Method for testing static SOC-OCV of lithium ion battery

Abstract

The invention relates to the technical field of lithium battery manufacturing, in particular to a lithium ion battery static SOC-OCV testing method, which comprises the following steps of (1) DCIR testing and SOC partitioning, (2) determining OCV steady-state time of different SOC intervals, and (3) adjusting load and testing OCV at different temperatures. By introducing a partitioning strategy based on direct current internal resistance (DCIR) difference of different SOC points, and determining standing time required for the OCV voltage of each partition to be stable, the defect that unified standing time is adopted for all SOC points in the prior art is overcome, the difference of the polarization relaxation characteristics of the battery in different SOC states is fully considered, and the stability of the battery is improved. Therefore, the accuracy of OCV test data is obviously improved.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery technology, specifically relating to a test method for static SOC-OCV of lithium-ion batteries. Background Technology

[0002] Lithium-ion batteries, due to their high specific energy and long cycle life, have been widely used in consumer electronics, electric bicycles, new energy vehicles, and energy storage systems. The battery's state of charge (SOC), as a key parameter reflecting its current remaining capacity, directly affects the battery management system's judgment of driving range, energy status, and safety limits. Under certain conditions, there is a correlation between the battery's open circuit voltage (OCV) and SOC. Therefore, accurately obtaining the OCV values ​​corresponding to different SOC points, especially static SOC-OCV data under multiple temperature conditions, is of great significance for improving the accuracy of SOC estimation and optimizing vehicle energy management.

[0003] Currently, there are various SOC-OCV testing methods for lithium-ion batteries in the industry, but they still have certain limitations: For example, patent CN112014752A proposes a lithium battery SOC-OCV testing method, which involves multiple capacity calibrations of the battery at different temperatures, and then gradually adjusting the SOC and testing the OCV at the same temperature. This method suffers from long testing cycles and low efficiency due to the need for repeated capacity calibration and SOC adjustments at different temperatures. Furthermore, the short resting time before the OCV test (e.g., 30 minutes) means the battery voltage has not yet fully stabilized, affecting the accuracy of the test results.

[0004] Patent CN112130080B discloses a method for measuring the SOC-OCV curve of a power lithium-ion battery at low temperatures. This method attempts to make the dynamic and static OCV curves match by comparing them and adjusting the current. However, due to the objective existence of battery polarization effects, the dynamic OCV and static OCV are difficult to completely coincide. The theoretical basis of this method deviates from the actual physical characteristics, thus limiting the accuracy of its measurement results.

[0005] Patent CN113985286A discloses a method for SOC-OCV testing at different temperatures. It calibrates the capacity at room temperature but adjusts the SOC by charging and discharging at different temperatures. Since the actual discharge capacity of a battery varies significantly with temperature, this method is prone to failing to discharge the full calibrated capacity under low-temperature conditions due to prematurely reaching the voltage cutoff point, thus introducing systematic errors.

[0006] The patent with publication number CN115128478A proposes adjusting the SOC at room temperature, then allowing the battery to stand at various temperature points and testing the OCV. This method is somewhat similar to the concept of this invention. However, it has significant shortcomings: First, it does not fully consider the time required for the battery to reach thermal equilibrium at low temperatures, and it sets a fixed standing time (e.g., 2-5 hours) for all SOC points, failing to consider the differences in the polarization relaxation process of the battery under different SOC states; second, by repeatedly placing and testing the same battery sample at different temperatures, the battery capacity may change during temperature cycling, causing a shift in the correspondence between the final measured OCV and the initial calibrated SOC, affecting the accuracy of the data.

[0007] In summary, existing SOC-OCV testing methods generally suffer from problems such as low testing efficiency, insufficient consideration of polarization differences and relaxation characteristics at different SOC points, unreasonable setting of resting time, and potential introduction of capacity drift errors during the testing process. Summary of the Invention

[0008] The purpose of this invention is to provide a test method for static SOC-OCV of lithium-ion batteries, so as to better meet the needs of battery management systems for accurate SOC estimation.

