Method for evaluating internal resistance condition of each battery unit in battery pack

Abstract

The invention discloses a method for evaluating the internal resistance condition of each battery unit in a battery pack, and belongs to the technical field of batteries, the battery pack is formed by connecting a plurality of battery units in series, each battery unit is provided with a voltage sensor, and each battery unit is electrically connected with a discharge resistor through an equalization electronic switch to form an equalization circuit; resistance values of discharge resistors on all equalization circuits are equal, and the method comprises the following steps: before a battery pack leaves a factory, at different environment temperatures, testing voltage drop reference rates in different intervals in the equalization process of battery units, and forming a look-up table corresponding to different environment temperatures; and after the battery pack is in service, when the equalization circuits of one or more battery units are switched on, recording the current environment temperature, and respectively evaluating the internal resistance condition of each battery unit executing equalization. The invention solves the problems that the existing battery pack internal resistance assessment technical equipment and operation cost is high and the internal resistance of the whole battery pack and each unit cannot be conveniently and simultaneously assessed.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a method for evaluating the internal resistance of each battery cell within a battery pack. Background Technology

[0002] With the widespread application of battery technology in new energy vehicles, energy storage systems, and other fields, battery packs consisting of multiple battery cells connected in series have become the mainstream configuration. During the service life of a battery pack, the battery cells gradually age due to internal physicochemical changes, leading to increased internal resistance and performance degradation, which in turn affects the overall performance and safety of the battery pack. Therefore, various technologies have been developed to evaluate the internal resistance of battery packs and to diagnose faults when the internal resistance is high.

[0003] Existing internal resistance assessment techniques have the following drawbacks: First, assessing changes in battery internal resistance requires specialized instruments, complex data analysis methods, or specialized testing procedures. This often translates to high equipment and operating costs and can also impact the daily use of the battery pack. For example, Chinese patent CN202110236318.X discloses a method for diagnosing the end-of-life of an electric vehicle battery pack in conjunction with a charging pile. This method requires measuring the battery pack's internal resistance using a specialized pulse discharge test method and comparing it with a preset value. Second, it is not convenient to simultaneously assess the overall internal resistance of the battery pack and the internal resistance of each individual cell that makes up the battery pack. In fact, if only a few individual battery cells have high internal resistance, the overall internal resistance of the battery pack will increase only slightly. However, this will still cause a large amount of heat generation and performance degradation in that individual battery cell, thus creating a "weak link" in the battery pack and causing performance deterioration and safety hazards.

[0004] In view of this, the present invention provides a method for evaluating the internal resistance of each battery cell in a battery pack. Summary of the Invention

[0005] The purpose of this invention is to provide a method for evaluating the internal resistance of each battery cell in a battery pack, which solves the problems of high equipment and operating costs in existing battery pack internal resistance evaluation technologies and the inability to conveniently evaluate the internal resistance of the entire battery pack and each cell at the same time.

[0006] The above-mentioned technical objective of the present invention is achieved through the following technical solution.

[0007] A method for evaluating the internal resistance of each battery cell in a battery pack, wherein the battery pack is composed of multiple battery cells connected in series, each battery cell is equipped with a voltage sensor, and each battery cell is electrically connected to a discharge resistor through an equalization electronic switch to form an equalization circuit, wherein the resistance values ​​of the discharge resistors in all equalization circuits are equal, comprising the following steps: Before the battery pack leaves the factory, the voltage drop reference rate of the battery cells in different ranges during the equalization process is tested under different ambient temperatures, and a lookup table corresponding to different ambient temperatures is generated. After the battery pack is put into service, when the equalization circuit of one or more battery cells is turned on, the current ambient temperature is recorded, and the internal resistance of each battery cell performing equalization is evaluated. The evaluation of the internal resistance of each battery cell undergoing equalization includes the following steps: Step S21: Record the initial terminal voltage, the final terminal voltage, and the actual equalization time of the battery cell equalization process; Step S22: Select the corresponding lookup table based on the current ambient temperature, and calculate the reference equalization time based on the initial terminal voltage and the termination terminal voltage; Step S23: Calculate the ratio of the actual equalization time to the reference equalization time, and use it as the internal resistance index of the battery cell.

