Method for detecting discharge performance of power battery

Abstract

This invention discloses a method for testing the discharge performance of a power battery. The invention relates to the field of battery analysis technology and includes the following steps: acquiring the battery surface temperature signal through a constant current discharge test, plotting a battery charge-surface temperature coupling curve, and comparing it with an ideal standard curve to preliminarily determine the discharge performance; when the performance meets the requirements, determining several sub-test intervals by combining the surface temperature and the initial and final values ​​of the battery charge, acquiring voltage change data and discharge duration within each interval, analyzing the ohmic internal resistance based on the voltage change data, determining the unit internal resistance change by combining the discharge amount, obtaining the instantaneous voltage change, obtaining the relaxation time constant, and comprehensively evaluating the battery discharge performance based on the internal resistance change rate to determine whether it meets the standard, thus improving the completeness and systematic nature of battery performance evaluation.

Description

Technical Field

[0001] This invention relates to the field of battery analysis technology, specifically a method for testing the discharge performance of a power battery. Background Technology

[0002] With the rapid development of electric vehicles and renewable energy storage systems, the widespread application of power batteries has made their performance testing particularly important. The performance of power batteries not only directly affects the driving range, safety, and economy of electric vehicles, but also plays a crucial role in the storage and utilization of renewable energy.

[0003] However, as the demand for power batteries in these scenarios continues to grow, the evaluation and analysis of their performance has become increasingly important. The performance of power batteries directly affects the range, safety, and lifespan of devices. In particular, the analysis of internal resistance and dynamic response characteristics, such as relaxation time, provides crucial information for battery state assessment and management. Existing analytical methods lack a systematic design in data acquisition and processing. Some methods focus only on a single characteristic, such as internal resistance or capacity, failing to comprehensively analyze the overall performance state of lithium batteries based on the relaxation process. This limitation makes it difficult for lithium battery performance evaluation to meet the needs of complex application scenarios.

[0004] In the prior art, CN117214753A discloses a lithium battery analysis method, apparatus, storage medium, system, and terminal based on the relaxation process, including the following steps: acquiring the voltage of the lithium battery during the relaxation process; calculating the relaxation voltage change of the lithium battery based on the voltage of the lithium battery during the start and end times of the relaxation process, and calculating the internal resistance of the lithium battery based on the relaxation voltage change and the current during charging or discharging; calculating the relaxation time constant of the lithium battery based on the voltage acquired by the lithium battery during the relaxation process, thereby achieving comprehensive analysis of the lithium battery through the relaxation process.

[0005] However, this approach fails to effectively address the temperature behavior of the battery during charging and discharging. Lithium battery performance analysis methods mainly focus on the measurement and evaluation of parameters such as voltage, current, and internal resistance. Since the temperature rise of the battery is usually closely related to the internal resistance heating and the intensity of the electrochemical reaction, the temperature change of the battery during discharge is an important indicator for battery thermal management and safety assessment. The unified overall analysis ignores the influence of different temperatures during discharge, thus reducing the accuracy and effectiveness of the detection system.

[0006] The information disclosed in the background section is only intended to enhance the understanding of the background of this disclosure, and therefore may include information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0007] The purpose of this invention is to provide a method for testing the discharge performance of a power battery, so as to solve the problems mentioned in the background art.

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] A method for testing the discharge performance of a power battery, comprising the following steps:

[0010] Used for constant current discharge testing of power batteries under test, the surface temperature signal of the battery pack is collected at a fixed sampling frequency during the complete discharge process, the battery charge-surface temperature coupling curve is plotted, and it is compared with the ideal standard curve. Based on the degree of difference, it is preliminarily judged whether the discharge performance meets the requirements.

[0011] This is used to determine several sub-test intervals when the initial judgment of the discharge performance meets the requirements, by combining the battery surface temperature and the battery charge value at the beginning and end of the constant current discharge test. Within the sub-test intervals, the power battery under test is subjected to constant current discharge test again, and the voltage change data and discharge duration of the sub-test intervals are collected.

[0012] The internal resistance of the power battery under test in the sub-test interval is analyzed based on voltage change data, and the discharge amount of the power battery under test in the sub-test interval is analyzed based on discharge duration. The change in unit internal resistance of the power battery under test in the sub-test interval is determined by combining the internal resistance and discharge amount.

[0013] This method is used to obtain the instantaneous voltage change of the power battery under test after the termination of the sub-test interval, plot the instantaneous voltage recovery curve, analyze the recovery curve to obtain the relaxation time constant, and comprehensively characterize the discharge performance of the power battery under test in each sub-test interval based on the relaxation time constant and the unit internal resistance change, so as to determine whether the discharge performance of the power battery under test meets the standard.

[0014] Furthermore, the constant current discharge test specifically refers to discharging the battery with a constant current value from the initial value of the battery charge until the battery charge reaches the final value of the battery charge. This complete process is recorded as a complete discharge process, and the constant current is less than the battery's maximum output current.

[0015] The logic behind acquiring the surface temperature signal of the battery pack during the complete discharge process at a fixed sampling frequency is as follows: several temperature detection points are set on the surface of the power battery to be tested, and the average temperature of each temperature detection point is used as the surface temperature signal of the power battery pack to be tested at the corresponding time.

