Method for assessing the lifetime consistency of rechargeable cells

Abstract

This invention relates to the field of battery life technology, specifically a method for evaluating the life consistency of rechargeable cells. By performing calendar life tests on a battery system, fitting the capacity decay curve, calculating the capacity decay percentage and lifespan of individual cells in the system, and normalizing the lifespan of individual cells to obtain a life consistency characterization parameter Ca, where Ca follows a normal distribution with a mean of 1. Several individual cells are selected for other life tests, and the lifespan mean under corresponding conditions is calculated. Combined with the life consistency characterization parameter Ca, the lifespan distribution under those conditions can be analyzed, and the battery life consistency under those conditions can be evaluated. This invention improves the accuracy of life consistency evaluation results while significantly reducing the equipment and human resources required for testing.

Description

Technical Field

[0001] This invention relates to the field of battery management technology, specifically a method for evaluating the lifespan consistency of rechargeable battery cells. Background Technology

[0002] The lifespan of the same type of battery cell can vary under the same conditions. A common method for assessing lifespan consistency is to conduct lifespan tests on multiple parallel samples and then observe the lifespan range.

[0003] Conventional methods suffer from several drawbacks. First, the limited number of samples leads to significant randomness in the test results. For example, a normal distribution typically requires more than 30 samples to achieve a good distribution fit, but it is difficult to arrange so many parallel samples in actual testing. Second, large sample sizes require substantial testing resources and manpower. Finally, conventional methods lack sufficient accuracy in their test results.

[0004] Therefore, a highly accurate method for assessing the consistency of battery cell lifespan is needed. Summary of the Invention

[0005] To address the aforementioned issues, this invention provides a method for evaluating the lifespan consistency of rechargeable battery cells, which offers high accuracy.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] The method for assessing the lifespan consistency of rechargeable battery cells includes the following steps:

[0008] S1, perform a life test on the battery system, causing the battery system to enter a state of rapid degradation.

[0009] S1 specifically includes the following steps:

[0010] S101, Measure the initial capacity of the battery system;

[0011] S102 uses stepped charging to charge the battery system to 100% SOC;

[0012] S103, storing the battery system in a high-temperature environment;

[0013] S104, at set high-temperature storage times, perform room temperature capacity recovery rate tests and charge the battery system to 100% SOC;

[0014] S105, repeat steps S103-S104 to bring the battery system into a rapid degradation state.

[0015] S2, Fit the capacity decay curve of the battery system and calculate the chemical system coefficients in the decay curve;

[0016] S3, Remove the individual cells from the battery system and perform capacity testing;

[0017] S4. Based on the individual cell capacity and the initial capacity of the individual cell obtained in step S3, calculate the capacity decay percentage of each individual cell.

[0018] S5, calculate the lifespan of each individual cell to obtain the lifespan and average lifespan of the individual cell.

[0019] S6, normalize the mean of the lifespan of all individual cells in the system to obtain a consistency standard parameter Ca with a mean of 1 and a standard deviation of σ.

[0020] S7. Select several individual cells for life testing in different ways, and perform life consistency analysis and calculation.

[0021] Stepped charging specifically involves charging the battery system with a constant current in multiple stages until the battery system is charged to 100% SOC. Between each stage, the charging current gradually decreases.

[0022] Optionally, in one embodiment of the present invention, the specific steps of the stepped charging in step S102 are as follows:

[0023] S1021 uses a high current to charge the battery system at a constant current until it reaches 80% SOC.

[0024] S1022 uses a small current to charge the battery system at a constant current until it reaches 100% SOC.

[0025] Optionally, in one embodiment of the present invention, the high-temperature storage time in step S104 is ≤30 days. In the initial stage, the battery degrades faster, and in the later stage, the battery degrades slower. Therefore, in the initial stage, the high-temperature storage time interval is shorter, generally 7 days, and in the later stage, the high-temperature storage time interval is longer, generally 30 days.

