Lithium ion battery self-discharge high-precision screening method

Abstract

The invention discloses a lithium ion battery self-discharge high-precision screening method. The method comprises the following steps: S1, obtaining a self-discharge rate parameter K and a residual capacity C of a battery; s2, detecting whether the battery capacity retention ratio C / C0 is smaller than a preset threshold value or not; if yes, the battery is judged as an unqualified battery; otherwise, executing the step S3; s3, calculating a battery comprehensive evaluation function F (K, C); s4, detecting whether the F value is within a preset gray area range or not; if yes, the battery is judged to be a to-be-rechecked battery, and the steps from S11 to S17 are executed again; if the F value is larger than the maximum value of the range, the battery is judged as an unqualified battery; and if the F value is smaller than the range minimum value, the battery is judged to be a qualified battery. According to the method, the residual capacity C obtained by directly metering the self-discharge rate parameter K and the electric quantity is combined, and the battery cell is quantitatively judged by utilizing the comprehensive evaluation function, so that the misjudgment and omission ratio in a voltage platform flat system is remarkably reduced, and the judgment reliability is ensured.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery testing and screening technology, and in particular to a high-precision screening method for lithium-ion battery self-discharge. Background Technology

[0002] Currently, lithium-ion batteries have been widely used in digital products, electric vehicles, and energy storage due to their advantages such as high energy density, good cycle performance, and being green and pollution-free.

[0003] The self-discharge characteristics of lithium-ion batteries are an important indicator for measuring cell consistency and reliability. For batteries with a flat SOC (state of charge) - OCV (open circuit voltage) characteristic, such as lithium titanate (LTO) and lithium iron phosphate (LFP), the voltage change in the middle SOC region is minimal. Traditional methods for determining battery self-discharge have low sensitivity and are prone to missed detections or false positives.

[0004] It should be noted that the flat SOC (remaining charge)-OCV (open circuit voltage) characteristic of a battery refers to the flatness of the SOC-OCV curve. This means that within a specific range of the battery's state of charge (SOC), the open circuit voltage (OCV) changes very little. This typically occurs in the middle SOC range of the battery (e.g., 20% to 80%).

[0005] Current methods for determining battery self-discharge all rely solely on voltage as the determining factor. This single-dimensional approach cannot effectively identify various types of self-discharge defects in lithium-ion batteries with flat SOC-OCV characteristics: it misses both continuous capacity loss within the battery and transient electrical anomalies.

[0006] Therefore, there is an urgent need to develop a method that can solve the above-mentioned technical problems. Summary of the Invention

[0007] The purpose of this invention is to address the technical deficiencies of existing technologies by providing a high-precision screening method for the self-discharge of lithium-ion batteries.

[0008] Therefore, the present invention provides a high-precision screening method for self-discharge of lithium-ion batteries, comprising the following steps: Step S1: For the battery to be tested, obtain the battery's self-discharge rate parameter K and the battery's residual capacity C. The acquisition process includes steps S11 to S17. Step S2, perform the initial judgment operation: check whether the battery capacity retention ratio C / C0 is less than a preset threshold; where C0 is the initial reference capacity; If so, the battery is determined to be a self-discharge defective battery; Otherwise, proceed to step S3; Step S3: Calculate the battery's comprehensive evaluation function F(K,C) based on the battery's self-discharge rate parameter K and the battery's residual capacity C. Step S4: Detect whether the F value is within the preset gray area range; If so, the battery is identified as a battery to be re-inspected, and the process returns to steps S11 to S17. After the re-inspection, the final judgment is made according to the preset secondary judgment operation. If the F value is greater than the maximum value of the preset gray area range, the battery is determined to be a self-discharge defective battery. If the F value is less than the minimum value of the preset gray area range, the battery is determined to be a qualified self-discharge battery.

