A Method for Estimating the State of Charge of Lithium Iron Phosphate Batteries Based on Static Stress and DEIS

Abstract

The present invention relates to the technical field of battery SOC estimation, and discloses a method for estimating the SOC of a lithium iron phosphate battery based on static stress and DEIS, including the following steps: collecting the total stress F and the full-frequency dynamic impedance spectrum EIS of the lithium iron phosphate battery during operation, performing equidistant charging of the SOC of the lithium iron phosphate battery at different charging currents I, standing for 2 hours after each interval charging is completed, and then performing the next equidistant charging until the SOC reaches 100%, and calculating the dynamic stress F generated by charging in each SOC interval. D ; By defining static stress and dynamic stress, this method realizes accurate separation of static stress to estimate SOC. Compared with existing methods, this method overcomes the defects of high model accuracy requirements and inaccurate estimation in the voltage platform region, provides a solution that is more sensitive to charge and discharge history and current, and establishes a t-F S -SOC estimator to estimate SOC based on the obtained static stress FS.

Description

Technical Field

[0001] The present invention relates to the technical field of battery SOC estimation, and specifically to a method for estimating the SOC of a lithium iron phosphate battery based on static stress and DEIS. Background Art

[0002] Lithium iron phosphate batteries (LiFePO4) are known for their excellent safety and long life. Compared with other lithium-ion batteries, lithium iron phosphate batteries have better thermal stability and can withstand high temperatures without thermal runaway. At the same time, its cycle life is very long and can exceed thousands of charge and discharge cycles. In addition, the material cost of lithium iron phosphate batteries is relatively low, does not contain rare metals, and has environmental protection advantages. However, with the growth of market demand and the development of technology, the requirements for lithium batteries are also getting higher and higher, not only for their energy density and charge and discharge rate, but also for their safety and life management capabilities. Among these requirements, SOC reflects the available electric quantity in a lithium-ion battery and measures the endurance of the lithium-ion battery. The estimation of the SOC of a lithium battery is particularly important and has become one of the key factors affecting the performance and safety of electric vehicles.

[0003] Traditional methods for estimating the SOC of lithium-ion batteries include the ampere-hour counting method and model-based methods. The ampere-hour counting method depends on the accuracy of the initial SOC. However, under the flat open-circuit voltage (OCV) characteristics of lithium iron phosphate batteries, it is difficult to accurately obtain the initial SOC. For model-based methods, such as equivalent circuit models (ECM), data-driven models, or electrochemical models, although they can improve the estimation accuracy, their computational complexity and the characteristics of difficult parameter identification pose challenges in practical applications. In addition, the flatness and hysteresis effect of the OCV-SOC curve further limit the accuracy of voltage-based estimation methods. Therefore, previous studies have found that there is a significant relationship between the stress energy generated during the charge and discharge process of the battery and the SOC. However, since the stress change obtained during the charge and discharge process is non-linear, it has become a difficult problem to establish a direct correspondence between the stress and the SOC, and the non-linear problem of stress change needs to be solved. In addition, there is also the use of electrochemical impedance spectroscopy to estimate the SOC of lithium iron phosphate batteries. Although electrochemical impedance spectroscopy (EIS) can provide detailed information on the internal electrochemical process of the battery, traditional EIS methods still have some limitations. Traditional EIS mainly relies on low-frequency and high-frequency data to estimate the SOC, but the EIS parameters in the intermediate frequency region are often affected by multiple factors and it is difficult to obtain clear regular changes, resulting in inaccurate SOC estimation results. Therefore, a method for estimating the SOC of a lithium iron phosphate battery based on static stress and DEIS is proposed to solve the above problems. Summary of the Invention

[0004] In view of the deficiencies of the prior art, the present invention provides a method for estimating the state of charge (SOC) of a lithium iron phosphate battery based on static stress and DEIS, which has the advantages of accurately estimating the SOC of the lithium iron phosphate battery, etc., and solves the problem that it is difficult to obtain the clear regular changes of the EIS parameters in the intermediate frequency region, resulting in the inaccuracy of the SOC estimation result.

[0005] To achieve the above-mentioned accurate purpose of estimating the SOC of the lithium iron phosphate battery, the present invention provides the following technical solutions: A method for estimating the SOC of a lithium iron phosphate battery based on static stress and DEIS, comprising the following steps:

[0006] S1: Collect the total stress F and the full-frequency dynamic impedance spectrum DEIS of the lithium iron phosphate battery during operation.