[0026] Optionally, in one embodiment of the present invention, the room temperature recovery rate test in step S104 is specifically as follows;

[0027] S1041, Before high-temperature storage, the battery system is tested for capacity, and the battery capacity is found to be C0.

[0028] S1042, After high-temperature storage, the battery system is taken out and discharged at room temperature, including two discharge stages: the first stage, the discharge capacity is C1; the second stage, the battery system is charged to 100% SOC and then discharged, the discharge capacity is C2.

[0029] S1043, calculate the capacity recovery rate and capacity retention rate. Capacity recovery rate = C2 / C0 * 100%, capacity retention rate = C1 / C0 * 100%.

[0030] Optionally, in one embodiment of the present invention, the relationship between the capacity decay percentage and storage time in step S4 is as follows:

[0031] Q = a * t z

[0032] Where Q represents the capacity decay percentage, which is variable and varies depending on the product; t represents the storage time; Z is a coefficient determined by the chemical system; and a is the constant to be fitted.

[0033] Q = a * t z

[0034] Taking the natural logarithm of both sides transforms it into a linear equation, as follows:

[0035] Q = a * t z

[0036] This can be transformed into In(Q) = z*In(t) + In(a); Let y = In(Q), X = In(t), b = In(a), k = z, then In(Q) = z*In(t) + In(a) can be expressed as a linear equation y = kx + b with slope k and intercept In(a).

[0037] Specific explanation: During the battery system calendar life test, the percentage of capacity decay and storage time are logarithmicd to obtain In(Q) and In(t). Let In(Q) be the vertical axis and In(t) be the horizontal axis to draw a scatter plot. The curves of In(Q) and In(t) are linearly fitted, and the resulting slope is the z value.

[0038] Since the batteries in a battery system are of the same model, they have the same chemical system, so the z-value of a single cell in the battery system is the same as the z-value of the battery system.

[0039] Optionally, in one embodiment of the present invention, the calculation of the lifespan of a single battery cell in step S5 specifically includes the following steps:

[0040] S501, through step S4, the capacity decay percentages Q1, Q2, Q3...Qn of each individual cell are obtained;

[0041] S502, according to Q=a*t z It is derived that Substitute the capacity decay percentages Q1, Q2, Q3...Qn of each individual cell into... The fitting constants a1, a2, a3...an corresponding to each individual battery cell are obtained;

[0042] S503, according to Q=a*t z Derivation Substitute the percentage of capacity decay at the end of the life of a single battery cell into... The predicted lifespan of a single cell in the battery system can be obtained. L represents the predicted lifespan of a single battery cell, with the lifespans of each cell designated as L1, L2, L3...Ln, and the average lifespan being L. 均值 .

[0043] Optionally, in one embodiment of the present invention, the method for calculating the consistency standard parameter Ca in step S6 specifically includes the following steps:

[0044] S601, lifespan of each individual cell substituted Where Ca is the normalized lifetime value, the normalized lifetime values ​​Ca1, Ca2, Ca3...Can corresponding to each individual cell can be obtained;

[0045] S602, the normalized lifetime value Ca follows a normal distribution with a mean of 1 and a standard deviation of σ. The standard deviation σ is calculated from the normalized lifetime value and denoted as Ca ~ N(1,σ). 2 );

[0046] S603, the lifetime consistency parameter Ca is quantitatively characterized by a distribution in which the lifetime consistency of a single cell is represented. Its 3σ lifetime lower limit is: (1-3σ)*L 均值 .

[0047] Optionally, in one embodiment of the present invention, the average lifespan of a single battery cell can be obtained in step S7 as L. 均值 The lifespan distribution of a single battery cell is L = Ca * L 均值 .

[0048] Beneficial effects of the invention

[0049] 1. The rechargeable cell lifespan consistency assessment method of the present invention obtains the capacity decay percentage of each individual cell and calculates the lifespan of each individual cell. The consistency standard parameter Ca is obtained by normalizing the average lifespan. Lifespan consistency analysis and calculation are performed under other conditions, which improves the accuracy of lifespan assessment.

