Square lithium ion battery grading method

Abstract

The invention relates to the technical field of lithium batteries, in particular to a square lithium ion battery grade matching method. Comprising the following steps: firstly, discharging a plurality of square lithium ion batteries to 2.0-2.5 V at a constant current of 0.5-1 C, and then placing the square lithium ion batteries in an environment of 25 + / -2 DEG C for 24-72 hours to stabilize the voltage; testing the open-circuit voltage of each battery, and classifying the batteries of which the voltage difference is less than or equal to 5mV into the same voltage section; equalization processing is carried out for different voltage sections, active equalization not lower than 500mA is adopted for a high voltage section, and passive equalization not higher than 100mA is adopted for a low voltage section; storing the same section of batteries for 7-14 days under the conditions that the temperature is 25 + / -2 DEG C and the humidity is 50% + / -5%, carrying out a self-discharge test, and rejecting the batteries with the daily average voltage drop exceeding 1mV; and finally, combining the capacity parameters, and assembling the batteries with the capacity difference within + / -1% into a lithium battery pack with high consistency. According to the method, through multi-stage accurate sorting and balancing, the influence of polarization and self-discharge is effectively reduced, the grouping consistency and the matching accuracy are remarkably improved, and the method is suitable for large-scale production.

Description

Technical Field

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

[0002] In the field of lithium battery technology, prismatic lithium-ion batteries are widely used in electric vehicles, large-scale energy storage systems, and high-end portable electronic devices due to their stable structure, high energy density, long cycle life, and excellent safety performance. However, the overall performance of batteries in a battery pack is often constrained by the consistency between individual cells. How to achieve efficient and precise matching to build a battery pack with balanced performance has become a key technical challenge for the industry. Currently, prismatic lithium-ion batteries have the following main technical defects in the matching process: First, the most prominent problem is voltage inconsistency. Due to inherent process dispersions such as batch fluctuations in raw materials, microscopic differences in electrode coating thickness, and uneven electrolyte wetting during battery manufacturing, coupled with the influence of external factors such as charge / discharge history and ambient temperature during use, there are significant deviations in the open-circuit voltage between individual cells. This voltage inconsistency means that the usable capacity of the battery pack in series depends on the cell with the lowest voltage, resulting in a significant decrease in system energy utilization. More seriously, it may cause overcharging or over-discharging of some cells at the end of the charge / discharge cycle, which not only accelerates battery aging but also brings safety hazards such as thermal runaway. Secondly, the difference in self-discharge rate is often overlooked in the traditional matching process. Different individual cells have different self-discharge rates due to differences in internal impurity content, separator microstructure, SEI film stability, etc. In long-term static or float charging application scenarios, this difference will cause the state of charge of each individual cell to gradually diverge, exacerbating the imbalance inside the battery pack. Batteries with high self-discharge rates will lose power faster, which will not only increase the maintenance cost and balancing burden of the system, but also shorten the effective life of the entire battery pack.

[0003] Furthermore, existing equalization technologies have significant limitations. Most existing equalization strategies adopt a "one-size-fits-all" approach, lacking differentiated equalization solutions for batteries with different voltage states, and thus failing to achieve refined and adaptive adjustment. Therefore, current technologies have not effectively solved the problem of multi-dimensional parameter coordination and matching, lacking a complete technical solution that can simultaneously take into account voltage state calibration, self-discharge characteristic assessment, differentiated equalization processing, and efficient and accurate matching. This results in limited overall performance improvement of battery packs and persistently high life-cycle costs. Summary of the Invention

[0004] Based on the problems existing in the above-mentioned background technology, the present invention proposes a method for matching square lithium-ion batteries, the steps of which are as follows.

[0005] Step S1: Discharge multiple square lithium-ion batteries by capacity division, and then place them in an open-circuit environment at room temperature for 24-72 hours;

[0006] Step S2: Test the open-circuit voltage of each battery, and classify batteries whose absolute value of the open-circuit voltage difference is less than or equal to a preset threshold into the same voltage range.

[0007] Step S3: Apply passive or active equalization processing to batteries in different voltage ranges for a preset duration;

[0008] Step S4: After equalization, lithium-ion batteries belonging to the same voltage range are stored under the same temperature and humidity conditions for self-discharge testing, and unqualified batteries are removed based on the self-discharge test results.

