A method for battery state of health based on reversible and irreversible thermal analysis

Abstract

The present invention discloses a method for battery state of health based on reversible and irreversible thermal analysis, including: Step S1, for the battery to be tested, analyze and obtain the characteristic SOC at which the capacity of the battery system to be tested decays, and analyze and establish an offline database for reversible and irreversible heat generation analysis of the battery system to be tested; Step S2, at the characteristic SOC, perform preset reversible and irreversible thermal test operations on the battery to be tested to obtain the reversible and irreversible heat of the battery to be tested; Step S3, obtain the growth rates of the reversible and irreversible heat of the battery to be tested according to the reversible and irreversible heat of the fresh battery and the battery to be tested; Step S4, calculate and obtain the capacity retention rate of the battery to be tested. By performing symmetric charge and discharge tests on the battery, the present invention makes full use of the battery temperature data collected during the test, deduces and calculates the reversible and irreversible heat, and compares and analyzes it with the offline database, so as to obtain the state of health (capacity retention rate) of the battery.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium - ion batteries, and particularly to a method for analyzing the state of health of a battery based on reversible and irreversible heat analysis. Background Art

[0002] As the usage time of lithium - ion batteries extends, their performance will decline. Therefore, for lithium - ion batteries used in electric vehicles and energy storage, accurately evaluating the state of health of the battery through on - line monitoring can not only ensure the normal performance of the battery, but also effectively prevent safety accidents caused by battery abuse.

[0003] Existing state - of - health monitoring methods usually extract parameters such as voltage and current during the charge - discharge cycle of the battery, analyze the health characteristics related to battery capacity degradation of these parameters, and then realize the prediction of the state of health of the battery through the established model and related data analysis methods. The selection of model parameters in the model building process directly affects the accuracy of predicting the state of health of the battery.

[0004] Currently, in most model building, voltage and current data are used, and the utilization of temperature data is relatively less, and it is limited to the analysis of the temperature rise data during the charge - discharge process of the battery. As is well known, lithium - ion batteries generate heat during both charge and discharge processes, and the amount of heat generated can directly characterize the performance of the battery and affect the state of health of the lithium - ion battery.

[0005] However, there is currently no method that can analyze the state of health of lithium - ion batteries based on the amount of heat generated during the charge - discharge process of lithium - ion batteries. Summary of the Invention

[0006] The object of the present invention is to provide a method for analyzing the state of health of a battery based on reversible and irreversible heat analysis in view of the technical defects existing in the prior art.

[0007] To this end, the present invention provides a method for analyzing the state of health of a battery based on reversible and irreversible heat analysis, including the following steps:

[0008] Step S1, for a battery under test whose state of health needs to be evaluated, a fresh battery and one or more reference batteries are pre - selected. Then, the preset reversible and irreversible heat test operations are respectively performed on the fresh battery and each reference battery. The reversible and irreversible heat of the fresh battery and each reference battery are obtained through the test. Then, based on the reversible and irreversible heat of the fresh battery and each reference battery, the characteristic state of charge (SOC) at which the battery system under test experiences capacity decay is analyzed, and based on the characteristic SOC at which the battery system under test experiences capacity decay, an off - line database for reversible and irreversible heat generation analysis of the battery system under test is established through analysis.

[0009] Among them, a fresh battery is a battery that belongs to the same battery system as the battery under test and has not experienced capacity attenuation;

[0010] Multiple reference batteries are multiple batteries that belong to the same battery system as the lithium-ion battery under test, have experienced capacity attenuation after cycling, and have different capacity retention rates;

[0011] Step S2: At the characteristic SOC at which the battery system under test obtained in Step S1 experiences capacity attenuation, perform a preset reversible and irreversible thermal test operation on the battery under test to obtain the reversible and irreversible heat of the battery under test;

[0012] Step S3: Calculate the growth rates of the reversible and irreversible heat of the battery under test based on the reversible and irreversible heat of the fresh battery obtained in Step S1 and the reversible and irreversible heat of the battery under test obtained in Step S2;

[0013] Step S4: Calculate the capacity retention rate of the battery under test based on the fitting relationship between the growth rates of the reversible and irreversible heat of the battery and the capacity retention rate of the battery in the offline database of the reversible and irreversible heat generation analysis of the battery system under test and the growth rates of the reversible and irreversible heat of the battery under test.

[0014] As can be seen from the technical solutions provided by the present invention above, compared with the prior art, the present invention provides a method for analyzing the battery health state based on reversible and irreversible heat. Its design is scientific. By performing symmetric charge and discharge tests on the battery, making full use of the battery temperature data collected during the test, deducing and calculating the reversible and irreversible heat, and comparing and analyzing it with the offline database, the health state of the battery (i.e., reflected as the capacity retention rate) can be obtained, which has great practical significance.

[0015] In addition, for the present invention, in view of the different generation mechanisms of reversible and irreversible heat, by distinguishing and testing the two, it can assist in analyzing the reasons for battery attenuation.

[0016] In addition, since the method provided by the present invention is limited to testing and analyzing within a characteristic interval, the test efficiency can be greatly improved, and it is easy to be implemented online in a battery management system. Description of the Drawings

[0017] Figure 1 It is a flowchart of a method for analyzing the battery health state based on reversible and irreversible heat provided by the present invention;

[0018] Figure 2 It is a schematic diagram of the reversible heat generation amounts measured for a fresh battery and a battery after cyclic attenuation at different SOCs;

[0019] Figure 3Schematic diagram of irreversible heat generation measured for fresh batteries and batteries after cyclic degradation at different SOCs;

[0020] Figure 4 Schematic diagram of the reversible and irreversible heat growth rates of a reference battery relative to a fresh battery at different SOCs;

[0021] Figure 5 Graph showing the relationship between the reversible and irreversible heat growth rates of a reference battery with respect to the capacity retention rates of each cell in the battery system under test and the battery capacity retention rate. Detailed implementation manners

