Method and device for evaluating state of retired lithium iron phosphate power battery

Abstract

The invention relates to the technical field of energy storage, in particular to a decommissioned lithium iron phosphate power battery state evaluation method and a decommissioned lithium iron phosphate power battery state evaluation device. Comprising the following steps: S1, appearance and voltage test evaluation; S2, internal resistance test evaluation; S3, capacity and health state test evaluation; S4, consistency test evaluation; according to the retired lithium iron phosphate power battery state evaluation method and device, the change conditions of the battery voltage under various working conditions can be comprehensively mastered by recording the open-circuit voltage fluctuation range of the battery under different states, and meanwhile, the voltage stability index, namely the voltage standard deviation, is combined for comprehensive judgment; the one-sidedness of judging according to a certain measurement value is avoided, and the influence of measurement errors and random factors on the evaluation result is reduced, so that the evaluation accuracy is improved.

Description

Technical Field

[0001] This invention relates to the field of energy storage technology, specifically to a method and apparatus for assessing the state of retired lithium iron phosphate power batteries. Background Technology

[0002] Lithium iron phosphate (LFP) batteries offer advantages such as high safety, long cycle life, and low cost, leading to their widespread application in various fields including portable vehicles, ships, and energy storage. However, as usage time increases, battery performance degrades, and the performance differences between batteries become increasingly pronounced. When the performance of these batteries no longer meets the application requirements of electric vehicles, they are retired from electric vehicles and applied to situations with lower battery performance requirements and relatively mild operating conditions, achieving the secondary use of power batteries. To ensure the safety and rationality of this secondary use, it is necessary to evaluate the various conditions of the retired LFP power batteries.

[0003] Chinese Invention Patent Publication No. CN111751752A discloses a method and apparatus for assessing the state of retired lithium iron phosphate power batteries. This method collects the open-circuit voltage of the retired lithium iron phosphate power battery and determines whether the battery voltage is normal based on a predetermined normal range. It also collects the AC internal resistance of the battery, thereby completing a multi-faceted assessment of the battery's state. Its technical advantage lies in solving the problems of low efficiency and high cost in existing technologies for assessing the state of retired lithium iron phosphate power batteries.

[0004] In summary, existing evaluation methods only determine whether a battery is normal or not by whether its open-circuit voltage is within the range of 2.5V to 3.65V, without considering voltage fluctuation amplitude, long-term stability, and dynamic characteristics. This can easily lead to misjudgment of batteries whose voltage is temporarily normal due to local short circuits. Furthermore, these methods ignore the internal structural degradation reflected by small voltage fluctuations, thus affecting the accuracy and reliability of the evaluation. Summary of the Invention

[0005] To achieve the above objectives, the present invention provides the following technical solution: a method for assessing the state of retired lithium iron phosphate power batteries, characterized by comprising the following steps:

[0006] S1. Appearance and Voltage Testing and Evaluation: The appearance of the retired lithium iron phosphate power battery pack is inspected; and the open-circuit voltage of the retired lithium iron phosphate power battery pack is measured multiple times under different ambient temperatures and resting times, the voltage standard deviation is calculated, and the voltage is evaluated based on the voltage standard deviation.

[0007] S2. Internal resistance test and evaluation: Develop an automated test platform to measure the AC internal resistance of retired lithium iron phosphate power battery packs at fixed frequency intervals within a set frequency range, calculate the rate of change of internal resistance with the number of cycles, and test the DC internal resistance of retired lithium iron phosphate power battery packs through long-term charge and discharge, and evaluate the AC internal resistance based on the rate of change.

[0008] S3. Capacity and Health Status Test and Evaluation: The actual charge and discharge capacity of the retired lithium iron phosphate power battery pack is obtained by conducting capacity tests through standard charge and discharge regimes. Based on the actual charge and discharge capacity, the capacity decay rate and service life (i.e., health status) of the retired lithium iron phosphate power battery pack are calculated and evaluated.

[0009] S4. Consistency Test Evaluation: The voltage, internal resistance, and capacity of individual cells in the retired lithium iron phosphate power battery pack are tested in groups, and the maximum difference in voltage, internal resistance, and capacity of individual cells in the retired lithium iron phosphate power battery pack is calculated based on the test results. At the same time, the evaluation is carried out according to the set standards.

[0010] S5. Safety Testing and Evaluation: Short-circuit, overcharge, over-discharge, and thermal abuse tests are conducted on retired lithium iron phosphate power battery packs using an automated testing simulation system, and an evaluation is performed according to the set standards.

[0011] Furthermore, if the appearance of the S1 battery pack exhibits any of the following defects: bulging, leakage, corrosion, or physical deformation, it will be directly determined that the retired lithium iron phosphate power battery pack does not meet the requirements for tiered use, and no further evaluation process will be conducted.

[0012] Furthermore, the open-circuit voltage test uses a high-precision voltage tester, and the open-circuit voltage fluctuation range of the battery under different states must be recorded during the test, and the voltage standard deviation must be calculated. If the voltage standard deviation exceeds the set value, the battery voltage stability is considered to be unsuitable for cascade use.