[0008] A further preferred method is as follows: In step S22, a corresponding lookup table is selected based on the current ambient temperature, and all intervals between the initial terminal voltage and the terminal voltage and the voltage drop reference rate corresponding to each interval are obtained from the lookup table. The total time required for the equalization process from the initial terminal voltage to the terminal voltage is calculated in intervals and used as the reference equalization time.

[0009] A further preferred embodiment is: the method for calculating the baseline equalization duration is as follows: Find all intervals between the initial terminal voltage and the final terminal voltage from the lookup table; If the voltage at the initial terminal and the voltage at the termination terminal are within a certain range, then the difference between the voltages at both terminals is divided by the voltage drop reference rate within that range to obtain the reference equalization time. If the initial voltage and the termination voltage are in different voltage ranges, calculate the time from the initial voltage to the lower limit of the voltage range, the time between the upper and lower limits of each intermediate range, and the time from the upper limit of the voltage range of the termination voltage range to the termination voltage. Then add all the times together to obtain the reference equalization time.

[0010] A further preferred embodiment is that the voltage drop reference rate test step includes: Step S11: Charge the battery pack to full power; Step S12: Place the battery pack at a specified ambient temperature; Step S13: Close all equalization electronic switches and discharge each battery cell to the discharge cutoff voltage through its discharge resistor. Step S14: Based on the change curve of the terminal voltage of each battery cell over time during the discharge process, divide the range of terminal voltage change into multiple adjacent voltage intervals, and calculate the rate of decrease of the terminal voltage of each battery cell in each interval. Step S15: Calculate the average of the terminal voltage drop rate of all battery cells in each voltage range to obtain the voltage drop reference rate for different voltage ranges under the ambient temperature.

[0011] A further preferred embodiment is that, in step S12, the battery pack is placed at a specified ambient temperature for a duration of not less than 1 hour.

[0012] A further preferred embodiment is that the different ambient temperatures are expressed in terms of T. min For the lower limit, T max For the upper limit, T is a temperature sequence with equally spaced steps, where T min and T max These are the minimum and maximum allowable charging ambient temperatures for the battery pack, respectively. T=p(T max -T min ), where p is a coefficient between 0.05 and 0.2.

[0013] A further preferred embodiment is that the method further includes: determining the fault based on the internal resistance index obtained from the most recent equalization process of each battery cell.

[0014] A further preferred embodiment is that the fault determination includes: If the internal resistance index of a battery cell exceeds the preset threshold m, a fault of excessive internal resistance of the battery cell is determined and the number of the battery cell is output. If the difference between the maximum and minimum values ​​of the internal resistance index of each battery cell exceeds the preset threshold n, it is determined that an internal resistance deviation fault has occurred. If the sum of the internal resistance indices of each battery cell exceeds a preset threshold r, a fault of excessive internal resistance in the battery pack is determined to have occurred.

[0015] More preferably, the preset threshold m is between 2 and 10, and the preset threshold n is between 0.5 and 5.

[0016] A further preferred embodiment is that the preset threshold r is k times the total number of battery cells in the battery pack, where k is between 1.5 and 10.

[0017] In summary, the present invention has the following beneficial effects: (1) This invention, from the perspective of equalization time, compares the actual equalization time of the battery cells after the battery pack has been put into service with the preset benchmark equalization time measured in the initial state before leaving the factory to obtain the internal resistance index as a parameter. It also fully considers the environmental factors and voltage range factors that affect the voltage drop benchmark rate to ensure its accuracy and ultimately obtain a highly reliable internal resistance index. The larger the ratio of the actual equalization time of the battery cell to the benchmark equalization time, the larger the ratio of the current internal resistance to the internal resistance in the initial state before leaving the factory, and the larger the increase in the internal resistance of the battery cell. This evaluation method is scientific and reasonable, with high reliability and accuracy. The evaluation method adopted is based on the existing hardware conditions of the battery pack with passive equalization. The main approach is to collect its voltage change data and record the total equalization time when one or more battery cells in the battery pack perform equalization. On this basis, calculations are performed and compared with preset values ​​to evaluate the internal resistance change. Therefore, it is simple and easy to implement, widely applicable, and does not affect the normal use of the battery pack itself.

