A method and system for evaluating the stable discharge capability of lithium-ion batteries

Abstract

This invention discloses a method and system for evaluating the stable discharge capability of lithium-ion batteries, relating to the field of battery evaluation technology. The method includes the following steps: acquiring voltage parameters at sampling time points during the discharge process; plotting voltage change curves based on the voltage parameters corresponding to the sampling time points, and determining a relatively stable region based on the voltage change curves; determining discharge capability evaluation indicators, and constructing a discharge capability evaluation model based on the discharge capability evaluation indicators; and evaluating the parameters of the relatively stable region using the discharge capability evaluation model to obtain the battery's stable discharge capability evaluation result. This invention can more precisely evaluate the discharge capability of lithium-ion batteries and more accurately understand the specific performance of lithium-ion batteries during the discharge process.

Description

Technical Field

[0001] This invention relates to the field of battery evaluation technology, and in particular to a method and system for evaluating the stable discharge capability of lithium-ion batteries. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] The capacity of a lithium-ion battery is one of the most important parameters for evaluating a battery. Battery capacity is divided into rated capacity and actual capacity. The rated capacity refers to the minimum amount of electricity the battery should discharge under certain discharge conditions, as specified in the battery's design and manufacturing. The actual capacity, however, depends on the battery's usage. The actual discharge capacity of a lithium-ion battery is affected by various factors, such as temperature, humidity, charge / discharge rate, and stress. Even under relatively mild experimental conditions (temperature: 25±2℃, relative humidity: 15%~90%RH), different discharge methods result in different discharge capacities. Generally, the capacity of a lithium-ion battery is evaluated by subjecting it to constant current discharge. During constant current discharge, the battery voltage undergoes three stages: a decrease, relative stabilization, and another decrease. The period of relative stabilization is the longest. The longer the relative stabilization time, the longer the battery can operate stably.

[0004] There are many methods for testing the discharge capacity of lithium-ion batteries. Currently, the test method in the national standard GB / T 31486-2015, "Electrical Performance Requirements and Test Methods for Power Batteries for Electric Vehicles," is as follows:

[0005] a) Initial charging of individual cells;

[0006] b) At room temperature, the battery is discharged at a current of 1I1(A) until it is discharged to the discharge termination voltage specified in the company's technical specifications;

[0007] c) Measure the discharge capacity (in Ah) and calculate the discharge specific energy (in Wh / kg);

[0008] d) Repeat steps a) to c) 5 times. When the range of the results of 3 consecutive tests is less than 3% of the rated capacity, the test can be terminated in advance, and the average value of the last 3 test results is taken.

[0009] Existing patent literature contains relevant testing methods, such as the Chinese invention patent CN114089191 B, entitled "A Method for Estimating the Health Status of Composite Lithium-ion Batteries." This invention introduces an enhanced single-particle model (eSPM), extracts parameter features directly related to the state of health (SOH) of individual cells, and uses real-time charging conditions (including fast charging) to obtain model parameter features. This allows for the establishment of an electrochemical model for composite lithium-ion batteries to estimate their health status. Furthermore, it provides a cycle aging experiment for LMO-NMC-graphite composite lithium-ion batteries to verify the feasibility and effectiveness of the novel battery model and SOH estimation method.

[0010] Chinese invention patent CN115951247A, entitled "A Method, Apparatus, Terminal Equipment and Storage Medium for Capacity Assessment of Lithium-ion Batteries," discloses the technical feature of discharging at different rates, plotting different discharge curves, further obtaining a combined rate discharge curve, and determining the total discharge capacity of the lithium-ion battery based on the combined rate discharge curve.

[0011] The invention, entitled "A Method, System and Readable Storage Medium for Predicting Battery Discharge Capacity," published by Chinese Patent No. CN112098848B, obtains an incremental capacity curve representing the relationship between battery charging voltage and discharge capacity, processes it using a prediction model, and obtains the predicted result of the battery's discharge capacity.