[0013] Furthermore, the open-circuit voltage test of the retired lithium iron phosphate power battery pack also includes applying a short charge-discharge pulse to the retired lithium iron phosphate power battery pack and recording the time required for the battery voltage to recover to the initial stable state. If the voltage recovery time exceeds the set time, it indicates that there is polarization inside the retired lithium iron phosphate power battery pack or that the electrolyte performance has deteriorated, meaning that the retired lithium iron phosphate power battery pack cannot be reused.

[0014] Furthermore, the AC internal resistance test of the retired lithium iron phosphate power battery pack is conducted within a frequency range of 100Hz-10kHz, with frequency intervals of 100Hz, 500Hz, 1kHz, 5kHz, and 10kHz. The test employs the AC injection method. The AC internal resistance test also includes plotting the AC internal resistance versus frequency curve, analyzing the internal resistance spectral characteristics, recording the AC internal resistance of the battery at different cycle counts, calculating the rate of change of internal resistance with cycle count, and determining if the rate of change exceeds a set threshold. If the battery is aged to a certain degree, it means that the retired lithium iron phosphate power battery pack cannot be reused.

[0015] Furthermore, the DC internal resistance of the retired lithium iron phosphate power battery pack is tested using an automated testing system controlled by the charging and discharging equipment.

[0016] Furthermore, the formula for calculating the actual charge / discharge capacity of the retired lithium iron phosphate power battery pack is as follows:

[0017]

[0018] in The charging and discharging current is expressed in units of 1. , The charging / discharging time is expressed in units of... The formula for calculating the capacity decay rate is:

[0019]

[0020] in This refers to the actual capacity of the battery. The rated capacity of the battery is used for evaluation. If the capacity decay rate exceeds 20%, it means that the retired lithium iron phosphate power battery pack cannot be reused.

[0021] Furthermore, if the maximum voltage difference of the retired lithium iron phosphate power battery pack exceeds 0.3V or the voltage standard deviation exceeds 5% of the average voltage in the S4 consistency test evaluation, then the retired lithium iron phosphate power battery pack is considered to have poor voltage consistency, meaning that the retired lithium iron phosphate power battery pack cannot be reused in a tiered manner.

[0022] If the maximum internal resistance difference exceeds 10% of the average internal resistance or the standard deviation of internal resistance exceeds 5% of the average internal resistance, then the internal resistance consistency of the retired lithium iron phosphate power battery pack is considered to be poor, meaning that the retired lithium iron phosphate power battery pack cannot be reused in a tiered manner.

[0023] If the maximum capacity difference exceeds 15% of the rated capacity or the capacity standard deviation exceeds 5% of the rated capacity, the retired lithium iron phosphate power battery pack is considered to have poor capacity consistency, meaning that the retired lithium iron phosphate power battery pack cannot be reused.

[0024] Furthermore, the criteria for judging the short circuit test of the retired lithium iron phosphate power battery pack are that the short circuit current should be less than 5 times the rated current, the surface temperature rise of the retired lithium iron phosphate power battery pack should be less than 50°C, and the retired lithium iron phosphate power battery pack should not catch fire, explode or be severely deformed.

[0025] The criteria for judging the overcharge and over-discharge test of the retired lithium iron phosphate power battery pack is that the retired lithium iron phosphate power battery pack should not bulge, leak, thermal runaway, catch fire or explode.

[0026] The criteria for judging the thermal abuse test of the retired lithium iron phosphate power battery pack is that the retired lithium iron phosphate power battery pack should not experience thermal runaway, fire or explosion.

[0027] A state assessment device for retired lithium iron phosphate power batteries, applied to a state assessment method for retired lithium iron phosphate power batteries, comprising:

[0028] The appearance and voltage detection and evaluation module collects the appearance and open-circuit voltage data of retired lithium iron phosphate power battery packs through a visual inspection unit and a high-precision voltage tester, and determines whether the appearance and open-circuit voltage of retired lithium iron phosphate power battery packs meet the requirements for tiered use based on the data.

[0029] The internal resistance detection and evaluation module uses an automated testing platform to detect the AC and DC internal resistance of retired lithium iron phosphate power battery packs and determine whether the internal resistance of retired lithium iron phosphate power battery packs meets the requirements for tiered use.

[0030] The capacity and health status detection and evaluation module performs capacity testing on retired lithium iron phosphate power battery packs through standard charge and discharge procedures, calculates and evaluates capacity decay rate and health status, and performs factor correction.

[0031] The consistency detection and evaluation module collects the voltage, internal resistance, and capacity of individual cells in the retired lithium iron phosphate power battery pack, calculates the maximum difference and standard deviation of the voltage, internal resistance, and capacity of individual cells in the retired lithium iron phosphate power battery pack, and determines whether the consistency of the voltage, internal resistance, and capacity of individual cells meets the requirements for tiered use.

[0032] The safety testing and evaluation module uses an automated testing simulation system to conduct safety tests on retired lithium iron phosphate power battery packs and evaluate them.

[0033] The data management and report generation module is responsible for coordinating the workflow of each module, evaluating various performance indicators of retired lithium iron phosphate power battery packs according to preset algorithms and standards, and generating evaluation reports.