[0009] Step S5: After screening, the batteries are matched according to their capacity parameters to form a lithium battery pack.

[0010] Preferably, in step S1, the capacity discharge is a constant current discharge at a rate of 0.5C-1C until the voltage is 2.0V-2.5V; the ambient temperature is 25±2℃.

[0011] Preferably, in step S1, the capacity discharge is performed with a constant current at a rate of 0.7C-0.9C, and the circuit is left open for 48 hours at room temperature.

[0012] Preferably, in step S2, the preset threshold value of the open-circuit voltage difference is 5mV.

[0013] Preferably, in step S3, applying passive or active equalization to batteries in different voltage ranges specifically means: after dividing the battery into n voltage ranges in step S2, n is a natural number ≥1;

[0014] When n=1, the open-circuit voltage difference of all batteries is ≤5mV, and the system directly enters the multi-parameter collaborative equilibrium stage.

[0015] When n=2, the voltage is divided into a high-voltage segment and a low-voltage segment. Active equalization is used for the high-voltage segment and passive equalization is used for the low-voltage segment.

[0016] When n≥3, the system is divided into high voltage, medium voltage and low voltage segments. Active balancing is used for the high voltage segment, passive balancing is used for the low voltage segment, and multi-parameter collaborative balancing is used for the medium voltage segment.

[0017] Preferably, in step S3, the multi-parameter collaborative balancing step involves first calculating the capacity range ΔC, which is the difference between the maximum and minimum capacity of the battery within the voltage range, and the standard deviation σ of the open-circuit voltage of the battery within the voltage range.

[0018] If the capacity range ΔC ≤ 2% of the rated capacity and the standard deviation σ ≤ 2mV are satisfied at the same time, it indicates that the voltage range itself has good consistency and active equalization is used for efficient fine-tuning.

[0019] If one or more of the capacity range ΔC and standard deviation σ conditions are not met, passive balancing is adopted to achieve intra-segment consistency in a robust and low-cost manner.

[0020] Preferably, in step S3, the active balancing is performed by a capacitor or inductor energy transfer circuit with a current of not less than 500mA for a duration of 0.5-1.5 hours; the passive balancing is performed by discharging a parallel resistor with a current of not more than 100mA for a duration of 1-3 hours.

[0021] Preferably, in step S3, the active balancing current is 500mA and lasts for 1 hour; the passive balancing current is 100mA and lasts for 2 hours.

[0022] Preferably, in step S4, the specific steps for storing the lithium-ion battery under the same temperature and humidity conditions for self-discharge testing are as follows: store the lithium-ion battery at a temperature of 25±2℃ and a relative humidity of 50%±5% for 7-14 days, and calculate the voltage drop rate to eliminate batteries with a daily average voltage drop exceeding 1mV.

[0023] Preferably, in step S5, the final matching based on capacity parameters refers to matching batteries in the same voltage range after screening in step S4 with capacity as the final parameter, and classifying batteries whose actual capacity difference does not exceed ±1% of the rated capacity into the same matching group, thereby forming a highly consistent lithium battery pack.

[0024] Compared with existing technologies, the beneficial effects of this invention are as follows: By combining capacity-controlled discharge with long-term open-circuit storage, the differences in battery polarization voltage are effectively eliminated, providing a reliable benchmark for subsequent accurate voltage sorting; a differentiated equalization strategy based on voltage range is adopted, implementing high-efficiency active equalization for high-voltage ranges and low-cost passive equalization for low-voltage ranges, achieving an optimal balance between equalization efficiency and economic benefits; long-term self-discharge testing is conducted based on a unified voltage benchmark and equalization processing, which can accurately identify and eliminate batteries with abnormal self-discharge rates, fundamentally improving the long-term storage consistency of battery packs; finally, refined matching is performed based on capacity parameters, forming a multi-dimensional parameter collaborative screening mechanism to ensure that the batteries in the pack maintain a high degree of consistency in terms of voltage, self-discharge rate, and capacity, thereby significantly improving the overall energy utilization rate, cycle life, and safety reliability of the battery pack. At the same time, the method has a clear process, is controllable in operation, and has good prospects for industrial application. Detailed Implementation

[0025] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0026] Example 1

[0027] A method for consistent matching of square lithium-ion batteries includes the following steps:

[0028] Step S1: Perform capacity discharge on multiple square lithium-ion batteries. Capacity discharge is performed by discharging at a constant current of 0.8C rate until the voltage reaches 2.0V. The ambient temperature is 25±2℃. Then, the batteries are placed in an open-circuit environment at room temperature for 48 hours.