[0009] As can be seen from the technical solutions provided by the present invention above, compared with the prior art, the present invention provides a high-precision screening method for self-discharge of lithium-ion batteries. It is a high-precision screening method for self-discharge of lithium-ion batteries with a gray zone determination mechanism. This method combines the self-discharge rate parameter K and the residual capacity C obtained by direct measurement of the charge, and uses a comprehensive evaluation function to quantitatively determine the battery cell. This significantly reduces the false positive and false negative rates in a system with a flat voltage platform, and ensures the reliability of the determination through the gray zone re-inspection mechanism. It has significant practical significance.

[0010] The high-precision screening method for self-discharge of lithium-ion batteries of the present invention introduces the direct measurement dimension of electric charge on the basis of the traditional K-value method and establishes a gray zone re-inspection mechanism. Through the collaborative judgment of dual indicators, a robust quality control system with high-precision screening and low misjudgment is realized.

[0011] The method of this invention is a self-discharge assessment method based on dual-parameter collaborative judgment, that is, simultaneously considering the battery's K value and capacity decay C / C0. The K value reflects the battery's voltage stability during rest, while C / C0 directly reflects the battery's capacity loss. By combining these two seemingly independent but intrinsically related indicators, the battery's self-discharge level can be assessed more comprehensively and accurately.

[0012] The core idea of ​​this invention is that for batteries with minor self-discharge defects, both the K value and C / C0 typically exhibit abnormalities simultaneously. Therefore, by constructing a comprehensive evaluation function F(K,C) = α·K + β·(1-C / C0), the two indicators can be integrated into a single evaluation value, thereby achieving a more reliable judgment. Here, α and β are weighting coefficients that can be optimized based on actual production data.

[0013] Furthermore, this invention introduces an intermediate state of "pending re-inspection." Batteries with F-values ​​at ambiguous boundaries can be marked as "pending re-inspection" and re-evaluated. This "grayscale" processing method avoids hasty judgments on boundary samples and significantly reduces the false judgment rate.

[0014] Practical testing has shown that the technical solution of this invention has strong industrial feasibility, maintains high screening accuracy without the need for precise environmental compensation or temperature correction, and is suitable for large-scale production testing lines.

[0015] This invention is a high-precision self-discharge screening method for lithium-ion batteries with flat SOC-OCV characteristics. It is applicable to lithium-ion battery systems with flat SOC-OCV characteristics, including lithium titanate batteries, lithium iron phosphate batteries and their composite systems, as well as other battery systems with similar voltage platform characteristics. Attached Figure Description

[0016] Figure 1 A basic flowchart of a high-precision screening method for self-discharge of lithium-ion batteries provided by the present invention; Figure 2 The overall flowchart of a high-precision screening method for self-discharge of lithium-ion batteries provided by the present invention; Figure 3 This is a two-dimensional distribution diagram of the electrical K value and capacity ratio (i.e., capacity ratio, also called capacity retention ratio) in Embodiment 1 of the present invention. Detailed Implementation

[0017] 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, and 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.

[0018] To enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0019] See Figures 1 to 3 This invention provides a high-precision screening method for self-discharge of lithium-ion batteries, comprising the following steps: Step S1: For the battery to be tested, obtain the battery's self-discharge rate parameter K and the battery's residual capacity C. It should be noted that lithium-ion batteries have a flat SOC–OCV characteristic curve.

[0020] Step S2, perform the initial judgment operation: check whether the battery capacity retention ratio C / C0 is less than a preset threshold (e.g., 95%). Wherein, C0 is the initial charged capacity. If so, the battery is determined to be a self-discharge defective battery; Otherwise, proceed to step S3; It should be noted that if the battery's capacity retention ratio (C / C0) is less than a preset threshold, the battery will be judged as a self-discharge failure battery. It should be noted that the initial baseline capacity C0 is a baseline capacity value used for capacity calculation and comparison in the self-discharge test, and does not represent the capacity result obtained by actual measurement during the test.