[0018] (2) This invention achieves the evaluation of the internal resistance index at the battery cell level. The total internal resistance of the battery pack is obtained by summing the internal resistance indices of each battery cell, and further evaluation is carried out from three aspects: excessive internal resistance of battery cells, excessive internal resistance deviation, and excessive total internal resistance of the battery pack. After obtaining the internal resistance index of each battery cell, the internal resistance index of each battery cell is compared with the specified parameters to determine whether the battery cell has failed. When an excessive internal resistance deviation is found, it may mean that the consistency of the battery cells is poor, which may cause local overheating / underheating and performance differences between battery cells. Timely detection of such faults is beneficial for operation and maintenance personnel to pay attention to and deal with them in a timely manner. The method for evaluating the internal resistance of each battery cell in the battery pack of this invention is simple, easy to implement, scientific and reasonable, reliable and accurate, and does not affect the daily use of the battery pack. It solves the problems of high equipment and operating costs of existing battery pack internal resistance evaluation technology and the inability to conveniently evaluate the internal resistance of the entire battery pack and each cell at the same time. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the battery pack and its equalization circuit in an embodiment of the present invention; Figure 2 This is a flowchart of a method for evaluating the internal resistance of each battery cell in a battery pack according to an embodiment of the present invention.

[0020] In the diagram, 1 is the battery cell; 11 is the positive terminal; 12 is the negative terminal; 2 is the equalization electronic switch; 3 is the discharge resistor; 4 is the wire; 5 is the main positive terminal of the battery pack; and 6 is the main negative terminal of the battery pack. Detailed Implementation

[0021] The invention will now be further described with reference to the accompanying drawings.

[0022] A method for evaluating the internal resistance of individual battery cells within a battery pack, such as... Figure 1 As shown, the battery pack consists of multiple battery cells 1 connected in series, and each battery cell 1 is equipped with a voltage sensor. Each battery cell 1 is electrically connected to a discharge resistor 3 through an equalization electronic switch 2 to form an equalization circuit. The resistance values ​​of the discharge resistors 3 in all equalization circuits are equal. The equalization electronic switch 2 is normally open. When a certain equalization electronic switch 2 is closed, the equalization process of the battery cell 1 in the same closed circuit is initiated.

[0023] Preferably, the battery cell 1 is provided with a positive terminal 11 and a negative terminal 12, and multiple battery cells 1 are connected in series end to end through their positive terminal 11 and negative terminal 12. The first and last battery cells 1 are respectively connected to the main positive terminal 5 and the main negative terminal 6 of the battery pack wire 4.

[0024] Preferably, the battery pack is composed of lithium-ion batteries, and a temperature sensor is provided on the battery pack to collect ambient temperature.

[0025] like Figure 2 As shown, the method for evaluating the internal resistance of a battery pack includes the following steps: Step S1: Before the battery pack leaves the factory, test the voltage drop reference rate of battery cell 1 in different ranges during the equalization process under different ambient temperatures, and form a lookup table corresponding to different ambient temperatures.

[0026] The following steps were used to test the voltage drop reference rate in different intervals during the equalization process of battery cell 1: Step S11: Charge the battery pack to full power; Step S12: Place the battery pack at the specified ambient temperature for at least 1 hour; Step S13: Close all equalization electronic switches and discharge each battery cell 1 through its discharge resistor to the discharge cutoff voltage. Step S14: For the voltage change curve of each battery cell 1 terminal during the discharge process, divide it into multiple adjacent voltage intervals according to the range of terminal voltage change, and calculate the rate of decrease of the voltage of each battery cell 1 terminal in each interval. Step S15: Calculate the average voltage drop rate of all battery cells 1 in each voltage range to obtain the voltage drop reference rate for different voltage ranges under the ambient temperature.

[0027] Preferably, the different ambient temperatures are defined as T. min For the lower limit, T max For the upper limit, T is a temperature sequence with equally spaced steps, where T min and T maxThese are the minimum and maximum allowable charging ambient temperatures for the battery pack, respectively. T=p(T max -T min ), where p is a coefficient between 0.05 and 0.2.