[0029] Step S2: Test the open-circuit voltage of each battery, set the preset threshold for the open-circuit voltage difference to 5mV, and classify batteries whose absolute value of the open-circuit voltage difference is less than or equal to the preset threshold into the same voltage range.

[0030] Step S3: Apply passive or active equalization processing to batteries in different voltage ranges for a preset duration;

[0031] S3.1 After dividing the battery into n voltage segments in step S2, n is a natural number ≥ 1;

[0032] When n=1, the open-circuit voltage difference of all batteries is ≤5mV, and the system directly enters the multi-parameter collaborative equilibrium stage.

[0033] When n=2, the voltage is divided into a high-voltage segment and a low-voltage segment. Active equalization is used for the high-voltage segment and passive equalization is used for the low-voltage segment.

[0034] When n≥3, the voltage is divided into high voltage end, medium voltage segment and low voltage segment. Active equalization is used for high voltage segment, passive equalization is used for low voltage segment, and multi-parameter collaborative equalization is used for medium voltage segment.

[0035] S3.2 Multi-parameter collaborative equalization steps: First, calculate the capacity range ΔC, which is the difference between the maximum and minimum capacity of the battery within the voltage range, and the standard deviation σ of the open-circuit voltage of the battery within the voltage range;

[0036] If the capacity range ΔC ≤ 2% of the rated capacity and the standard deviation σ ≤ 2mV are satisfied at the same time, it indicates that the voltage range itself has good consistency and active equalization is used for efficient fine-tuning.

[0037] If one or more of the capacity range ΔC and standard deviation σ conditions are not met, passive balancing is adopted to achieve intra-segment consistency in a robust and low-cost manner.

[0038] S3.3 Active balancing is performed using a capacitor or inductor energy transfer circuit with a current of 500mA for 1 hour; passive balancing is performed using a parallel resistor discharge with a current of 100mA for 2 hours.

[0039] Step S4: Store the lithium-ion batteries that belong to the same voltage range after equalization at a temperature of 25°C and a relative humidity of 50% for 10 days, and calculate the voltage drop rate to remove batteries with a daily average voltage drop of more than 1mV.

[0040] Step S5: Among the batteries in the same voltage range after screening in step S4, the capacity is used as the final parameter for matching. Batteries with actual capacity differences not exceeding ±1% of rated capacity are grouped into the same matching group to form a highly consistent lithium battery pack.

[0041] Example 1 achieves precise battery matching through a phased balancing strategy and dynamic detection. In the voltage segmentation stage, batteries are divided into n voltage segments using a 5mV threshold. When n=1, multi-parameter collaborative balancing is directly initiated. When n≥2, the high-voltage segment uses 500mA active balancing for 1 hour for rapid voltage adjustment, the low-voltage segment uses 100mA passive balancing for 2 hours for robust voltage adjustment, and the medium-voltage segment is determined by the detection results of a capacity range ΔC≤2% and a standard deviation σ≤2mV—if both conditions are met, active balancing is used for efficient fine-tuning; otherwise, passive balancing is used for low-cost processing. Polarization differences are eliminated through capacity-based discharge, and abnormal batteries with a daily average voltage drop >1mV are removed by self-discharge testing. The final matched capacity difference is ≤±1%, improving battery pack voltage consistency by 40% and extending cycle life by 15%, achieving dual optimization of energy utilization and safety. This significantly improves the overall energy utilization, cycle life, and safety reliability of the battery pack. Furthermore, this method has a clear process, is controllable, and has good prospects for industrial application.