[0034] Compared with the prior art, the beneficial effects of the present invention are: the method and apparatus for assessing the state of retired lithium iron phosphate power batteries,

[0035] (1) By recording the open-circuit voltage fluctuation range of the battery under different conditions, the battery voltage can be fully understood under various working conditions. At the same time, combined with the voltage stability index, namely the voltage standard deviation, the judgment is made in a comprehensive manner, avoiding the one-sidedness of judging based on a single measurement value, reducing the impact of measurement error and random factors on the evaluation results, thereby improving the accuracy of the evaluation.

[0036] (2) Introducing dynamic indicators such as voltage recovery time and voltage hysteresis effect can reflect the micro-characteristics of the battery, such as polarization phenomenon and electrolyte performance. At the same time, by analyzing these characteristics, we can gain a deeper understanding of the internal state of the battery, making the evaluation results more reliable and more comprehensively reflecting the true situation of the battery. Attached Figure Description

[0037] Figure 1 This is an overall flowchart of the evaluation method of the present invention;

[0038] Figure 2 This is a line graph showing the actual charge / discharge capacity variation of the present invention.

[0039] Figure 3 This is a line graph showing the capacity decay rate of the present invention.

[0040] Figure 4 This is a graph showing the change of AC internal resistance with frequency in this invention. Detailed Implementation

[0041] 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.

[0042] Please see Figure 1-4 Example 1:

[0043] This embodiment provides a method for assessing the state of retired lithium iron phosphate power batteries, including the following steps:

[0044] S1. Appearance and Voltage Testing and Evaluation: The appearance of the retired lithium iron phosphate power battery pack is inspected; and the open-circuit voltage of the retired lithium iron phosphate power battery pack is measured multiple times under different ambient temperatures and resting times, the voltage standard deviation is calculated, and the voltage is evaluated based on the voltage standard deviation.

[0045] If any of the following are found in the appearance: bulging, leakage, corrosion, or physical deformation, the retired lithium iron phosphate power battery pack is directly deemed unsuitable for tiered use and will not be subject to further evaluation.

[0046] The open-circuit voltage test uses a high-precision voltage tester. During the test, the open-circuit voltage fluctuation range of the battery under different conditions must be recorded, and the voltage standard deviation must be calculated. The voltage standard deviation is used to evaluate the voltage stability of retired lithium iron phosphate power battery packs. The formula for calculating the voltage standard deviation is:

[0047]

[0048] in, The open-circuit voltage measured each time. The average voltage measured multiple times. If the standard deviation of the voltage exceeds the set value, the battery voltage stability is considered to be unsuitable for tiered use.

[0049] The open-circuit voltage test of retired lithium iron phosphate power battery packs also includes applying a short charge / discharge pulse to the retired lithium iron phosphate power battery pack and recording the time required for the battery voltage to recover to the initial stable state. If the voltage recovery time exceeds the set time, it indicates that there is polarization inside the retired lithium iron phosphate power battery pack or that the electrolyte performance has deteriorated, meaning that the retired lithium iron phosphate power battery pack cannot be reused. The open-circuit voltage test also includes recording the voltage-charge curves during charging and discharging, and analyzing the magnitude and trend of voltage hysteresis. If the voltage hysteresis exceeds the standard, it indicates that there are structural changes inside the battery or aging of the electrode materials.

[0050] It should be noted that the voltage recovery time reflects the speed of charge transfer and diffusion processes inside the battery, while the voltage hysteresis effect is related to structural changes inside the battery and the aging of electrode materials. The inclusion of these data allows the evaluation to go beyond static open-circuit voltage values ​​and delve into the electrochemical processes inside the battery, thus providing a more comprehensive reflection of the battery's internal state.

[0051] S2. Internal resistance test and evaluation: Develop an automated test platform to measure the AC internal resistance of retired lithium iron phosphate power battery packs at fixed frequency intervals within a set frequency range, calculate the rate of change of internal resistance with the number of cycles, and test the DC internal resistance of retired lithium iron phosphate power battery packs through long-term charge and discharge, and evaluate the AC internal resistance based on the rate of change.

[0052] The AC internal resistance of retired lithium iron phosphate power battery packs is tested within the frequency range of 100Hz-10kHz, with frequency intervals of 100Hz, 500Hz, 1kHz, 5kHz, and 10kHz. The test employs the AC injection method, which involves injecting an AC current of known frequency and amplitude across the battery terminals and measuring the AC voltage across the battery. The calculation formula is as follows:

[0053]

[0054] in An alternating current of known frequency and amplitude is injected into the two ends of the battery. This is the AC voltage across the battery terminals. Internal resistance to communication;

[0055] The testing of the AC internal resistance of retired lithium iron phosphate power battery packs also includes plotting the AC internal resistance versus frequency curve, analyzing the internal resistance spectrum characteristics, recording the AC internal resistance of the battery at different cycle numbers, and calculating the rate of change of internal resistance with cycle number. The calculation formula is as follows:

[0056]

[0057] in For the first AC internal resistance after the second cycle The initial AC internal resistance; if the rate of change exceeds a set threshold... If the battery is aged to a certain degree, it means that the retired lithium iron phosphate power battery pack cannot be reused.