[0016] After obtaining the time-series data of the surface temperature of the power battery pack to be tested, a battery capacity-surface temperature coupling curve is plotted with battery capacity as the independent variable and battery surface temperature as the dependent variable.

[0017] Furthermore, the ideal standard curve specifically refers to a healthy power battery of the same model as the power battery under test, and the battery charge-surface temperature coupling curve is plotted under the conditions of selecting the initial and final values ​​of the battery charge corresponding to the power battery under test, performing constant current discharge test with the same constant current, and the same ambient temperature.

[0018] Furthermore, the battery charge-surface temperature coupling curve is compared with an ideal standard curve. The specific steps include: extracting the characteristic parameters of the battery charge-surface temperature coupling curve, including the temperature corresponding to different charges, the temperature peak, and the integral value of the curve with respect to the independent variable; simultaneously extracting the corresponding characteristic parameters of the ideal standard curve; calculating the difference coefficient based on the relative difference of the characteristic parameters of the two curves; and characterizing the degree of difference through the difference coefficient.

[0019] Furthermore, the specific logic underlying the calculation of the difference coefficient includes:

[0020] Obtain the surface temperature corresponding to different charge points of the battery charge-surface temperature coupling curve of the power battery under test, calculate the absolute difference between the surface temperature corresponding to different charge points and the surface temperature of the corresponding charge point in the ideal standard curve, normalize the absolute difference by the surface temperature of the corresponding charge point in the ideal standard curve to obtain the error normalization value, calculate the mean of the error normalization value of different charge points, and record the mean as the first difference coefficient.

[0021] The temperature peaks in the battery charge-surface temperature coupling curve and the ideal standard curve of the power battery under test are obtained. The absolute difference between the two temperature peaks is calculated, and the absolute difference between the temperature peaks is normalized by the temperature peak in the ideal standard curve to obtain the second difference coefficient.

[0022] Meanwhile, the normalized value of the absolute error of the area enclosed by the battery charge-surface temperature coupling curve and the ideal standard curve of the power battery under test and the horizontal axis is used as the third difference coefficient.

[0023] The difference coefficient is obtained by summing the values ​​of the first, second, and third difference coefficients.

[0024] Furthermore, based on the degree of difference, a preliminary judgment is made as to whether the discharge performance meets the requirements, specifically:

[0025] Set a threshold for the degree of difference. Compare the difference coefficient with the threshold for the degree of difference. If the difference coefficient is less than the threshold for the degree of difference, it is preliminarily judged that the discharge performance meets the requirements. Otherwise, it is preliminarily judged that the discharge performance does not meet the requirements.

[0026] Furthermore, the specific logic for dividing the test intervals is as follows: the interval formed by the initial and final battery charge values ​​is denoted as the charge distribution interval. A charge step size is set, and several sampling points are determined within the charge distribution interval based on the charge step size. The initial battery charge value is used as the starting point of the first sub-test interval, and the surface temperature corresponding to the starting point is recorded. Each sampling point is traversed, and for any sampling point, the temperature change amplitude and temperature change slope between the sampling point and the starting point of the latest sub-test interval are calculated. If the temperature change amplitude is less than a preset threshold or the temperature change slope is less than a preset threshold, then this sampling point is taken as the end point of the latest sub-test interval and as the starting point of the next sub-test interval. The temperature change slope is calculated using the finite difference method.

[0027] Furthermore, based on the absolute difference of voltage change data in different sub-test intervals and the set constant discharge current, the ohmic internal resistance of the power battery under test during constant current discharge in the sub-test interval is calculated using Ohm's law.

[0028] The steps for determining the discharge quantity within a sub-test interval include: recording the discharge start time and discharge end time of each sub-test interval; characterizing the discharge quantity of each sub-test interval by multiplying the discharge voltage value at different times by the constant discharge current and the integral value over the time interval between the discharge start time and discharge end time of each sub-test interval.

[0029] The change in unit internal resistance is specifically characterized by the ratio of the ohmic internal resistance within a sub-test interval to the discharge quantity within that sub-test interval. The change in unit internal resistance is used to represent the internal resistance corresponding to a unit discharge quantity.

[0030] Furthermore, the logic for obtaining the relaxation time constant is as follows: obtain the instantaneous voltage change of the power battery under test after the end of the sub-test interval, and perform first-order exponential fitting on the voltage recovery curve after the end of each sub-test interval to obtain the relaxation time constant.

[0031] When the error between the fitted relaxation voltage and the actual relaxation voltage is less than the set voltage error threshold, the corresponding time constant is the relaxation time constant.

[0032] The comprehensive evaluation coefficient is calculated based on the relaxation time constant and the rate of change of internal resistance. This comprehensive evaluation coefficient comprehensively characterizes the discharge performance during the corresponding sub-test interval. The specific logic underlying the calculation of the comprehensive evaluation coefficient is as follows:

[0033] Obtain the unit internal resistance change in different sub-test intervals, calculate the absolute difference between the unit internal resistance change in different sub-test intervals and the reference value of the unit internal resistance change, and normalize the absolute difference of the internal resistance change using the reference value of the unit internal resistance change. Take the square root of the normalized value and record it as the first evaluation coefficient.