[0012] The test methods in the aforementioned national standards and patents measure the actual capacity of a battery under test conditions through specific charge-discharge methods, reflecting the overall discharge capacity of the battery. However, in actual battery use, the amount of electricity released during the initial and final stages of discharge, when the voltage drops rapidly, is very small, accounting for approximately 2% to 3% of the total discharge capacity. The amount of electricity released during these two periods of rapid voltage drop has little impact on the overall discharge capacity. Therefore, focusing on the battery's discharge capacity during relatively stable periods is necessary for evaluating the overall discharge capacity of the battery. However, the aforementioned methods only focus on the overall discharge capacity of the battery, neglecting the details of the discharge process. This overly simplistic assessment of the battery's discharge capability leads to inaccurate results, which is also a problem encountered in actual battery charge-discharge testing. Summary of the Invention

[0013] To address the shortcomings of existing technologies, the purpose of this invention is to provide a method and system for evaluating the stable discharge capability of lithium-ion batteries. By establishing a comprehensive discharge capability evaluation model, the discharge capability of lithium-ion batteries can be evaluated more precisely, and the specific performance of lithium-ion batteries during the discharge process can be understood more accurately.

[0014] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0015] The first aspect of this invention provides a method for evaluating the stable discharge capability of a lithium-ion battery, comprising the following steps:

[0016] Obtain the voltage parameters at sampling time points during the discharge process;

[0017] Based on the voltage parameters corresponding to the sampling time points, a voltage change curve is plotted. The relatively stable region is then determined based on the voltage change curve. Specifically, the slope of the linear regression line is calculated for all two adjacent time points and their corresponding voltages, yielding the slope of each data point on the voltage change curve, which is the first derivative of the voltage-time change function. The slope of the linear regression line is then calculated for all two adjacent time points and their corresponding voltage change slopes, yielding the second derivative of the voltage-time change function. The starting time point of the relatively stable region and the relative stable region itself are then determined based on the second derivative of the voltage-time change function.

[0018] Determine the discharge capability assessment indicators and construct a discharge capability assessment model based on the discharge capability assessment indicators;

[0019] The parameters of the relatively stable region are evaluated using a discharge capacity assessment model to obtain the battery's stable discharge capacity assessment results.

[0020] Furthermore, the sampling interval at each sampling time point is less than or equal to 5 seconds.

[0021] Furthermore, the specific steps for determining the start time point of the relatively stable region based on the second derivative of the voltage-time change function are as follows:

[0022] The second derivative of the voltage-time variation function from the start of discharge to the point when it decreases to the first threshold is the first rapid decline region, and the point when the second derivative of the voltage-time variation function reaches the first threshold is the start of the relatively stable region.

[0023] The second derivative of the voltage-time variation function from the second threshold time point to the discharge end time point constitutes the second rapid decline region, and the second derivative of the voltage-time variation function at the second threshold time point marks the end of the relatively stable region.

[0024] Furthermore, the discharge capability evaluation indicators include the capacity in the relatively stable region, the voltage change rate in the relatively stable region, and the voltage difference in the relatively stable region.

[0025] Furthermore, the discharge capability assessment model outputs discharge capability in three levels: good discharge capability, average discharge capability, and poor discharge capability.

[0026] A second aspect of the present invention provides a lithium-ion battery stable discharge capability evaluation system, comprising:

[0027] The data acquisition module is configured to acquire voltage parameters at sampling time points during the discharge process;

[0028] The parameter extraction module is configured to plot voltage change curves based on the voltage parameters corresponding to the sampling time points, and determine the relatively stable region based on the voltage change curves. Specifically, it calculates the slope of the linear regression line for all two adjacent time points and their corresponding voltages, obtaining the slope of each data point on the voltage change curve, which is the first derivative of the voltage-time change function. It also calculates the slope of the linear regression line for all two adjacent time points and their corresponding voltage change slopes, which is the second derivative of the voltage-time change function. Finally, it determines the start time point of the relatively stable region and the relative stable region based on the second derivative of the voltage-time change function.

[0029] The model building module is configured to determine the discharge capability assessment index and build a discharge capability assessment model based on the discharge capability assessment index.

[0030] The discharge capability assessment module is configured to evaluate the parameters of the relatively stable region using a discharge capability assessment model to obtain the battery's stable discharge capability assessment results.

[0031] Furthermore, during the sampling process, the sampling interval of the data acquisition module is less than or equal to 5 seconds.

[0032] Furthermore, the parameter extraction module also includes a time point determination module, which is configured as follows:

[0033] The second derivative of the voltage-time variation function from the start of discharge to the point when it decreases to the first threshold is the first rapid decline region, and the point when the second derivative of the voltage-time variation function reaches the first threshold is the start of the relatively stable region.