[0007] S2: Charge the lithium iron phosphate battery at equal intervals of SOC under different charging currents I. After each charging interval is completed, let it stand for 2 hours, and then perform the next equal-interval charging until the SOC reaches 100%. Calculate the dynamic stress F generated by charging in each SOC interval D , and obtain the dynamic stress change rate ΔF of each SOC interval D . Under each SOC interval, fit the relationship between the charging current I and the dynamic stress change rate ΔF D to obtain the fitting expression ΔF of the dynamic stress change rate and the current D (I).

[0008] S3: Charge the lithium iron phosphate battery at equal intervals of SOC under different charging currents I in different charging processes. After each charging interval is completed, let it stand, and then perform the next equal-interval charging until the SOC reaches 100%. Fit the relationship expressions of the dynamic stress change rate ΔF D，t , the current I, and the previous dynamic stress F D,t-1 to obtain the fitting expression F of each SOC interval D,t-1 (ΔF D , I).

[0009] S4: Calculate the static stress F through the total stress F and the dynamic stress F D , and establish an F S -SOC estimator through the look-up table method. Due to the non-linearity of the static stress F S , use the slope of the static stress F S and the impedance spectrum EIS as discrimination conditions to accurately estimate the SOC. S Preferably, the step S2 is specifically as follows:

[0010] Preferably, the step S2 is specifically as follows:

[0011] S2.1: The lithium iron phosphate battery is charged at intervals of 10% SOC at different charging currents I, and the total stress F1 after charging and the total stress F2 after standing are obtained at different SOC intervals and different charging currents I. The dynamic stress F generated by each 10% SOC charging is calculated. D , the expression is:

[0012] ;

[0013] Among them, F1 is the total stress after charging, and F2 is the total stress after standing;

[0014] S2.2: Dynamic stress F generated by charging in each SOC interval D The dynamic stress change rate ΔF of each SOC interval is calculated D , the expression is:

[0015] ;

[0016] in, It is the dynamic stress after the SOC interval is fully charged. is the dynamic stress at the start SOC moment of the SOC interval, is the charging time in the SOC interval;

[0017] S2.3: Use the dynamic stress change rate ΔF generated by different charging currents I in each SOC interval calculated in step S2.2 D Fitting is performed to obtain the dynamic stress change rate ΔF D The relationship between ΔF and charging current I D (I), the specific expression is;

[0018] ;

[0019] Where I is the charging current, α1, α2, and α3 are all fitting expressions ΔF D Fitting parameters in (I).

[0020] Preferably, the step S3 is specifically:

[0021] S3.1: Set different charging processes. The same SOC interval is charged using different charging processes at different charging currents I. Different charging processes will generate different pre-dynamic stress F D，t-1 ;

[0022] S3.2: Repeat step S3.1 for each SOC interval to obtain the dynamic stress F at different front ends. D，t-1 Dynamic stress change rate ΔF generated by using different charging currents I D,t, the dynamic stress change rate ΔF in each SOC interval is obtained by using the fitting method D,t , the charging current I, and the previous dynamic stress F D,t-1 of the fitting expression F D,t-1 (ΔF D , I), and the expression is:

[0023] ;

[0024] where F D,t-1 is the previous dynamic stress at time t, I is the charging current, and ΔF D,t is the dynamic stress change rate at time t, are all fitting parameters in the fitting expression.

[0025] Preferably, the specific steps in step S4 are:

[0026] S4.1: In each SOC interval, the dynamic stress change rate ΔF at the current moment is calculated by the charging current I at the current moment and the fitting relationship ΔF D (I), and the previous dynamic stress F at the current moment is calculated according to the fitting relationship F D,t (ΔF D,t-1 (ΔF D , I), and the previous dynamic stress F at each moment D,t-1 is the dynamic stress F at the previous moment D,t-1 , and the dynamic stress F at each moment can be calculated D ; D ;

[0027] S4.2: The static stress F D is calculated by the total stress F and the dynamic stress F S , and the expression is:

[0028] ;

[0029] S4.3: According to the measured real-time total stress F data, combined with the calculated dynamic stress F D , the static stress F S data per second is calculated. According to the real SOC data of the battery per second, a look-up table method corresponding to time t, static stress F S , and real SOC is established to obtain a t-F S -SOC estimator;

[0030] S4.4: The slope K S of the static stress F S is added as a supplementary discrimination condition, and the t-F S in the t-F SThe expression and find the first derivative of the static stress to obtain the static stress slope K S The expression is:

[0031] ;

[0032] ;

[0033] ;

[0034] where F S is the static stress, is the first derivative of the static stress, K S is the static stress slope, is the fitting parameter in the fitting relationship between the static stress and time.