[0050] 2. Lifetime consistency under other conditions can be derived from the test results;

[0051] 3. As can be seen from the above, this method improves accuracy while reducing the required time, allowing more time to be devoted to research and development, thus increasing the speed of research and development. Attached Figure Description

[0052] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0053] Figure 1 Schematic diagram of the overall process of Embodiment 1 of the present invention;

[0054] Figure 2 Schematic diagram of the life test process in Embodiment 1 of the present invention;

[0055] Figure 3 Schematic diagram of the capacity recovery rate test process in Embodiment 1 of the present invention;

[0056] Figure 4 Example 1 of this invention: Lifetime calculation steps for a single battery cell;

[0057] Figure 5 The calculation steps of the consistency standard parameter Ca in Embodiment 1 of the present invention. Detailed Implementation

[0058] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structure, features and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided below.

[0059] Example 1

[0060] The conventional method for assessing battery life consistency involves conducting life tests on multiple parallel samples and then observing the life range. However, the accuracy of conventional battery cell life assessment methods is not high and is limited by the number of samples. Setting up a large number of samples requires a significant amount of testing resources and manpower.

[0061] Cell lifespan under identical testing conditions is a random quantity. Reliability analysis of cell lifespan requires consistency analysis of cell lifespan under the same conditions and quantitative characterization using statistical methods such as lifespan distribution analysis.

[0062] Therefore, a method for evaluating the lifespan of battery cells has been developed, which is the rechargeable battery cell lifespan consistency evaluation method of this application.

[0063] This method is generally divided into five steps: battery system calendar life test - fitting of battery system capacity decay curve - individual cell capacity test - individual cell life calculation - calculation of life consistency parameters. This evaluation method is not only applicable to battery calendar life test, but also to other life tests such as cycle life test and operating condition life test. In comparison, the reason for choosing calendar life test is that during cycle life, there are voltage and temperature differences between individual cells when the battery system is cycled, which will interfere with the cell life and cannot accurately reflect the life consistency of the cell itself. However, during calendar life, the voltage and temperature differences are very small and will not cause additional interference, so it most accurately reflects the consistency of cell life. In addition, high temperature test is shorter than room temperature test.

[0064] Therefore, in this method, a high-temperature calendar life test is used for the battery system.

[0065] The evaluation method specifically includes the following steps:

[0066] S1, conducts a high-temperature calendar life test on the battery system, causing the battery system to enter a rapid degradation state.

[0067] S1 specifically includes the following steps:

[0068] S101, Measure the initial capacity of the battery system;

[0069] S102 uses stepped charging to charge the battery system to 100% SOC;

[0070] The stepped charging in step S102 specifically involves performing constant current charging on the battery system in multiple stages until the battery system is charged to 100% SOC. Between stages, the charging current gradually decreases. In this embodiment, the battery system is charged in two stages, and the specific steps are as follows:

[0071] S1021 uses a high current to charge the battery system at a constant current until it reaches 80% SOC.

[0072] S1022 uses a small current to charge the battery system at a constant current until it reaches 100% SOC.

[0073] For example:

[0074] A certain LFP / Gra lithium-ion battery system has the following stages: Stage 1, charging at 75A to the maximum single-cell voltage reaches 3.55V; Stage 2, charging at 35A to the maximum single-cell voltage reaches 3.60V; Stage 3, charging at 10A to the maximum single-cell voltage reaches 3.65V.

[0075] There is no limit to the number of charging stages in a stepped charging system; it can be divided according to actual needs.

[0076] S103, storing the battery system in a high-temperature environment;

[0077] S104, at set high-temperature storage times, perform room temperature capacity recovery rate tests and charge the battery system to 100% SOC;

[0078] In step S104, the high-temperature storage time is ≤30 days. In the initial stage, the battery degrades faster, while in the later stage, the battery degrades slower. Therefore, in the initial stage, the high-temperature storage interval is shorter, generally 7 days, and in the later stage, the high-temperature storage interval is longer, generally 30 days. If it is necessary to improve the accuracy of the data, the shorter the interval, the more accurate the data, but it requires a lot of manpower.