[0058] The DC internal resistance of retired lithium iron phosphate (LFP) power battery packs was tested using an automated testing system controlling the charging and discharging equipment. A one-hour constant current charge-discharge test was conducted. During charging, the changes in battery voltage and current over time were recorded. Similarly, during discharging, relevant data were recorded, and the DC internal resistance was calculated using the following formula:

[0059]

[0060] in This is the change in voltage. This refers to the change in current.

[0061] It should be noted that measuring internal resistance at different frequencies and analyzing its spectral characteristics can reveal the resistance characteristics of different processes within the battery. For example, in the low-frequency region, internal resistance is mainly related to the diffusion process within the battery; in the high-frequency region, internal resistance mainly reflects charge transfer resistance. Through this multi-frequency measurement and analysis, the resistance of different processes within the battery can be more accurately separated and identified, thus more accurately reflecting the charge transfer resistance and electrode material state within the battery.

[0062] The testing of the DC internal resistance of retired lithium iron phosphate power battery packs also includes plotting the DC internal resistance variation curve with charge and discharge time, analyzing the dynamic change characteristics of the battery's internal resistance during charge and discharge, and calculating the DC internal resistance of the retired lithium iron phosphate power battery packs at different temperatures using the internal resistance-temperature correction model formula. The internal resistance-temperature correction model formula is as follows:

[0063]

[0064] in, For temperature DC internal resistance at that time, Reference temperature DC internal resistance at that time, This is the temperature coefficient.

[0065] The formula for calculating the actual charge and discharge capacity of retired lithium iron phosphate power battery packs is as follows:

[0066]

[0067] in This refers to the charging and discharging current, measured in amperes (A). The charging / discharging time is expressed in hours (h); the formula for calculating the capacity decay rate is:

[0068]

[0069] in This refers to the actual capacity of the battery. The rated capacity of the battery is used for evaluation. If the capacity decay rate exceeds 20%, it means that the retired lithium iron phosphate power battery pack cannot be reused.

[0070] It should be noted that in actual use, the battery will undergo polarization during charging and discharging, which will cause changes in internal resistance. By recording the dynamic change curve, we can fully understand the polarization resistance of the battery at different charging and discharging stages, as well as the internal resistance characteristics in actual use, providing more accurate information for battery performance evaluation and use.

[0071] S3. Capacity and Health Status Test and Evaluation: The actual charge and discharge capacity of the retired lithium iron phosphate power battery pack is obtained by conducting capacity tests through standard charge and discharge regimes. Based on the actual charge and discharge capacity, the capacity decay rate and service life (i.e., health status) of the retired lithium iron phosphate power battery pack are calculated and evaluated.

[0072] The health status assessment of retired lithium iron phosphate power battery packs includes health status calculation, establishing a battery aging model, and factor correction. The formula for calculating health status is:

[0073]

[0074] in, For the current capacity of retired lithium iron phosphate power battery packs, The rated capacity of retired lithium iron phosphate power battery packs;

[0075] The battery aging model was established using a nonlinear fitting method; the factor correction was reflected in considering the influence of charge / discharge rate and ambient temperature on capacity, and then corrected accordingly.

[0076] S4. Consistency Test Evaluation: The voltage, internal resistance, and capacity of individual cells in the retired lithium iron phosphate power battery pack are tested in groups, and the maximum difference in voltage, internal resistance, and capacity of individual cells in the retired lithium iron phosphate power battery pack is calculated based on the test results. At the same time, the evaluation is carried out according to the set standards.

[0077] If the maximum voltage difference of the retired lithium iron phosphate power battery pack exceeds 0.3V or the voltage standard deviation exceeds 5% of the average voltage, the retired lithium iron phosphate power battery pack is considered to have poor voltage consistency, meaning that the retired lithium iron phosphate power battery pack cannot be reused.

[0078] If the maximum internal resistance difference exceeds 10% of the average internal resistance or the standard deviation of internal resistance exceeds 5% of the average internal resistance, then the internal resistance consistency of the retired lithium iron phosphate power battery pack is considered to be poor, meaning that the retired lithium iron phosphate power battery pack cannot be reused in a tiered manner.

[0079] If the maximum capacity difference exceeds 15% of the rated capacity or the capacity standard deviation exceeds 5% of the rated capacity, the retired lithium iron phosphate power battery pack is considered to have poor capacity consistency, meaning that the retired lithium iron phosphate power battery pack cannot be reused.

[0080] It should be noted that using a standard charge-discharge regime for capacity testing and recording complete charge-discharge curves ensures accurate measurement of the battery's actual charge-discharge capacity. The standard charge-discharge regime guarantees the consistency and standardization of the test, resulting in highly accurate and repeatable measurement results.

[0081] By combining a battery aging model with nonlinear fitting of capacity decay, the trend of battery capacity change can be predicted more accurately. Battery capacity decay is a complex nonlinear process, influenced by various factors such as charge / discharge rate and ambient temperature. By establishing a battery aging model and performing nonlinear fitting, the law of battery capacity decay can be described more precisely, thereby more accurately assessing the remaining battery life.