[0021] The initial reference capacity C0 is obtained by directly reading the nominal rated capacity in the battery specification sheet of the battery to be tested and multiplying it by a preset coefficient. That is: initial reference capacity C0 = nominal rated capacity × preset coefficient. The nominal rated capacity refers to the parameters that have been measured and solidified in the existing factory testing of the battery; this invention does not introduce new capacity measurement steps. The preset coefficient is preferably in the range of 30%–80%, more preferably 50%.

[0022] Step S3: Calculate the battery's comprehensive evaluation function F(K,C) based on the battery's self-discharge rate parameter K and the battery's residual capacity C. Step S4: Detect whether the F value is within the preset gray area range (i.e., the preset numerical range); If so, the battery is identified as a battery to be re-inspected, and the process returns to steps S11 to S17 (i.e., enters the re-inspection workflow), and after the re-inspection, the final judgment is made according to the preset secondary judgment operation. It should be noted that the F value, i.e., the value of F(K,C); If the F value is greater than the maximum value of the preset gray area range (i.e., the preset numerical range), the battery is determined to be a self-discharge unqualified battery. If the F value is less than the minimum value of the preset gray area range (i.e., the preset numerical range), the battery is determined to be a qualified self-discharge battery.

[0023] It should be noted that, for this invention, when the F value is within a preset gray area range (i.e., a preset numerical range), the battery is identified as a battery to be re-inspected and enters the re-inspection management area for secondary confirmation (re-executing the test process for obtaining the self-discharge rate parameter K and residual capacity C). This method combines the sensitivity of the K value with the reliability of capacity measurement, and can significantly reduce the rate of missed detections and false judgments under a flat voltage platform system, achieving high-precision screening of cell self-discharge, and is suitable for quality control of power battery and energy storage battery production lines.

[0024] In step S4, the preset secondary judgment operation is as follows: if the battery to be re-inspected, after returning to execute steps S11 to S17 and re-executing step S3, obtains an F value that is less than the minimum value of the preset gray area range (i.e., the preset numerical range) and meets the conditions that the battery's self-discharge rate parameter K < 0.04 mV / h and the capacity retention ratio C / C0 > 95%, then it is judged as a qualified battery (i.e., the judgment is restored to a qualified battery); otherwise, the battery is judged as an unqualified battery and is a scrapped battery.

[0025] In this invention, in specific implementation, step S1 specifically includes the following operations: Step S11, Pre-treatment operation: Perform charge and discharge operation on the battery (i.e., first fully charge and then discharge), and then charge the battery at a constant current of 1C to half of the rated capacity. Step S12, stand: Place the battery in a 45℃ environment for 72 hours; Step S13, room temperature equilibration: Place the battery in a 25℃ environment for 24 hours, and measure and record the battery voltage V1 at this time; Step S14, stand at room temperature: Place the battery in an environment of 25℃ for 72 hours, and measure and record the battery voltage V2 at this time; Step S15: Obtain the battery's self-discharge rate parameter K according to the calculation formula for the self-discharge rate parameter K. Step S16, Long-term storage: Place the battery in a 25°C environment and leave it for 21 days; Step S17, Discharge Measurement: Discharge the battery at a constant current rate of 1C to the discharge cutoff voltage (e.g., 1.5V) and measure the residual capacity C.

[0026] In step S11, the battery is charged and discharged at a rate of 0.5C (i.e., fully charged first, and then discharged completely).

[0027] In step S15, the formula for calculating the self-discharge rate parameter K is as follows: K = ΔOCV / Δt; Where ΔOCV = OCV_initial - OCV_final; ΔOCV is the voltage drop, measured in mV; OCV_initial, or V1, is the battery open-circuit voltage before self-discharge begins; OCV_final, or V2, is the battery open-circuit voltage during the discharged storage time Δt (i.e., discharge duration). Δt = t_storage, where Δt is the room temperature standing time between steps S13 and S14, preferably 72h.

[0028] It should be noted that the formula for calculating the self-discharge rate K is the change in battery voltage divided by the change in time.