[0009] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for testing the static SOC-OCV of lithium-ion batteries, comprising the following steps: 1. DCIR testing and SOC partitioning: 1.1 At room temperature T1, the battery under test is subjected to charge-discharge cycles to determine its nominal capacity C0 at room temperature T1; 1.2 Measure the charging DC internal resistance and discharging DC internal resistance of the battery under test at different SOC points; 1.3. Using the charging DC internal resistance value at 50% SOC as a benchmark, calculate the difference between the charging DC internal resistance values ​​at other SOC points and this benchmark value, and divide all SOC points into N charging SOC intervals according to a preset difference threshold range; and, Using the discharge DC internal resistance value of 50% SOC as a benchmark, calculate the difference between the discharge DC internal resistance values ​​of other SOC points and this benchmark value, and divide all SOC points into M discharge SOC intervals according to the preset difference threshold range. 2. Determine the steady-state time of OCV in different SOC ranges: 2.1 Select one representative SOC point from each of the N charging SOC intervals as N target charging SOC points; select one representative SOC point from each of the M discharging SOC intervals as M target discharging SOC points; 2.2 Prepare multiple sets of battery samples, each set containing at least two batteries, and adjust one or more sets of batteries to the same target charging SOC point or target discharging SOC point; 2.3 At room temperature T1, each group of batteries at the same target SOC point was left to stand for a long time, and the OCV values ​​of each battery at multiple time points were recorded. 2.4. Using the OCV value at the end of the resting period as a reference, calculate the difference between the OCV value at each time point and the reference value. When the OCV difference of all batteries in the same group at a certain time point is less than or equal to the preset voltage stability threshold, then the time point is determined as the OCV steady-state time tN of the SOC interval to which the target SOC point belongs. 3. Adjust the load and test the OCV at different temperatures: 3.1 Prepare multiple battery samples and adjust them to multiple different SOC states at room temperature T1. Each SOC state corresponds to at least two parallel samples. 3.2. Place samples in different SOC states simultaneously in an environment at the first test temperature T2 and let them stand for a first preset time. The first preset time is the sum of the thermal equilibrium time tx at temperature T2 and the OCV steady-state time tN of each SOC state interval. 3.3 After the settling period, measure and record the OCV values ​​of all samples within the specified time (2-48 hours). Take the average value of the OCV values ​​measured for samples with the same SOC state as the OCV value of that SOC state at temperature T2. 3.4. Change to the second test temperature T3, and repeat steps 3.2 to 3.3 to obtain the OCV value of this SOC state at temperature T3; 3.5 Repeat steps 3.1 to 3.4 to obtain the OCV values ​​of different SOC states at multiple temperatures during the charging and discharging processes, thereby obtaining a complete static SOC-OCV data table.

[0010] Preferably, in step 1.3, the preset difference threshold range is as follows: difference ≤ 5% is divided into the first interval, 5% < difference ≤ 10% is divided into the second interval, and so on.

[0011] Preferably, in step 2.4, the long-term static time is not less than 48 hours, and the voltage stability threshold is 2mV.

[0012] Preferably, in step 3.1, adjusting the SOC state includes: for the discharge process, discharging the battery to multiple discrete points between 0% SOC and 95% SOC at room temperature T1; and for the charging process, charging the battery to multiple discrete points between 5% SOC and 100% SOC at room temperature T1.

[0013] Preferably, the room temperature T1 is 25±3℃.

[0014] Preferably, the values ​​of the temperatures to be measured, T2 and T3, range from -35℃ to 60℃.

[0015] Preferably, the method for measuring the DC internal resistance value in step 1.2 includes: a. After adjusting the battery to a specific SOC point and allowing it to stand until thermal equilibrium is reached, record its first OCV value; b. Apply a pulse discharge or pulse charge current lasting for a first duration Δt1, and record the second OCV value at the end of the pulse; c. Calculate the discharge DC internal resistance or charging DC internal resistance at the SOC point based on the first OCV value, the second OCV value, and the amplitude of the pulse current.