[0028] Step S2: After the battery pack is put into service, when the equalization circuit of one or more battery cells 1 is turned on, record the current ambient temperature and evaluate the internal resistance of each battery cell 1 that is performing equalization.

[0029] For each battery cell 1 undergoing equalization, the internal resistance is evaluated according to the following steps: Step S21: Record the initial terminal voltage, the final terminal voltage, and the actual equalization time of battery cell 1 during the equalization process; Step S22: Select the corresponding lookup table based on the current ambient temperature, and calculate the reference equalization time based on the initial terminal voltage and the final terminal voltage; Step S23: Calculate the ratio of the actual equalization time to the reference equalization time, and use it as the internal resistance index of the battery cell 1.

[0030] Preferably, in step S22, an existing lookup table is found at the ambient temperature closest to the current ambient temperature. Then, all intervals between the initial terminal voltage and the terminal voltage in step S21 and the voltage drop reference rate corresponding to each interval are found and obtained from the lookup table. The total time required for the equalization process from the initial terminal voltage to the terminal voltage is calculated in intervals and used as the reference equalization time.

[0031] More preferably, the method for calculating the baseline equalization duration is as follows: Find all the intervals between the initial terminal voltage and the final terminal voltage from the lookup table; If the voltage from the initial terminal to the terminal is within a certain range, then the difference between the two voltages is divided by the voltage drop reference rate within that range to obtain the reference equalization time. If the initial voltage and the termination voltage are in different voltage ranges, calculate the time from the initial voltage to the lower limit of the voltage range, the time between the upper and lower limits of each intermediate range, and the time from the upper limit of the voltage range of the termination voltage range to the termination voltage. Then add all the times together to obtain the reference equalization time.

[0032] Specifically, if the initial terminal voltage to the terminal voltage are in different intervals, then the initial terminal voltage and the terminal voltage are recorded as being in the first interval and the last interval, respectively, and there are several intermediate intervals between the first interval and the last interval. The reference duration of the first interval is obtained by dividing the difference between the initial terminal voltage and the lower limit of the first interval voltage by the reference rate of voltage drop in that interval. For all intermediate intervals, the reference duration of each intermediate interval is obtained by dividing the difference between the upper and lower limit voltages of each interval by the corresponding voltage drop reference rate; the reference duration of the last interval is obtained by dividing the difference between the upper limit voltage of the last interval and the termination voltage by the voltage drop reference rate of that interval. The baseline equilibrium duration is obtained by adding the baseline duration of the first interval, the baseline duration of each intermediate interval, and the baseline duration of the last interval in sequence.

[0033] Step S3: Determine the fault based on the internal resistance index obtained from the most recent equalization process of each battery cell 1.

[0034] Specifically, fault diagnosis includes: If the internal resistance index of a battery cell 1 exceeds the preset threshold m, it is determined that the internal resistance of the battery cell 1 is too high and the number of the battery cell 1 is output. If the difference between the maximum and minimum values ​​of the internal resistance index of each battery cell 1 exceeds the preset threshold n, it is determined that an internal resistance deviation fault has occurred. If the sum of the internal resistance indices of each battery cell 1 exceeds the preset threshold r, it is determined that a fault of excessive internal resistance of the battery pack has occurred.

[0035] Preferably, the preset threshold m is between 2 and 10, and the preset threshold n is between 0.5 and 5.

[0036] Preferably, the preset threshold r is k times the total number of battery cells 1 in the battery pack, where k is between 1.5 and 10.

[0037] Specifically, the voltage acquisition, equalization execution, data storage, and analysis calculations in the evaluation process of this invention are all performed by existing battery pack management systems (BMS).

[0038] Specifically, each battery cell 1 has its own unique number to facilitate its identification and location.

[0039] In the actual process of equalizing the battery pack, only some battery cells 1 may undergo equalization. Therefore, in further evaluation, the internal resistance index obtained from the most recent equalization process of each battery cell 1 is used as the evaluation basis. For example, if battery cell 1# undergoes equalization while battery cell 2# does not, then in the further evaluation after this equalization, the internal resistance index of battery cell 1# is calculated based on the equalization record of battery cell 1# in this instance; while for battery cell 2#, the internal resistance index obtained from its most recent historical equalization process is used in this evaluation.