[0034] The natural logarithm of the sum of the relaxation time constant and the integer 1 is used as the second evaluation coefficient for different sub-test intervals;

[0035] The sum of the first evaluation coefficient and the second evaluation coefficient is used as the comprehensive evaluation coefficient of the sub-test interval.

[0036] Furthermore, based on the comprehensive evaluation coefficient of the sub-test interval, the specific logic for completing the discharge performance test of the power battery under test is as follows: set the performance evaluation threshold for the corresponding sub-test interval, compare the comprehensive evaluation coefficient of the corresponding sub-test interval with the performance evaluation threshold, if the comprehensive evaluation coefficient is greater than the performance evaluation threshold, it is determined that the discharge performance in the sub-test interval does not meet the requirements, if the comprehensive evaluation coefficient is not greater than the performance evaluation threshold, it is determined that the discharge performance in the sub-test interval meets the requirements, and when the discharge performance in all sub-test intervals meets the requirements, it is determined that the discharge performance of the power battery under test meets the standards.

[0037] Compared with the prior art, the beneficial effects of the present invention are:

[0038] First, this invention solves the problem of neglecting temperature changes in the prior art by collecting surface temperature data of the power battery in real time during the complete discharge process and plotting the battery charge-surface temperature coupling curve at a fixed sampling frequency. Furthermore, by comparing with an ideal standard curve, it can promptly determine whether there are abnormal temperature behaviors of the battery, ensuring high-precision capture of thermal behavior during discharge and effectively improving the comprehensiveness of power battery performance testing.

[0039] Secondly, based on temperature monitoring results, this invention divides the complete discharge process into several sub-test intervals and collects voltage change data and duration for each interval. This partitioned monitoring method solves the limitation of existing technologies that only focus on the overall discharge process and cannot capture detailed issues. It can deeply analyze the dynamic performance of the power battery at different stages through the division of sub-test intervals, enhance the accuracy of dynamic analysis of the internal resistance of the power battery, and provide a scientific basis for the comprehensive evaluation of battery health status, energy loss and aging degree.

[0040] Furthermore, this invention combines the dynamic analysis of voltage recovery characteristics and relaxation process, characterizes the speed at which the battery recovers to equilibrium state through the relaxation time constant, and evaluates the discharge performance within the sub-test interval by combining the internal resistance change rate. This invention can comprehensively analyze the dynamic characteristics of the power battery in each interval, improving the completeness and systematic nature of battery performance evaluation. Attached Figure Description

[0041] Figure 1 This is a schematic diagram of the overall method flow of the present invention;

[0042] Figure 2This is a diagram showing the surface temperature distribution of the power battery.

[0043] Figure 3 The fitting curve of ohmic internal resistance-interval discharge duration;

[0044] Figure 4 A bar chart showing the statistical changes in unit internal resistance. Detailed Implementation

[0045] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0046] It should be noted that, unless otherwise defined, the technical or scientific terms used in this invention should have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0047] Example:

[0048] Please see Figures 1-4 The present invention provides a technical solution:

[0049] A method for testing the discharge performance of a power battery, specifically including:

[0050] Step 1: Perform a constant current discharge test on the power battery to be tested. Collect the surface temperature signal of the battery pack during the complete discharge process at a fixed sampling frequency, plot the battery charge-surface temperature coupling curve, and compare it with the ideal standard curve. Based on the degree of difference, make a preliminary judgment on whether the discharge performance meets the requirements.

[0051] A constant current discharge test is performed on the power battery to be tested. The constant current discharge test specifically refers to discharging the battery with a constant current value from the initial value of the battery charge until the battery charge reaches the final value of the battery charge. Generally, the initial value of the battery charge can be 98% and the final value of the battery charge can be 2%. This complete process is recorded as a complete discharge process. The constant current is set based on the application scenario of the power battery to be tested, and the set constant current is less than the maximum output current of the battery.

[0052] Different application scenarios, such as electric vehicles and energy storage systems, have different requirements for the discharge characteristics of power batteries. The set constant current should reflect the actual usage under actual working conditions in order to more accurately evaluate the actual performance of the battery. For example, electric vehicles may experience a higher instantaneous discharge current when accelerating, while maintaining a lower constant current when cruising.

[0053] The logic behind acquiring the surface temperature signal of the battery pack during the complete discharge process at a fixed sampling frequency is as follows: several temperature detection points are set on the surface of the power battery to be tested, and the average temperature of each temperature detection point is used as the surface temperature signal of the power battery pack to be tested at the corresponding time.

[0054] These detection points are randomly distributed at different locations on the battery pack to comprehensively monitor the temperature changes on the battery pack surface. Since the battery temperature changes due to electrochemical reactions and internal resistance during discharge, but the surface temperature of the battery may have spatial distribution differences, setting up multiple temperature detection points can improve the accuracy and completeness of temperature monitoring.

[0055] The number and distribution of temperature detection points are optimized based on the size, shape, and internal structure of the battery pack, so that each detection point can represent the temperature changes in its area. For example, more temperature detection points are set in key areas on the surface of the power battery to be tested. These locations include the central area, edge area, and other areas prone to temperature anomalies, such as connection points or locations in contact with heat sources, to ensure that the comprehensive distribution of battery surface temperature can be captured.