[0035] S4.5: By real-time monitoring the dynamic impedance spectrum DEIS during the charging of the lithium iron phosphate battery and extracting the charge transfer resistance , double-layer capacitance , equivalent series capacitance Three key parameters, expressed as:

[0036] ;

[0037] ;

[0038] ;

[0039] where are the real part values at the low-frequency end and high-frequency end of the semicircle curve in the intermediate frequency region of the impedance spectrum respectively, is the pi, is the frequency corresponding to the maximum imaginary part value of the semicircle curve in the intermediate frequency region of the impedance spectrum, is the scanning frequency in the intermediate frequency region of the impedance spectrum.

[0040] Compared with the prior art, the present invention provides a method for estimating the SOC of a lithium iron phosphate battery based on static stress and DEIS, having the following beneficial effects:

[0041] 1. The method for estimating the SOC of a lithium iron phosphate battery based on static stress and DEIS realizes accurate separation of static stress to estimate the SOC by defining static stress and dynamic stress. Compared with the existing methods, this method overcomes the defects of high model accuracy requirements and inaccurate estimation in the voltage plateau region, provides a solution that is more sensitive to the charge and discharge history and current, and establishes a t - F S -SOC estimator to estimate the SOC according to the obtained static stress FS.

[0042] 2. The SOC estimation method for lithium iron phosphate batteries based on static stress and DEIS combines the static stress slope K S As a supplementary discrimination method for solving the static stress F S Corresponding to multiple SOC values, and based on the change of dynamic impedance spectrum parameters of lithium iron phosphate batteries, the equivalent series capacitance of EIS parameters in the intermediate frequency region is proposed As a further discrimination method for SOC estimation, it successfully solves the problem of using the static stress slope K S Under the non-linear state of S And the equivalent series capacitance As the SOC estimation discrimination method, it realizes the accurate estimation of the SOC of lithium iron phosphate batteries, making up for the deficiency of the non-linearity of stress change in the previous methods. Brief Description of the Drawings

[0043] Figure 1 It is a schematic diagram of the overall process of a SOC estimation method for lithium iron phosphate batteries based on static stress and DEIS proposed by the present invention;

[0044] Figure 2 It is a schematic diagram of the charging curve at intervals of 10% SOC under different currents of a SOC estimation method for lithium iron phosphate batteries based on static stress and DEIS proposed by the present invention;

[0045] Figure 3 It is the equivalent series capacitance obtained by sweeping the frequency at 100 mHz in the intermediate frequency region under different SOCs of a SOC estimation method for lithium iron phosphate batteries based on static stress and DEIS proposed by the present invention Trend chart of change;

[0046] Figure 4 It is a schematic diagram of the situation where one static stress Fs corresponds to multiple SOC values of a SOC estimation method for lithium iron phosphate batteries based on static stress and DEIS proposed by the present invention. Detailed Embodiment

[0047] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0048] Please refer to Figures 1-4 , a SOC estimation method for lithium iron phosphate batteries based on static stress and DEIS, includes the following steps:

[0049] S1: Collect the total stress F and the full - frequency dynamic impedance spectrum DEIS of the lithium iron phosphate battery during operation.

[0050] S2: Charge the lithium iron phosphate battery at equal SOC intervals with different charging currents I. After each charging interval is completed, let it stand for 2 hours. After standing, perform the next equal - interval charging until the SOC reaches 100%. Calculate the dynamic stress F generated during charging in each SOC interval D , and obtain the dynamic stress change rate ΔF for each SOC interval D . For each SOC interval, fit the relationship between the charging current I and the dynamic stress change rate ΔF D to obtain the fitting expression ΔF D (I) of the dynamic stress change rate versus the current.