[0042] Example 2

[0043] A method for consistent matching of square lithium-ion batteries includes the following steps:

[0044] Step S1: Perform capacity discharge on multiple square lithium-ion batteries. Capacity discharge is performed by discharging at a constant current of 0.7C rate until the voltage reaches 2.5V. The ambient temperature is 25±2℃. Then, the batteries are placed in an open-circuit environment at room temperature for 60 hours.

[0045] Step S2: Test the open-circuit voltage of each battery, set the preset threshold for the open-circuit voltage difference to 5mV, and classify batteries whose absolute value of the open-circuit voltage difference is less than or equal to the preset threshold into the same voltage range.

[0046] Step S3: Apply passive or active equalization processing to batteries in different voltage ranges for a preset duration;

[0047] S3.1 After dividing the battery into n voltage segments in step S2, n is a natural number ≥ 1;

[0048] When n=1, the open-circuit voltage difference of all batteries is ≤5mV, and the system directly enters the multi-parameter collaborative equilibrium stage.

[0049] When n=2, the voltage is divided into a high-voltage segment and a low-voltage segment. Active equalization is used for the high-voltage segment and passive equalization is used for the low-voltage segment.

[0050] When n≥3, the voltage is divided into high voltage end, medium voltage segment and low voltage segment. Active equalization is used for high voltage segment, passive equalization is used for low voltage segment, and multi-parameter collaborative equalization is used for medium voltage segment.

[0051] S3.2 Multi-parameter collaborative equalization steps: First, calculate the capacity range ΔC, which is the difference between the maximum and minimum capacity of the battery within the voltage range, and the standard deviation σ of the open-circuit voltage of the battery within the voltage range;

[0052] If the capacity range ΔC ≤ 2% of the rated capacity and the standard deviation σ ≤ 2mV are satisfied at the same time, it indicates that the voltage range itself has good consistency and active equalization is used for efficient fine-tuning.

[0053] If one or more of the capacity range ΔC and standard deviation σ conditions are not met, passive balancing is adopted to achieve intra-segment consistency in a robust and low-cost manner.

[0054] S3.3 Active balancing is performed using a capacitor or inductor energy transfer circuit with a current of not less than 500mA for 1 hour; passive balancing is performed using a parallel resistor discharge with a current of not more than 100mA for 2 hours.

[0055] Step S4: Store the lithium-ion batteries that belong to the same voltage range after equalization for 7 days at a temperature of 25±2℃ and a relative humidity of 45%, and calculate the voltage drop rate to remove batteries with a daily average voltage drop of more than 1mV.

[0056] Step S5: Among the batteries in the same voltage range after screening in step S4, the capacity is used as the final parameter for matching. Batteries with actual capacity differences not exceeding ±1% of rated capacity are grouped into the same matching group to form a highly consistent lithium battery pack.

[0057] Example 3

[0058] A method for consistent matching of square lithium-ion batteries includes the following steps:

[0059] Step S1: Perform capacity discharge on multiple square lithium-ion batteries. The capacity discharge is performed by discharging at a constant current of 0.9C rate until the voltage is 2.2V. The ambient temperature is 25±2℃. Then, place them in an open-circuit environment at room temperature for 36 hours.

[0060] Step S2: Test the open-circuit voltage of each battery, set the preset threshold for the open-circuit voltage difference to 5mV, and classify batteries whose absolute value of the open-circuit voltage difference is less than or equal to the preset threshold into the same voltage range.

[0061] Step S3: Apply passive or active equalization processing to batteries in different voltage ranges for a preset duration;

[0062] S3.1 After dividing the battery into n voltage segments in step S2, n is a natural number ≥ 1;

[0063] When n=1, the open-circuit voltage difference of all batteries is ≤5mV, and the system directly enters the multi-parameter collaborative equilibrium stage.

[0064] When n=2, the voltage is divided into a high-voltage segment and a low-voltage segment. Active equalization is used for the high-voltage segment and passive equalization is used for the low-voltage segment.

[0065] When n≥3, the voltage is divided into high voltage end, medium voltage segment and low voltage segment. Active equalization is used for high voltage segment, passive equalization is used for low voltage segment, and multi-parameter collaborative equalization is used for medium voltage segment.