[0029] It should be noted that in step S1, the remaining battery capacity C is obtained through a direct energy metering method. This direct energy metering method is a method for obtaining energy consumption by directly reading the displayed values ​​of dedicated instruments such as energy meters. It is characterized by its ease of operation and real-time performance.

[0030] In step S17, specifically, the residual capacity C is obtained by discharging the battery at a constant current rate of 1C to the discharge cutoff voltage (e.g., 1.5V).

[0031] In step S3, specifically, the comprehensive evaluation function F(K,C) of the battery is calculated as follows: F(K,C)=α·K+β·(1-C / C0); In practice, the weighting coefficients α and β are set to values ​​between 0.5 and 0.7 and between 0.3 and 0.5, respectively, and can be determined by regression analysis based on the characteristics of historical samples.

[0032] In this invention, the parameters included in the comprehensive evaluation function F(K,C) for the battery are described as follows: F(K,C) is the Comprehensive Self-discharge Evaluation Index, an engineering judgment value used to characterize the self-discharge risk level of a battery. The smaller the value, the better the self-discharge characteristics. F(K,C) is obtained by calculating K and C / C0 according to weights, and is only used for relative comparison between samples.

[0033] K is the self-discharge rate, which is the rate at which the open-circuit voltage of a battery decays per unit time during storage. The self-discharge rate K is obtained by calculating the change in open-circuit voltage over a test period, i.e., K = ΔOCV / Δt, with units of mV / h.

[0034] C stands for Residual Capacity, which is the actual capacity released by the battery during discharge after the storage test is completed. The residual capacity C is obtained by discharging the battery at a set current to the cutoff voltage at the end of the test, and measured by a charge / discharge device. The unit is Ah or mAh.

[0035] C / C0 is the capacity retention ratio of a battery, a dimensionless proportional parameter used to characterize the level of capacity retention during storage. The capacity retention ratio C / C0 is obtained by calculating the measured residual capacity C and the baseline capacity C0.

[0036] α is the Voltage Decay Weighting Coefficient, used to adjust the influence of the self-discharge rate K in the overall evaluation function. α is obtained by setting values ​​based on sample statistics or experience; the preferred value range for α is 0.5–0.7, and α also helps to eliminate the dimension of K.

[0037] β is the Capacity Loss Weighting Coefficient, used to adjust the influence of the capacity retention ratio in the overall evaluation function. β is obtained by setting values ​​based on sample statistics or experience; the preferred range for β is 0.3–0.5.

[0038] In step S4, specifically, the preset gray area range (i.e., preset numerical range) of the F value is determined by statistical analysis; In step S4, specifically, the preset gray area range (i.e., preset numerical range) of the F value is preferably F1≤F≤F2, where F1=0.04 and F2=0.06; It should be noted that when F > F2, the battery is deemed to be unqualified for self-discharge.

[0039] In a specific implementation of this invention, a temperature drift correction step may also be included: based on real-time monitored temperature data, the K value and battery capacity loss value are corrected to the equivalent results at standard temperature using the well-known Arrhenius equation.

[0040] In specific implementation, for this invention, the charging and discharging operation of the battery is performed by a charging and discharging device with an accuracy of not less than ±0.1%, and the test environment temperature is controlled at 25℃±2℃.

[0041] To better understand the technical solution of the present invention, the following specific embodiments will be used to illustrate the technical solution of the present invention.

[0042] Example 1.