[0016] Preferably, the first duration Δt1 is 10 seconds, the amplitude of the pulse discharge current is the maximum pulse discharge current of the battery at the corresponding SOC point, and the amplitude of the pulse charging current is the maximum pulse charging current of the battery at the corresponding SOC point.

[0017] Preferably, in steps 3.2 and 3.4, for the 0% SOC state, the OCV steady-state time tN is determined with reference to the tN value determined for the 5% SOC discharge range; for the 100% SOC state, the OCV steady-state time tN is determined with reference to the tN value determined for the 95% SOC charging range.

[0018] Preferably, after completing the OCV test at all temperatures, all test samples are recalibrated at room temperature T1 to verify whether the battery capacity changes significantly during the test.

[0019] The beneficial effects of this invention are as follows: First, by introducing a partitioning strategy based on the difference in DC internal resistance (DCIR) at different SOC points, and determining the resting time required for the OCV voltage to stabilize for each partition, this overcomes the drawback of the existing technology that uses a uniform resting time for all SOC points. It fully considers the differences in battery polarization relaxation characteristics under different SOC states, thereby significantly improving the accuracy of OCV test data. Second, by using a scheme that uniformly adjusts multiple battery samples to different SOC states at room temperature and then performs parallel tests at each test temperature, the overall test cycle is significantly shortened through batch testing, improving efficiency. Furthermore, it avoids the capacity drift problem that may be caused by repeated resting and testing of a single sample at different temperatures, ensuring the stability and reliability of the test results. Third, this method combines the necessary DCIR test data with SOC-OCV testing, eliminating the need for additional complex test items. The process is clear, highly operable, and easy to implement in battery R&D and quality inspection. Attached Figure Description

[0020] Figure 1 This is a flowchart of the present invention; Figure 2 The tx values ​​are the recommended values ​​for different temperatures in this invention. Detailed Implementation

[0021] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings and embodiments. It should be noted that the following embodiments are only used to explain the present invention and do not constitute a limitation on the scope of protection of the present invention.

[0022] like Figure 1 As shown, a test method for static SOC-OCV of lithium-ion batteries includes... 1. DCIR testing and SOC partitioning 1.1 At room temperature T1, allow the battery to stand for 2 hours until thermal equilibrium is reached. Discharge at a constant current of 1 / 3C or 1C to the cutoff voltage U0, and allow it to stand for 30 / 60 minutes. Then charge at a constant current of 1 / 3C to 1C to the battery cutoff voltage U1. Switch to constant voltage charging until the current drops to I1, then stop charging and allow it to stand for 30 / 60 minutes. Repeat this process 5 times. If the capacity difference between the last 3 tests does not exceed 3% of the rated capacity, the test can be stopped early. Take the average discharge capacity of the last 3 tests as the nominal capacity C0 of the cell at room temperature T1. 1.2. At room temperature T1, allow the system to stand for 120 minutes until thermal equilibrium is reached. Discharge at a constant current of 1 / 3C or 1C to 95% SOC, and allow it to stand for 120 minutes until thermal equilibrium is reached. Record the voltage value OCV1 at the 120-minute mark. Discharge with discharge current m1 for 10 seconds and record the voltage value OCV2 at the 10-second mark. Charge with a constant current of 1 / 3C or 1C for t2 seconds, then reduce the SOC to 95% SOC. Allow it to stand for 120 minutes until thermal equilibrium is reached, and record the voltage value OCV3 at the 120-minute mark. Charge with charging current m2 for 10 seconds and record the voltage value OCV4 at the 10-second mark. Calculate the discharge DCIR at 95% SOC as DCIR95%discharge = (OCV1 - OCV2) / m1 * 1000, and the charging DCIR at 95% SOC as DCIR95%charge = (OCV4 - OCV3) / m2 * 1000. The unit of DCIR is mΩ.