[0079] The room temperature recovery rate test in step S104 is as follows;

[0080] S1041, Before high-temperature storage, the battery system is tested for capacity, and the battery capacity is found to be C0.

[0081] S1042, After high-temperature storage, the battery system is taken out and discharged at room temperature, including two discharge stages: the first stage, the discharge capacity is C1; the second stage, the battery system is charged to 100% SOC and then discharged, the discharge capacity is C2.

[0082] S1043, calculate the capacity recovery rate and capacity retention rate. Capacity recovery rate = C2 / C0 * 100%, capacity retention rate = C1 / C0 * 100%.

[0083] S105, repeat steps S103-S104 to bring the battery system into a rapid degradation state, which is the end of the battery system's lifespan. The data is most accurate when the battery system degrades to EOL. Even if the capacity has not degraded to EOL, the predicted lifespan of the battery cell can be calculated according to the calculation formulas in S501 to S503.

[0084] Among them, steps S101-S105 are in the life test stage, which in this embodiment is the high temperature calendar life test.

[0085] S2, fit the capacity decay curve of the battery system and calculate the chemical system coefficient in the decay curve, which is the curve of capacity decay percentage versus storage time.

[0086] S3, Remove the individual cells from the battery system and perform capacity testing;

[0087] S4. Based on the individual cell capacity and the initial capacity of the individual cell obtained in step S3, calculate the capacity decay percentage of each individual cell. Here, the individual cell capacity obtained in step S3 is represented by B, and the initial capacity of the individual cell is represented by A. The capacity decay percentage Q = B / A*100%, where the initial capacity of the individual cell is the capacity obtained in the capacity grading process, which is known data.

[0088] The capacity decay mechanism of lithium-ion batteries under normal use is mainly due to the growth of the SEI film leading to the consumption of active lithium ions. The relationship between the capacity decay percentage and storage time in step S4 is as follows:

[0089] Q = a * t z

[0090] Where Q is the capacity decay percentage, which is variable and varies depending on the product; t is the storage time; Z is a coefficient determined by the chemical system; and a is the constant to be fitted.

[0091] right

[0092] Q = a * t z

[0093] Taking the natural logarithm of both sides transforms it into a linear equation, as follows:

[0094] Q = a * t z

[0095] This can be transformed into In(Q) = z*In(t) + In(a); Let y = In(Q), X = In(t), b = In(a), k = z, then In(Q) = z*In(t) + In(a) can be expressed as a linear equation y = kx + b with slope k and intercept In(a).

[0096] Specific explanation: During the battery system calendar life test, the percentage of capacity decay and storage time are logarithmicd to obtain In(Q) and In(t). Let In(Q) be the vertical axis and In(t) be the horizontal axis to draw a scatter plot. The curves of In(Q) and In(t) are linearly fitted, and the resulting slope is the z value.

[0097] Since the batteries in a battery system are of the same model, they have the same chemical system, so the z-value of a single cell in the battery system is the same as the z-value of the battery system.

[0098] S5, calculate the lifespan of each individual cell to obtain the lifespan and average lifespan of the individual cell.

[0099] The lifespan calculation for a single battery cell in step S5 specifically includes the following steps:

[0100] S501, through step S4, the capacity decay percentages Q1, Q2, Q3...Qn of each individual cell are obtained;

[0101] S502, according to Q=a*t z It is derived that Substitute the capacity decay percentages Q1, Q2, Q3...Qn of each individual cell into... The fitting constants a1, a2, a3...an corresponding to each individual battery cell are obtained;

[0102] S503, according to Q=a*t z Derivation Substitute the percentage of capacity decay at the end of the life of a single battery cell. Here, we calculate the predicted lifespan of each individual cell in the test system based on a typical capacity degradation percentage of 20%. L represents the predicted lifespan of a single battery cell, with the lifespans of each cell designated as L1, L2, L3...Ln, and the average lifespan being L. 均值 .