[0056] Real-time temperature data at each temperature detection point is collected using temperature sensors, such as thermocouples or infrared temperature sensors, at a fixed sampling frequency. This fixed sampling frequency ensures continuous recording of the dynamic changes in the battery surface temperature, avoiding the omission of critical temperature information; specifically, a sampling frequency of once per second is recommended.

[0057] The surface temperature time-series data of the power battery pack to be tested is acquired. The acquired surface temperature time-series data is preprocessed, including outlier removal and missing value imputation. Specifically, the outlier removal process uses the standard deviation method.

[0058] The specific logic is as follows: assuming the data follows a normal distribution, calculate the mean and standard deviation of the temperature data. Typically, 95.4% of the data from a normal distribution will fall within the range of the mean ± 2 times the standard deviation. Therefore, the range is set as the temperature mean. Temperature data exceeding twice the standard deviation are excluded as outliers.

[0059] The logic behind the missing value imputation process is as follows: the average surface temperature of adjacent sampling points is calculated using the surface temperature signals of adjacent sampling points, and the average surface temperature is used as the missing data for imputation. Based on the preprocessed temperature data, a battery charge-surface temperature coupling curve is plotted with battery charge as the independent variable and battery surface temperature as the dependent variable.

[0060] The battery charge-surface temperature coupling curve is compared with an ideal standard curve. The specific steps include: extracting characteristic parameters from the battery charge-surface temperature coupling curve, including the temperature corresponding to different charges, the peak temperature, and the integral value of the curve with respect to the independent variable; simultaneously extracting the corresponding characteristic parameters from the ideal standard curve; calculating the difference coefficient based on the relative differences in the characteristic parameters of the two curves; and characterizing the degree of difference through the difference coefficient. The specific logic underlying the calculation of the difference coefficient includes:

[0061] Obtain the surface temperature corresponding to different charge points of the battery charge-surface temperature coupling curve of the power battery under test, calculate the absolute difference between the surface temperature corresponding to different charge points and the surface temperature of the corresponding charge point in the ideal standard curve, normalize the absolute difference by the surface temperature of the corresponding charge point in the ideal standard curve to obtain the error normalization value, calculate the mean of the error normalization value of different charge points, and record the mean as the first difference coefficient.

[0062] The temperature peaks in the battery charge-surface temperature coupling curve and the ideal standard curve of the power battery under test are obtained. The absolute difference between the two temperature peaks is calculated, and the absolute difference between the temperature peaks is normalized by the temperature peak in the ideal standard curve to obtain the second difference coefficient.

[0063] Meanwhile, the normalized value of the absolute error of the area enclosed by the battery charge-surface temperature coupling curve and the ideal standard curve of the power battery under test and the horizontal axis is used as the third difference coefficient.

[0064] The sum of the first, second, and third difference coefficients yields the difference coefficient, which is calculated using the following formula:

[0065]

[0066] In the formula, The coefficient of variation is... The battery charge-surface temperature coupling curve of the power battery under test is shown in the first... The surface temperature corresponding to a randomly selected electrical charge point The surface temperature at the corresponding charge point in the ideal standard curve. That is, the first difference coefficient. The temperature peak of the battery charge-surface temperature coupling curve of the power battery under test. The peak temperature in the ideal standard curve. This represents the second difference coefficient. The normalized value of the integral difference between the two curves with respect to the independent variable is the third difference coefficient, where... To randomly select the index of the electricity point, n is the total number of randomly selected power points;

[0067] It should be noted that the coefficient of difference This index is used to quantify the difference between the battery charge-surface temperature coupling curve and the ideal standard curve of the power battery under test. It comprehensively considers the errors of multiple characteristic parameters and is used to evaluate the degree of conformity between the actual battery performance and the ideal state. The larger the value, the greater the difference between the temperature curve of the power battery under test and the ideal standard curve. Specifically, it means that the surface temperature of the actual battery at different charge levels is significantly higher or lower than the ideal state, or the temperature peak performance is not as expected, which may indicate that the battery has overheating, poor heat dissipation or other performance problems.

[0068] in, For the power battery under test in the first The surface temperature corresponding to each charge point is a measured temperature value, used to reflect the thermal state of the battery under specific charge conditions. It is the surface temperature at the corresponding charge point in the ideal standard curve, a theoretically expected temperature value, based on standard test or design specifications, and using relative error. In this form, the influence of temperature dimensions is eliminated, allowing for comparison of temperature changes in different ranges, and the relative error can more accurately reflect the difference between the actual temperature and the ideal state.

[0069] By comparing the relative errors of temperature peaks It can assess whether the maximum thermal state of the battery during discharge meets expectations, reflecting the stability of the battery design and manufacturing, and determining whether the battery meets the requirements.

[0070] The integral difference reflects whether heat generation and dissipation within a specific power range meet design expectations. By comparing the overall area, the heat dissipation capacity under different operating conditions can be identified. The normalized value of the integral difference... This method is used to quantify the overall difference between two curves over the integration range. By integrating the independent variable (electricity), it is possible to effectively assess whether the trend of temperature change is consistent over the entire electricity range.

[0071] The normalized value of the integral difference between the two curves with respect to the independent variable is calculated. The formula for the third difference coefficient is as follows:

[0072]

[0073] In the formula, The power battery under test has a charge level of [missing information]. Surface temperature at that time The battery capacity in the ideal standard curve is The surface temperature of the power battery at that time This is the initial battery charge value. This represents the final battery charge level, where This represents the battery charge variable between the initial and final battery charge levels.