[0051] S3: Charge the lithium iron phosphate battery at equal SOC intervals with different charging currents I under different charging processes. After each charging interval is completed, let it stand, and then perform the next equal - interval charging until the SOC reaches 100%. Fit the relationship between the dynamic stress change rate ΔF D，t , the current I, and the previous dynamic stress F D,t-1 to obtain the fitting expression F D,t-1 (ΔF D , I) for each SOC interval.

[0052] S4: Calculate the static stress F from the total stress F and the dynamic stress F D . Establish an F - SOC estimator through the look - up table method. Due to the non - linearity of the static stress F S , use the slope of the static stress F S and the impedance spectrum EIS as discrimination conditions to accurately estimate the SOC. S S D Obtain the relationship of ΔF

[0053] (I) for each 10% SOC interval, that is, knowing the charging current I in different SOC intervals, the dynamic stress change rate ΔF can be calculated D as shown in Table 1. D， As shown in Table 1.

[0054] Table 1

[0055]

[0056] Specifically, step S2 is as follows:

[0057] S2.1: Charge the lithium iron phosphate battery at intervals of 10% SOC under different charging currents I, obtain the total stress F1 after charging in different SOC intervals and under different charging currents I, and the total stress F2 after standing, and calculate the dynamic stress F generated by each 10% SOC charge. D , the expression is:

[0058] ;

[0059] Among them, F1 is the total stress after charging, and F2 is the total stress after standing;

[0060] S2.2: Calculate the dynamic stress change rate ΔF of each SOC interval through the dynamic stress F generated by charging in each SOC interval D , and the expression is: D , the expression is:

[0061] ;

[0062] Among them, is the dynamic stress after charging in the SOC interval, is the dynamic stress at the starting SOC moment of the SOC interval, is the charging time of the SOC interval;

[0063] S2.3: Use the dynamic stress change rate ΔF generated by different charging currents I in each SOC interval calculated in step S2.2 D for fitting to obtain the relationship ΔF D between the dynamic stress change rate ΔF and the charging current I, and the specific expression is; D (I), the specific expression is;

[0064] ;

[0065] Among them, I is the charging current, and α1, α2, and α3 are all fitting parameters in the fitting expression ΔF D (I).

[0066] Step S3 is specifically:

[0067] S3.1: Set different charging processes. In the same SOC interval, charge with different charging processes under different charging currents I to establish different pre-dynamic stresses F D，t-1 ;

[0068] Set four different charging processes. Taking the 30%-40% SOC interval as an example, please refer to Table II.

[0069] Table II

[0070]

[0071] Each static state is for obtaining the total stress before and after the static state according to the real-time acquisition of the stress sensor, so as to calculate the dynamic stress F. D 。

[0072] In Process 1, since the battery is directly charged from 30% to 40% SOC, and it was static for 2 hours after being charged from 0% to 30% SOC before, therefore, in Overcharge 1, the previous dynamic stress F in the 30%-40% SOC D，30%SOC is 0.

[0073] In Process 2, during the process of charging the battery from 0% to 40% SOC, the previous dynamic stress in the 30%-40% SOC part accumulates the dynamic stress of the previous 0%-30% SOC. Therefore, the previous dynamic stress F in the 30%-40% SOC in Process 2 D，30%SOC is not 0. The process of charging the battery from 0% to 30% SOC and then static is for calculating the accumulation of the dynamic stress of 0%-30% SOC, and this accumulated dynamic stress is used as the previous dynamic stress F of 30%-40% SOC later. D，30%SOC 。

[0074] Process 3 is the same as Process 2. The previous dynamic stress in the 30%-40% SOC part accumulates the dynamic stress of the previous 10%-30% SOC. Therefore, the previous dynamic stress F in the 30%-40% SOC in Process 2 D，30%SOC is also not 0. The difference from Process 2 is that the battery is charged from 10% to 40% SOC, that is, the previous dynamic stress only accumulates the dynamic stress of 10%-30% SOC. The process of charging the battery from 10% to 30% SOC and then static is for calculating the accumulation of the dynamic stress of 10%-30% SOC, and this accumulated dynamic stress is used as the previous dynamic stress F of 30%-40% SOC later. D，30%SOC 。