[0040] This invention applies to battery packs with passive equalization functionality. To improve voltage consistency among battery cells 1 during battery pack service, passive resistance discharge is commonly used for equalization. This involves each battery cell 1 forming an equalization circuit with a normally open electronic switch and a discharge resistor. When the voltage of some battery cells 1 is too high, their corresponding electronic switch is closed, activating the equalization circuit and allowing these cells 1 to discharge individually through the discharge resistor, thereby consuming their energy and reducing their voltage to the required level. For safety, stability, and cost control, the equalization current of battery cells 1 is generally small, thus requiring a relatively long time to complete the equalization process. The total resistance of the equalization circuit for battery cells 1 consists of internal resistance and an external discharge resistor. The external discharge resistor maintains a constant resistance, while the internal resistance tends to increase with battery pack aging. Furthermore, changes in battery capacity during the equalization process can be characterized by voltage changes. Therefore, the time required to equalize the same voltage drop is closely related to the internal resistance: a higher internal resistance results in a higher total resistance of the equalization circuit, a smaller equalization current, and a longer time required to achieve the same voltage and capacity reduction. Therefore, from the perspective of equalization time, this invention proposes to obtain the internal resistance index by comparing the actual equalization time of battery cell 1 after the battery pack has been put into service with the preset benchmark equalization time measured in the initial state before leaving the factory: the larger the ratio of the actual equalization time of battery cell 1 to the benchmark equalization time, the larger the ratio of the current internal resistance to the internal resistance in the initial state before leaving the factory, and the greater the increase in the internal resistance of battery cell 1.

[0041] To improve the reliability and accuracy of the assessment, this invention fully considers environmental factors and voltage range factors affecting the voltage drop reference rate, ensuring high accuracy of the reference rate across a wide range of ambient temperature and voltage variations. For the former, lookup tables corresponding to different ambient temperatures are measured and obtained beforehand. For the latter, the voltage range is divided into several sequentially adjacent voltage ranges, and the voltage drop rate of each battery cell 1 within each range is calculated. Based on this premise, battery cells 1 undergoing equalization after the battery pack enters service can have their total reference duration calculated segmentally based on their current ambient temperature and the voltage change curve during the equalization process, ultimately obtaining a highly reliable internal resistance index. Therefore, the internal resistance assessment method of this invention is scientifically sound, reasonable, and highly reliable and accurate.

[0042] This invention is based on the existing hardware of a battery pack with passive equalization. The main approach is to collect voltage change data and record the total equalization time when one or more battery cells 1 in the battery pack perform equalization. Based on this, calculations are performed and compared with preset values ​​to evaluate the internal resistance change. Therefore, it is simple, easy to implement, widely applicable, and does not affect the normal use of the battery pack itself.

[0043] This invention achieves internal resistance index evaluation at the battery cell level, and the total internal resistance of the battery pack is obtained by summing the internal resistance indices of each battery cell 1. Based on this, three criteria related to internal resistance faults are further proposed: if the internal resistance index of one battery cell 1 is large, a fault of excessive internal resistance in battery cell 1 is determined, and its number is output; if the difference between the maximum and minimum values ​​of the internal resistance indices of each battery cell 1 is too large, a fault of excessive internal resistance deviation is determined; if the sum of the internal resistance indices of each battery cell 1 is too large, a fault of excessive internal resistance of the entire battery pack is determined.

[0044] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0045] Example: A method for evaluating the internal resistance of each battery cell within a battery pack, such as... Figure 1 , 2 As shown, a certain energy storage battery pack consists of 1 parallel and 60 series lithium iron phosphate battery cells, with each battery cell forming a battery cell 1. The discharge cutoff voltage of each battery cell 1 is 2.7V. Under normal circumstances, this battery pack needs to be left to stand for more than 10 minutes to allow the voltage to stabilize before equalization is performed.