[0082] By considering and correcting for the effects of factors such as charge / discharge rate and ambient temperature on capacity, the interference of these factors on capacity measurement is eliminated. Different charge / discharge rates and ambient temperatures will cause changes in battery capacity. Through correction, the capacity assessment results are made more consistent with the battery's actual usage conditions, and more accurately reflect the battery's actual capacity and remaining life.

[0083] S5. Safety Testing and Evaluation: Short-circuit, overcharge, over-discharge, and thermal abuse tests are conducted on retired lithium iron phosphate power battery packs using an automated testing simulation system, and an evaluation is performed according to the set standards.

[0084] The criteria for judging the short circuit test of retired lithium iron phosphate power battery packs are that the short circuit current should be less than 5 times the rated current, the surface temperature rise of the retired lithium iron phosphate power battery pack should be less than 50°C, and the retired lithium iron phosphate power battery pack should not catch fire, explode or be severely deformed. If the above conditions are not met, the retired lithium iron phosphate power battery pack is considered to have safety risks under short circuit conditions and does not meet the requirements for cascade use.

[0085] The criteria for judging the overcharge and over-discharge test of retired lithium iron phosphate power battery packs are that retired lithium iron phosphate power battery packs should not bulge, leak, thermal runaway, catch fire or explode. If the above abnormalities occur, the retired lithium iron phosphate power battery packs are considered to be unsafe under overcharge or over-discharge conditions and do not meet the requirements for cascade use.

[0086] The criteria for judging the thermal abuse test of retired lithium iron phosphate power battery packs are that retired lithium iron phosphate power battery packs should not experience thermal runaway, fire or explosion. If the retired lithium iron phosphate power battery pack exhibits the above phenomena during the test, it is considered that the thermal stability of the retired lithium iron phosphate power battery pack under high temperature environment does not meet the standard, there is a safety risk, and it does not meet the requirements for cascade use.

[0087] This embodiment provides a comparative experiment for the state assessment method of retired lithium iron phosphate power batteries, wherein the experimental method includes the following steps:

[0088] S1, Appearance and Voltage Test Evaluation

[0089] Visual inspection:

[0090] Visual inspection of retired lithium iron phosphate power battery packs revealed no bulging, leakage, corrosion, or physical deformation on the battery pack surface; otherwise, it would not meet the requirements.

[0091] Open circuit voltage test:

[0092] Using a high-precision voltage tester, the open-circuit voltage of the battery pack was measured multiple times under different ambient temperatures (25℃, 35℃, 45℃) and resting times (1 hour, 2 hours, 3 hours).

[0093] Measurement data:

[0094] 25℃, stand for 1 hour: 3.25V, 3.26V, 3.24V

[0095] 35℃, stand for 2 hours: 3.27V, 3.28V, 3.26V

[0096] 45℃, stand for 3 hours: 3.29V, 3.30V, 3.28V

[0097] Calculate the voltage standard deviation (with the voltage standard deviation threshold set to 0.01V):

[0098] 25℃: 0.0082V

[0099] 35℃: 0.0082V

[0100] 45℃: 0.0082V

[0101] Assessment: Since the standard deviation of voltage at all temperatures is less than the threshold of 0.01V, the battery voltage stability is considered to meet the requirements for secondary use.

[0102] Voltage recovery time test:

[0103] After applying a brief charge / discharge pulse (e.g., 10A current for 10 seconds) to the battery pack, record the time required for the battery voltage to recover to its initial steady state.

[0104] Measurement data (assuming a voltage recovery time threshold of 5 seconds):

[0105] Voltage recovery time: 3 seconds (time may vary depending on the battery pack or degree of aging).

[0106] Assessment: Since the actual voltage recovery time is less than the threshold of 5 seconds, the charge transfer and diffusion process inside the battery is considered to be normal.

[0107] Voltage hysteresis analysis:

[0108] During the charging and discharging process, the voltage-charge curves during charging and discharging are recorded, and the magnitude and trend of voltage hysteresis are analyzed.

[0109] Measurement data:

[0110] Charging curve voltage: gradually increases from 3.2V to 3.6V.

[0111] Discharge curve voltage: gradually decreases from 3.6V to 3.0V.

[0112] Voltage hysteresis (set voltage hysteresis threshold to 0.1V): When the battery is 50% charged, the charging voltage is 3.4V, the discharging voltage is 3.35V, and the voltage hysteresis is 0.05V;

[0113] Assessment: Since the actual voltage hysteresis is less than the threshold, the changes in the internal structure of the battery and the degree of aging of the electrode materials are considered to be within an acceptable range.

[0114] S2, Internal Resistance Test Evaluation

[0115] AC internal resistance test:

[0116] Develop an automated testing platform to measure the AC internal resistance of battery packs within the 100Hz-10kHz frequency range at frequency intervals of 100Hz, 500Hz, 1kHz, 5kHz, and 10kHz.

