present invention, the constant temperature environment is between 20°C and 30°C. Temperature is a key factor affecting battery voltage. For example, for lithium iron phosphate batteries, a 1°C change in temperature results in a voltage deviation of approximately 0.5mV. A constant temperature range of 20°C to 30°C can control the temperature-induced voltage error within 5mV, ensuring the comparability of voltage data from different batteries and avoiding screening failures caused by environmental interference. Furthermore, this temperature range is close to room temperature, eliminating the need for deep cooling or heating, resulting in low industrial implementation costs and suitability for large-scale production.

[0038] In embodiments of the present invention, the classification of lithium-ion batteries in step S6 further includes at least one of the following operations: classifying based on the difference between V2 and V1, utilizing the mapping relationship between voltage and SOC in the low SOC range to achieve indirect and rapid assessment of battery capacity, replacing the traditional time-consuming full charge and discharge capacity test; classifying based on the difference between V4 and V3 to achieve rapid screening of battery polarization internal resistance; classifying based on V5 to achieve assessment of battery ohmic internal resistance and interface state; classifying based on V6, with the same effect as classifying based on the difference between V2 and V1; and classifying based on the ratio of the difference between V8 and V9 to the resting time to achieve high-throughput, long-term monitoring of battery self-discharge rate, replacing the traditional time-consuming K-value test.

[0039] A specific embodiment of the present invention is described below.

[0040] One embodiment of the present invention uses lithium iron phosphate lithium-ion batteries for packing. The specific implementation steps of the packing method of the present invention are as follows: S1: Place the formed and aged battery in a constant temperature environment of 25±2.5℃. Perform constant current charging at any charging rate from 0.33C to 0.5C (e.g., 0.4C) until the battery reaches 30% SOC. Record the charging start voltage V1 and end voltage V2. Calculate the voltage difference ΔV1 = V2 - V1. Divide the voltage ranges based on ΔV1, resulting in the following voltage levels: 16.5mV-17.5mV, 18mV-20mV, and 20mV-21.5mV. In this embodiment, batteries with ΔV1 between 16.5mV and 17.5mV are classified into one voltage level.

[0041] S2: Let the battery rest for 3 minutes. The resting time can be selected from 1 minute to 5 minutes. Note that the resting time in this step has no impact on the result; it is mainly for the consideration of the total time of the entire test step. Then, charge the lithium-ion battery at a high rate. In this embodiment, charge the battery at a constant current rate of 2C for 30 seconds. Record the charging start voltage V3 and end voltage V4. Calculate the voltage difference ΔV2 = V4 - V3. Based on ΔV2, classify the batteries. Batteries with ΔV2 in the range of 95mV to 105mV are judged as abnormal batteries because the polarization internal resistance of batteries in this range is too high. Other batteries are selected as candidate batteries.

[0042] S3: Let the battery rest for 10 minutes to eliminate the polarization effect caused by high-current charging. Note that this resting time can be selected from 1 minute to 10 minutes. If you are considering charging the battery at a high rate, you can set the resting time slightly longer. Then, discharge the battery at a constant current to the cutoff voltage at any discharge rate from 0.33C to 0.5C (e.g., 0.4C). According to the characteristics of lithium iron phosphate batteries, the battery discharge cutoff voltage is generally set to 2.0V.

[0043] S4: After discharging, allow the battery to stand for 2 minutes and record the rebound voltage V5 at the end of the standing period. Note that the standing time in this step is set to a relatively short period, between 1 and 5 minutes. Then, based on V5, batteries with V5 values ​​of 2.54V-2.56V, 2.56V-2.6V, and 2.6V-2.65V can be divided into three categories. In this embodiment, batteries with V5 values ​​between 2.54V and 2.56V are selected as candidate batteries. The battery is then charged again at a constant current to 20% SOC using any charging rate between 0.33C and 0.5C (e.g., 0.4C), and the charging end voltage V6 is recorded. Note that the charging end SOC is selected to be suitable for screening self-discharge. By using V6 for grading, batteries with voltage ratings of 3.34V-3.35V, 3.35V-3.36V, and 3.36V-3.37V can be divided into three gradations. In this embodiment, batteries with V6 at 3.34V to 3.35V are selected as candidate batteries.

[0044] S5: Let the battery rest for 5 minutes and record the voltage V7 at the end of the resting period. This V7 is the actual open-circuit voltage of the battery after completing SOC adjustment and eliminating charging polarization, which provides an initial reference voltage for self-discharge screening. Then let the battery rest for 7 days, recording the voltage V8 after day 1 and the voltage V9 after day 7. Calculate the self-discharge rate K = (V8 - V9) / resting time, and group batteries with similar K values ​​into the same category, selecting batteries with a specific K value as candidate batteries.

[0045] S6: Based on the grading results of the above steps, select the candidate batteries for assembly into the same battery pack.

[0046] like Figure 2 As shown, the battery cell B selected using this method is placed into the battery pack housing A in a series-parallel manner to form the final cell pack.

[0047] Solution Validation To verify the effectiveness of the present invention, a comparative experiment was conducted, using 50 battery packs assembled by voltage grading (defined as voltage groups) and 50 battery packs assembled by capacity grading (defined as capacity groups).