[0023] 1.3. At room temperature T1, allow the system to stand for 120 minutes until thermal equilibrium is reached. Discharge at a constant current of 1 / 3C or 1C to 90% SOC, and allow it to stand for 120 minutes until thermal equilibrium is reached. Record the voltage value OCV1 at the 120-minute mark. Discharge with discharge current m1 for 10 seconds and record the voltage value OCV2 at the 10-second mark. Charge with a constant current of 1 / 3C or 1C for t2 seconds, then reduce the SOC to 90% SOC. Allow the system to stand for 120 minutes until thermal equilibrium is reached, and record the voltage value OCV3 at the 120-minute mark. Charge with charging current m2 for 10 seconds and record the voltage value OCV4 at the 10-second mark. Calculate the discharge DCIR at 90% SOC as DCIR90%discharge = (OCV1 - OCV2) / m1 * 1000, and the charging DCIR at 90% SOC as DCIR90%charge = (OCV4 - OCV3) / m2 * 1000. The unit of DCIR is mΩ.

[0024] 1.4 Repeat steps 1.2 and 1.3 to obtain discharge DCIR and charge DCIR at 80%, 70%, 20%, 10%, and 5% SOC.

[0025] 1.5 Based on the discharge DCIR under different SOCs, using the discharge DCIR at 50% SOC as the benchmark, calculate the difference between the discharge DCIR at other SOCs and the discharge DCIR at 50% SOC. Divide the DCIR difference into 5% zones, i.e., DCIR difference ≤ 5% is considered as the same steady-state zone, 5% < DCIR difference ≤ 10% is considered as the same steady-state zone, and so on, until all SOCs are divided into zones, resulting in N discharge SOC intervals.

[0026] 1.6. Based on the charging DCIR under different SOCs, using the charging DCIR at 50% SOC as the benchmark, calculate the difference between the charging DCIR of other SOCs and the charging DCIR at 50% SOC. Divide the DCIR difference into 5% zones, i.e., DCIR difference ≤ 5% is considered as the same steady-state zone, 5% < DCIR difference ≤ 10% is considered as the same steady-state zone, and so on, until all SOCs are divided, resulting in N charging SOC intervals.