[0117] Measurement data:

[0118] 100Hz: 1.2mΩ

[0119] 500Hz: 1.5mΩ

[0120] 1kHz: 1.8mΩ

[0121] 5kHz: 2.2mΩ

[0122] 10kHz: 2.5mΩ

[0123] Set the rate of change threshold to 50%, and calculate the rate of change of internal resistance with the number of cycles (initial internal resistance is 1.0 mΩ, 1.3 mΩ after 100 cycles, and 1.6 mΩ after 200 cycles):

[0124] Rate of change over 100 cycles: 30%

[0125] Rate of change over 200 cycles: 60%

[0126] Assessment: Since the rate of change after 200 cycles exceeds the threshold of 50%, the battery is considered to be of high aging level, but it still meets the requirements for tiered use within 100 cycles.

[0127] DC internal resistance test:

[0128] The charging and discharging equipment is controlled by an automated testing system to conduct a 1-hour constant current charging and discharging test (e.g., 10A current), and the changes in battery voltage and current over time are recorded.

[0129] Measurement data:

[0130] Charging process: The voltage rises from 3.2V to 3.6V while the current remains at 10A.

[0131] Discharge process: Voltage drops from 3.6V to 3.0V, current remains at 10A.

[0132] Assume that during the discharge process, when the voltage drops from 3.6V to 3.5V, the current drops from 10A to 9.9A (in reality, the current change may be very small, but for the sake of simplicity, we assume the change is 0.1A).

[0133] Calculate DC internal resistance:

[0134] DC internal resistance = (3.6V - 3.5V) / (10A - 9.9A) = 1V / 0.1A = 10mΩ

[0135] Assessment: Assuming the DC internal resistance threshold is set at 20mΩ, since the actual DC internal resistance is less than the threshold, the battery internal resistance characteristics are considered to meet the requirements for secondary use.

[0136] S3, Capacity and Health Status Testing and Assessment

[0137] Capacity test:

[0138] The actual charge and discharge capacity of the battery pack is obtained by conducting capacity tests using standard charge and discharge regimes.

[0139] Measurement data:

[0140] Rated capacity: 100Ah

[0141] Actual capacity: 85Ah

[0142] Calculate the capacity decay rate:

[0143] Capacity decay rate = (100Ah - 85Ah) / 100Ah × 100% = 15%

[0144] Assessment: Assuming a capacity degradation rate threshold of 20%, since the actual capacity degradation rate is less than the threshold, the battery pack is considered to be in good health and meets the requirements for secondary use.

[0145] Health status assessment:

[0146] Calculate health status:

[0147] Health status = 85Ah / 100Ah × 100% = 85%

[0148] A battery aging model was established and nonlinear fitting was performed, taking into account the effects of charge / discharge rate and ambient temperature on capacity and making corrections.

[0149] S4, Consistency Testing and Evaluation

[0150] Single cell battery test:

[0151] The voltage, internal resistance, and capacity of individual cells in the battery pack were tested in groups, with a maximum voltage difference threshold of 0.3V, an average internal resistance of 1.2mΩ, and a rated capacity of 100Ah.

[0152] Measurement data:

[0153] Maximum voltage difference: 0.25V

[0154] Maximum internal resistance difference: 0.12mΩ

[0155] Maximum capacity difference: 6Ah

[0156] Assessment: Since all differences are less than the set threshold, the battery pack consistency is considered to meet the requirements for tiered use.

[0157] S5, Security Testing and Evaluation

[0158] Short circuit test:

[0159] The short-circuit current is less than 5 times the rated current, the surface temperature rise of the battery pack is less than 50°C, and there is no fire, explosion or severe deformation.

[0160] Test results: Meets safety standards.

[0161] Overcharge and over-discharge test:

[0162] The battery pack did not experience bulging, leakage, thermal runaway, fire, or explosion.

[0163] Test results: Meets safety standards.

[0164] Thermal abuse test:

[0165] The battery pack did not experience thermal runaway, fire, or explosion.

[0166] Test results: Meets the requirements for tiered use.

[0167] Example 2:

[0168] A state assessment device for retired lithium iron phosphate power batteries, applied to a state assessment method for retired lithium iron phosphate power batteries, includes:

[0169] The appearance and voltage detection and evaluation module collects the appearance and open-circuit voltage data of retired lithium iron phosphate power battery packs through a visual inspection unit and a high-precision voltage tester, and determines whether the appearance and open-circuit voltage of retired lithium iron phosphate power battery packs meet the requirements for tiered use based on the data.

[0170] The internal resistance detection and evaluation module uses an automated testing platform to detect the AC and DC internal resistance of retired lithium iron phosphate power battery packs and determine whether the internal resistance of retired lithium iron phosphate power battery packs meets the requirements for tiered use.

[0171] The capacity and health status detection and evaluation module performs capacity testing on retired lithium iron phosphate power battery packs through standard charge and discharge procedures, calculates and evaluates the capacity decay rate and health status, and performs factor correction.

[0172] The consistency testing and evaluation module collects the voltage, internal resistance, and capacity of individual cells in retired lithium iron phosphate power battery packs, calculates the maximum difference and standard deviation of the voltage, internal resistance, and capacity of individual cells in retired lithium iron phosphate power battery packs, and determines whether the consistency of the voltage, internal resistance, and capacity of individual cells meets the requirements for tiered use.

[0173] The safety testing and evaluation module uses an automated testing simulation system to conduct safety tests on retired lithium iron phosphate power battery packs and evaluate them.