[0048] The two battery packs maintained consistency in cell quantity, production batch, assembly method, and testing conditions. The assembled battery packs underwent electrical performance testing to evaluate the overall discharge capacity of the batteries within the pack, as well as the voltage difference at the charging and discharging ends, thereby demonstrating the overall consistency of the batteries within the pack. Test results are as follows: Figures 3 to 6 As shown.

[0049] As one of the most important parameters of a battery pack, discharge capacity reflects whether the battery pack can meet the overall energy density requirements. For example... Figure 3 As shown, there is no significant difference in the total discharge capacity between the voltage group and the capacity group.

[0050] DCR, as a key parameter for evaluating the overall resistance of the battery pack, can identify whether there are abnormalities in the overall pack welding and whether there are abnormalities in the internal resistance of the batteries within the pack. Figure 4 As shown, there is no significant difference in the DC resistance (DCR) distribution between the two battery packs (the DCR of some batteries in the voltage group is higher, which may be due to differences between the batteries, but all are below the process requirements).

[0051] The electrical test pack is charged using a constant current charging method, and the voltage difference at the end of the charging process is recorded. The higher the consistency of the contents of the entire pack, the smaller the voltage difference. For example... Figure 5 As shown, the voltage difference between the two battery packs at the end of charging is at the same low level, indicating good consistency within the battery packs.

[0052] The battery pack was discharged using a constant current charging method, and the voltage difference at the end of the discharge was recorded. Batteries with poor consistency throughout the pack showed a larger voltage difference, which can be used to assess the consistency of battery capacity within the pack. Figure 6 As shown, the voltage difference distribution of the two battery packs remains at the same level, indicating that the consistency of the batteries in the two battery packs is at the same level and there is no significant difference.

[0053] The comparison of the above electrical test results shows that the lithium-ion battery packing method of the present invention is fast and yields stable and reliable results. These results fully demonstrate that the voltage-based packing method provided by the present invention can effectively replace the traditional capacity grading method, significantly improving production efficiency and reducing costs while ensuring battery pack performance consistency.

Claims

1. A method for grouping lithium-ion batteries, characterized in that, Under constant temperature conditions, perform the following operations on lithium-ion batteries after formation and aging: S1. Charge the lithium-ion battery to the first SOC level at the first charging rate, and record the charging start voltage V1 and end voltage V2. S2. Let the lithium-ion battery rest for a first time, then charge the lithium-ion battery at the second charging rate for a second time, and record the charging start voltage V3 and end voltage V4. S3. Let the lithium-ion battery stand for a third time, and then discharge the lithium-ion battery to a specific cutoff voltage at the first discharge rate. S4. Let the lithium-ion battery rest for four hours and record the resting end voltage V5. Then charge the lithium-ion battery to the second SOC level at the third charging rate and record the charging end voltage V6. S5. Let the lithium-ion battery rest for five hours and record the resting end voltage V7. Then, let the lithium-ion battery that has completed the above charging and discharging steps rest and record the voltage V8 after the first day of rest and the voltage V9 after the seventh day of rest. S6. Based on at least one of voltages V1 to V9, or the voltage difference calculated from any two of voltages V1 to V9, classify the lithium-ion batteries into different grades, and group the lithium-ion batteries classified into the same grade into a lithium-ion battery pack. Wherein, the first charging rate, the third charging rate, and the first discharging rate are all less than the second charging rate.

2. The lithium-ion battery packing method according to claim 1, characterized in that, The first charging rate in step S1 is 0.33C-0.5C.

3. The lithium-ion battery packing method according to claim 2, characterized in that, The first charging rate in step S1, the first discharging rate in step S3, and the third charging rate in step S4 are the same.

4. The lithium-ion battery packing method according to claim 2, characterized in that, The first SOC level is 20% SOC to 50% SOC.

5. The lithium-ion battery packing method according to claim 4, characterized in that, The second SOC level in step S4 is 20% SOC to 50% SOC.

6. The lithium-ion battery packing method according to claim 1, characterized in that, The second charging rate in step S2 is 1C to 3C.

7. The lithium-ion battery packing method according to claim 1, characterized in that, The second duration is 20 to 60 seconds, the first and fourth durations are 1 to 5 minutes, the third duration is 1 to 10 minutes, and the fifth duration is 3 to 10 minutes.

8. The lithium-ion battery packing method according to claim 1, characterized in that, The specific cutoff voltage in step S3 is associated with the battery material type and ranges from 2.0V to 3.2V.

9. The lithium-ion battery packing method according to claim 1, characterized in that, The temperature of the constant temperature environment is 20°C to 30°C.

10. The lithium-ion battery packing method according to any one of claims 1 to 9, characterized in that, Step S6, which involves classifying lithium-ion batteries into different tiers, further includes at least one of the following operations: The classification is based on the difference between V2 and V1; Tiers are determined based on the difference between V4 and V3; Tier division is based on V5; Tier division based on V6; The grading is based on the ratio of the difference between V8 and V9 to the settling time.

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