[0034] The second derivative of the voltage-time variation function from the second threshold time point to the discharge end time point constitutes the second rapid decline region, and the second derivative of the voltage-time variation function at the second threshold time point marks the end of the relatively stable region.

[0035] Furthermore, in the model building module, the discharge capability evaluation indicators include the capacity of the relatively stable region, the voltage change rate of the relatively stable region, and the voltage difference of the relatively stable region.

[0036] Furthermore, the discharge capability assessment model outputs discharge capability in three levels: good discharge capability, average discharge capability, and poor discharge capability.

[0037] The above one or more technical solutions have the following beneficial effects:

[0038] This invention provides a method and system for evaluating the stable discharge capability of lithium-ion batteries. By processing some parameters during the discharge process, selecting data that meets the requirements as the relatively stable region, and comprehensively evaluating the charge in the relatively stable region from multiple dimensions and angles, a thorough and specific evaluation of the battery's stable discharge capability is achieved.

[0039] Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0040] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0041] Figure 1 This is a flowchart of the lithium-ion battery stable discharge capability evaluation method in Embodiment 1 of the present invention;

[0042] Figure 2 This is a schematic diagram of the voltage change curve in Embodiment 1 of the present invention. Detailed Implementation

[0043] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0044] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0045] Example 1:

[0046] Embodiment 1 of the present invention provides a method for evaluating the stable discharge capability of lithium-ion batteries. By plotting the voltage change over time during the 1C discharge process of a fully charged battery, the following can be obtained: Figure 2The voltage-time curve shown is roughly divided into three parts, representing three stages of battery voltage change over time: rapid decrease, relative stability, and rapid decrease again. Compared to the two rapid decrease regions, the relatively stable region is characterized by its long duration, large discharge capacity, and small voltage change, playing a decisive role in the battery's discharge capacity throughout the entire discharge time. This embodiment analyzes the relatively stable region, improving the accuracy of discharge capacity assessment. Figure 1 As shown, the specific steps include:

[0047] Step 1: Obtain the voltage parameters at the sampling time points during the discharge process.

[0048] Step 2: Plot the voltage change curve based on the voltage parameters corresponding to the sampling time points, and determine the relatively stable region based on the voltage change curve.

[0049] Step 3: Determine the discharge capability assessment index and construct the discharge capability assessment model based on the discharge capability assessment index.

[0050] Step 4: Use the discharge capacity assessment model to evaluate the parameters of the relatively stable region and obtain the battery stable discharge capacity assessment results.

[0051] In step 1, a fully charged battery is selected for evaluation. The fully charged battery is discharged using an IC to the discharge cutoff voltage, and the process data is recorded. The smaller the sampling interval during the entire discharge process, the better; the sampling interval at each sampling time point should be less than or equal to 5 seconds.

[0052] In step 2, the specific steps for determining the relatively stable region based on the voltage change curve are as follows:

[0053] Step 2.1: Calculate the slope of the linear regression line for all two adjacent time points and the corresponding voltages to obtain the slope of each data point on the voltage change curve, which is the first derivative of the voltage-time change function.

[0054] Step 2.2: Calculate the slope of the linear regression line for all two adjacent time points and the corresponding voltage change slopes, which is the second derivative of the voltage-time change function.

[0055] Step 2.3: Determine the start time point of the relatively stable region and the relative stable region based on the second derivative of the voltage-time change function, and thus determine the relatively stable region.

[0056] Wherein, the second derivative of the voltage relative to time change function from the discharge start time point to the time point when it decreases to the first threshold is the first rapid decline region, and the time point when the second derivative of the voltage relative to time change function is the first threshold is the start time point of the relatively stable region; in this embodiment, the first threshold is set to 0.0001.

[0057] The second derivative of the voltage-time variation function, from the time point of the second threshold to the time point of discharge termination, constitutes the second rapid decline region. The time point where the second derivative of the voltage-time variation function equals the second threshold is the end time point of the relatively stable region. In this embodiment, the second threshold is set to -0.0001.

[0058] In this embodiment, the first threshold and the second threshold are inflection points on the curve, indicating the turning point where the voltage change from fast to slow over time, with units of V*s. -2 .