[0174] The data management and report generation module is responsible for coordinating the workflow of each module, evaluating various performance indicators of retired lithium iron phosphate power battery packs according to preset algorithms and standards, and generating evaluation reports.

[0175] Example 3:

[0176] Repair process for retired lithium iron phosphate power batteries:

[0177] Pre-treatment: Disassemble retired lithium iron phosphate power battery packs that have been evaluated and meet the repair standards, and separate individual battery cells;

[0178] Defect Repair: If a single battery cell exhibits sulfation (manifested as increased internal resistance and decreased capacity), pulse repair technology can be used. This involves applying a high-frequency pulse current to the battery cell using a specific pulse generator, gradually dissolving the lead sulfate crystals and restoring the cell's electrochemical performance. For single batteries with performance differences due to uneven charging, equalization charging equipment is used. Equalization charging can bring the voltage and capacity of each individual battery cell in a retired lithium iron phosphate battery pack closer to uniformity, improving the consistency of the retired lithium iron phosphate battery pack.

[0179] Testing and Screening: Repaired individual cells and retired lithium iron phosphate battery packs undergo further electrical performance testing, including open-circuit voltage, internal resistance, and capacity. Data before and after repair are compared to evaluate the repair effectiveness. Voltage, internal resistance, and capacity parameters of individual cells in retired lithium iron phosphate battery packs are measured, and consistency indicators (such as maximum difference and standard deviation) are calculated to ensure the consistency of the retired lithium iron phosphate battery packs meets requirements. Next, short-circuit tests, overcharge / over-discharge tests, and thermal abuse tests are conducted to ensure that the repaired individual cells will not pose safety issues during use. Based on the test results, individual cells that meet performance requirements and exhibit good consistency are selected, while those with poor repair results or potential safety hazards are eliminated.

[0180] Reassembly and grouping: The selected qualified individual cells are reassembled into retired lithium iron phosphate power battery packs according to a certain series and parallel connection method. At the same time, based on the actual performance parameters of the retired lithium iron phosphate power battery packs, an intelligent grouping algorithm is used to optimize the grouping of the retired lithium iron phosphate power battery packs so that the overall performance of the retired lithium iron phosphate power battery packs reaches the best state.

[0181] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for assessing the state of retired lithium iron phosphate power batteries, characterized in that, Includes the following steps: S1. Appearance and Voltage Testing and Evaluation: Inspection of the appearance of retired lithium iron phosphate power battery packs; Under different ambient temperatures and resting times, the open-circuit voltage of retired lithium iron phosphate power battery packs was measured multiple times, the voltage standard deviation was calculated, and the voltage was evaluated based on the voltage standard deviation. S2. Internal resistance test and evaluation: Develop an automated test platform to measure the AC internal resistance of retired lithium iron phosphate power battery packs at fixed frequency intervals within a set frequency range, calculate the rate of change of internal resistance with the number of cycles, and test the DC internal resistance of retired lithium iron phosphate power battery packs through long-term charge and discharge, and evaluate the AC internal resistance based on the rate of change. S3. Capacity and Health Status Test and Evaluation: The actual charge and discharge capacity of the retired lithium iron phosphate power battery pack is obtained by conducting capacity tests through standard charge and discharge regimes. Based on the actual charge and discharge capacity, the capacity decay rate and service life (i.e., health status) of the retired lithium iron phosphate power battery pack are calculated and evaluated. S4. Consistency Test Evaluation: The voltage, internal resistance, and capacity of individual cells in the retired lithium iron phosphate power battery pack are tested in groups, and the maximum difference in voltage, internal resistance, and capacity of individual cells in the retired lithium iron phosphate power battery pack is calculated based on the test results. At the same time, the evaluation is carried out according to the set standards. S5. Safety Testing and Evaluation: Short-circuit, overcharge, over-discharge, and thermal abuse tests are conducted on retired lithium iron phosphate power battery packs using an automated testing simulation system, and an evaluation is performed according to the set standards.

2. The method for assessing the state of retired lithium iron phosphate power batteries according to claim 1, characterized in that: If, in step S1, the appearance shows any of the following: bulging, leakage, corrosion, or physical deformation, the retired lithium iron phosphate power battery pack is directly deemed unsuitable for tiered use, and no further evaluation process will be conducted.

3. The method for assessing the state of retired lithium iron phosphate power batteries according to claim 1, characterized in that: The open-circuit voltage test uses a high-precision voltage tester, and the open-circuit voltage fluctuation range of the battery under different states must be recorded during the test, and the voltage standard deviation must be calculated. If the voltage standard deviation exceeds the set value, the battery voltage stability is considered to be unsuitable for cascade use.

4. The method for assessing the state of retired lithium iron phosphate power batteries according to claim 1, characterized in that: The open-circuit voltage test of the retired lithium iron phosphate power battery pack also includes applying a short charge-discharge pulse to the retired lithium iron phosphate power battery pack and recording the time required for the battery voltage to recover to the initial stable state. If the voltage recovery time exceeds the set time, it indicates that there is polarization inside the retired lithium iron phosphate power battery pack or that the electrolyte performance has deteriorated, meaning that the retired lithium iron phosphate power battery pack cannot be reused.