[0074] It should be noted that the integral form represents the cumulative effect of temperature differences across the entire power range. By integrating, we can obtain the comprehensive differences across the entire power range, rather than just comparing a single data point. Furthermore, by using the integral of an ideal standard curve as the denominator, we ensure the relativity of the final result, making comparisons under different power ranges or different battery characteristics more reasonable.

[0075] The ideal standard curve mentioned above refers specifically to a healthy power battery of the same model as the power battery under test. The battery charge and surface temperature coupling curve is plotted by selecting the initial and final values ​​of the battery charge corresponding to the power battery under test, performing constant current discharge test with the same constant current, and under the same ambient temperature.

[0076] The logic behind the preliminary judgment of whether the discharge performance meets the requirements based on the degree of difference is as follows: a difference threshold is set, and the difference coefficient is compared with the difference threshold. If the difference coefficient is greater than or equal to the difference threshold, the discharge performance is preliminarily judged to be unacceptable; if the difference coefficient is less than the difference threshold, the discharge performance is preliminarily judged to be compliant. The difference threshold is specifically set based on battery design requirements and expert experience.

[0077] Step 2: When the discharge performance is initially determined to meet the requirements, several sub-test intervals are determined by combining the battery surface temperature and the battery charge at the beginning and end of the constant current discharge test. The constant current discharge test is then performed again on the power battery under test within the sub-test intervals, and the voltage change data and discharge duration of the sub-test intervals are collected.

[0078] The specific logic for dividing the test interval is as follows: the interval formed by the initial value of battery power and the final value of battery power is recorded as the power distribution interval. A power step size is set. Based on the power step size, several sampling points are determined within the power distribution interval. The initial value of battery power is used as the starting point of the first sub-test interval, and the surface temperature corresponding to the starting point is recorded. Each sampling point is traversed. For any sampling point, the temperature change amplitude and temperature change slope between the sampling point and the starting point of the latest sub-test interval are calculated. If the temperature change amplitude is less than a preset threshold or the temperature change slope is less than a preset threshold, then this sampling point is used as the end point of the latest sub-test interval and as the starting point of the next sub-test interval.

[0079] It should be noted that dividing the battery capacity into several sub-test intervals based on temperature changes makes the temperature more stable within each sub-test interval. Moreover, the temperature change of the battery is more consistent within a sub-test interval, which makes the analysis results more stable and reduces the uncertainty of external factors. Subdividing the intervals makes temperature monitoring and battery performance evaluation more accurate.

[0080] Within a sub-test interval, the relatively consistent temperature changes reduce the volatility between data points. This consistency results in more stable temperature data, effectively avoiding noise caused by short-term temperature fluctuations and providing a more reliable foundation for subsequent data analysis and model building. Simultaneously, consistent temperature changes indicate that the battery's thermal management performance and operating status are relatively stable within this interval; therefore, the analysis and evaluation results based on data from this interval are more reliable.

[0081] The slope of temperature change is calculated using the finite difference method; the specific formula used to calculate the slope of temperature change is as follows:

[0082]

[0083] In the formula, Let be the slope of the temperature change between the j-th sampling point and the starting point of the latest sub-test interval. Let J be the surface temperature of the j-th sampling point. The starting surface temperature is the starting point of the latest sub-test interval. Let the battery level be at the j-th sampling point. is the battery level at the starting point of the latest sub-test interval, and j is the index of the sampling point.

[0084] It should be noted that, Representing the The slope of temperature change at each sampling point reflects the rate of temperature change with charge. By calculating the slope, the thermal behavior of the battery during charging and discharging can be dynamically evaluated. The larger the value, the faster the temperature changes with the amount of electricity.

[0085] Step 3: Analyze the ohmic internal resistance of the power battery under test in the sub-test interval based on the voltage change data, and analyze the discharge amount of the power battery under test in the sub-test interval based on the discharge duration. Combine the ohmic internal resistance and the discharge amount to determine the unit internal resistance change of the power battery under test in the sub-test interval.

[0086] Based on the absolute difference of voltage change data in different sub-test intervals and the set constant discharge current, the ohmic internal resistance of the power battery under test during constant current discharge in the sub-test intervals is calculated using Ohm's law. The specific formula used is as follows:

[0087]

[0088] In the formula, Let be the ohmic internal resistance in the p-th sub-test interval. Let be the end voltage of the p-th sub-test interval. Let be the starting voltage of the p-th sub-test interval. The constant discharge current is set, where p is the index of the sub-test interval;

[0089] It's important to note that internal resistance is a crucial factor affecting battery performance during discharge. Calculating the ohmic internal resistance allows for dynamic assessment of the battery's state and timely understanding of its performance under different operating conditions. This formula is based on Ohm's law, which states that the voltage drop generated when current flows through the battery is directly proportional to the internal resistance. The internal resistance can be calculated by measuring voltage changes and applying a constant current.