[0027] 2. Test the steady-state time of OCV in different SOC ranges. 2.1. Take one SOC state from each of the N charging SOC intervals as the charging SOC for steady-state time testing to obtain N target charging SOCs; take one SOC state from each of the N discharging SOC intervals as the discharging SOC for steady-state time testing to obtain N target discharging SOCs. 2.2 Take 4N samples and let them stand at room temperature (T1) for 2 hours until thermal equilibrium is reached. Discharge them at a constant current of 1 / 3C or 1C to the cutoff voltage U0, and let them stand for 30 / 60 minutes. Then charge them at a constant current of 1 / 3C to 1C to the battery cutoff voltage U1, and switch to constant voltage charging until the current drops to I1. Stop charging and let them stand for 30 / 60 minutes. Repeat this process 5 times. If the capacity difference of 3 consecutive tests does not exceed 3% of the rated capacity, the test can be stopped early. Take the average discharge capacity of the last 3 tests as the nominal capacity C0 of each cell at room temperature (T1). 2.3 Take 2N samples, grouping them into sets of two. Allow them to stand at room temperature (T1) for 120 minutes until thermal equilibrium is reached. Then, discharge them at a constant current of 1 / 3C or 1C to the target SOC1, SOC2, ... SOCN, obtaining N groups of samples with discharged SOC states. 2.4 Take another 2N samples, grouping them into sets of two cells. Allow them to stand at room temperature (T1) for 120 minutes until thermal equilibrium is reached. Then charge them at a constant current of 1 / 3C or 1C to the target SOC1, SOC2, ..., SOCN, obtaining N groups of samples with different SOC states. 2.5. The N groups of discharged SOC samples and the N groups of charged SOC samples were left to stand at room temperature T1 for 48 hours, and the OCV values ​​of each sample were recorded at 2h, 4h, 8h, 12h, 24h, 36h and 48h. 2.6. Using the OCV value at 48h as the baseline OCV, calculate the difference S between the OCV values ​​at other time points and the baseline OCV. When the difference S between two samples in the same SOC state at a certain time point is within 2mV, that time point is considered to be the steady-state time of the OCV in the SOC interval where that SOC state is located. The steady-state time of N charging SOC intervals and the steady-state time of N discharging SOC intervals tN hours are obtained. 3. Adjust the load and test the OCV at different temperatures and SOCs. 3.1 Take 57 samples and let them stand at room temperature (T1) for 2 hours until thermal equilibrium is reached. Discharge them at a constant current of 1 / 3C or 1C to the cutoff voltage U0, and let them stand for 30 / 60 minutes. Then charge them at a constant current of 1 / 3C to 1C to the battery cutoff voltage U1, and switch to constant voltage charging until the current drops to I1. Stop charging and let them stand for 30 / 60 minutes. Repeat this process 5 times. If the capacity difference between the last 3 tests does not exceed 3% of the rated capacity, the test can be stopped early. Take the average discharge capacity of the last 3 tests as the nominal capacity C0 of the cell at room temperature (T1). 3.2. After standing at room temperature (T1) for tx + tN hours, the samples were removed and discharged at a constant current of 1 / 3C or 1C to 95% SOC, 90% SOC, 85% SOC, ... 10% SOC, 5% SOC, and 0% SOC, resulting in a total of 20 discharge SOC states. Three parallel samples were used for each SOC state. 3.3. Place all samples at the same time in the test temperature T2, let them stand for tx + tN hours, and take them out. Test and record the OCV value of each sample within 5 minutes. Take the average value of 3 samples with the same SOC state as the OCV value under that SOC, and obtain the OCV value of different discharge SOC states at T2 temperature. 3.4. Place all samples in the test temperature T3 at the same time, let them stand for tx + tN hours, and take them out. Test the OCV value of each sample within 5 minutes. Take the average value of 3 samples with the same SOC state as the OCV value under that SOC, and obtain the OCV value of different discharge SOC states at T3 temperature. 3.5. Place all samples at the test temperature TX simultaneously, let them stand for tx + tN hours, and then take them out. Test the OCV value of each sample within 5 minutes. Take the average value of 3 samples with the same SOC state as the OCV value under that SOC, and obtain the OCV value of different discharge SOC states at the TX temperature. 3.6 After all temperature tests are completed, the OCV values ​​of different discharge SOC states at each temperature are obtained; 3.7 Repeat step 3.1 to recalibrate the capacity C1 of all cells; 3.8. After standing at room temperature (T1) for tx + tN hours, the samples were removed and charged at a constant current of 1 / 3C or 1C to 5% SOC, 10% SOC, 15% SOC, ... 90% SOC, 95% SOC, and 100% SOC, resulting in a total of 20 charging SOC states. Three parallel samples were used for each SOC state. 3.9 Repeat steps 3.3, 3.4, and 3.5 to obtain the OCV values ​​for different charging SOC states at various temperatures; 3.10. Draw a corresponding table of SOC and OCV to obtain a complete table of SOC-OCV in the discharge state and SOC-OCV in the charge state.