[0075] The principle of Process 4 is the same as that of Process 3. The previous dynamic stress in the 30%-40% SOC part accumulates the dynamic stress of the previous 20%-30% SOC. Therefore, the previous dynamic stress F in the 30%-40% SOC in Process 2 D，30%SOC is also not 0. The difference from Process 2 is that the battery is charged from 20% to 40% SOC, that is, the previous dynamic stress only accumulates the dynamic stress of 20%-30% SOC. The process of charging the battery from 20% to 30% SOC and then static is for calculating the accumulation of the dynamic stress of 20%-30% SOC, and this accumulated dynamic stress is used as the previous dynamic stress F of 30%-40% SOC later. D，30%SOC 。

[0076] Obviously, the difference among the four processes lies in the magnitudes of the pre-dynamic stresses, which are used to obtain the influence of pre-dynamic stresses of different magnitudes on the dynamic stress change rate ΔF D .

[0077] S3.2: Repeat step S3.1 for each SOC interval to obtain the dynamic stress change rate ΔF D，t-1 generated by using different charging currents I under different pre-dynamic stresses F D,t . Use the fitting method to obtain the fitting expression F D,t of the dynamic stress change rate ΔF D,t-1 , charging current I, and pre-dynamic stress F D,t-1 at each SOC interval, and the expression is: D ;

[0078]

[0079] where F D,t-1 is the pre-dynamic stress at time t, I is the charging current, and ΔF D,t is the dynamic stress change rate at time t, and are all fitting parameters in the fitting expression.

[0080] The specific steps in step S4 are as follows:

[0081] S4.1: In each SOC interval, calculate the dynamic stress change rate ΔF D at the current time through the charging current I at the current time and the fitting relationship ΔF D,t (I), and calculate the pre-dynamic stress F D,t-1 at the current time according to the fitting relationship F D (ΔF D,t-1 , I). The pre-dynamic stress F D,t-1 at each moment is the dynamic stress F D at the previous moment, so the dynamic stress F D， at each moment can be calculated. Please refer to Table III.

[0082] Table III

[0083]

[0084] S4.2: Calculate the static stress F D through the total stress F and the dynamic stress F S , and the expression is:

[0085] ;

[0086] S4.3: According to the measured real-time total stress F data from the experiment, combined with the calculated dynamic stress F​D , the static stress F per second is calculated S Data. According to the true SOC data of the battery per second, establish a look-up table method corresponding to time t, static stress F S , and true SOC, and obtain an estimator;

[0087] S4.4: Due to the non-linearity of the static stress F S , some static stress F S values will correspond to multiple SOC values. Increase the slope K S of the static stress F S as a supplementary discrimination condition, and fit the expression of t-F in the estimator, and find the first derivative of the static stress S , so as to obtain the static stress slope K , and the expression is: S ;

[0088] ;

[0089] ;

[0090] ;

[0091] where F S is the static stress, is the first derivative of the static stress, K S is the static stress slope, is the fitting parameter in the fitting relationship between the static stress and time.

[0092] In Figure 4 , for example, when the obtained static stress F S value corresponds to two SOC values c and g, if the static stress slope K S is "zero" at this time, then the SOC is the value at point c. If the static stress slope K S is "positive" at this time, then the SOC is the value at point g. Similarly, the situation where the static stress F S value corresponds to two SOC values a and e can be discriminated. When the obtained static stress F S value corresponds to three SOC values b, d, and f, if the static stress slope K S is "negative" at this time, then the SOC is the value at point d. If the static stress slope K S is "positive" at this time, then the SOC is the value at point b or f.

[0093] By judging the "positive, negative, zero" situation of the static stress slope K S , the SOC can be further estimated;

[0094] S4.5: When a static stress F S value corresponds to multiple SOC values, and when the "positive, negative, zero" conditions of the static stress slope K S are the same, further discrimination is required to achieve accurate SOC estimation. By real-time monitoring the dynamic impedance spectrum (DEIS) of a lithium iron phosphate battery during charging and extracting the charge transfer resistance , double-layer capacitance , and equivalent series capacitance from the impedance spectrum, which are expressed as:

[0095] ;

[0096] ;

[0097] ;

[0098] where, are the real part values at the low-frequency end and high-frequency end of the semicircle curve in the middle-frequency region of the impedance spectrum respectively, is the pi, is the frequency corresponding to the maximum imaginary part value of the semicircle curve in the middle-frequency region of the impedance spectrum, is the scanning frequency in the middle-frequency region of the impedance spectrum.

