阅读说明：本技术 一种锂离子电池充电产热来源的估算方法 (Estimation method for charging heat production source of lithium ion battery ) 是由 甘小燕 孙昌业 王安 余俊锋 于 2020-06-09 设计创作，主要内容包括：本发明公开了一种锂离子电池充电产热来源的估算方法,在一定温度和倍率条件下,对电池进行恒流恒压充电测试、混合动力脉冲能力特性HPPC充电测试以及电动势温度变化系数dE/dT的测试；基于HPPC充电测试结果进行参数辨识,并将辨识结果同电动势温度变化系数dE/dT一起与SOC拟合；将各参数拟合结果输入在COMSOL中建立的电池有限元模型中,进行仿真验证,根据仿真结果可绘出各热源产生的热功率变化图,对各热源进行时间积分可得各热源在整个充电过程中产生的热量及其占比。(The invention discloses a method for estimating a charging heat production source of a lithium ion battery, which comprises the steps of carrying out a constant-current constant-voltage charging test, a hybrid power pulse capability characteristic HPPC charging test and an electromotive force temperature change coefficient dE/dT test on the battery under the conditions of certain temperature and multiplying power; performing parameter identification based on the HPPC charging test result, and fitting the identification result and the electromotive force temperature change coefficient dE/dT together with the SOC; and inputting the fitting result of each parameter into a finite element model of the battery established in COMSOL, performing simulation verification, drawing a thermal power change diagram generated by each heat source according to the simulation result, and performing time integration on each heat source to obtain the heat generated by each heat source in the whole charging process and the ratio of the heat.)

1. A method for estimating a heat source generated by charging a lithium ion battery is characterized by comprising the following steps:

1) under the conditions of certain temperature and multiplying power, performing a constant-current and constant-voltage charging test, a hybrid power pulse capability characteristic HPPC charging test and an electromotive force temperature change coefficient dE/dT test on the battery;

2) data obtained by constant-current constant-voltage charging test and hybrid power pulse capability characteristic HPPC charging test are collated, ohmic internal resistance and polarization internal resistance parameters are identified, and ohmic internal resistance R is used₀Internal polarization resistance R_p1And R_p2Calculating the identification result and the electromotive force temperature change coefficient dE/dT to obtain the heat generation power of the battery in the charging process;

wherein q is the thermal power of the battery in unit volume, V is the battery volume, I is the charge-discharge current of the battery, and T is the battery temperature;

3) establishing a battery geometric model according to the geometric parameters of the battery in the COMSOL software, and converting the ohmic internal resistance R into the ohmic internal resistance R₀And polarization internal resistance R_p1And R_p2Fitting the identification result and the SOC of the battery, and inputting the fitting result into a COMSOL model to serve as sources of ohmic heat and polarization heat of the battery; fitting the temperature change coefficient dE/dT of the measured electromotive force with the SOC, and inputting the fitted electromotive force temperature change coefficient dE/dT and SOC into a COMSOL model to serve as a battery reaction heat source; drawing the change of the thermal power generated by each heat source along with time in the COMSOL, determining the change rule of each heat source, and calculating the total heat generation amount of each heat source in the charging process by using a time integral operator timeint (t1, t2, expr);

4) the heat exchange between the battery surface and the environment adopts a third type boundary condition, the thermal power q obtained by calculation in a heat generation power equation is substituted into a heat conduction equation for transient calculation, the actual measured temperature value of the battery surface is compared with the temperature simulation result at the same position of the battery, and the simulation accuracy is verified;

wherein the transfer process of the heat generated by the battery is described by a three-dimensional unsteady heat conduction differential equation:

in the formula, ρ represents a cell density; c_pRepresents the specific heat capacity of the battery; lambda [ alpha ]_r、

2. The method for estimating the heat source generated by charging the lithium ion battery according to claim 1, wherein the battery to be measured in the step 1 needs to be kept still in a normal temperature environment to reach heat balance, then standard constant current discharge is carried out to cut-off voltage with the environmental temperature kept unchanged, and then the battery is kept still for 1 hour.

3. The method for estimating the heat source generated by charging the lithium ion battery according to claim 1, wherein the steps of the constant-current constant-voltage charging test, the hybrid power pulse capability characteristic HPPC charging test and the electromotive force temperature change coefficient dE/dT test in step 1 comprise the following steps:

1) attaching a thermocouple to the center of the surface of the battery, connecting the battery to be tested with a clamp of a battery charging and discharging system, and then placing the battery in a thermostat;

2) closing the thermostat door, connecting the circuit, and editing the testing step;

3) and setting parameters of the thermostat, and starting a charge and discharge test system to start testing after the temperature is stable.

4. The method for estimating the heat source generated by charging the lithium ion battery according to claim 1, wherein in the step 2, ohmic internal resistance and polarization internal resistance parameters of the battery are identified based on a second-order Thevenin equivalent circuit model.

5. The method for estimating heat source generated during charging of lithium ion battery according to claim 1, wherein the specific heat capacity C of the battery in step 4_pAnd coefficient of thermal conductivity lambda_x、λ_y、λ_zThe specific heat capacity and the heat conductivity coefficient of the anode and cathode materials, the electrolyte, the diaphragm and the anode and cathode current collector materials in the battery are respectively obtained by weighted average.

Technical Field

The invention belongs to the technical field of lithium ion batteries, and particularly relates to a method for estimating a charging heat production source of a lithium ion battery.

Background

The environmental pollution problems such as greenhouse effect, haze, acid rain, ozone layer holes and the like caused by the exhaust emission of the traditional fuel automobile directly or indirectly have seriously influenced the living environment of human beings, and the new energy automobile can replace the traditional fuel automobile to better solve the problems of energy exhaustion and environmental pollution. In the field of new energy automobiles, lithium ion batteries have been widely used due to their advantages of high specific energy, long cycle life, etc., but the problem of exposure to safety has also attracted widespread social attention. Statistics show that thermal runaway accidents of electric automobiles in the charging process account for a high proportion of total accidents.