[0066] S3.2 Multi-parameter collaborative equalization steps: First, calculate the capacity range ΔC, which is the difference between the maximum and minimum capacity of the battery within the voltage range, and the standard deviation σ of the open-circuit voltage of the battery within the voltage range;

[0067] If the capacity range ΔC ≤ 2% of the rated capacity and the standard deviation σ ≤ 2mV are satisfied at the same time, it indicates that the voltage range itself has good consistency and active equalization is used for efficient fine-tuning.

[0068] If one or more of the capacity range ΔC and standard deviation σ conditions are not met, passive balancing is adopted to achieve intra-segment consistency in a robust and low-cost manner.

[0069] S3.3 Active balancing is performed through a capacitor or inductor energy transfer circuit with a current of not less than 500mA for a duration of 0.5 hours; passive balancing is performed through a parallel resistor discharge with a current of not more than 100mA for a duration of 1 hour.

[0070] Step S4: Store the lithium-ion batteries that belong to the same voltage range after equalization treatment for 14 days at a temperature of 25±2℃ and a relative humidity of 55%, and calculate the voltage drop rate to remove batteries with a daily average voltage drop of more than 1mV.

[0071] Step S5: Among the batteries in the same voltage range after screening in step S4, the capacity is used as the final parameter for matching. Batteries with actual capacity differences not exceeding ±1% of rated capacity are grouped into the same matching group to form a highly consistent lithium battery pack.

[0072] Example 4

[0073] A method for consistent matching of square lithium-ion batteries includes the following steps:

[0074] Step S1: Perform capacity discharge on multiple square lithium-ion batteries. The capacity discharge is performed by discharging at a constant current of 0.5C rate until the voltage reaches 2.3V. The ambient temperature is 25±2℃. Then, place them in an open-circuit environment at room temperature for 24 hours.

[0075] Step S2: Test the open-circuit voltage of each battery, set the preset threshold for the open-circuit voltage difference to 5mV, and classify batteries whose absolute value of the open-circuit voltage difference is less than or equal to the preset threshold into the same voltage range.

[0076] Step S3: Apply passive or active equalization processing to batteries in different voltage ranges for a preset duration;

[0077] S3.1 After dividing the battery into n voltage segments in step S2, n is a natural number ≥ 1;

[0078] When n=1, the open-circuit voltage difference of all batteries is ≤5mV, and the system directly enters the multi-parameter collaborative equilibrium stage.

[0079] When n=2, the voltage is divided into a high-voltage segment and a low-voltage segment. Active equalization is used for the high-voltage segment and passive equalization is used for the low-voltage segment.

[0080] When n≥3, the voltage is divided into high voltage end, medium voltage segment and low voltage segment. Active equalization is used for high voltage segment, passive equalization is used for low voltage segment, and multi-parameter collaborative equalization is used for medium voltage segment.

[0081] S3.2 Multi-parameter collaborative equalization steps: First, calculate the capacity range ΔC, which is the difference between the maximum and minimum capacity of the battery within the voltage range, and the standard deviation σ of the open-circuit voltage of the battery within the voltage range;

[0082] If the capacity range ΔC ≤ 2% of the rated capacity and the standard deviation σ ≤ 2mV are satisfied at the same time, it indicates that the voltage range itself has good consistency and active equalization is used for efficient fine-tuning.

[0083] If one or more of the capacity range ΔC and standard deviation σ conditions are not met, passive balancing is adopted to achieve intra-segment consistency in a robust and low-cost manner.

[0084] S3.3 Active balancing is performed using a capacitor or inductor energy transfer circuit with a current of not less than 500mA for 1.5 hours; passive balancing is performed using a parallel resistor discharge with a current of not more than 100mA for 3 hours.

[0085] Step S4: After equalization, lithium-ion batteries belonging to the same voltage range are stored for 8 days at a temperature of 25±2℃ and a relative humidity of 50%, and the voltage drop rate is calculated to remove batteries with a daily average voltage drop exceeding 1mV.

[0086] Step S5: Among the batteries in the same voltage range after screening in step S4, the capacity is used as the final parameter for matching. Batteries with actual capacity differences not exceeding ±1% of rated capacity are grouped into the same matching group to form a highly consistent lithium battery pack.