[0022] To enable those skilled in the art to better understand the solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0023] See Figure 1 , the present invention provides a method for analyzing the state of health of a battery based on reversible and irreversible heat, including the following steps:

[0024] Step S1, for the battery under test (which is a lithium-ion battery) whose state of health needs to be evaluated, a fresh battery and one or more reference batteries are pre-selected. Then, preset reversible and irreversible heat test operations are respectively performed on the fresh battery and each reference battery, and the reversible and irreversible heat of the fresh battery and each reference battery are obtained through testing. Then, based on the reversible and irreversible heat of the fresh battery and each reference battery, the characteristic SOC at which the battery system under test undergoes capacity decay is analyzed, and based on the characteristic SOC at which the battery system under test undergoes capacity decay, an offline database for reversible and irreversible heat generation analysis of the battery system under test is established;

[0025] Among them, the fresh battery is a battery that belongs to the same battery system as the battery under test and has not undergone capacity decay (i.e., a battery with a capacity retention rate of 100%);

[0026] The multiple reference batteries are multiple batteries that belong to the same battery system as the lithium-ion battery under test, have undergone capacity decay after cycling, and have different capacity retention rates;

[0027] It should be noted that heat is generated during both the charging and discharging processes of lithium-ion batteries. The amount of heat generated by lithium-ion batteries can directly characterize the quality of battery performance. The heat generated by lithium-ion batteries includes two parts: reversible heat and irreversible heat. Among them, since reversible heat is mainly generated by the entropy change during the lithium insertion / extraction process of the positive and negative electrode materials in the electrochemical reaction, while irreversible heat is mainly generated by the internal impedance of the battery, manifested as heat dissipated to the outside (i.e., heat release). Therefore, differentiating the heat generation of lithium-ion batteries into reversible heat and irreversible heat is conducive to analyzing the main reasons for battery decay.

[0028] In step S1, specifically, the reference battery and the lithium-ion battery to be tested are batteries of the same batch and the same model, and are batteries of the same system;

[0029] In step S1, specifically, the preset reversible and irreversible heat test operations specifically include the following steps S11 to S12:

[0030] Step S11: Perform a symmetrical charge-discharge test on the battery to obtain the battery surface temperature before charging, the battery surface temperature at the end of charging, the battery surface temperature before discharging, and the battery surface temperature at the end of discharging;

[0031] In step S11, specifically, place the battery in a high and low temperature test chamber set to a constant temperature. A thermocouple (such as a PT100 thermocouple) is fixedly arranged on the surface of the battery to collect battery surface temperature data;

[0032] The battery with the thermocouple is sealed and wrapped with an adiabatic material film, so as to reduce the rapid heat exchange between the battery and the surrounding environment.

[0033] In step S11, specifically, connect the battery to existing conventional charge-discharge equipment to perform a charge-discharge test on the battery.

[0034] In step S11, specifically, when the battery has not reached the preset state of charge (SOC) before performing the symmetrical charge-discharge test, if the current state of charge (SOC) of the battery is greater than the preset state of charge (SOC), then use a small current of a preset size (such as 0.3C) to perform a constant current discharge on the lithium-ion battery until the lithium-ion battery reaches the preset state of charge (SOC);

[0035] If the current state of charge (SOC) of the lithium-ion battery is less than the preset state of charge (SOC), then use a small current of a preset size (such as 0.3C) to perform a constant current charge on the lithium-ion battery until the lithium-ion battery reaches the preset state of charge (SOC).

[0036] In step S11, specifically, the symmetrical charge-discharge test on the battery specifically includes the following operations:

[0037] First, charge the battery at a preset charging current for a preset charging duration;

[0038] Then, discharge the battery at a preset discharging current for a preset discharging duration;

[0039] Among them, the magnitudes of the charging current and the discharging current are equal, both equal to the current I;

[0040] The preset charging duration and the preset discharging duration are equal in magnitude and both equal the duration t.

[0041] In terms of specific implementation, the value range of the current I is preferably 0.25C to 1C.

[0042] In terms of specific implementation, during the symmetric charge-discharge test, the total charging power and the total discharging power of the battery are equal;

[0043] During the symmetric charge-discharge test, the total charging power and the total discharging power of the battery are both preferably equal to 1% - 10% of the SOC of the battery.

[0044] It should be noted that during the symmetric charge-discharge test, the total charging power of the battery is equal to the product of the charging current of the battery and the charging duration; the total discharging power of the battery is equal to the product of the discharging current of the battery and the discharging duration.

[0045] It should be noted that in the present invention, with the symmetric charge-discharge test method, the battery is charged and discharged within the range of 1% - 10% SOC.

[0046] It should be noted that the so-called symmetric charge-discharge means that both the current I and the duration t of the battery are consistent during the charging and discharging processes. Therefore, the total charging power during the charging process and the total discharging power during the discharging process of the battery are the same.

[0047] It should be noted that after the symmetric charge-discharge, that is, after charging or discharging, the battery needs to be left standing for 1h - 4h to let the temperature rise caused by charging or discharging drop and reach stability. After performing the symmetric charge-discharge test on the battery at a certain preset state of charge (SOC), through subsequent calculation operations (i.e., the second step), the reversible heat and irreversible heat of the battery at this preset state of charge can be obtained. Since the difference between the total charging power and the total discharging power during the symmetric charge-discharge process is zero, therefore, when it is necessary to test the voltage temperature coefficient of the lithium-ion battery at another state of charge (SOC, defined as the second state of charge), the battery is then charged or discharged with a small current for a certain period of time until the battery reaches the second state of charge. By repeating the symmetric charge-discharge test process, the reversible heat and irreversible heat at the second state of charge can be calculated. By repeating this way, the reversible heat and irreversible heat of the battery at any state of charge can be obtained.

[0048] In the present invention, considering that the lithium-ion battery generates less heat at low rates and there may be a large calculation error due to the low battery temperature rise, the current I is preferably above 0.2C; at the same time, considering that when the battery is charged and discharged at a large current, the irreversible heat in its generated heat dominates, which may affect the test of the reversible heat, the current is preferably below 1C. Therefore, the current I during the charging process and the discharging process is preferably between 0.25C and 1C.