[0090] The steps for determining the discharge quantity within a sub-test interval include: recording the discharge start and end times of each sub-test interval; characterizing the discharge quantity of each sub-test interval by integrating the product of the discharge voltage and constant discharge current at different times over the time interval between the discharge start and end times; that is, the formula for calculating the discharge quantity within a sub-test interval is as follows:

[0091]

[0092] In the formula, Let p be the discharge amount in the p-th sub-test interval. Let be the time variable of the discharge process in the p-th sub-test interval. for Voltage value at time, The discharge start time of the p-th sub-test interval is... This represents the final discharge time of the p-th sub-test interval;

[0093] The discharge amount is calculated using an integral form, which reflects the dynamic characteristics of the battery discharge process within a given time period. This integral form represents the cumulative effect of the product of current and voltage over time throughout the entire discharge period, reflecting the total amount of electricity released by the battery during this period.

[0094] The change in unit internal resistance is specifically characterized by the ratio of the ohmic internal resistance within a sub-test interval to the discharge quantity within that sub-test interval. The change in unit internal resistance represents the internal resistance corresponding to a unit discharge quantity. The formula used to calculate the change in unit internal resistance is as follows:

[0095]

[0096] In the formula, This represents the unit internal resistance change in the p-th sub-test interval, used to indicate the internal resistance corresponding to a unit discharge charge.

[0097] It should be noted that this formula represents the unit discharge capacity. The change in internal resistance can be understood as the degree of change in internal resistance corresponding to the discharge of 1 ampere-hour of charge. Divide by discharge amount This allows us to obtain the internal resistance characteristics per unit charge. By calculating the change in unit internal resistance, we can better quantify the battery's performance during discharge. This indicator helps to evaluate the battery's efficiency and health status.

[0098] Step 4: Obtain the instantaneous voltage change of the power battery under test after the termination of the sub-test interval, plot the instantaneous voltage recovery curve, analyze the recovery curve to obtain the relaxation time constant, and comprehensively characterize the discharge performance of the power battery under test in each sub-test interval based on the relaxation time constant and the unit internal resistance change, so as to determine whether the discharge performance of the power battery under test meets the standard.

[0099] The logic underlying the determination of the relaxation time constant is as follows: Obtain the instantaneous voltage change of the battery under test after the end of each sub-test interval; perform a first-order exponential fit on the voltage recovery curve after the end of each sub-test interval to obtain the relaxation time constant. Specifically, the formula used is:

[0100]

[0101] In the formula, To fit the relaxation voltage, This is the stable voltage at the end of relaxation. This is the voltage value at the moment the discharge stops. To indicate the duration of relaxation, The relaxation time constant;

[0102] When the error between the fitted relaxation voltage and the actual relaxation voltage is less than the set voltage error threshold, the corresponding That is, the relaxation time constant;

[0103] This formula, based on a first-order exponential decay model, describes the voltage recovery process of a battery after discharge has ceased. Voltage recovers from its initial value. Gradually recovering to the final stable voltage Fitting the exponential form of voltage change can effectively capture the voltage change pattern after battery discharge, and the relaxation time constant can be determined through data fitting.

[0104] The comprehensive evaluation coefficient is calculated based on the relaxation time constant and the rate of change of internal resistance. This comprehensive evaluation coefficient comprehensively characterizes the discharge performance during the corresponding sub-test interval. The specific logic underlying the calculation of the comprehensive evaluation coefficient is as follows:

[0105] Obtain the unit internal resistance change in different sub-test intervals, calculate the absolute difference between the unit internal resistance change in different sub-test intervals and the reference value of the unit internal resistance change, and normalize the absolute difference of the internal resistance change using the reference value of the unit internal resistance change. Take the square root of the normalized value and record it as the first evaluation coefficient.

[0106] The natural logarithm of the sum of the relaxation time constant and the integer 1 is used as the second evaluation coefficient for different sub-test intervals;

[0107] The sum of the first evaluation coefficient and the second evaluation coefficient is taken as the comprehensive evaluation coefficient of the sub-test interval. The specific formula for calculating the comprehensive evaluation coefficient is as follows:

[0108]

[0109] In the formula, Let be the comprehensive evaluation coefficient for the p-th sub-test interval. This is a reference value for the unit internal resistance change. The normalized value of the relaxation time constant of the p-th sub-test interval is given. Specifically, the normalized value of the relaxation time constant of the p-th sub-test interval refers to taking the maximum and minimum relaxation time constants of all sub-test intervals as the endpoints of the normalization interval, and normalizing the relaxation time constant of a single sub-test interval within the normalization interval.

[0110] It should be noted that the comprehensive evaluation coefficient Combining the battery's internal resistance change and relaxation time constant, it provides a comprehensive performance index that reflects the battery's actual performance under discharge conditions, comprehensively considering the battery's energy output efficiency and recovery capability. A higher value usually means that the battery exhibits a larger change in internal resistance and / or a slower voltage recovery capability during discharge, which indicates that the battery's discharge efficiency is reduced and its discharge performance is worse.

[0111] Among them when When the value increases, it causes Part 1 This increase indicates that a greater difference in the battery's internal resistance relative to the reference value may signify a decline in battery performance, hence the higher... This will lead to a higher overall evaluation coefficient. An increase reflects a decrease in battery discharge performance; the square root function is used to smooth out the effects of changes in internal resistance. Through square root transformation, this change can be mitigated, making the change smoother and less prone to excessive amplification.