[0087] Example 5

[0088] A method for consistent matching of square lithium-ion batteries includes the following steps:

[0089] Step S1: Perform capacity discharge on multiple square lithium-ion batteries. The capacity discharge is performed by discharging at a constant current of 1C rate until the voltage is 2.4V. The ambient temperature is 25±2℃. Then, place them in an open-circuit environment at room temperature for 72 hours.

[0090] Step S2: Test the open-circuit voltage of each battery, set the preset threshold for the open-circuit voltage difference to 5mV, and classify batteries whose absolute value of the open-circuit voltage difference is less than or equal to the preset threshold into the same voltage range.

[0091] Step S3: Apply passive or active equalization processing to batteries in different voltage ranges for a preset duration;

[0092] S3.1 After dividing the battery into n voltage segments in step S2, n is a natural number ≥ 1;

[0093] When n=1, the open-circuit voltage difference of all batteries is ≤5mV, and the system directly enters the multi-parameter collaborative equilibrium stage.

[0094] When n=2, the voltage is divided into a high-voltage segment and a low-voltage segment. Active equalization is used for the high-voltage segment and passive equalization is used for the low-voltage segment.

[0095] When n≥3, the voltage is divided into high voltage end, medium voltage segment and low voltage segment. Active equalization is used for high voltage segment, passive equalization is used for low voltage segment, and multi-parameter collaborative equalization is used for medium voltage segment.

[0096] S3.2 Multi-parameter collaborative equalization steps: First, calculate the capacity range ΔC, which is the difference between the maximum and minimum capacity of the battery within the voltage range, and the standard deviation σ of the open-circuit voltage of the battery within the voltage range;

[0097] If the capacity range ΔC ≤ 2% of the rated capacity and the standard deviation σ ≤ 2mV are satisfied at the same time, it indicates that the voltage range itself has good consistency and active equalization is used for efficient fine-tuning.

[0098] If one or more of the capacity range ΔC and standard deviation σ conditions are not met, passive balancing is adopted to achieve intra-segment consistency in a robust and low-cost manner.

[0099] S3.3 Active balancing is performed using a capacitor or inductor energy transfer circuit with a current of not less than 500mA for 1 hour; passive balancing is performed using a parallel resistor discharge with a current of not more than 100mA for 3 hours.

[0100] Step S4: After equalization, lithium-ion batteries belonging to the same voltage range are stored for 12 days at a temperature of 25±2℃ and a relative humidity of 50%, and the voltage drop rate is calculated to remove batteries with a daily average voltage drop exceeding 1mV.

[0101] Step S5: Among the batteries in the same voltage range after screening in step S4, the capacity is used as the final parameter for matching. Batteries with actual capacity differences not exceeding ±1% of rated capacity are grouped into the same matching group to form a highly consistent lithium battery pack.

[0102] Comparative Example 1

[0103] A threshold of 10mV voltage range was used. For the high-voltage range, only 300mA passive equalization was performed for 2 hours, while for the low-voltage range, direct matching was performed without processing. All other steps were identical to those in Example 1. Results showed that the battery pack voltage consistency deviation reached 5mV, cycle life was shortened by 8%, and the self-discharge test rejection rate increased by 15%. Compared with the 5mV threshold and differentiated equalization strategy of Example 1, this invention demonstrates its advantages in precision control and performance optimization.

[0104] Comparative Example 2

[0105] After capacity grading, batteries were directly grouped according to their capacity and internal resistance parameters. Batteries with capacity differences controlled within ±2% and internal resistance differences controlled within ±5% were grouped into the same group, completely omitting the voltage segmentation and dynamic balancing processes. The remaining steps were exactly the same as in Example 1. The results showed that after 30 days of rest, the voltage inconsistency of this battery pack significantly increased to 40mV. The difference in self-discharge led to severe differentiation in the state of charge. The battery with the lowest capacity in the group triggered the protection mechanism prematurely during cycling, resulting in the actual usable capacity of the entire battery pack being only 92% of the initial capacity, which could not meet the requirements for long-term consistent use.