[0049] In step S12, according to the battery surface temperatures before charging, at the end of charging, before discharging, and at the end of discharging of the battery obtained in step S11 during the symmetric charge-discharge test, the reversible heat and irreversible heat generated by the battery during the symmetric charge-discharge test are calculated.

[0050] In step S12, specifically, target data extraction and analysis are performed based on the temperature data of the battery measured in step S11 during the symmetric charge-discharge process. The battery surface temperature before charging is T OC , the battery surface temperature at the end of charging is T EC ; the battery surface temperature before discharging is T OD , and the battery surface temperature at the end of discharging is T ED . Then, the temperature rise of the battery during charging is ΔT C = T EC - T OC , and the temperature rise of the battery during discharging is ΔT D = T ED - T OD . Thus, the total heat generation Q C of the battery during charging is calculated as follows:

[0051] Q C = C p ·m·ΔT C , formula (1);

[0052] The total heat generation Q D of the battery during discharging is calculated as follows:

[0053] Q D = C p ·m·ΔT D , formula (2);

[0054] In the above formulas (1) and (2), C p is the specific heat of the battery, with the unit of J·g -1 ·K -1 ;

[0055] m is the mass of the battery, with the unit of g;

[0056] ΔT C is the temperature rise of the battery during charging;

[0057] ΔT D is the temperature rise of the battery during discharging.

[0058] It should be noted that the heat generation of the battery during charging and discharging is equal to the reversible heat Q 可逆and the irreversible heat Q 不可逆 The sum of the irreversible heat always appears as heat release and is positive, while the irreversible heat changes with the sign of the current. The current is positive during charging and negative during discharging.

[0059] It should be noted that according to the Bernadi heat generation model, the battery heat generation power is q = I(E - U) - I·T·dE / dT, where I is the current during battery charging and discharging, U is the working voltage of the battery, E is the open-circuit voltage; T is the ambient temperature of the battery, and dE / dT is the voltage temperature coefficient. The term I(E - U) is the heat generation caused by the battery impedance and polarization, and I(E - U) = I 2 R, where R is the total battery impedance including the ohmic resistance and the polarization resistance. Therefore, it appears as heat release during both the charging and discharging processes of the battery, which is the irreversible heat; the term -I·T·dE / dT is the reaction heat (entropy change heat) of the positive and negative electrodes of the lithium-ion battery. The performance of this reaction heat is opposite during the charging and discharging processes of the battery. If it appears as heat release during charging, it appears as heat absorption during discharging, or if it appears as heat absorption during charging, it appears as heat release during discharging, which is the reversible heat.

[0060] It should be noted that the reversible heat generation power is -I·T·dE / dT. Here, I is the charging or discharging current. Generally, the current I is taken as positive during battery charging and negative during battery discharging. Therefore, the heat generation performance of the irreversible heat is opposite during the charging and discharging processes.

[0061] Therefore, the total heat generation Q of the battery during the charging process C is the sum of the reversible heat Q 可逆 and the irreversible heat Q 不可逆 , that is, Q C = Q 不可逆 + Q 可逆 ; while the total heat generation Q of the discharging process D is the difference between the reversible heat Q 可逆 and the irreversible heat Q 不可逆 , that is, Q D = Q 不可逆 - Q 可逆 . Therefore, through mathematical derivation, the calculation formulas for the irreversible heat Q 不可逆 and the reversible heat Q 可逆 are as follows:

[0062] Q 不可逆 =(Q C + Q D ) / 2, formula (3);

[0063] Q 可逆 =(Q C - Q D ) / 2, formula (4);

[0064] In the above formulas (3) and (4), Q C is the total heat generation of the battery during charging;

[0065] Q D is the total heat generation of the battery during discharging.

[0066] In step S1, first, using the symmetric charge-discharge method, the reversible and irreversible heat of a fresh battery and a reference battery (i.e., the battery after cycle attenuation) at each state of charge are tested (i.e., the fresh battery and a reference battery are subjected to symmetric charge-discharge tests to obtain the reversible and irreversible heat generated during the symmetric charge-discharge test process of the battery), and then the characteristic SOC at which the capacity attenuation of the battery system to be tested occurs is analyzed, specifically including the following steps S13:

[0067] Step S13: Compare and analyze the reversible and irreversible heat of a fresh battery and a reference battery (i.e., the battery after cycle attenuation) at different states of charge (SOC), determine the SOC at which the reversible and irreversible heat shows a regular change with the decay of battery performance, and then use this SOC interval as the characteristic SOC at which the capacity attenuation of the battery system to be tested occurs;

[0068] The said step S13 specifically includes the following operations:

[0069] Step S131: Based on the reversible and irreversible heat of the fresh battery, combined with the reversible and irreversible heat of a reference battery, calculate the growth rate of the reversible and irreversible heat of a reference battery (i.e., the battery after cycle attenuation), and the specific calculation formula is as follows:

[0070] Growth rate of reversible heat = (Reversible heat of reference battery - Reversible heat of fresh battery) / Reversible heat of fresh battery;

[0071] Growth rate of irreversible heat = (Irreversible heat of reference battery - Irreversible heat of fresh battery) / Irreversible heat of fresh battery;

[0072] Step S132: Using the growth rate of the reversible and irreversible heat of a reference battery (i.e., the battery after cycle attenuation) as the ordinate and the multiple different states of charge SOC of a reference battery as the abscissa to plot a graph, obtain the first curve, analyze this first curve, and select the SOC with relatively high growth rates of both reversible and irreversible heat as the characteristic SOC of the battery cycle attenuation of the battery system to be tested.

