Battery equalization method based on EIS characteristics
By using impedance slope screening and equalization operations based on EIS characteristics, the problem of equalization difficulties in lithium iron phosphate batteries is solved, improving the accuracy and consistency of battery packs. This technology is suitable for existing equipment and large-scale battery modules.
Patent Information
- Application Number
- CN202511343104.7
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-09-19
- Publication Date
- 2025-11-18
AI Technical Summary
Existing battery balancing methods based on open-circuit voltage are not effective for lithium iron phosphate batteries, resulting in large SOC estimation errors and affecting balancing accuracy and battery pack consistency.
Electrochemical impedance spectroscopy (EIS) is used to calculate the impedance slope of a single cell by applying a low-frequency constant current excitation, screen the cells to be equalized, and perform charge-discharge equalization operation based on the impedance slope change curve until an equalization state is reached.
It significantly improves the balancing accuracy and consistency of lithium iron phosphate batteries, is suitable for existing equipment, requires no hardware modification, reduces upgrade costs, and is suitable for balancing maintenance of large-scale battery modules.
Smart Images
Figure CN120978951A_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The present application relates to the technical field of lithium ion battery manufacturing and detection, and more particularly, to a method for battery equalization based on EIS characteristics. BACKGROUND
[0002] With the emphasis on environmental protection worldwide, new energy vehicles gradually replace traditional fuel vehicles as the mainstream. Lithium ion batteries are widely used in the power systems of new energy vehicles due to their high energy density, long service life and environmental protection. In practical applications, lithium ion batteries are usually in the form of modules or battery packs, which are composed of multiple battery monomers in series. However, due to the discreteness of the manufacturing process and the complexity of the use environment, there are differences in parameters such as capacity and internal resistance among battery monomers, which can lead to performance degradation, shortened service life and even safety accidents of battery packs.
[0003] The common battery equalization method at present is mainly based on open circuit voltage (OCV), which adjusts the voltage of each monomer to achieve consistency of state of charge (SOC). This method is effective for ternary material batteries, but has obvious shortcomings for lithium iron phosphate (LFP) batteries. LFP batteries have a flat open circuit voltage platform and a significant hysteresis effect, which leads to an unobvious change in OCV with SOC in the platform area. From the attached Figure 2 As can be seen, even if the voltage sampling accuracy reaches 1mV, the SOC estimation error based on OCV can still reach 5%-10%, which seriously affects the equalization accuracy and consistency of the battery pack.
[0004] To overcome the above problems, electrochemical impedance spectroscopy (EIS) technology is introduced into battery state estimation. EIS can reflect the internal dynamics of the battery by applying a small amplitude alternating current excitation and measuring the voltage response. In particular, in the low frequency region below 0.1Hz, the EIS impedance characteristics are closely related to the diffusion behavior of lithium ions in the electrode material, which can indirectly reflect the SOC state. Therefore, using EIS characteristics for battery equalization, especially for battery systems with obvious voltage platform such as LFP, has important research value and application prospect. SUMMARY
[0005] To overcome the above-mentioned defects of the prior art, embodiments of the present application provide a method for battery equalization based on EIS characteristics to solve the problems raised in the background art.
[0006] To achieve the above object, the present application provides the following technical scheme: a method for battery equalization based on EIS characteristics, comprising the following steps: step S1: applying low-frequency constant current excitation to a fully rested module to be equalized; step S2: calculating the impedance slope k of each single battery and screening out the single battery to be equalized according to the deviation from the median; step S3: obtaining the impedance slope-time variation curve of the single battery not needing equalization within 1 hour after being rested after charging and discharging as a standard reference curve; step S4: performing equalization operation on the screened single battery to be equalized and applying low-frequency constant current excitation to obtain the impedance slope k after the operation; and step S5: judging whether the current single battery reaches the equalization state according to the obtained impedance slope k, and if not, continuing to perform the equalization operation.
[0007] In a preferred embodiment, in step S1, the current size of the low-frequency constant current excitation is 0.01C, and the excitation frequency points are 11 points satisfying the logarithmic distribution in the range of 0.1Hz-0.01Hz.
[0008] In a preferred embodiment, in step S1, the impedance slope k is obtained by linear fitting of the Nyquist diagram of the frequency band of 0.1Hz-0.01Hz, the fitting method is the least square method, and the fitting equation is y=kx+b.
[0009] In a preferred embodiment, the impedance points in the Nyquist diagram are smoothed before fitting, and the smoothing is performed by the three-point average method.
[0010] In a preferred embodiment, in step S2, the standard for screening the single battery to be equalized is that if the impedance slope of a single battery deviates from the median of the module by more than ±5%, it is determined that the single battery needs to be equalized.