[0103] S6, the consistency standard parameter Ca is obtained by normalizing the average lifespan of individual cells;

[0104] The calculation method for the consistency standard parameter Ca in step S6 specifically includes the following steps:

[0105] S601, lifespan of each individual cell substituted Where Ca is the normalized lifetime value, the normalized lifetime values ​​Ca1, Ca2, Ca3...Can corresponding to each individual cell can be obtained;

[0106] S602, the normalized lifetime value Ca follows a normal distribution with a mean of 1 and a standard deviation of σ. The standard deviation σ is calculated from the normalized lifetime value and is denoted as Ca~N(1,σ2).

[0107] S603, the lifetime consistency parameter Ca is quantitatively characterized by a distribution in which the lifetime consistency of a single cell is represented. Its 3σ lifetime lower limit is: (1-3σ)*L 均值 .

[0108] S7. Select several individual cells for further life tests and perform life consistency analysis calculations.

[0109] In step S7, the average lifespan of a single battery cell can be obtained as L. 均值 The lifespan distribution of a single battery cell is L = Ca * L 均值 .

[0110] The following example illustrates step S7:

[0111] For example, if the standard deviation σ in step S6 is calculated to be 0.1, and then 5 individual cells are selected for cyclic testing at room temperature (25℃), with an average cycle life of 2000 cycles, then its 3σ lifespan lower limit is (1-3*0.1)*2000=1400 cycles; assuming the average cycle lifespan is 1000 cycles, its 3σ lifespan lower limit is (1-3*0.1)*1000=700 cycles. This example uses the number of cycles for easier understanding.

[0112] The following is an example illustrating steps S6 and S7:

[0113] ① Take two batteries of a certain model and measure their lifespans under condition A, let them be X1 and X2. The average lifespan is... Its standard deviation σ is calculated as follows:

[0114]

[0115] ② Assuming the acceleration factor of condition B relative to condition A is 2, meaning the lifetime of condition 2 is twice that of condition 1, then its test lifetimes are 2X1 and 2X2, and its average lifetime is Its standard deviation σ1 is calculated as follows:

[0116]

[0117] ③In summary, if the product lifespan under condition A follows a mean of... A normal distribution with standard deviation σ.

[0118] Then, under condition B, the product lifespan follows a mean of . A normal distribution with a standard deviation of 2σ can be characterized as:

[0119] like but That is, when the mean lifespan of the same product increases or decreases, its standard deviation also increases or decreases proportionally.

[0120] Example 2

[0121] In this embodiment, the calendar life test in S1 of Example 1 is changed to a cycle life test. The specific steps are as follows:

[0122] S101, Measure the initial capacity of the battery system;

[0123] S102 uses stepped charging to charge the battery system to 100% SOC;

[0124] The stepped charging in step S102 specifically involves performing constant current charging on the battery system in multiple stages until the battery system is charged to 100% SOC. Between stages, the charging current gradually decreases. In this embodiment, the battery system is charged in two stages, and the specific steps are as follows:

[0125] S1021 uses a high current to charge the battery system at a constant current until it reaches 80% SOC.

[0126] S1022 uses a small current to charge the battery system at a constant current until it reaches 100% SOC.

[0127] S103, the battery system is put on hold;

[0128] S104 performs constant current discharge on the battery system and then sets it aside after discharging to the cutoff voltage.

[0129] The resting time in steps S103 and S104 is generally 30 min to 12 h.

[0130] S105, repeating steps S102-S104, causes the battery system to enter a rapid degradation state, which is the end of the battery system's lifespan; in Example 2, t is the number of cycles x, and other parameters remain unchanged.