[0073] Among them, the SOC with relatively high reversible and irreversible heat growth rates refers to that among the reversible heat growth rates corresponding to multiple different states of charge (SOC) of a reference battery, the value of the reversible heat growth rate of this SOC ranks among the top few (for example, among the top four) when sorted by magnitude, and among the irreversible heat growth rates corresponding to multiple different states of charge (SOC) of a reference battery, the value of the irreversible heat growth rate of this SOC ranks among the top few (for example, among the top four) when sorted by magnitude;

[0074] It should be noted that the relatively high reversible and irreversible heat growth rates mean that, compared with other SOCs, at the characteristic SOC, both the reversible heat growth rate and the irreversible heat growth rate of the battery are relatively significant. Therefore, it is relatively sensitive to characterize the battery cycle attenuation, and since both the reversible heat and the irreversible heat growth rate are significant, it can be used to distinguish and reflect the attenuation degrees caused by two factors, namely, the entropy change of the battery material and the impedance change.

[0075] When determining the characteristic SOC, first select the highest value, the second highest value, the third highest value,... of the reversible heat growth rate (such as Figure 4 each point marked by the large circles in the figure, which are respectively the highest value to the fourth highest value of the reversible heat growth rate); and select the highest value, the second highest value, the third highest value,... of the irreversible heat growth rate (such as Figure 4 each point marked by the squares in the figure, which are respectively the highest value to the fourth highest value of the irreversible heat growth rate); from these high-value points, select the SOC where the high values of the reversible heat growth rate and the irreversible heat growth rate overlap, that is, the reversible and irreversible heat growth rates are both relatively high (that is, both the reversible and irreversible heat growth rates rank among the top few when sorted by magnitude).

[0076] It should be noted that from a mechanism analysis, the irreversible heat of the battery shows a gradually increasing trend as its attenuation degree increases; while the reversible heat comes from the entropy change of the material, and the deterioration of the material properties generally causes an increase in entropy, so the reversible heat will also increase.

[0077] In step S1, specifically, based on the characteristic SOC of the battery system under test where capacity attenuation occurs, an offline database for reversible and irreversible heat generation analysis of the battery system under test is established through analysis, which specifically includes the following steps:

[0078] Step S14: At the characteristic SOC of the battery cycle attenuation of the battery system to be measured, use the symmetric charge-discharge method to perform reversible and irreversible heat tests on the fresh battery and the batteries with different capacity retention rates after cycling attenuation (i.e., reference batteries) (that is, perform symmetric charge-discharge tests on the fresh battery and multiple reference batteries to obtain the reversible heat and irreversible heat generated by the battery during the symmetric charge-discharge test), obtain the reversible and irreversible heat of the fresh battery and reference batteries with different capacity retention rates, and calculate the reversible and irreversible heat growth rates of multiple reference batteries with different capacity retention rates based on the reversible and irreversible heat of the fresh battery;

[0079] Among them, the reversible heat growth rate = (the reversible heat of the reference battery - the reversible heat of the fresh battery) / the reversible heat of the fresh battery;

[0080] The irreversible heat growth rate = (the irreversible heat of the reference battery - the irreversible heat of the fresh battery) / the irreversible heat of the fresh battery;

[0081] Step S15: Use the capacity retention rates of multiple reference batteries as the ordinate and the reversible and irreversible heat growth rates of multiple reference batteries with different capacity retention rates as the abscissa to make a scatter plot and perform linear fitting. The obtained fitting relationship is used as the calculation formula for obtaining the capacity retention rate of the battery to be measured based on the reversible and irreversible heat generation of the battery to be measured in the battery system to be measured, that is, the fitting relationship between the reversible and irreversible heat growth rates of the battery and the capacity retention rate of the battery system to be measured in the offline database;

[0082] Among them, the reversible and irreversible heat of the fresh battery and reference batteries with different capacity retention rates are included in the offline comparison database for battery health state analysis, that is, used as the offline database for reversible and irreversible heat generation analysis of the battery system to be measured.

[0083] That is to say, the fitting relationship expresses the relationship between the reversible and irreversible heat generation data of the battery to be measured and the capacity retention rate in the battery system to be measured;

[0084] It should be noted that for the present invention, a scatter plot is made with the capacity retention rate of the battery as the ordinate and the growth rates of the reversible and irreversible heat of the batteries with different capacity retention rates as the abscissa, and relevant fitting is performed. From theoretical analysis, the reversible heat increases linearly with the decrease of the battery capacity retention rate, and the irreversible heat also increases linearly with the decrease of the battery capacity retention rate.

[0085] In the present invention, specifically, taking the capacity retention rate of reference cells with different capacity retention rates as the ordinate and the growth rates of reversible and irreversible heat of multiple reference cells with different capacity retention rates as the abscissa, a scatter plot is made and linearly fitted. The obtained fitting relationship is y = kx + b, where x is the growth rate of reversible or irreversible heat of a cell with a different capacity retention rate (such as a reference cell or a cell to be tested), and y is the capacity retention rate of a cell with a different capacity retention rate (such as a reference cell or a cell to be tested).

[0086] Step S2: At the characteristic SOC where the capacity of the battery system to be tested obtained in step S1 decays, perform a preset reversible and irreversible heat test operation on the battery to be tested to obtain the reversible and irreversible heat of the battery to be tested;

[0087] It should be noted that the preset reversible and irreversible heat test operation in step S2, as described above, specifically includes the aforementioned steps S11 to S12. Since it has been described above, it will not be elaborated here again.

[0088] In step S2, specifically, when the battery to be tested is not at the characteristic SOC where the capacity of the battery system to be tested decays before performing the preset reversible and irreversible heat test operation, the battery is discharged to the cut-off voltage, and then a small current of a preset magnitude (such as 0.3C) is used to perform constant current charging on the battery to be tested until the state of charge (SOC) of the battery to be tested reaches the characteristic SOC where the capacity of the battery system to be tested decays;

[0089] Step S3: Calculate the growth rates of reversible and irreversible heat of the battery to be tested based on the reversible and irreversible heat of the fresh battery obtained in step S1 and the reversible and irreversible heat of the battery to be tested obtained in step S2;

[0090] It should be noted that the test values of reversible and irreversible heat of the fresh battery (non-decayed battery) at the characteristic SOC are extracted from the offline database, namely Q 可逆R and Q 不可逆R . Based on this, the growth rate R 可逆T of the reversible heat and the growth rate R 不可逆T of the irreversible heat of the battery to be tested are calculated as follows:

[0091] R 可逆T =(Q 可逆T / Q 可逆R )×100%;

[0092] R 不可逆T =(Q 不可逆T / Q 不可逆R )×100%;

[0093] where Q 可逆Tis the reversible heat generated by the battery under test during the reversible and irreversible heat test operations (specifically during the symmetric charge-discharge test process);

[0094] Q 不可逆T is the irreversible heat generated by the battery under test during the reversible and irreversible heat test operations (specifically during the symmetric charge-discharge test process).