[0028] This embodiment uses a 166Ah battery sample (lithium iron phosphate system, 2.0~3.8V) as the test object. 1. DCIR testing and SOC partitioning 1.1 At room temperature (25±3℃), allow the battery to stand for 120 minutes until thermal equilibrium is reached. Discharge at a constant current of 1 / 3C until the cutoff voltage reaches 2.0V, then allow it to stand for 30 minutes. Charge at a constant current of 1 / 3C until the battery cutoff voltage reaches 3.8V, then switch to constant voltage charging until the current drops to 0.05C. Stop charging and allow it to stand for 30 minutes. Repeat this process 5 times, and take the average discharge capacity of the last 3 times as the nominal capacity C0 of the cell. 1.2. At room temperature (25±3℃), allow the system to stand for 120 minutes until thermal equilibrium is reached. Discharge at a constant current of 1 / 3C to 95% SOC, allowing it to stand for another 120 minutes until thermal equilibrium is reached. Record the voltage value OCV1 at the 120-minute mark. Discharge at a current of 5C for 10 seconds, recording the voltage value OCV2 at the 10-second mark. Charge at a constant current of 1 / 3C for 151.5 seconds, then reduce the SOC to 95% SOC. Allow the system to stand for another 120 minutes until thermal equilibrium is reached, recording the voltage value OCV3 at the 120-minute mark. Charge at a current of 3.75C for 10 seconds, recording the voltage value OCV4 at the 10-second mark. Calculate the discharge DCIR at 95% SOC as DCIR95%discharge = (OCV1 - OCV2) / 3C * 1000, and the charging DCIR at 95% SOC as DCIR95%charge = (OCV4 - OCV3) / 2C * 1000. The unit of DCIR is mΩ.

[0029] 1.3. At room temperature (25±3℃), allow the system to stand for 120 minutes until thermal equilibrium is reached. Discharge at a constant current of 1 / 3C to 90% SOC, and allow to stand for another 120 minutes until thermal equilibrium is reached. Record the voltage value OCV1 at the 120-minute mark. Discharge at a current of 5C for 10 seconds, and record the voltage value OCV2 at the 10-second mark. Charge at a constant current of 1 / 3C or 1C for 151.5 seconds, then reduce the SOC to 90% SOC. Allow the system to stand for another 120 minutes until thermal equilibrium is reached, and record the voltage value OCV3 at the 120-minute mark. Charge at a current of 3.75C for 10 seconds, and record the voltage value OCV4 at the 10-second mark. Calculate the discharge DCIR at 90% SOC as DCIR90%discharge = (OCV1 - OCV2) / m1 * 1000, and the charging DCIR at 90% SOC as DCIR90%charge = (OCV4 - OCV3) / m2 * 1000. The unit of DCIR is mΩ.

[0030] 1.4 Repeat steps 1.2 and 1.3 to obtain charging DCIR and discharging DCIR at 80%, 70%, 20%, 10%, and 5% SOC.

[0031] 1.5 Based on the charging DCIR under different SOCs, using the charging DCIR at 50% SOC as a benchmark, calculate the difference between the charging DCIR at other SOCs and the charging DCIR at 50% SOC. Divide the DCIR difference into 5% intervals to obtain 3 charging SOC intervals.

[0032] 1.6 Following the method in step 1.5, five discharge SOC intervals are obtained.