[0059] The relevant parameters and evaluation of the relatively stable region: The capacity of the relatively stable region accounts for approximately 97% to 98% of the total battery discharge capacity. Therefore, the larger the capacity of the relatively stable region, the greater the capacity the battery can discharge. Within the relatively stable region, the smaller the voltage change rate, the more stable the battery discharge. During battery discharge, the start and end points of the relatively stable region on the macroscopic curve represent the highest and lowest voltage points, respectively. The smaller the voltage difference between these two points, the more stable the battery discharge.

[0060] To more intuitively demonstrate the impact of the above parameters on discharge capacity, this embodiment conducts the following experiment: Three cells A, B, and C with a rated capacity of 73Ah are taken, fully charged first, and then subjected to a 1C discharge test. The voltage and cumulative discharge capacity parameters during the discharge process are recorded, with a sampling interval of 5 seconds. The voltage data is then processed simply in Excel software.

[0061] The SLOPE function is used to calculate the slope of the linear regression line for all two adjacent time points and their corresponding voltages, thus obtaining the slope of each data point on the curve, which is the first derivative of the voltage-time change function.

[0062] The slope of the linear regression line is calculated for all two adjacent time points and the corresponding voltage change slopes, which is the second derivative of the voltage-time change function.

[0063] Find the sampling point where the second derivative ranges from greater than 0.0001 to less than or equal to 0.0001; this is the starting point of the relatively stable discharge region.

[0064] Find the sampling point where the second derivative is greater than or equal to -0.0001 to less than -0.0001; this is the end point of the relatively stable discharge region.

[0065] Record the voltage V1 and cumulative discharge capacity C1 at the beginning of the relatively stable discharge region, and the voltage V2 and cumulative discharge capacity C2 at the end of the relatively stable discharge region. The discharge capacity Cflat of the relatively stable region can be obtained from C2 - C1. The average slope of each data point on the curve within the relatively stable region is taken as the voltage change rate of the relatively stable region. The voltage difference ΔV of the relatively stable region is obtained from V1 - V2, thus yielding Table 1:

[0066] Table 1. Main voltage and cumulative capacity parameters of cells A, B, and C

[0067]

[0068] Evaluation of the relative stability region of the three cells:

[0069] (1) Capacity of the relatively stable region

[0070] In terms of capacity, cell C has the highest stable discharge capability, followed by cell A, and cell B has the worst.

[0071] (2) Voltage change rate in the relatively stable region

[0072] From the perspective of voltage change rate, cell A has the highest stable discharge capability, followed by cell B, and cell C has the worst.

[0073] (3) Voltage difference in the relatively stable region

[0074] From the perspective of voltage difference, cell A has the highest stable discharge capability, followed by cell B, and cell C has the worst.

[0075] Based on a comprehensive evaluation of capacity, voltage change rate, and voltage difference, cell A has the best stable discharge capability.

[0076] Therefore, it can be seen that the discharge capacity of a battery is affected by a variety of factors. The evaluation criteria of existing methods are too singular and cannot accurately reflect its true discharge capacity. Based on the above problems, this invention constructs a discharge capacity evaluation model to comprehensively evaluate the battery discharge capacity, so as to overcome the problem of insufficient accuracy in discharge capacity testing in the existing technology.

[0077] In step 3, the discharge capacity evaluation index is determined based on the above experimental results and analysis. The discharge capacity evaluation index includes the capacity of the relatively stable region, the voltage change rate of the relatively stable region, and the voltage difference of the relatively stable region.

[0078] The capacity of the relatively stable region accounts for about 97% to 98% of the total discharge capacity of the battery. Compared with the total discharge capacity, it not only reflects the overall discharge capacity of the battery, but also its ability to discharge stably.

[0079] The voltage change rate in the relatively stable region mainly refers to the average voltage change rate in the relatively stable region. In an ideal discharge curve, the voltage change rate in the relatively stable region is close to 0. The voltage change rate can clearly reflect the stability of the battery during the discharge process. The smaller the absolute value of the voltage change rate, the more stable the battery is.

[0080] The voltage difference in the relatively stable region mainly refers to the difference between the start voltage and the end voltage of the relatively stable region. The larger the voltage difference, the more stable the battery discharge, which can intuitively reflect the battery's ability to discharge stably for a long time.

[0081] Based on the above indicators, this embodiment constructs a discharge capacity evaluation model, which quantifies the stable discharge capacity of lithium-ion batteries by evaluating the parameters of the relatively stable region of the battery under test.