[0046] It should be noted that, Figure 1 Only three battery units 1 are shown in the diagram for illustration purposes; the remaining battery units 1 are omitted using the “…” symbol.

[0047] In this embodiment, the minimum allowable charging ambient temperature T of the battery pack is... min =0℃, maximum charging ambient temperature T max =50℃, coefficient p=0.2, uniform interval step size T=p(T max -T min =0.2×(50-0)=10℃, parameter m=3, parameter n=1, parameter k=2, parameter r=2×60=120.

[0048] Before the battery pack leaves the factory, the voltage drop reference rate during the equalization process of the battery pack is tested at ambient temperatures of 0℃, 10℃, 20℃, 30℃, 40℃ and 50℃, and a lookup table corresponding to the above six different ambient temperatures is generated. Taking the 20℃ environment as an example, the lookup table is shown in Table 1.

[0049] Table 1. Reference Rate of Voltage Drop at 20℃ Taking the interval number 3 in Table 1 as an example, the average time for all battery cells 1 to drop from 3.30V to 3.25V during the equalization process is 119 minutes. Therefore, the voltage drop reference rate corresponding to the voltage range of 3.25V to 3.30V is (3.30-3.25) / 119 = 0.42mV / min.

[0050] After the battery pack entered service, the battery management system initiated equalization, activating the equalization circuits for all 52 battery cells 1. The current ambient temperature was 23°C, and the closest reference table for voltage drop rate was found at an ambient temperature of 20°C. Based on the initial and final voltages of each of the 52 battery cells 1 during the equalization process, all intervals between the initial and final voltages and their corresponding voltage drop rates were retrieved from the lookup table. The total time required for the equalization process from the initial to the final voltage was then calculated as the reference equalization time, segmented by interval.

[0051] For example, taking one of the battery cells, cell 1, as an example, its initial terminal voltage is 3.362V, its terminal voltage is 3.298V, and its actual equalization time is 89.2 minutes. Therefore, it involves three voltage ranges: the first range is 3.35V~3.40V, the middle range is 3.30V~3.35V, and the final range is 3.25V~3.30V. The calculation yields: The baseline duration for the first interval is (3.362 - 3.35) × 1000 / 0.95 = 12.6 min; The baseline duration of the intermediate interval = (3.35 - 3.30) × 1000 / 0.87 = 57.5 min; The base duration of the final interval = (3.30 - 3.298) × 1000 / 0.42 = 4.8 min; Total baseline balancing time = 12.6 + 57.5 + 4.8 = 74.9 min; Internal resistance index = 89.2 / 74.9 = 1.2.

[0052] The remaining five battery cells were not balanced this time, and their internal resistance index, obtained from their most recent balancing process, was used in this evaluation.

[0053] Further evaluation based on the internal resistance index obtained from the most recent equalization process of each battery cell 1 revealed the following: (1) The internal resistance index of all battery cells 1 does not exceed 3, so it is determined that there is no fault of excessive internal resistance of battery cells 1. (2) The maximum and minimum values ​​of the internal resistance index of each battery cell 1 are 2.4 and 1.3 respectively. The difference between the two is 1.1 and exceeds 1. Therefore, it is determined that there is a fault of excessive internal resistance deviation. (3) The sum of the internal resistance index of each battery cell 1 is 84, which does not exceed 120, so it is determined that there is no fault of excessive internal resistance of the battery pack.

[0054] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principle of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A method for evaluating the internal resistance of each battery cell in a battery pack, wherein the battery pack is composed of multiple battery cells connected in series, each battery cell is equipped with a voltage sensor, each battery cell is electrically connected to a discharge resistor via an equalization electronic switch to form an equalization circuit, and the resistance values ​​of the discharge resistors in all equalization circuits are equal, characterized in that: Includes the following steps: Before the battery pack leaves the factory, the voltage drop reference rate of the battery cells in different ranges during the equalization process is tested under different ambient temperatures, and a lookup table corresponding to different ambient temperatures is generated. After the battery pack is put into service, when the equalization circuit of one or more battery cells is turned on, the current ambient temperature is recorded, and the internal resistance of each battery cell performing equalization is evaluated. The evaluation of the internal resistance of each battery cell undergoing equalization includes the following steps: Step S21: Record the initial terminal voltage, the final terminal voltage, and the actual equalization time of the battery cell equalization process; Step S22: Select the corresponding lookup table based on the current ambient temperature, and calculate the reference equalization time based on the initial terminal voltage and the termination terminal voltage; Step S23: Calculate the ratio of the actual equalization time to the reference equalization time, and use it as the internal resistance index of the battery cell.