[0095] Step S4: According to the fitting relationship between the reversible and irreversible heat growth rates and the battery capacity retention rate of the battery system under test in the offline database for reversible and irreversible heat generation analysis of the battery system under test, and the growth rates of the reversible and irreversible heats of the battery under test, calculate the capacity retention rate of the battery under test;

[0096] It should be noted that according to the fitting relationship between the reversible and irreversible heat growth rates and the battery capacity retention rate of the battery system under test in the offline database obtained in step S15, calculate the capacity retention rate of the battery under test.

[0097] It should be noted that for the present invention, it is also possible to simultaneously analyze the main factors causing battery decay by comparing the reversible and irreversible heat growth rates.

[0098] In the present invention, in step S4, the following analysis steps are further included:

[0099] Compare and analyze the growth rates of the reversible and irreversible heats of the battery under test. If the irreversible heat growth rate of the battery under test is significantly higher than the reversible heat growth rate, it is determined that the main factors causing the decay of the battery under test are the increase in battery resistance and polarization. If the reversible heat growth rate of the battery under test is significantly higher than the irreversible heat growth rate, it is determined that the main reason for the decay of the battery under test is the deterioration of the electrode active material;

[0100] Among them, the irreversible heat growth rate of the battery under test being significantly higher than the reversible heat growth rate means that the difference between the irreversible heat growth rate and the reversible heat growth rate is greater than a preset value (for example, greater than 5%);

[0101] The reversible heat growth rate of the battery under test being significantly higher than the irreversible heat growth rate means that the difference between the reversible heat growth rate and the irreversible heat growth rate is greater than a preset value (for example, greater than 5%).

[0102] It should be noted that for the present invention, the method of the present invention first analyzes and determines the characteristic SOC at which the battery performance decays according to the reversible and irreversible heat data of the battery at each SOC with different decay degrees.

[0103] For the present invention, through the acquisition of temperature data during the symmetric charge and discharge process, the reversible and irreversible heat of the battery can be calculated simultaneously. The reversible heat mainly comes from the entropy change of the positive and negative active materials inside the battery. Therefore, its change rate can reflect the attenuation degree of the electrode active materials. The irreversible heat mainly comes from the battery resistance and polarization impedance. Therefore, its change rate can reflect the growth of the battery resistance and impedance. The separate test and analysis of the reversible and irreversible heat can provide a reference for analyzing the reasons for battery attenuation.

[0104] For the present invention, by establishing an offline database of reversible and irreversible heat, it can provide a basis for analyzing the health state of the battery. At the same time, the distinction between reversible and irreversible heat can provide a reference for analyzing the reasons for battery attenuation.

[0105] Based on the above technical solutions, in order to make full use of battery temperature data for battery health state analysis, the present invention provides a method for analyzing the health state of lithium-ion batteries based on reversible and irreversible heat generation. First, through symmetric charge and discharge tests on batteries with different attenuation degrees, the reversible and irreversible heat generation data of the battery are calculated through temperature data. The characteristic state of charge SOC strongly related to battery attenuation is investigated and locked. A fitting relationship between reversible and irreversible heat and the battery capacity retention rate is established as an offline database for analyzing the health state of the battery under test. When analyzing the health state of the battery under test, the same method is used to perform symmetric charge and discharge on the battery at the characteristic SOC, calculate the reversible and irreversible heat, calculate the growth rate of reversible and irreversible heat based on the initial values in the offline database, and then calculate the capacity retention rate of the battery under test according to the fitting relationship between reversible and irreversible heat and the battery capacity retention rate in the offline database.

[0106] To more clearly understand the technical solutions of the present invention, the following specific embodiments are used to illustrate the technical solutions of the present invention.

[0107] Example 1.

[0108] A commercial 21700 lithium-ion battery with a capacity of 4.8 Ah is selected for testing. In this embodiment, an Arbin BT2000 charge and discharge test system is used to perform charge and discharge tests on the battery. An ECT-408 high and low temperature test chamber is used to provide a constant temperature environment for the battery test. The acquisition of the battery surface temperature data is completed through a Fluke 2638A data collector and a PT100 thermocouple connected to the data collector.

[0109] In this embodiment, the method for analyzing the health state of the battery based on reversible and irreversible heat provided by the present invention specifically includes the following steps:

[0110] Step S1: For the battery to be tested whose health status needs to be evaluated, a fresh battery and a battery with a capacity retention rate of 95% after cycle testing are preselected as reference batteries. Then, the preset reversible and irreversible thermal test operations are performed on the fresh battery and the reference batteries respectively.

[0111] Specifically, place the fresh battery and the reference batteries in a thermostat at 25 ± 1°C and connect them to a conventional charge and discharge device (such as the Arbin BT2000 charge and discharge test system). Fix a PT100 thermocouple on the surface of each battery to collect the surface temperature data of the battery. Seal and wrap the battery with heat-insulating materials to reduce the rapid heat exchange between the battery and the surrounding environment.