[0112] The relaxation time constant reflects the battery's ability to recover voltage after discharge stops. A larger relaxation time constant indicates a longer time for the battery to recover to a steady state, which is usually associated with higher internal resistance or poorer battery health. This indicates that the battery takes longer to recover to a stable voltage after discharging stops, which usually indicates impaired battery performance. Therefore, it is directly proportional to the overall evaluation coefficient and is a logarithmic function. Able to emphasize a large The impact of the value is reduced, while the sensitivity to small changes is decreased, making the overall assessment more stable.

[0113] The logic for completing the discharge performance test of the power battery under test based on the comprehensive evaluation coefficient of the p-th sub-test interval is as follows: Set the performance evaluation threshold for the corresponding sub-test interval, compare the comprehensive evaluation coefficient of the corresponding sub-test interval with the performance evaluation threshold, if the comprehensive evaluation coefficient is greater than the performance evaluation threshold, it is determined that the discharge performance in the sub-test interval does not meet the requirements, if the comprehensive evaluation coefficient is not greater than the performance evaluation threshold, it is determined that the discharge performance in the sub-test interval meets the requirements, and when the discharge performance in all sub-test intervals meets the requirements, it is determined that the discharge performance of the power battery under test meets the standards.

[0114] The performance evaluation thresholds for each sub-test interval are determined based on historical data. The specific method is as follows: by collecting data on the unit internal resistance change and relaxation time constant of several healthy power batteries of the same model in each sub-test interval, the average value and standard deviation of the comprehensive evaluation coefficient of all healthy power batteries are calculated. The average value of the comprehensive evaluation coefficient plus three times the standard deviation is used as the reference value of the comprehensive evaluation coefficient. Based on the reference value of the comprehensive evaluation coefficient and combined with expert experience, fine-tuning is performed to obtain the performance evaluation thresholds for each sub-test interval.

[0115] The above formulas are all dimensionless calculations. The formulas are derived from software simulations based on a large amount of collected data to obtain the most recent real-world results. The preset parameters in the formulas are set by those skilled in the art according to the actual situation.

[0116] The above embodiments can be implemented, in whole or in part, by software, hardware, firmware, or any other combination thereof. When implemented in software, the above embodiments can be implemented, in whole or in part, as a computer program product. Those skilled in the art will recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented by electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution.

[0117] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0118] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application.

Claims

1. A method for testing the discharge performance of a power battery, characterized in that, include: A constant current discharge test is performed on the power battery to be tested. The surface temperature signal of the battery pack during the complete discharge process is collected at a fixed sampling frequency. The battery charge-surface temperature coupling curve is plotted and compared with the ideal standard curve. The discharge performance is preliminarily judged based on the degree of difference. When the initial judgment is that the discharge performance meets the requirements, several sub-test intervals are determined by combining the battery surface temperature and the battery charge value at the beginning and end of the constant current discharge test. The constant current discharge test is carried out again on the power battery under test in the sub-test intervals, and the voltage change data and discharge duration of the sub-test intervals are collected. The internal resistance of the power battery under test in the sub-test interval is analyzed based on voltage change data, and the discharge amount of the power battery under test in the sub-test interval is analyzed based on discharge duration. The change in unit internal resistance of the power battery under test in the sub-test interval is determined by combining the internal resistance and discharge amount. The instantaneous voltage change of the power battery under test after the termination of the sub-test interval is obtained, the instantaneous voltage recovery curve is plotted, the recovery curve is analyzed to obtain the relaxation time constant, and the discharge performance of the power battery under test in each sub-test interval is comprehensively characterized by the relaxation time constant and the unit internal resistance change, so as to determine whether the discharge performance of the power battery under test meets the standard. The change in unit internal resistance is specifically characterized by the ratio of the ohmic internal resistance within a sub-test interval to the discharge quantity within that sub-test interval. The change in unit internal resistance is used to represent the internal resistance corresponding to a unit discharge quantity.

2. The method for testing the discharge performance of a power battery according to claim 1, characterized in that: The constant current discharge test specifically refers to discharging the battery with a constant current value from the initial value of the battery charge until the battery charge reaches the final value of the battery charge. This complete process is recorded as a complete discharge process, and the constant current is less than the battery's maximum output current. The logic behind acquiring the surface temperature signal of the battery pack during the complete discharge process at a fixed sampling frequency is as follows: several temperature detection points are set on the surface of the power battery to be tested, and the average temperature of each temperature detection point is used as the surface temperature signal of the power battery pack to be tested at the corresponding time. After obtaining the time-series data of the surface temperature of the power battery pack to be tested, a battery capacity-surface temperature coupling curve is plotted with battery capacity as the independent variable and battery surface temperature as the dependent variable.

3. The method for testing the discharge performance of a power battery according to claim 2, characterized in that: The ideal standard curve refers specifically to a healthy power battery of the same model as the power battery under test. The battery charge and surface temperature coupling curve is plotted under the conditions of selecting the initial and final values ​​of the battery charge corresponding to the power battery under test, performing constant current discharge test with the same constant current, and the same ambient temperature.