[0082] After extensive experimental verification, the following three evaluation criteria are generally used: (1) If the ratio of the discharge capacity of the relatively stable region to the total discharge capacity of the lithium-ion battery is not less than 97%, its stable discharge capability is considered to be good; if the ratio is between 95% and 97%, its stable discharge capability is considered to be average; and if the ratio is less than 95%, its stable discharge capability is considered to be poor. (2) If the average voltage change rate of the relatively stable region of the lithium-ion battery is greater than -0.0003V / s, its stable discharge capability is considered to be good; and if it is less than -0.0003V / s, its stable discharge capability is considered to be poor. (3) If the voltage difference of the relatively stable region of the lithium-ion battery is less than 0.75V, its stable discharge capability is considered to be good; and if it is greater than 0.75V, its stable discharge capability is considered to be poor.

[0083] Therefore, based on the aforementioned standards, the discharge capacity assessment model outputs three levels: good discharge capacity, average discharge capacity, and poor discharge capacity. Specifically, it identifies indicators based on the characteristics of the input data, matches the corresponding evaluation standards to the indicators, outputs the corresponding discharge capacity level, and further generates a table, thereby achieving a multi-faceted assessment of the stability of lithium-ion batteries.

[0084] Example 2:

[0085] Embodiment 2 of the present invention provides a lithium-ion battery stable discharge capability evaluation system, comprising:

[0086] The data acquisition module is configured to acquire voltage parameters at sampling time points during the discharge process;

[0087] The parameter extraction module is configured to plot voltage change curves based on the voltage parameters corresponding to the sampling time points, and determine the relatively stable region based on the voltage change curves. Specifically, it calculates the slope of the linear regression line for all two adjacent time points and their corresponding voltages, obtaining the slope of each data point on the voltage change curve, which is the first derivative of the voltage-time change function. It also calculates the slope of the linear regression line for all two adjacent time points and their corresponding voltage change slopes, which is the second derivative of the voltage-time change function. Finally, it determines the start time point of the relatively stable region and the relative stable region based on the second derivative of the voltage-time change function.

[0088] The model building module is configured to determine the discharge capability assessment index and build a discharge capability assessment model based on the discharge capability assessment index.

[0089] The discharge capability assessment module is configured to evaluate the parameters of the relatively stable region using a discharge capability assessment model to obtain the battery's stable discharge capability assessment results.

[0090] During the sampling process, the sampling interval of the data acquisition module is less than or equal to 5 seconds.

[0091] The parameter extraction module also includes a time point determination module, which is configured as follows:

[0092] The second derivative of the voltage-time variation function from the start of discharge to the point when it decreases to the first threshold is the first rapid decline region, and the point when the second derivative of the voltage-time variation function reaches the first threshold is the start of the relatively stable region.

[0093] The second derivative of the voltage-time variation function from the second threshold time point to the discharge end time point constitutes the second rapid decline region, and the second derivative of the voltage-time variation function at the second threshold time point marks the end of the relatively stable region.

[0094] In the model building module, the discharge capability evaluation indicators include the capacity of the relatively stable region, the voltage change rate of the relatively stable region, and the voltage difference of the relatively stable region. Based on the evaluation criteria of these indicators, the discharge capability evaluation model outputs three levels: good discharge capability, average discharge capability, and poor discharge capability.

[0095] The steps and methods involved in the above embodiment two correspond to those in embodiment one. For specific implementation details, please refer to the relevant description section of embodiment one.

[0096] Those skilled in the art will understand that the modules or steps of the present invention described above can be implemented using general-purpose computer devices. Optionally, they can be implemented using computer-executable program code, thereby allowing them to be stored in a storage device for execution by a computer device, or they can be fabricated as separate integrated circuit modules, or multiple modules or steps can be fabricated as a single integrated circuit module. The present invention is not limited to any particular combination of hardware and software.