[0112] Specifically, after the battery is discharged to 2.5V, charge the battery at a current of 0.3C = 1.44A for 600s, then the battery reaches a state of charge of 5% (i.e., a state of charge to be tested). Then, perform charge and discharge within the 5% SOC range on the battery using the symmetric charge and discharge method, that is, charge at 0.3C for 600s, then let it stand for 2h, and then discharge at 0.3C for 600s, and then let it stand for 2h. At this time, the symmetric charge and discharge test of the battery at 5% SOC is completed. Extract and analyze the target data from the temperature data measured during the symmetric charge and discharge process of the battery. The surface temperature of the battery before charging is T OC , the surface temperature of the battery at the end of charging is T EC ; the surface temperature of the battery before discharging is T OD , the surface temperature of the battery at the end of discharging is T ED , then the temperature rise of the battery during charging is ΔT C = T EC - T OC , the temperature rise of the battery during discharging is ΔT D = T ED - T OD , thus the total heat generation Q C during the charging process of the battery is calculated as follows:

[0113] Q C = C p ·m·ΔT C , formula (1);

[0114] The total heat generation Q D during the discharging process of the battery is calculated as follows:

[0115] Q D = C p ·m·ΔT D , formula (2);

[0116] In the above formulas (1) and (2), Cp Cp is the specific heat of the battery, with the unit of J·g -1 ·K -1 , for this type of battery, Cp = 1.0 J·g -1 ·K -1 ;

[0117] m is the mass of the battery, with the unit of g;

[0118] ΔT C is the temperature rise of the battery during charging;

[0119] ΔT D is the temperature rise of the battery during discharging.

[0120] Therefore, the total heat generation Q of the battery during charging C is the sum of the reversible heat Q 可逆 and the irreversible heat Q 不可逆 , that is, Q C = Q 不可逆 + Q 可逆 ; and the total heat generation Q of the battery during discharging D is the difference between the reversible heat Q 可逆 and the irreversible heat Q 不可逆 , that is, Q D = Q 不可逆 - Q 可逆 , therefore, through mathematical derivation, the calculation formulas for the irreversible heat Q 不可逆 and the reversible heat Q 可逆 are as follows:

[0121] Q 不可逆 = (Q C + Q D ) / 2, formula (3);

[0122] Q 可逆 = (Q C - Q D ) / 2, formula (4);

[0123] In the above formulas (3) and (4), Q C is the total heat generation of the battery during charging;

[0124] Q D is the total heat generation of the battery during discharging.

[0125] Table 1: Reversible and irreversible heat test data of a fresh battery and a reference battery at 15% SOC;

[0126]

[0127] Thus, continue to charge the battery at a current of 0.3C = 1.44A to the state of charge of 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80% in sequence, and through the symmetric charge-discharge method test, complete the reversible and irreversible heat tests of the battery under the corresponding state of charge.

[0128] The reversible and irreversible heat of the fresh battery and the reference battery are obtained under different states of charge SOC, as Figure 2 and Figure 3 shown. Then, calculate the growth rates of the reversible and irreversible heat of the reference battery relative to the fresh battery at each SOC:

[0129] Reversible heat growth rate = (Reversible heat of the reference battery - Reversible heat of the fresh battery) / Reversible heat of the fresh battery × 100%;

[0130] Irreversible heat growth rate = (Irreversible heat of the reference battery - Irreversible heat of the fresh battery) / Irreversible heat of the fresh battery × 100%;

[0131] For example, at 15% SOC, the measured reversible heat of the fresh battery is -1.86 J, the irreversible heat is 32.52 J, while the reversible heat of the reference battery is -11.11 J, and the irreversible heat is 39.27 J. Then at 15% SOC, the reversible heat growth rate of the reference battery is (-11.11 - (-1.86)) ÷ (-1.86) × 100% = 498.2%, and the irreversible heat growth rate of the reference battery is (39.27 - 32.52) ÷ 32.52 × 100% = 20.7%. Similarly, calculate the reversible and irreversible heat growth rates at other states of charge, and plot with the reversible and irreversible heat growth rates as the vertical axis and the state of charge SOC of the battery as the horizontal axis to obtain Figure 4 .

[0132] In Figure 4 , the data points circled by the large circles are the highest, second-highest, third-highest, and fourth-highest points of the reversible heat growth rate. The data points circled by the rectangles are the highest, second-highest, third-highest, and fourth-highest points of the irreversible heat growth rate. Looking at the top 4 data points with higher values, the reversible heat and the irreversible heat overlap at 15% SOC, that is, the reversible and irreversible heat growth rates at this SOC are both relatively high, and can be used simultaneously to reflect the attenuation of the battery due to material entropy change and impedance increase.

[0133] From Figure 4 analysis, taking the principle that both the reversible and irreversible heat growth rates are relatively high, select 15% SOC as the characteristic SOC for the capacity attenuation of the battery in this system.

[0134] Furthermore, a fresh battery (with a capacity retention rate of 100%) and two reference batteries with different capacity retention rates (97.7% and 96.9%) after cycle attenuation were selected for symmetric charge-discharge tests at the characteristic SOC to obtain the reversible and irreversible heat of each battery, as shown in Table 2;

[0135] Table 2: Reversible and irreversible heat test data of fresh batteries and multiple reference batteries with different capacity retention rates;

[0136]

[0137] Taking the reversible and irreversible heat of the fresh battery as the benchmark, the growth rates of the reversible and irreversible heat of each battery with a certain capacity retention rate were calculated. Taking the growth rates of the reversible and irreversible heat of each battery with a certain capacity retention rate as the abscissa and the battery capacity retention rate as the ordinate, a scatter plot was made and linearly fitted, as Figure 5 , and the relationships were obtained as y1 = -0.0043x1 + 0.9996 and y2 = -0.1079x2 + 0.9999.

[0138] x1 is the growth rate of the reversible heat of the battery, and y1 is the battery capacity retention rate calculated based on the reversible heat; x2 is the growth rate of the irreversible heat of the battery, and y2 is the battery capacity retention rate calculated based on the irreversible heat.

[0139] Thus, an offline database for reversible and irreversible heat generation analysis of the battery system to be measured was obtained, including the reversible and irreversible heat of the fresh battery and batteries with different capacity retention rates (such as reference batteries), as well as the growth rates of the reversible and irreversible heat of batteries with different capacity retention rates (such as reference batteries).