[0011] In a preferred embodiment, in step S3, the standard reference curve is obtained by the following method:
[0012] Constant current charging or discharging is performed on the module, the current is not more than 0.01C, and the duration is 1 hour;
[0013] Low-frequency impedance test is performed every 10 minutes after charging and discharging, a total of 6 times;
[0014] The median of the impedance slope of the single battery not needing equalization at each time is taken to form a standard k-t curve.
[0015] In a preferred embodiment, the maximum value of the standard charging k-t curve is defined as the equalization peak top calibration value k ↑ , and the minimum value of the standard discharging k-t curve is defined as the equalization valley bottom calibration value k ↓ , which is used as the subsequent equalization judgment reference.
[0016] In a preferred embodiment, in step S4, the balancing operation includes charging balancing or discharging balancing, the operating current is the same as in step S3, and the duration of each operation is not less than 1 hour.
[0017] In a preferred embodiment, the condition for determining that the equalization is complete in step S5 is:
[0018] If discharge equalization is performed, the minimum value k of the discharge kt curve of the current cell will be... * It should not be greater than k ↓ ;
[0019] If charging equalization is performed, the maximum value k of the charging kt curve of the current cell will be... * It should be no less than k ↑ .
[0020] The technical effects and advantages of this invention are as follows:
[0021] 1. By converting SOC into the impedance slope characteristic of EIS, the equalization difficulties caused by the voltage plateau and hysteresis effect of LFP batteries are effectively avoided, and the equalization accuracy and consistency are significantly improved.
[0022] 2. This invention can be used in conjunction with the traditional OCV equalization method, performing coarse equalization first and then fine equalization, thereby improving the overall equalization efficiency without modifying the existing BMS hardware structure and reducing upgrade costs.
[0023] 3. It only requires the collection of low-frequency impedance data below 0.1Hz, which has low requirements for the sampling frequency of the equipment and is suitable for most existing systems and maintenance equipment that support active balancing.
[0024] 4. This method is simple to operate and makes clear judgments. It is suitable for the balanced maintenance of large-scale battery modules and has good potential for industrial application. Attached Figure Description
[0025] Figure 1 This is a flowchart of the method of the present invention;
[0026] Figure 2 Typical SOC-OCV curves and SOC-dOCV / dSOC curves for lithium iron phosphate batteries are shown.
[0027] Figure 3 This is a schematic diagram of the experimental module for an example of the present invention. Detailed Implementation
[0028] With reference to the drawings of the embodiments of the present application, the technical solutions in the embodiments of the present application will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments of the present application, all the other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.
[0029] With reference to Figures 1-3 A method and device for battery equalization based on EIS characteristics, comprising the following steps:
[0030] Step S1: applying a low-frequency constant current excitation to a fully rested module to be equalized;
[0031] In step S1, the excitation current size is determined according to the battery capacity size, and is usually set to 0.01C, where C is the charge-discharge rate. The impedance excitation frequency points are 11 points in logarithmic distribution of 0.1Hz-0.01Hz, and the above working condition is referred to as a first low-frequency impedance test.
[0032] In step S1, the Nyquist diagram of 0.1Hz-0.01Hz frequency can be approximated as a straight line, so the least square method is used for linear fitting of the impedance points to obtain a straight line in the form of y=kx+b, where the slope k is defined as the impedance slope of the battery.
[0033] In step S1, in order to reduce external interference, the Nyquist diagram needs to be smoothed. The three-point average method can be used to smooth the impedance points on the complex plane to improve the accuracy of linear fitting.
[0034] Step S2: calculating the impedance slope k and selecting the single cells to be equalized;
[0035] In the above step S2, it is assumed that the impedance slopes of all single cells of the module are k1, k2, …, ki, …, kn, and the median of k1, k2, …, ki, …, kn is k. n mid For each single cell i, if k then the i-th single cell SOC is low and needs to be charged and equalized; if k then the i-th single cell SOC is high and needs to be discharged and equalized; if k then it is considered that the i-th single cell does not need to be equalized. The battery set that needs to be equalized is denoted as C1, and the battery set that does not need to be equalized is denoted as C0.
[0036] Step S3: obtaining the impedance slope-time change curve of the single cell that does not need to be equalized after being rested for 1 hour after being charged and discharged;
[0037] In step S3, the module to be balanced is charged with constant current, and then low-frequency impedance test is performed on the module every 10 minutes, and the impedance slope-time variation curve (hereinafter referred to as the charging k-t curve) of 1 hour after charging is obtained by repeating 6 times, and then the module is discharged with constant current, and then low-frequency impedance test is performed on the module every 10 minutes, and the impedance slope-time variation curve (hereinafter referred to as the discharging k-t curve) of 1 hour after discharging is obtained by repeating 6 times.