[0097] While the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, this is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A method for evaluating the stable discharge capability of a lithium-ion battery, characterized in that, Includes the following steps: Obtain the voltage parameters at sampling time points during the discharge process; Based on the voltage parameters corresponding to the sampling time points, a voltage change curve is plotted. The relatively stable region is then determined based on the voltage change curve. Specifically, the slope of the linear regression line is calculated for all two adjacent time points and their corresponding voltages, yielding the slope of each data point on the voltage change curve, which is the first derivative of the voltage-time change function. The slope of the linear regression line is then calculated for all two adjacent time points and their corresponding voltage change slopes, yielding the second derivative of the voltage-time change function. The starting time point of the relatively stable region and the relative stable region itself are then determined based on the second derivative of the voltage-time change function. The specific steps for determining the start time point of the relatively stable region based on the second derivative of the voltage-time change function are as follows: The second derivative of the voltage-time variation function from the start of discharge to the point when it decreases to the first threshold is the first rapid decline region, and the point when the second derivative of the voltage-time variation function reaches the first threshold is the start of the relatively stable region. The second derivative of the voltage-time variation function from the second threshold time point to the discharge end time point is the second rapid decline region, and the second derivative of the voltage-time variation function at the second threshold time point is the end time point of the relatively stable region. Determine the discharge capability assessment indicators and construct a discharge capability assessment model based on the discharge capability assessment indicators; The parameters of the relatively stable region are evaluated using a discharge capacity assessment model to obtain the battery's stable discharge capacity assessment results.

2. The method for evaluating the stable discharge capability of a lithium-ion battery as described in claim 1, characterized in that, The sampling interval at each sampling time point is less than or equal to 5 seconds.

3. The method for evaluating the stable discharge capability of a lithium-ion battery as described in claim 1, characterized in that, Discharge capacity evaluation indicators include the capacity in the relatively stable region, the voltage change rate in the relatively stable region, and the voltage difference in the relatively stable region.

4. The method for evaluating the stable discharge capability of a lithium-ion battery as described in claim 1, characterized in that, The discharge capability assessment model outputs discharge capability in three levels: good discharge capability, average discharge capability, and poor discharge capability.

5. A lithium-ion battery stable discharge capability evaluation system, characterized in that, include: The data acquisition module is configured to acquire voltage parameters at sampling time points during the discharge process; The parameter extraction module is configured to plot voltage change curves based on the voltage parameters corresponding to the sampling time points, and determine the relatively stable region based on the voltage change curves. Specifically, it calculates the slope of the linear regression line for all two adjacent time points and their corresponding voltages, obtaining the slope of each data point on the voltage change curve, which is the first derivative of the voltage-time change function. It also calculates the slope of the linear regression line for all two adjacent time points and their corresponding voltage change slopes, which is the second derivative of the voltage-time change function. Finally, it determines the start time point of the relatively stable region and the relative stable region based on the second derivative of the voltage-time change function. The specific steps for determining the start time point of the relatively stable region based on the second derivative of the voltage-time change function are as follows: The second derivative of the voltage-time variation function from the start of discharge to the point when it decreases to the first threshold is the first rapid decline region, and the point when the second derivative of the voltage-time variation function reaches the first threshold is the start of the relatively stable region. The second derivative of the voltage-time variation function from the second threshold time point to the discharge end time point is the second rapid decline region, and the second derivative of the voltage-time variation function at the second threshold time point is the end time point of the relatively stable region. The model building module is configured to determine the discharge capability assessment index and build a discharge capability assessment model based on the discharge capability assessment index. The discharge capability assessment module is configured to evaluate the parameters of the relatively stable region using a discharge capability assessment model to obtain the battery's stable discharge capability assessment results.

6. The lithium-ion battery stable discharge capability evaluation system as described in claim 5, characterized in that, During the sampling process, the sampling interval of the data acquisition module is less than or equal to 5 seconds.

7. The lithium-ion battery stable discharge capability evaluation system as described in claim 5, characterized in that, The parameter extraction module also includes a time point determination module, which is configured as follows: The second derivative of the voltage-time variation function from the start of discharge to the point when it decreases to the first threshold is the first rapid decline region, and the point when the second derivative of the voltage-time variation function reaches the first threshold is the start of the relatively stable region. The second derivative of the voltage-time variation function from the second threshold time point to the discharge end time point constitutes the second rapid decline region, and the second derivative of the voltage-time variation function at the second threshold time point marks the end of the relatively stable region.

8. The lithium-ion battery stable discharge capability evaluation system as described in claim 5, characterized in that, In the model building module, the discharge capability evaluation indicators include the capacity of the relatively stable region, the voltage change rate of the relatively stable region, and the voltage difference of the relatively stable region.

9. The lithium-ion battery stable discharge capability evaluation system as described in claim 5, characterized in that, The discharge capability assessment model outputs discharge capability in three levels: good discharge capability, average discharge capability, and poor discharge capability.

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