[0140] Step S2, at the characteristic SOC where the battery system to be measured obtained in step S1 undergoes capacity attenuation, perform a preset reversible and irreversible heat test operation on the battery to be measured to obtain the reversible and irreversible heat of the battery to be measured; in this embodiment, the reversible and irreversible heat test operation was performed on the battery to be measured at 15% SOC, and the data obtained are shown in Table 2. Thus, the reversible heat of the battery to be measured was -10.23 J, and the irreversible heat was 41.70 J.

[0141] Step S3, calculate the growth rates of the reversible and irreversible heat of the battery to be measured based on the reversible and irreversible heat of the fresh battery obtained in Table 2;

[0142] Growth rate of reversible heat = (-11.42 - (-1.86)) ÷ (-1.86) × 100% = 514.8%, and the growth rate of the irreversible heat of the reference battery was (39.58 - 32.52) ÷ 32.52 × 100% = 21.7%;

[0143] Step S4: Based on the fitting relationship between the reversible and irreversible heat growth rates and the battery capacity retention rate of the battery system to be measured in the off-line database, and the reversible and irreversible heat growth rates of the battery to be measured, calculate the capacity retention rate of the battery to be measured.

[0144] Calculated based on the reversible heat growth rate: The capacity retention rate y1 of the battery = -0.0043x1 + 0.9996 = -0.0043×514.8% + 0.9996 = 97.7%;

[0145] Calculated based on the irreversible heat growth rate: y2 = -0.1079x2 + 0.9999 = -0.1079×21.7% + 0.9999 = 97.6%.

[0146] It can be concluded that the capacity retention rate of the battery to be measured is between 97.6% and 97.7%. This indicates that from the aspects of the entropy change of the positive and negative electrode materials and the growth of battery impedance and polarization, the capacity attenuation law of the battery to be measured conforms to the cycle attenuation law of the batteries in this system. And from the comparative analysis of the reversible and irreversible heat growth rates of the battery to be measured, its reversible heat growth rate is 514.8%, while the irreversible heat growth rate is 21.7%. The reversible heat growth rate is significantly higher than the irreversible heat growth rate. Therefore, it is analyzed that the main factor causing the capacity attenuation of the battery to be measured is the relatively high entropy change degree of its positive and negative electrode materials. Therefore, when improving the performance of the batteries in this system, materials with better entropy change stability in the electrochemical reaction should be considered.

[0147] In summary, compared with the prior art, a method for analyzing the battery health state based on reversible and irreversible heat provided by the present invention is scientifically designed. By performing symmetric charge and discharge tests on the battery, making full use of the battery temperature data collected during the test, deducing and calculating the reversible and irreversible heat, and comparing and analyzing with the off-line database, the health state of the battery (i.e., reflected as the capacity retention rate) can be obtained, which has great practical significance.

[0148] In addition, for the present invention, in view of the different generation mechanisms of reversible and irreversible heat, by distinguishing and testing the two, it can assist in analyzing the reasons for battery attenuation.

[0149] In addition, since the method provided by the present invention is limited to testing and analyzing at the characteristic SOC, the testing efficiency can be greatly improved, and it is easy to be implemented online in the battery management system.

[0150] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A method for the health state of a battery based on reversible and irreversible thermal analysis, characterized in that, The method includes the following steps: Step S1: For the battery under test whose health status needs to be evaluated, a fresh battery and one or more reference batteries are preselected. Then, preset reversible and irreversible heat test operations are respectively performed on the fresh battery and each reference battery to obtain the reversible and irreversible heat of the fresh battery and each reference battery. Then, based on the reversible and irreversible heat of the fresh battery and each reference battery, the characteristic state of charge (SOC) at which the capacity decay occurs in the battery system under test is analyzed, and based on the characteristic SOC of the battery system under test at which the capacity decay occurs, an off-line database for reversible and irreversible heat generation analysis of the battery system under test is established by analysis. The fresh battery is a battery that belongs to the same battery system as the battery under test and has not experienced capacity decay. The multiple reference batteries are multiple batteries that belong to the same battery system as the lithium-ion battery under test, have experienced capacity decay after cycling, and have different capacity retention rates. In step S1, the analysis of obtaining the characteristic SOC at which the capacity decay occurs in the battery system under test includes step S13: Step S13: Compare and analyze the reversible and irreversible heat of the fresh battery and a reference battery at multiple different states of charge (SOC), determine the SOC at which the reversible and irreversible heat shows a regular change with the decay of battery performance, and then use this SOC as the characteristic SOC at which the capacity decay occurs in the battery system under test. The step S13 specifically includes the following operations: Step S131: Based on the reversible and irreversible heat of the fresh battery, combined with the reversible and irreversible heat of a reference battery, calculate the growth rate of the reversible and irreversible heat of the reference battery. Step S132: Use the growth rate of the reversible and irreversible heat of a reference battery as the ordinate and the multiple different states of charge (SOC) of the reference battery as the abscissa to plot a graph, obtain the first curve, analyze the first curve, and select the SOC with relatively high growth rates of both reversible and irreversible heat as the characteristic SOC of the battery cycle decay of the battery system under test. The SOC with relatively high growth rates of both reversible and irreversible heat means that among the growth rates of reversible heat corresponding to multiple different states of charge (SOC) of a reference battery, the numerical value of the growth rate of reversible heat at this SOC ranks among the top several in the preset order, and among the growth rates of irreversible heat corresponding to multiple different states of charge (SOC) of a reference battery, the numerical value of the growth rate of irreversible heat at this SOC ranks among the top several in the preset order. Step S2: At the characteristic SOC at which the capacity decay occurs in the battery system under test obtained in step S1, perform preset reversible and irreversible heat test operations on the battery under test to obtain the reversible and irreversible heat of the battery under test. Step S3: Based on the reversible and irreversible heat of the fresh battery obtained in step S1 and the reversible and irreversible heat of the battery under test obtained in step S2, calculate the growth rate of the reversible and irreversible heat of the battery under test. Step S4: Calculate the capacity retention rate of the battery under test based on the fitting relationship between the reversible and irreversible heat growth rates and the capacity retention rate of the battery in the offline database for the battery system under test, as well as the growth rates of the reversible and irreversible heat of the battery under test.