[0033] 2. Test the steady-state time of OCV in different SOC ranges. 2.1. Take 5% SOC, 60% SOC and 95% SOC in 3 charging SOC intervals as the charging SOC for steady-state time test; take 5% SOC, 20% SOC, 60% SOC, 80% SOC and 90% SOC in 5 discharging SOC intervals as the discharging SOC for steady-state time test, and obtain 5 target discharging SOCs. 2.2 Take 32 samples and let them stand at room temperature (25±3℃) for 2 hours until thermal equilibrium is reached. Discharge them at a constant current of 1 / 3C until the cutoff voltage is 2.0V, let them stand for 30 minutes, charge them at a constant current of 1 / 3C until the battery cutoff voltage is 3.8V, switch to constant voltage charging until the current drops to 0.05C and stop charging, let them stand for 30 minutes. Repeat this process 5 times, and take the average discharge capacity of the last 3 times as the nominal capacity C0 of each cell at room temperature (25±3℃). 2.3 Take 16 samples, divide them into groups of 2, and let them stand at room temperature (25±3℃) for 120 minutes until thermal equilibrium is reached. Then discharge them at a constant current of 1 / 3C to the target SOC: 5% SOC, 20% SOC, 60% SOC, 80% SOC, and 90% SOC, respectively, to obtain 5 groups of samples with discharge state SOC. 2.4 Take another 16 samples, with 2 cells as a group, and let them stand at room temperature (25±3℃) for 120 minutes until thermal equilibrium is reached. Then charge them at a constant current of 1 / 3C to the target SOC: 5% SOC, 60% SOC, and 95% SOC, respectively, to obtain 3 groups of samples with SOC in the charging state. 2.5. Five groups of discharged SOC samples and three groups of charged SOC samples were left to stand at room temperature (25±3℃) for 48 hours, and the OCV values ​​of each sample were recorded at 2h, 4h, 8h, 12h, 24h, 36h and 48h. 2.6 Using the OCV value at 48h as the baseline OCV, calculate the difference between the OCV values ​​at other time points and the baseline OCV. When the difference between two samples in the same SOC state at a certain time point is within 2mV, the time point is considered to be the steady-state time of the OCV in the SOC interval where the SOC state is located. The steady-state time of 3 charging SOC intervals and the steady-state time of 5 discharging SOC intervals tN hours are obtained. 3. Adjust the load and test the OCV at different temperatures and SOCs. 3.1. Based on the number of target SOC points, take three times the number of samples, let them stand at room temperature (25±3℃) for 2 hours until thermal equilibrium is reached, discharge at 1 / 3C constant current to the cutoff voltage of 2.0V, let them stand for 30 minutes, charge at 1 / 3C constant current to the battery cutoff voltage of 3.8V, switch to constant voltage charging until the current drops to 0.05C and stop charging, let them stand for 30 minutes. Repeat this process 5 times, and take the average discharge capacity of the last 3 times as the nominal capacity C0 of the cell at room temperature (25±3℃). 3.2. At room temperature (25±3℃), the samples were allowed to stand for 120 minutes until thermal equilibrium was reached. Then, they were discharged at a constant current of 1 / 3C to 95% SOC, 90% SOC, 85% SOC, ... 10% SOC, 5% SOC, and 0% SOC, resulting in a total of 20 discharge SOC states. Three parallel samples were used for each SOC state. 3.3. Place all samples in the test temperature -30℃ at the same time, let them stand for tx + tN hours, and then take them out. Test the OCV value of each sample within 5 minutes. Take the average value of 3 samples with the same SOC state as the OCV value under that SOC, and obtain the OCV value of different discharge SOC states at -30℃. 3.4. Test the OCV values ​​of different discharge SOC states at -20℃, 0℃, and 45℃ according to the method in 3.3. 3.5 Repeat step 3.1 to recalibrate the capacity C1 of the 57 cells; 3.6. At room temperature (25±3℃), the samples were allowed to stand for 120 minutes until thermal equilibrium was reached. They were then charged at a constant current of 1 / 3C to 5% SOC, 10% SOC, 15% SOC, ..., 90% SOC, 95% SOC, and 100% SOC, resulting in a total of 20 charging SOC states. Three parallel samples were used for each SOC state. 3.7 Repeat steps 3.3 and 3.4 to obtain the OCV values ​​for different charging SOC states at various temperatures; 3.8. Draw corresponding tables for SOC and OCV to obtain complete discharge-state SOC-OCV tables and charge-state SOC-OCV tables, as shown in Table 1 and Table 2 below:

[0034]

[0035] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for testing the static SOC-OCV of a lithium-ion battery, characterized by comprising the following steps:

1. DCIR testing and SOC partitioning: 1.1 At room temperature T1, the battery under test is subjected to charge-discharge cycles to determine its nominal capacity C0 at room temperature T1; 1.2 Measure the charging DC internal resistance and discharging DC internal resistance of the battery under test at different SOC points; 1.

3. Using the charging DC internal resistance value of 50% SOC as a benchmark, calculate the difference between the charging DC internal resistance value of other SOC points and this benchmark value, and divide all SOC points into N charging SOC intervals according to the preset difference threshold range. as well as, Using the discharge DC internal resistance value of 50% SOC as a benchmark, calculate the difference between the discharge DC internal resistance values ​​of other SOC points and this benchmark value, and divide all SOC points into M discharge SOC intervals according to the preset difference threshold range.