2. The method for evaluating the internal resistance of each battery cell in a battery pack according to claim 1, characterized in that: In step S22, a corresponding lookup table is selected based on the current ambient temperature. All intervals between the initial terminal voltage and the terminal voltage and the voltage drop reference rate corresponding to each interval are obtained from the lookup table. The total time required for the equalization process from the initial terminal voltage to the terminal voltage is calculated in intervals and used as the reference equalization time.

3. The method for evaluating the internal resistance of each battery cell in a battery pack according to claim 2, characterized in that: The method for calculating the baseline equalization duration is as follows: Find all intervals between the initial terminal voltage and the final terminal voltage from the lookup table; If the voltage at the initial terminal and the voltage at the termination terminal are within a certain range, then the difference between the voltages at both terminals is divided by the voltage drop reference rate within that range to obtain the reference equalization time. If the initial voltage and the termination voltage are in different voltage ranges, calculate the time from the initial voltage to the lower limit of the voltage range, the time between the upper and lower limits of each intermediate range, and the time from the upper limit of the voltage range of the termination voltage range to the termination voltage. Then add all the times together to obtain the reference equalization time.

4. The method for evaluating the internal resistance of each battery cell in a battery pack according to claim 1, characterized in that: The voltage drop reference rate test procedure includes: Step S11: Charge the battery pack to full power; Step S12: Place the battery pack at a specified ambient temperature; Step S13: Close all equalization electronic switches and discharge each battery cell to the discharge cutoff voltage through its discharge resistor. Step S14: Based on the change curve of the terminal voltage of each battery cell over time during the discharge process, divide the range of terminal voltage change into multiple adjacent voltage intervals, and calculate the rate of decrease of the terminal voltage of each battery cell in each interval. Step S15: Calculate the average of the terminal voltage drop rate of all battery cells in each voltage range to obtain the voltage drop reference rate for different voltage ranges under the ambient temperature.

5. The method for evaluating the internal resistance of each battery cell in a battery pack according to claim 4, characterized in that: In step S12, the battery pack is placed at a specified ambient temperature for a duration of not less than 1 hour.

6. The method for evaluating the internal resistance of each battery cell in a battery pack according to claim 1, characterized in that: The different ambient temperatures are in T. min For the lower limit, T max For the upper limit, T is a temperature sequence with equally spaced steps, where T min and T max These are the minimum and maximum allowable charging ambient temperatures for the battery pack, respectively. T=p(T max -T min ), where p is a coefficient between 0.05 and 0.

2.

7. The method for evaluating the internal resistance of each battery cell in a battery pack according to claim 1, characterized in that: The method also includes: determining faults based on the internal resistance index obtained from the most recent equalization process of each battery cell.

8. The method for evaluating the internal resistance of each battery cell in a battery pack according to claim 7, characterized in that: The fault diagnosis includes: If the internal resistance index of a battery cell exceeds the preset threshold m, a fault of excessive internal resistance of the battery cell is determined and the number of the battery cell is output. If the difference between the maximum and minimum values ​​of the internal resistance index of each battery cell exceeds the preset threshold n, it is determined that an internal resistance deviation fault has occurred. If the sum of the internal resistance indices of each battery cell exceeds a preset threshold r, a fault of excessive internal resistance in the battery pack is determined to have occurred.

9. The method for evaluating the internal resistance of each battery cell in a battery pack according to claim 8, characterized in that: The preset threshold m is between 2 and 10, and the preset threshold n is between 0.5 and 5.

10. The method for evaluating the internal resistance of each battery cell in a battery pack according to claim 8, characterized in that: The preset threshold r is k times the total number of battery cells in the battery pack, where k is between 1.5 and 10.

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