2. The method for battery state of health based on reversible and irreversible thermal analysis according to claim 1, wherein In step S1, the preset reversible and irreversible heat test operations specifically include the following steps S11 to S12: Step S11: Conduct a symmetric charge-discharge test on the battery to obtain the battery surface temperature before charging, the battery surface temperature at the end of charging, the battery surface temperature before discharging, and the battery surface temperature at the end of discharging. Step S12: Calculate the reversible heat and irreversible heat generated by the battery during the symmetric charge-discharge test based on the battery surface temperature before charging, the battery surface temperature at the end of charging, the battery surface temperature before discharging, and the battery surface temperature at the end of discharging obtained in step S11.

3. The method for battery state of health based on reversible and irreversible thermal analysis according to claim 2, wherein, In step S11, the symmetric charge-discharge test on the battery specifically includes the following operations: First, charge the battery at a constant current of a preset magnitude for a preset charging duration. Then, discharge the battery at a constant current of a preset magnitude for a preset discharging duration. Among them, the magnitudes of the charging current and the discharging current are equal, both equal to current I. The magnitudes of the preset charging duration and the preset discharging duration are equal, both equal to duration t.

4. The method for battery state of health based on reversible and irreversible thermal analysis according to claim 3, wherein The value range of current I is 0.25C to 1C. During the symmetric charge-discharge test, the total charging charge and the total discharging charge of the battery are equal. During the symmetric charge-discharge test, the total charging charge and the total discharging charge of the battery are both equal to 1% - 10% of the SOC of the battery.

5. The method for battery state of health based on reversible and irreversible thermal analysis according to claim 2, characterized in that, In step S12, the irreversible heat Q 不可逆 and the reversible heat Q 可逆 are calculated by the following formulas: Q 不可逆 = (Q C + Q D ) / 2; Q 可逆 = (Q C - Q D ) / 2; In the above formula, Q C is the total heat generation of the battery during charging; Q D is the total heat generation of the battery during discharge; Among them, Q C The calculation formula is as follows: Q C = C p ·m·ΔT C ; Q D The calculation formula is as follows: Q D = C p ·m·ΔT D ; In the above formula, C p is the specific heat of the battery, with the unit of J·g -1 ·K -1 ; m is the mass of the battery, with the unit of g. ΔT C is the temperature rise of the battery during charging, which is equal to the difference between the battery surface temperature at the end of charging and the battery surface temperature before charging; ΔT D is the temperature rise of the battery during discharge, which is equal to the difference between the battery surface temperature at the end of discharge and the battery surface temperature before discharge.

6. The method for battery state of health based on reversible and irreversible thermal analysis according to claim 1, wherein In step S131, the specific calculation formulas for the growth rates of the reversible and irreversible heat of the reference battery are as follows: Reversible heat growth rate = (Reversible heat of the reference battery - Reversible heat of the fresh battery) / Reversible heat of the fresh battery Irreversible heat growth rate = (Irreversible heat of the reference battery - Irreversible heat of the fresh battery) / Irreversible heat of the fresh battery 7. The method for battery state of health based on reversible and irreversible thermal analysis according to claim 1, wherein In step S1, based on the characteristic SOC at which capacity decay occurs in the battery system under test, analyze and establish an offline database for reversible and irreversible heat generation analysis of the battery system under test, which specifically includes the following steps: Step S14: Conduct symmetric charge-discharge tests on the fresh battery and multiple reference batteries at the characteristic SOC of battery cycle decay in the battery system under test to obtain the reversible heat and irreversible heat generated by the fresh battery and multiple reference batteries with different capacity retention rates during the symmetric charge-discharge test. Based on the reversible and irreversible heat of the fresh battery, calculate the reversible and irreversible heat growth rates of multiple reference batteries with different capacity retention rates. In step S15, a scatter plot is made with the capacity retention rate of multiple reference cells as the ordinate and the reversible and irreversible heat growth rates of the reference cells with different capacity retention rates as the abscissa, and linear fitting is performed. The obtained fitting relationship is used as the fitting relationship between the reversible and irreversible heat growth rates and the capacity retention rate of the battery system to be measured in the offline database.

8. The method for battery state of health based on reversible and irreversible thermal analysis according to claim 7, wherein The specific fitting relationship is as follows: y = kx + b, where x is the growth rate of the reversible or irreversible heat of the battery with different capacity retention rates; y is the capacity retention rate of the battery with different capacity retention rates.

9. The method for battery state of health based on reversible and irreversible thermal analysis according to claim 1, wherein In step S2, when the battery to be measured does not have the characteristic SOC of capacity attenuation in the battery system to be measured before performing the preset reversible and irreversible heat test operations, the battery is discharged to the cut-off voltage, and then a small current of a preset magnitude is used to charge the battery to be measured at a constant current until the state of charge of the battery to be measured reaches the characteristic SOC of capacity attenuation in the battery system to be measured.

10. The method for battery state of health based on reversible and irreversible thermal analysis according to claim 1, wherein In step S4, the following analysis steps are also included: Comparatively analyze the growth rates of the reversible and irreversible heat of the battery to be measured. If the irreversible heat growth rate of the battery to be measured is significantly higher than the reversible heat growth rate, it is determined that the main factor causing the attenuation of the battery to be measured is the increase in battery resistance and polarization. If the reversible heat growth rate of the battery to be measured is significantly higher than the irreversible heat growth rate, it is determined that the main reason for the attenuation of the battery to be measured is the deterioration of the electrode active material; Among them, the irreversible heat growth rate of the battery to be measured being significantly higher than the reversible heat growth rate means that the difference between the irreversible heat growth rate and the reversible heat growth rate is greater than a preset value; The reversible heat growth rate of the battery to be measured being significantly higher than the irreversible heat growth rate means that the difference between the reversible heat growth rate and the irreversible heat growth rate is greater than a preset value.

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