[0131] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for evaluating the lifespan consistency of rechargeable battery cells, characterized in that... This includes the following steps: S1, Perform a life test on the battery system to induce a rapid degradation state in the battery system; S2, plot the capacity decay curve of the battery system when it enters the rapid decay state, fit the capacity decay curve, and calculate the chemical system coefficient in the capacity decay curve. S3, Remove individual battery cells from the battery system and perform capacity testing on each individual cell; S4, Calculate the capacity decay percentage of each individual cell based on multiple capacity data obtained from the capacity testing and the initial capacity of the individual cell; The relationship between the capacity decay percentage and storage time in step S4 is as follows: Where Q is the capacity decay percentage, t is the storage time, Z is the coefficient determined by the chemical system, and a is the constant to be fitted. S5, calculate the lifespan of each individual cell to obtain the lifespan and average lifespan of the individual cell. S6, normalize the mean of the lifespan of all individual cells in the battery system to obtain a consistency standard parameter Ca with a mean of 1 and a standard deviation of σ; the calculation method of the consistency standard parameter Ca in step S6 specifically includes the following steps: S601, lifespan of each individual cell substituted Where Ca is the normalized lifetime value, the normalized lifetime values ​​Ca1, Ca2, Ca3...Can for each individual cell can be obtained, and L is the predicted lifetime of the individual cell. This represents the average lifespan. S602, the normalized lifetime value Ca follows a normal distribution with a mean of 1 and a standard deviation of σ. The standard deviation σ is calculated from the normalized lifetime value and is denoted as σ. ; S603, the lifetime normalized value Ca quantitatively characterizes the lifetime consistency of a single cell in the form of a distribution, and its 3σ lifetime lower limit is: S7, Select several individual cells for life testing in different ways, and perform life consistency analysis and calculation.

2. The method for evaluating the lifespan consistency of rechargeable battery cells according to claim 1, characterized in that: The life test in S1 specifically includes the following steps: S101, Measure the initial capacity of the battery system; S102 uses stepped charging to charge the battery system to 100% SOC; S103, storing the battery system in a high-temperature environment; S104, at set high-temperature storage times, perform room temperature capacity recovery rate tests and charge the battery system to 100% SOC; S105, repeat steps S103-S104 to make the battery system work in a rapid degradation state.

3. The method for evaluating the lifespan consistency of rechargeable battery cells according to claim 2, characterized in that: The stepped charging in S102 specifically involves constant current charging of the battery system in multiple stages until the battery system is charged to 100% SOC. Between the stages, the charging current gradually decreases.

4. The method for evaluating the lifespan consistency of rechargeable battery cells according to claim 2, characterized in that: The high-temperature storage time in S104 is less than or equal to 30 days.

5. The method for evaluating the lifespan consistency of rechargeable battery cells according to claim 2, characterized in that: The room temperature recovery rate test in S104 specifically includes: S401, Before high-temperature storage, the battery system is tested for capacity, and the battery capacity is found to be C0. S402, After high-temperature storage, the battery system is taken out and discharged at room temperature, including two discharge stages: the first stage, the discharge capacity is C1; the second stage, the battery system is charged to 100% SOC and then discharged, the discharge capacity is C2. S403, calculate capacity recovery rate and capacity retention rate; Capacity recovery rate = C2 / C0 * 100%, Capacity retention rate = C1 / C0 * 100%.

6. The method for evaluating the lifespan consistency of rechargeable battery cells according to claim 1, characterized in that: The calculation of the lifespan of a single battery cell in S5 specifically includes the following steps: S501, obtain the capacity decay percentages Q1, Q2, Q3...Qn of each individual cell through step S4; S502, according to... It is derived that Substitute the capacity decay percentages Q1, Q2, Q3...Qn of each individual cell into the equation. The fitting constants a1, a2, a3...an corresponding to each individual cell are obtained; S503, according to Derivation Substitute the percentage of capacity degradation at the end of the life of a single cell into , can be obtained L represents the predicted lifespan of a single battery cell, and the lifespans of each single cell are L1, L2, L3...Ln, with an average lifespan of... .

7. The method for evaluating the lifespan consistency of rechargeable battery cells according to claim 1, characterized in that: The average lifespan of a single battery cell can be obtained from S7. The lifespan distribution of a single battery cell is L=Ca* .

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