[0038] In step S3, the current used in constant current charging or constant current discharging is not more than 0.01C, and the charging and discharging time is controlled to be 1 hour; the impedance slope at time t is determined by the median of the impedance slope of the battery in set C0 at time t. The median of the impedance slope of the battery in set C0 at each time is taken as the standard charging k-t curve or the standard discharging k-t curve of the entire module.
[0039] In step S3, the maximum value of the standard charging k-t curve is defined as k ↑ , which is called the equalization peak top calibration value of the module; and the minimum value of the standard discharging k-t curve is defined as k ↓ , which is called the equalization valley bottom calibration value of the module. The equalization strategy needs to be executed based on the peak top calibration value or the valley bottom calibration value in the future.
[0040] In step S4, the equalization operation is performed on the single cell to be balanced in S2, and the low-frequency constant current excitation is applied to obtain the impedance slope k;
[0041] In step S4, for each battery cell in set C1, if discharging is needed, discharging is performed with the same current as in step S3, which is called discharging equalization operation; if charging is needed, charging is performed with the same current as in step S3, which is called charging equalization operation, and the charging and discharging time is not less than 1 hour each time.
[0042] In the above step S4, after the single cell i completes the discharging equalization or charging equalization operation, low-frequency impedance test is performed every 10 minutes, and the discharging k-t curve or the charging k-t curve of the single cell i is obtained by repeating 6 times.
[0043] In step S5, the operation to be performed on the single cell in the future is determined according to the impedance slope k.
[0044] In the above step S5, if the single cell i performs discharging equalization operation, and the minimum value of the obtained discharging k-t curve is denoted as k * , if k * >k ↓ , it is considered that the SOC of the single cell i is still high, and the discharging equalization operation in step S4 needs to be continued, otherwise it is considered that the single cell i has been balanced.
[0045] In the above step S5, if the monomer i performs the charge equalization operation, the maximum value of the obtained charge k-t curve is assumed to be k * If k * <k ↑ , it is considered that the SOC of the monomer i is still low, and the charge equalization operation in step S4 needs to be continued, otherwise it is considered that the monomer i has been equalized.
[0046] Embodiment:
[0047] By using the method proposed in the above specific embodiment, a 280 Ah lithium iron phosphate energy storage battery module of 1P12S is selected as the experimental object, and the battery numbers are 1 to 12, which are divided into two groups. Among them, the No. 1 battery is the experimental group, and the No. 2 to No. 12 is the control group.
[0048] The specific division schematic diagram is shown in Figure 3 .
[0049] First, the SOC difference is constructed, and the experimental group is 10% more than the control group. Then, according to the method in the specific embodiment S1, a 5A constant current excitation is applied to the module, 11 points in the frequency range of 0.1Hz-0.01Hz satisfying the logarithmic distribution are collected, and the impedance spectrum of each monomer is obtained.
[0050] Then, the impedance spectrum is smoothed, and the linear regression model is used to fit the 11 points on the complex plane to obtain the slopes k1, k2, …, k 12 , defined as the impedance slope.
[0051] Next, the relative difference between the impedance slope k1 of the experimental group and the median k mid of the impedance slopes of the 12 monomers is calculated, and it is judged whether the equalization condition is met. Since The experimental group meets the discharge equalization trigger condition, and according to the method in step S3, the discharge k-t curve of the control group is collected, and the minimum value k ↓ of this curve is obtained. The experimental group is discharged at a constant current of 1A for 1% SOC capacity.
[0052] After discharging, the discharge k-t curve of the experimental group is collected and the minimum value k If , the experimental group has reached the equalization condition, and the equalization is ended, otherwise the experimental group is discharged for 1% SOC capacity and the discharge k-t curve of the experimental group is repeatedly collected until The equalization is ended.
[0053] The table below shows the change rule of k ↓ of the experimental group and the control group during the entire equalization process.
[0054]
[0055]
[0056] From the last set of results, when the SOC difference between the experimental group and the control group is 3%, the condition of is reached.
[0057] Therefore, it can be considered that the experimental group has been balanced at the current state, and the SOC difference with the control group is about 3%. From the SOC-dOCV / dSOC curve of Figure 2 , it can be seen that the dOCV / dSOC of the platform region of the lithium iron phosphate battery is generally below 0.2 mV / %, when the voltage sampling accuracy sent by the BMS is 1 mV, the SOC difference will exceed 5% using the traditional OCV-based balancing method, so the SOC difference based on the method of the present application is better than the OCV-based balancing method.