[0099] Under the middle-frequency region of the impedance spectrum of the lithium iron phosphate battery, it is found that the equivalent series capacitance shows a monotonically decreasing relationship with the increase of SOC. Therefore, it can be used as a discrimination condition for further SOC estimation to achieve accurate SOC estimation.

[0100] Set a threshold , and this threshold is the value of the equivalent series capacitance S at 100 mHz in the middle-frequency region of the impedance spectrum at any point within the decreasing region of the static stress F value. This value can be obtained through a pre-experiment. In the pre-experiment, when the static stress F S value starts to decrease during charging, use the impedance online measurement device to detect and calculate the value of the equivalent series capacitance at a point in real time, and use this as the threshold . Since shows a monotonically decreasing relationship with the increase of SOC, the value of the equivalent series capacitance S at 100 mHz in the middle-frequency region of the impedance spectrum in the increasing region before the F value decreases must be greater than the threshold , and the value of the equivalent series capacitance S at 100 mHz in the middle-frequency region of the impedance spectrum in the increasing region after the F value decreases must be less than the threshold .

[0101] In Figure 4 , when the obtained static stress F S value corresponds to the three SOC values of b, d, and f, if the static stress slope K S is "negative" at this time, the SOC is the value at point d. If the static stress slope K S is "positive" at this time, the SOC is the value at point b or point f. At this time, according to the equivalent series capacitance S in the intermediate frequency range of 100 mHz of the impedance spectrum corresponding to this static stress F , if is greater than the threshold , the SOC is the value at point b. If is less than the threshold , the SOC is the value at point f. Thus, the problem of accurately estimating the SOC when a static stress F S value corresponds to multiple SOC values is completely solved.

[0102] During use, by defining the static stress and dynamic stress, the static stress is accurately separated to estimate the SOC. Compared with the existing methods, this method overcomes the defects of high model accuracy requirements and inaccurate estimation in the voltage platform region, provides a solution that is more sensitive to the charge and discharge history and current, establishes a t - F S -SOC estimator, and estimates the SOC based on the obtained static stress FS.

[0103] In summary, the SOC estimation method for lithium iron phosphate batteries based on static stress and DEIS.

[0104] Moreover, by combining the static stress slope K S as a supplementary discrimination method for solving the problem that the static stress F S corresponds to multiple SOC values, and based on the change of the dynamic impedance spectrum parameters of lithium iron phosphate batteries, it is proposed to use the equivalent series capacitance in the intermediate frequency region of the EIS parameters as a further discrimination method for SOC estimation. It successfully solves the problem of using the static stress slope K S and the equivalent series capacitance S as SOC estimation discrimination methods under the non-linear state of the static stress F to achieve accurate estimation of the SOC of lithium iron phosphate batteries, making up for the deficiency of non-linear stress changes in previous methods.

[0105] It should be noted that the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, such that a process, method, article or apparatus comprising a series of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article or apparatus. Without further limitation, an element defined by the phrase "comprising an..." does not exclude the presence of additional identical elements in the process, method, article or apparatus comprising said element.

[0106] Although the embodiments of the present invention have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for estimating SOC of a lithium iron phosphate battery based on static stress and DEIS, characterized in that: The following steps are involved: S1: Collect the total stress F and full-frequency dynamic impedance spectrum DEIS of the lithium iron phosphate battery during operation; S2: The lithium iron phosphate battery is charged at equal intervals of SOC at different charging currents I. After each interval is charged, it is left to stand for 2 hours. After the standing time is completed, the next equal interval is charged again until the SOC reaches 100%. The dynamic stress F generated by charging in each SOC interval is calculated. D , and obtain the dynamic stress change rate ΔF for each SOC interval D , in each SOC range, the charging current I and the dynamic stress change rate ΔF D The relationship between the dynamic stress change rate and the current is fitted to obtain the fitting expression ΔF D (I); S3: Use different charging currents I to charge the lithium iron phosphate battery at equal intervals of SOC under different charging processes. After each interval is charged, let it stand, and then charge it at the next equal interval until the SOC reaches 100%. Fit the dynamic stress change rate ΔF under different charging processes D，t , current I, front dynamic stress F D,t-1 The relationship between the two equations is used to obtain the fitting expression F for each SOC interval. D,t-1 (ΔF D , I); S4: Through the total stress F and dynamic stress F D The static stress F is calculated S , establish F by table lookup method S -SOC estimator, due to static stress F S The nonlinearity of the static stress F S The slope and impedance spectrum EIS are used as discrimination conditions to accurately estimate SOC.