[0106] The above are merely preferred embodiments of the present invention. It should be noted that the above preferred embodiments should not be considered as limitations on the present invention, and the scope of protection of the present invention should be determined by the scope defined in the claims. For those skilled in the art, several improvements and modifications can be made without departing from the spirit and scope of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for matching square lithium-ion batteries, characterized in that: The steps are as follows: Step S1: Discharge multiple square lithium-ion batteries by capacity division, and then place them in an open-circuit environment at room temperature for 24-72 hours; Step S2: Test the open-circuit voltage of each battery, and classify batteries whose absolute value of the open-circuit voltage difference is less than or equal to a preset threshold into the same voltage range. Step S3: Apply passive or active equalization processing to batteries in different voltage ranges for a preset duration; Step S4: After equalization, lithium-ion batteries belonging to the same voltage range are stored under the same temperature and humidity conditions for self-discharge testing, and unqualified batteries are removed based on the self-discharge test results. Step S5: After screening, the batteries are matched according to their capacity parameters to form a lithium battery pack.

2. The method for matching square lithium-ion batteries according to claim 1, characterized in that: In step S1, the capacity discharge is a constant current discharge at a rate of 0.5C-1C until the voltage is 2.0V-2.5V; the ambient temperature is 25±2℃.

3. The method for matching square lithium-ion batteries according to claim 2, characterized in that: In step S1, the capacity discharge is performed with a constant current at a rate of 0.7C-0.9C, and the circuit is left open for 48 hours at room temperature.

4. The method for matching square lithium-ion batteries according to claim 1, characterized in that: In step S2, the preset threshold for the open-circuit voltage difference is 5mV.

5. The method for matching square lithium-ion batteries according to claim 1, characterized in that: In step S3, the passive or active equalization process applied to batteries in different voltage ranges is specifically as follows: after dividing the battery into n voltage ranges in step S2, n is a natural number ≥1. When n=1, the open-circuit voltage difference of all batteries is ≤5mV, and the system directly enters the multi-parameter collaborative equilibrium stage. When n=2, the voltage is divided into a high-voltage segment and a low-voltage segment. Active equalization is used for the high-voltage segment and passive equalization is used for the low-voltage segment. When n≥3, the system is divided into high voltage, medium voltage and low voltage segments. Active balancing is used for the high voltage segment, passive balancing is used for the low voltage segment, and multi-parameter collaborative balancing is used for the medium voltage segment.

6. The method for matching square lithium-ion batteries according to claim 5, characterized in that: In step S3, the multi-parameter collaborative balancing step involves first calculating the capacity range ΔC, which is the difference between the maximum and minimum capacity of the battery within the voltage range, and the standard deviation σ of the open-circuit voltage of the battery within the voltage range. If the capacity range ΔC ≤ 2% of the rated capacity and the standard deviation σ ≤ 2mV are satisfied at the same time, it indicates that the voltage range itself has good consistency and active equalization is used for efficient fine-tuning. If one or more of the capacity range ΔC and standard deviation σ conditions are not met, passive balancing is adopted to achieve intra-segment consistency in a robust and low-cost manner.

7. A method for matching square lithium-ion batteries according to claim 5, characterized in that: In step S3, the active balancing is performed by a capacitor or inductor energy transfer circuit with a current of not less than 500mA for a duration of 0.5-1.5 hours; the passive balancing is performed by discharging a parallel resistor with a current of not more than 100mA for a duration of 1-3 hours.

8. The method for matching square lithium-ion batteries according to claim 7, characterized in that: In step S3, the active balancing current is 500mA and lasts for 1 hour; the passive balancing current is 100mA and lasts for 2 hours.

9. A method for matching square lithium-ion batteries according to claim 1, characterized in that: In step S4, the specific steps for storing the lithium-ion battery under the same temperature and humidity conditions for self-discharge testing are as follows: store the lithium-ion battery at a temperature of 25±2℃ and a relative humidity of 50%±5% for 7-14 days, and calculate the voltage drop rate to eliminate batteries with a daily average voltage drop exceeding 1mV.

10. A method for matching square lithium-ion batteries according to claim 1, characterized in that: In step S5, the final matching based on capacity parameters refers to matching batteries in the same voltage range after screening in step S4 with capacity as the final parameter, and classifying batteries whose actual capacity difference does not exceed ±1% of the rated capacity into the same matching group, thereby forming a highly consistent lithium battery pack.