[0058] Based on the above, the advantages of the present application are that the present application gets rid of the traditional balancing method based on the size of open circuit voltage, and converts the SOC of the battery into the impedance slope, which is an EIS feature with obvious rules. For battery systems such as lithium iron phosphate that have voltage platform and hysteresis effect, this method makes up for the disadvantages of the open circuit voltage method, such as difficulty in balancing and poor consistency.
[0059] The present application can be compatible with traditional balancing methods and equipment, specifically, the module is first coarsely balanced using the open circuit voltage method, and then finely balanced using the method of the present application, and the combination of the two improves the balancing efficiency and consistency of the module.
[0060] The present application only needs to collect low-frequency impedance data below 0.1 Hz, and the sampling interval requirement for the equipment is very low, the existing BMS and balancing maintenance instrument equipment supporting active balancing can directly use the method of the present application, without the need to modify the hardware equipment, reducing the upgrade cost.
[0061] Finally, a few points should be noted: first, in the description of the present application, it should be noted that unless otherwise specified and limited, the terms "installation", "connection", "connection" should be broadly understood, which can be mechanical connection or electrical connection, or the communication between two elements, or direct connection, "up", "down", "left", "right" and the like are only used to indicate the relative positional relationship, when the absolute position of the described object changes, the relative positional relationship may change;
[0062] Secondly: the present application discloses the structure involved in the embodiment of the present application, other structures can refer to the usual design, and in the case of no conflict, the same embodiment and different embodiments of the present application can be combined with each other;
[0063] Finally: the above only for the preferred embodiments of the present application, and not for limiting the present application, any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application, should be included in the scope of protection of the present application.
Claims
1. A method for battery equalization based on EIS features, characterized in that; Includes the following steps: Step S1: Apply low-frequency constant current excitation to the fully rested module to be equalized; Step S2: Calculate the impedance slope k of each individual cell and select the cells to be balanced based on the deviation from the median. Step S3: Obtain the impedance slope-time change curve of the unequilibrated cell after charging and after discharging and resting for 1 hour, as a standard reference curve; Step S4: Perform equalization operation on the selected cells to be equalized, and apply low-frequency constant current excitation after the operation to obtain their impedance slope k; Step S5: Determine whether the current cell has reached the equilibrium state based on the obtained impedance slope k. If not, continue to perform the equilibrium operation.
2. The method for battery equalization based on EIS features according to claim 1, characterized in that: In step S1, the current magnitude of the low-frequency constant current excitation is 0.01C, and the excitation frequency points are 11 points within the range of 0.1Hz–0.01Hz that satisfy a logarithmic distribution.
3. The method for battery equalization based on EIS features according to claim 1, characterized in that: In step S1, the impedance slope k is obtained by linear fitting of the Nyquist plot in the 0.1Hz–0.01Hz frequency band. The fitting method is the least squares method, and the fitting equation is y=kx+b.
4. The method for battery equalization based on EIS features according to claim 3, characterized in that: Before fitting, the impedance points in the Nyquist plot were smoothed using a three-point averaging method.
5. The method for battery equalization based on EIS features according to claim 1, characterized in that: In step S2, the criterion for selecting the cell to be equalized is: if the impedance slope of a cell deviates from the median of the module by more than ±5%, then the cell is determined to need to be equalized.
6. The method for battery equalization based on EIS features according to claim 1, characterized in that: In step S3, the standard reference curve is obtained in the following way: The module is charged or discharged under constant current, with the current not exceeding 0.01C, for 1 hour. After charging and discharging, a low-frequency impedance test was performed every 10 minutes for a total of 6 times. The standard kt curve is constructed by taking the median of the impedance slope of the unbalanced unit at each time step.
7. The method for battery equalization based on EIS features according to claim 6, characterized in that: The maximum value of the standard charging kt curve is defined as the equilibrium peak calibration value k. ↑ The minimum value of the standard discharge kt curve is the equilibrium valley calibration value k. ↓ This serves as a benchmark for subsequent equilibrium judgment.
8. The method for battery equalization based on EIS features according to claim 1, characterized in that: In step S4, the balancing operation includes charging balancing or discharging balancing, the operating current is the same as in step S3, and the duration of each operation is not less than 1 hour.
9. The method for battery equalization based on EIS features according to claim 1, characterized in that: The condition for determining that the equilibrium is complete in step S5 is: If discharge equalization is performed, the minimum value k of the discharge kt curve of the current cell will be... * It should not be greater than k ↓ ; If charging equalization is performed, the maximum value k of the charging kt curve of the current cell will be... * It should be no less than k ↑ .