2. A method for estimating SOC of a lithium iron phosphate battery based on static stress and DEIS according to claim 1, characterized in that: The step S2 is specifically as follows: S2.1: The lithium iron phosphate battery is charged at intervals of 10% SOC at different charging currents I, and the total stress F1 after charging and the total stress F2 after standing are obtained at different SOC intervals and different charging currents I. The dynamic stress F generated by each 10% SOC charging is calculated. D , the expression is: ； Among them, F1 is the total stress after charging, and F2 is the total stress after standing; S2.2: Dynamic stress F generated by charging in each SOC interval D The dynamic stress change rate ΔF of each SOC interval is calculated D , the expression is: ； in, It is the dynamic stress after the SOC interval is charged. is the dynamic stress at the start SOC moment of the SOC interval, is the charging time in the SOC interval; S2.3: Use the dynamic stress change rate ΔF generated by different charging currents I in each SOC interval calculated in step S2.2 D Fitting is performed to obtain the dynamic stress change rate ΔF D The relationship between ΔF and charging current I D (I), the specific expression is; ； Where I is the charging current, α1, α2, and α3 are all fitting expressions ΔF D Fitting parameters in (I).

3. The method for estimating SOC of a lithium iron phosphate battery based on static stress and DEIS according to claim 1, characterized in that: The step S3 is specifically as follows: S3.1: Set different charging processes. The same SOC interval is charged using different charging processes at different charging currents I. Different charging processes will generate different pre-dynamic stress F D，t-1 ; S3.2: Repeat step S3.1 for each SOC interval to obtain the dynamic stress F at different front ends. D，t-1 Dynamic stress change rate ΔF generated by using different charging currents I D,t , the dynamic stress change rate ΔF in each SOC interval is obtained by fitting method D,t , charging current I, front dynamic stress F D,t-1 The fitting expression F of the three D,t-1 (ΔF D , I), the expression is: ； where F D,t-1 is the initial dynamic stress at time t, I is the charging current, ΔF D,t is the dynamic stress change rate at time t, are all fitting parameters in the fitting expression.

4. The method for estimating SOC of a lithium iron phosphate battery based on static stress and DEIS according to claim 1, characterized in that: The specific steps in step S4 are: S4.1: In each SOC interval, the current charging current I and the fitting relationship ΔF D (I) Calculate the dynamic stress change rate ΔF at the current moment D,t , and according to the fitting relationship F D,t-1 (ΔF D , I) calculate the current moment's forward dynamic stress F D,t-1 , the previous dynamic stress F at each moment D,t-1 The dynamic stress F at the previous moment D , the dynamic stress F at each moment can be calculated D ; S4.2: Through total stress F, dynamic stress F D The static stress F is calculated S , the expression is: ； S4.3: Based on the real-time total stress F data measured experimentally, the dynamic stress F is calculated and D , the static stress F per second can be calculated S Data, based on the real SOC data of the battery every second, establish time t, static stress F S , and the actual SOC correspond to the table method, and get a tF S -SOC estimator; S4.4: Increase static stress F S The slope K S As a supplementary discriminant condition, fitting tF S -tF in SOC estimator S and find the first-order derivative of static stress , thus obtaining the static stress slope K S , the expression is: ； ； ； Among them, F S is the static stress, is the first-order derivative of static stress, K S is the static stress slope, is the fitting parameter in the fitting relationship between static stress and time; S4.5: Real-time monitoring of the dynamic impedance spectrum DEIS of the lithium iron phosphate battery during charging, and extraction of the charge transfer resistance in the impedance spectrum , double layer capacitor , Equivalent Series Capacitance Three key parameters, expressed as: ； ； ； in, are the real values ​​of the low-frequency and high-frequency ends of the semicircular curve in the mid-frequency region of the impedance spectrum, is the circumference of a circle, is the frequency corresponding to the maximum value of the imaginary part of the semicircular curve in the mid-frequency region of the impedance spectrum, is the scanning frequency in the mid-frequency region of the impedance spectrum.

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