[0043] This invention provides a high-precision screening method for self-discharge of lithium-ion batteries. It tests the self-discharge characteristics of 50,000 lithium titanate batteries with a discharge cutoff voltage of 1.5V. Steps E1 to E8 in the following embodiments are only used to illustrate one specific implementation and do not constitute a limitation on the steps of the claims. Step E1, Pretreatment: After the battery has undergone a 0.5C charge-discharge cycle, it is charged at a constant current of 1C to half of its rated capacity; Step E2, stand: The battery is left to stand at 45℃ for 72 hours; then equilibrate at room temperature: it is left to stand at 25℃ for 24 hours, and the battery voltage V1 is measured and recorded at this time; Step E3, stand at room temperature: Place the battery in a 25℃ environment for 72 hours, and measure and record the battery voltage V2 at this time; Step E4, K value measurement: Based on the recorded V1 and V2, calculate K = ΔOCV / Δt according to the formula for calculating the self-discharge rate parameter K. Step E5, Long-term storage: Leave the battery at 25°C for 21 days; Step E6, Discharge Measurement: Discharge the battery at a constant current of 1C until the battery's discharge cutoff voltage of 1.5V, and measure the battery's residual capacity C. Step E7, Decision Logic: If C / C0 < 95%, it is considered unqualified; If F(K,C) falls between 0.04 and 0.06, it is classified as a battery to be re-examined (e.g., α=0.6, β=0.4). If F < 0.04, it is considered qualified.

[0044] In step E8, the battery to be re-inspected enters the re-inspection workflow, that is, it returns to the execution of steps E1 to E6, and performs a second judgment operation based on the re-inspection results.

[0045] It should be noted that in the secondary judgment operation, the battery must simultaneously meet the following conditions: K < 0.04mV / h and C / C0 > 95% to be restored as a qualified battery and allowed to re-enter the battery production line. Otherwise, it is judged as a scrapped battery.

[0046] To verify the effectiveness of the method of the present invention, a comprehensive test was conducted on 50,000 lithium titanate battery samples. Figure 3 This is a two-dimensional distribution diagram of the electrical K value and capacity ratio (i.e., capacity retention ratio) in Embodiment 1 of the present invention. See [link to diagram]. Figure 3 As shown, the battery sample included: 49,900 normal batteries (99.8%) and 100 batteries with abnormal internal self-discharge (0.2%). The test strictly followed the procedure of Example 1, and the F value was calculated using α=0.6 and β=0.4.

[0047] The test results for multiple batteries (e.g., 10 batteries) are shown in Tables 1 to 4.

[0048] Table 1: Preview of Typical Battery Sample Data

[0049] Table 2: Distribution of Initial Judgment Results (%)

[0050] Table 3: Results of Second MRB Re-inspection (for batteries with 0.5% gray area)

[0051] Table 4: Performance Statistics (Comparison with Traditional K-value Method)

[0052] In Table 4, the false negative rate is defined as the proportion of genuinely abnormal batteries that are mistakenly judged as qualified (the double-insurance mechanism of this invention ensures 0%); the false positive rate is defined as the proportion of normal batteries that are directly judged as unqualified (in the K-value method, 2% originates from voltage plateau fluctuations, which are captured and reworked by the gray zone mechanism of this invention); the final scrap rate refers to the proportion of batteries that fail twice under MRB (only 0.025%, with an error of <0.01% at the 95% confidence interval). Compared with the traditional K-value method, the method of this invention achieves a false negative rate of 0% and a false positive rate reduced by 75%, significantly improving screening robustness and industrial efficiency.

[0053] In summary, the technical solution of this invention establishes a complete system from data acquisition, feature calculation, gray zone determination to MRB re-inspection and handling, taking into account detection accuracy, false positive control, and industrial closed loop. Through dual-indicator synergy and gray zone re-inspection mechanism, the accuracy and stability of lithium-ion battery self-discharge screening are significantly improved, enabling high-consistency quality management in energy storage and power battery production.

[0054] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle 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 high-precision screening method for self-discharge of lithium-ion batteries, characterized in that, Includes the following steps: Step S1: For the battery to be tested, obtain the battery's self-discharge rate parameter K and the battery's residual capacity C. The acquisition process includes steps S11 to S17. Step S2, perform the initial judgment operation: check whether the battery capacity retention ratio C / C0 is less than a preset threshold; where C0 is the initial reference capacity; If so, the battery is determined to be a self-discharge defective battery; Otherwise, proceed to step S3; Step S3: Calculate the battery's comprehensive evaluation function F(K,C) based on the battery's self-discharge rate parameter K and the battery's residual capacity C. Step S4: Detect whether the F value is within the preset gray area range; If so, the battery is identified as a battery to be re-inspected, and the process returns to steps S11 to S17. After the re-inspection, the final judgment is made according to the preset secondary judgment operation. If the F value is greater than the maximum value of the preset gray area range, the battery is determined to be a self-discharge defective battery. If the F value is less than the minimum value of the preset gray area range, the battery is determined to be a qualified self-discharge battery.