4. The method for testing the discharge performance of a power battery according to claim 3, characterized in that: The battery charge-surface temperature coupling curve is compared with an ideal standard curve. Specific steps include: The characteristic parameters of the battery charge-surface temperature coupling curve are extracted. The characteristic parameters include the temperature corresponding to different charges, the temperature peak, and the integral value of the curve with respect to the independent variable. At the same time, the corresponding characteristic parameters of the ideal standard curve are extracted. The difference coefficient is calculated based on the relative difference of the characteristic parameters of the two curves, and the degree of difference is characterized by the difference coefficient. The specific logic underlying the calculation of the coefficient of difference includes: Obtain the surface temperature corresponding to different charge points of the battery charge-surface temperature coupling curve of the power battery under test, calculate the absolute difference between the surface temperature corresponding to different charge points and the surface temperature of the corresponding charge point in the ideal standard curve, normalize the absolute difference by the surface temperature of the corresponding charge point in the ideal standard curve to obtain the error normalization value, calculate the mean of the error normalization value of different charge points, and record the mean as the first difference coefficient. The temperature peaks in the battery charge-surface temperature coupling curve and the ideal standard curve of the power battery under test are obtained. The absolute difference between the two temperature peaks is calculated, and the absolute difference between the temperature peaks is normalized by the temperature peak in the ideal standard curve to obtain the second difference coefficient. Meanwhile, the normalized value of the absolute error of the area enclosed by the battery charge-surface temperature coupling curve and the ideal standard curve of the power battery under test and the horizontal axis is used as the third difference coefficient. The difference coefficient is obtained by summing the values ​​of the first, second, and third difference coefficients.

5. The method for testing the discharge performance of a power battery according to claim 4, characterized in that: Based on the degree of difference, a preliminary judgment is made as to whether the discharge performance meets the requirements, specifically: Set a threshold for the degree of difference. Compare the difference coefficient with the threshold for the degree of difference. If the difference coefficient is less than the threshold for the degree of difference, it is preliminarily judged that the discharge performance meets the requirements. Otherwise, it is preliminarily judged that the discharge performance does not meet the requirements.

6. The method for testing the discharge performance of a power battery according to claim 5, characterized in that: The specific logic behind dividing the test intervals is as follows: The interval between the initial and final battery charge values ​​is defined as the charge distribution interval. A charge step size is set, and several sampling points are determined within the charge distribution interval based on the charge step size. The initial battery charge value is used as the starting point of the first sub-test interval, and the surface temperature corresponding to the starting point is recorded. Each sampling point is traversed, and for any sampling point, the temperature change amplitude and temperature change slope between the sampling point and the starting point of the latest sub-test interval are calculated. If the temperature change amplitude is less than a preset threshold or the temperature change slope is less than a preset threshold, then this sampling point is used as the end point of the latest sub-test interval and as the starting point of the next sub-test interval. The slope of temperature change is calculated using the finite difference method.

7. The method for testing the discharge performance of a power battery according to claim 2, characterized in that: Based on the absolute difference of voltage change data in different sub-test intervals and the set constant discharge current, the internal resistance of the power battery under test during constant current discharge in the sub-test interval is calculated by Ohm's law. The steps for determining the discharge quantity within a sub-test interval include: recording the discharge start time and discharge end time of each sub-test interval; characterizing the discharge quantity of each sub-test interval by multiplying the discharge voltage value at different times by the constant discharge current and the integral value over the time interval between the discharge start time and discharge end time of each sub-test interval.

8. The method for testing the discharge performance of a power battery according to claim 7, characterized in that: The logic underlying the determination of the relaxation time constant is as follows: The instantaneous voltage change of the power battery under test after the end of the sub-test interval is obtained, and the voltage in the voltage recovery curve after the end of each sub-test interval is fitted with a first-order exponential fit to obtain the relaxation time constant. When the error between the fitted relaxation voltage and the actual relaxation voltage is less than the set voltage error threshold, the corresponding time constant is the relaxation time constant. The comprehensive evaluation coefficient is calculated based on the relaxation time constant and the rate of change of internal resistance. This comprehensive evaluation coefficient comprehensively characterizes the discharge performance during the corresponding sub-test interval. The specific logic underlying the calculation of the comprehensive evaluation coefficient is as follows: Obtain the unit internal resistance change in different sub-test intervals, calculate the absolute difference between the unit internal resistance change in different sub-test intervals and the reference value of the unit internal resistance change, and normalize the absolute difference of the internal resistance change using the reference value of the unit internal resistance change. Take the square root of the normalized value and record it as the first evaluation coefficient. The natural logarithm of the sum of the relaxation time constant and the integer 1 is used as the second evaluation coefficient for different sub-test intervals; The sum of the first evaluation coefficient and the second evaluation coefficient is used as the comprehensive evaluation coefficient of the sub-test interval.

9. The method for testing the discharge performance of a power battery according to claim 8, characterized in that: The logic for completing the discharge performance test of the power battery under test based on the comprehensive evaluation coefficient of the sub-test interval is as follows: Set the performance evaluation threshold for the corresponding sub-test interval, compare the comprehensive evaluation coefficient of the corresponding sub-test interval with the performance evaluation threshold. If the comprehensive evaluation coefficient is greater than the performance evaluation threshold, it is determined that the discharge performance in the sub-test interval does not meet the requirements. If the comprehensive evaluation coefficient is not greater than the performance evaluation threshold, it is determined that the discharge performance in the sub-test interval meets the requirements. When the discharge performance in all sub-test intervals meets the requirements, it is determined that the discharge performance of the power battery under test meets the standards.

Images