2. Determine the steady-state time of OCV in different SOC ranges: 2.1 Select one representative SOC point from each of the N charging SOC intervals as N target charging SOC points; select one representative SOC point from each of the M discharging SOC intervals as M target discharging SOC points; 2.2 Prepare multiple sets of battery samples, each set containing at least two batteries, and adjust one or more sets of batteries to the same target charging SOC point or target discharging SOC point; 2.3 At room temperature T1, each group of batteries at the same target SOC point was left to stand for a long time, and the OCV values ​​of each battery at multiple time points were recorded. 2.

4. Using the OCV value at the end of the resting period as a reference, calculate the difference between the OCV value at each time point and the reference value. When the OCV difference of all batteries in the same group at a certain time point is less than or equal to the preset voltage stability threshold, then the time point is determined as the OCV steady-state time tN of the SOC interval to which the target SOC point belongs.

3. Adjust the load and test the OCV at different temperatures: 3.1 Prepare multiple battery samples and adjust them to multiple different SOC states at room temperature T1. Each SOC state corresponds to at least two parallel samples. 3.

2. Place samples in different SOC states simultaneously in an environment at the first test temperature T2 and let them stand for a first preset time. The first preset time is the sum of the thermal equilibrium time tx at temperature T2 and the OCV steady-state time tN of each SOC state interval. 3.3 After the settling period, measure and record the OCV values ​​of all samples within the specified time. Take the average value of the OCV values ​​measured for samples with the same SOC state as the OCV value of that SOC state at temperature T2. 3.

4. Change to the second test temperature T3, and repeat steps 3.2 to 3.3 to obtain the OCV value of this SOC state at temperature T3; 3.5 Repeat steps 3.1 to 3.4 to obtain the OCV values ​​of different SOC states at multiple temperatures during the charging and discharging processes, thereby obtaining a complete static SOC-OCV data table.

2. The test method according to claim 1, characterized in that, In step 1.3, the preset difference threshold range is as follows: difference ≤ 5% is divided into the first interval, 5% < difference ≤ 10% is divided into the second interval, and so on.

3. The test method according to claim 1, characterized in that, In step 2.4, the long-term static time is not less than 48 hours, and the voltage stability threshold is 2mV.

4. The test method according to claim 1, characterized in that, In step 3.1, adjusting the SOC state includes: for the discharge process, discharging the battery to multiple discrete points between 0% SOC and 95% SOC at room temperature T1; and for the charging process, charging the battery to multiple discrete points between 5% SOC and 100% SOC at room temperature T1.

5. The test method according to claim 1, characterized in that, The room temperature T1 is 25±3℃.

6. The test method according to claim 1, characterized in that, The values ​​of the temperatures to be measured, T2 and T3, range from -35℃ to 60℃.

7. The test method according to claim 1, characterized in that, The method for measuring the DC internal resistance value in step 1.2 includes: a. After adjusting the battery to a specific SOC point and allowing it to stand until thermal equilibrium is reached, record its first OCV value; b. Apply a pulse discharge or pulse charge current lasting for a first duration Δt1, and record the second OCV value at the end of the pulse; c. Calculate the discharge DC internal resistance or charging DC internal resistance at the SOC point based on the first OCV value, the second OCV value, and the amplitude of the pulse current.

8. The test method according to claim 7, characterized in that, The first duration Δt1 is 10 seconds, the amplitude of the pulse discharge current is the maximum pulse discharge current of the battery at the corresponding SOC point, and the amplitude of the pulse charging current is the maximum pulse charging current of the battery at the corresponding SOC point.

9. The test method according to claim 1, characterized in that, In steps 3.2 and 3.4, for the 0% SOC state, the OCV steady-state time tN is determined with reference to the tN value determined for the 5% SOC discharge range; for the 100% SOC state, the OCV steady-state time tN is determined with reference to the tN value determined for the 95% SOC charging range.

10. The test method according to claim 1, characterized in that, After completing the OCV test at all temperatures, all test samples were recalibrated at room temperature T1 to verify whether the battery capacity changed significantly during the test.

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