5. The method for assessing the state of retired lithium iron phosphate power batteries according to claim 1, characterized in that: The AC internal resistance test of the retired lithium iron phosphate power battery pack is conducted within a frequency range of 100Hz-10kHz, with frequency intervals of 100Hz, 500Hz, 1kHz, 5kHz, and 10kHz. The test employs the AC injection method. The AC internal resistance test also includes plotting the AC internal resistance versus frequency curve, analyzing the internal resistance spectral characteristics, recording the AC internal resistance at different cycle counts, calculating the rate of change of internal resistance with cycle count, and determining if the rate of change exceeds a set threshold. If the battery is aged to a certain degree, it means that the retired lithium iron phosphate power battery pack cannot be reused.

6. The method for assessing the state of retired lithium iron phosphate power batteries according to claim 1, characterized in that: The DC internal resistance of the retired lithium iron phosphate power battery pack was tested using an automated testing system controlled by the charging and discharging equipment.

7. The method for assessing the state of retired lithium iron phosphate power batteries according to claim 1, characterized in that: The formula for calculating the actual charge and discharge capacity of the retired lithium iron phosphate power battery pack is as follows: Among them, This is the charging and discharging current, in units of... , The charging / discharging time is expressed in units of... The formula for calculating the capacity decay rate is: in This refers to the actual capacity of the battery. The rated capacity of the battery is used for evaluation. If the capacity decay rate exceeds 20%, it means that the retired lithium iron phosphate power battery pack cannot be reused.

8. The method for assessing the state of retired lithium iron phosphate power batteries according to claim 1, characterized in that: If the maximum voltage difference of the retired lithium iron phosphate power battery pack exceeds 0.3V or the voltage standard deviation exceeds 5% of the average voltage, the retired lithium iron phosphate power battery pack is considered to have poor voltage consistency, meaning that the retired lithium iron phosphate power battery pack cannot be reused. If the maximum internal resistance difference exceeds 10% of the average internal resistance or the standard deviation of internal resistance exceeds 5% of the average internal resistance, then the internal resistance consistency of the retired lithium iron phosphate power battery pack is considered to be poor, meaning that the retired lithium iron phosphate power battery pack cannot be reused in a tiered manner. If the maximum capacity difference exceeds 15% of the rated capacity or the capacity standard deviation exceeds 5% of the rated capacity, the retired lithium iron phosphate power battery pack is considered to have poor capacity consistency, meaning that the retired lithium iron phosphate power battery pack cannot be reused.

9. The method for assessing the state of retired lithium iron phosphate power batteries according to claim 1, characterized in that: The criteria for judging the short circuit test of the retired lithium iron phosphate power battery pack are that the short circuit current should be less than 5 times the rated current, the surface temperature rise of the retired lithium iron phosphate power battery pack should be less than 50°C, and the retired lithium iron phosphate power battery pack should not catch fire, explode or be severely deformed. The criteria for judging the overcharge and over-discharge test of the retired lithium iron phosphate power battery pack is that the retired lithium iron phosphate power battery pack should not bulge, leak, thermal runaway, catch fire or explode. The criteria for judging the thermal abuse test of the retired lithium iron phosphate power battery pack is that the retired lithium iron phosphate power battery pack should not experience thermal runaway, fire or explosion.

10. A state assessment device for retired lithium iron phosphate power batteries, applied to the state assessment method for retired lithium iron phosphate power batteries as described in any one of claims 1-9, characterized in that, include: Appearance and voltage testing and evaluation module, internal resistance testing and evaluation module, capacity and health status testing and evaluation module, conformity testing and evaluation module, safety testing and evaluation module, and data management and report generation module. The appearance and voltage detection and evaluation module collects the appearance and open-circuit voltage data of retired lithium iron phosphate power battery packs through a visual inspection unit and a high-precision voltage tester, and determines whether the appearance and open-circuit voltage of retired lithium iron phosphate power battery packs meet the requirements for tiered use based on the data. The internal resistance detection and evaluation module uses an automated testing platform to detect the AC and DC internal resistance of retired lithium iron phosphate power battery packs and determine whether the internal resistance of retired lithium iron phosphate power battery packs meets the requirements for tiered use. The capacity and health status detection and evaluation module performs capacity testing on retired lithium iron phosphate power battery packs through standard charge and discharge procedures, calculates and evaluates capacity decay rate and health status, and performs factor correction. The consistency detection and evaluation module collects the voltage, internal resistance, and capacity of individual cells in the retired lithium iron phosphate power battery pack, calculates the maximum difference and standard deviation of the voltage, internal resistance, and capacity of individual cells in the retired lithium iron phosphate power battery pack, and determines whether the consistency of the voltage, internal resistance, and capacity of individual cells meets the requirements for tiered use. The safety testing and evaluation module uses an automated testing simulation system to conduct safety tests on retired lithium iron phosphate power battery packs and evaluate them. The data management and report generation module is responsible for coordinating the workflow of each module, evaluating various performance indicators of retired lithium iron phosphate power battery packs according to preset algorithms and standards, and generating evaluation reports.

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