2. The high-precision screening method for self-discharge of lithium-ion batteries as described in claim 1, characterized in that, Initial baseline capacity C0 = nominal rated capacity × preset coefficient; The preset coefficient ranges from 30% to 80%.

3. The high-precision screening method for self-discharge of lithium-ion batteries as described in claim 1, characterized in that, Step S1 specifically includes the following operations: Step S11, Pre-treatment operation: Perform charge and discharge operation on the battery, and then charge the battery with a constant current at a 1C rate to half of the rated capacity. Step S12, stand: Place the battery in a 45℃ environment for 72 hours; Step S13, room temperature equilibration: Place the battery in a 25℃ environment for 24 hours, and measure and record the battery voltage V1 at this time; Step S14, stand at room temperature: Place the battery in an environment of 25℃ for 72 hours, and measure and record the battery voltage V2 at this time; Step S15: Obtain the battery's self-discharge rate parameter K according to the calculation formula for the self-discharge rate parameter K. Step S16, Long-term storage: Place the battery in a 25°C environment and leave it for 21 days; Step S17, Discharge Measurement: Discharge the battery at a constant current rate of 1C until the battery reaches its discharge cutoff voltage, and measure the residual capacity C.

4. The high-precision screening method for self-discharge of lithium-ion batteries as described in claim 3, characterized in that, In step S11, the battery is charged and discharged at a rate of 0.5C.

5. The high-precision screening method for self-discharge of lithium-ion batteries as described in claim 3, characterized in that, In step S15, the formula for calculating the self-discharge rate parameter K is as follows: K = ΔOCV / Δt; Where ΔOCV = OCV_initial - OCV_final; ΔOCV is the voltage drop, measured in mV; OCV_initial is the battery open-circuit voltage before self-discharge begins; OCV_final is the battery open-circuit voltage at the discharged storage time Δt. Δt is the storage time, in hours (h).

6. The high-precision screening method for self-discharge of lithium-ion batteries as described in claim 1, characterized in that, In step S2, the preset threshold is 95%.

7. The high-precision screening method for self-discharge of lithium-ion batteries as described in claim 1, characterized in that, In step S3, the comprehensive evaluation function F(K,C) of the battery is calculated as follows: F(K,C)=α·K+β·(1-C / C0); Where K is the self-discharge rate; C is the residual capacity; C / C0 is the battery capacity retention ratio; α is the voltage decay weighting coefficient; β is the capacity loss weighting coefficient.

8. The high-precision screening method for self-discharge of lithium-ion batteries as described in claim 7, characterized in that, The value of α ranges from 0.5 to 0.7; The value of β ranges from 0.3 to 0.

5.

9. The high-precision screening method for self-discharge of lithium-ion batteries as described in claim 1, characterized in that, In step S4, the preset gray area range of F value is: F1≤F≤F2, where F1=0.04 and F2=0.

06.

10. The high-precision screening method for self-discharge of lithium-ion batteries as described in any one of claims 1 to 9, characterized in that, In step S4, the preset secondary judgment operation is as follows: if the battery to be re-inspected, after returning to execute steps S11 to S17 and re-executing step S3, obtains an F value that is less than the minimum value of the preset gray area range and meets the conditions that the battery's self-discharge rate parameter K < 0.04 mV / h and capacity retention ratio C / C0 > 95%, then it is judged as a qualified battery; otherwise, the battery is judged as an unqualified battery.

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