The safety of the lithium battery is one of the important performances of the lithium battery, the heat generation rule of the lithium battery under different charging conditions and the change of the heat power generated by each heat generation source are determined through experiments and simulation, the design of a battery thermal management system can be effectively guided, the safety of the battery is improved, and a scientific theoretical basis is provided for the efficient and safe operation of the battery on an electric automobile.

At present, experiments and simulation researches are carried out on the heat generation rule of the lithium ion battery in the discharging process through experiments and various models at home and abroad, but the research on the heat generation rule and the change of each heat source in the charging process is less.

Disclosure of Invention

The invention aims to provide an estimation method of a lithium ion battery charging heat production source, which is used for analyzing the change of the sizes of heat sources and the heat production rule in the lithium ion battery charging process by calculating the heat production power of a battery on the basis of the parameter identification result of the lithium ion battery charging test.

In order to achieve the purpose, the technical scheme is as follows:

a method for estimating a heat source generated by charging a lithium ion battery comprises the following steps:

1) under the conditions of certain temperature and multiplying power, performing a constant-current and constant-voltage charging test, a hybrid power pulse capability characteristic HPPC charging test and an electromotive force temperature change coefficient dE/dT test on the battery;

2) data obtained by constant-current constant-voltage charging test and hybrid power pulse capability characteristic HPPC charging test are collated, ohmic internal resistance and polarization internal resistance parameters are identified, and ohmic internal resistance R is used₀In polarization ofResistance R_p1And R_p2Calculating the identification result and the electromotive force temperature change coefficient dE/dT to obtain the heat generation power of the battery in the charging process;

wherein q is the thermal power of the battery in unit volume, V is the battery volume, I is the charge-discharge current of the battery, and T is the battery temperature;

3) establishing a battery geometric model according to the geometric parameters of the battery in the COMSOL software, and converting the ohmic internal resistance R into the ohmic internal resistance R₀And polarization internal resistance R_p1And R_p2Fitting the identification result and the SOC of the battery, and inputting the fitting result into a COMSOL model to serve as sources of ohmic heat and polarization heat of the battery; fitting the temperature change coefficient dE/dT of the measured electromotive force with the SOC, and inputting the fitted electromotive force temperature change coefficient dE/dT and SOC into a COMSOL model to serve as a battery reaction heat source; drawing the change of the thermal power generated by each heat source along with time in the COMSOL, determining the change rule of each heat source, and calculating the total heat generation amount of each heat source in the charging process by using a time integral operator timeint (t1, t2, expr);

4) the heat exchange between the battery surface and the environment adopts a third type boundary condition, the thermal power q obtained by calculation in a heat generation power equation is substituted into a heat conduction equation for transient calculation, the actual measured temperature value of the battery surface is compared with the temperature simulation result at the same position of the battery, and the simulation accuracy is verified;

wherein the transfer process of the heat generated by the battery is described by a three-dimensional unsteady heat conduction differential equation:

in the formula, ρ represents a cell density; c_pRepresents the specific heat capacity of the battery; lambda [ alpha ]_r、And λ_zRepresenting the radial, circumferential and axial thermal conductivity in the battery active material, respectively.

According to the scheme, the battery to be tested in the step 1 needs to stand in a normal-temperature environment to reach thermal balance, then standard constant-current discharge is carried out to cut-off voltage by keeping the environmental temperature unchanged, and then standing is carried out for 1 h.

According to the scheme, the constant-current constant-voltage charging test, the hybrid power pulse capability characteristic HPPC charging test and the test of the electromotive force temperature change coefficient dE/dT in the step 1 comprise the following steps:

1) attaching a thermocouple to the center of the surface of the battery, connecting the battery to be tested with a clamp of a battery charging and discharging system, and then placing the battery in a thermostat;

2) closing the thermostat door, connecting the circuit, and editing the testing step;

3) and setting parameters of the thermostat, and starting a charge and discharge test system to start testing after the temperature is stable.

According to the scheme, in the step 2, the parameters of the ohmic resistance and the polarization internal resistance of the battery are identified based on a second-order Thevenin equivalent circuit model.

According to the scheme, the specific heat capacity C of the battery in the step 4_pAnd coefficient of thermal conductivity lambda_x、λ_y、λ_zThe specific heat capacity and the heat conductivity coefficient of the anode and cathode materials, the electrolyte, the diaphragm and the anode and cathode current collector materials in the battery are respectively obtained by weighted average.

Compared with the prior art, the invention has the beneficial effects that:

the change of the thermal power generated by each heat source along with time is drawn in the COMSOL, the change rule of each heat source can be determined, and the total heat generation amount of each heat source in the charging process can be calculated by using a time integral operator timeint (t1, t2, expr), so that the contribution of each heat generation factor to the total heat generation is determined.

The heat power of each heat source of the battery and the contribution of the heat power to the total heat production in the charging process of the lithium ion battery are estimated in a mode of combining experiments and simulation analysis.

The battery heat generation rule and the change of the battery heat generation factors have very important significance to the battery thermal management system; therefore, the invention has certain application prospect in the battery thermal management field.

Drawings

FIG. 1: a comparison graph of the battery temperature measured by the thermocouple during charging at 0.5C multiplying power at 20 ℃ and a simulation result;

FIG. 2: a comparison graph of the battery temperature measured by the thermocouple during 1.0C multiplying power charging at 30 ℃ and a simulation result;

FIG. 3: a curve graph of the heat generation power of each heat source and the total heat generation power of 0.5 ℃ at 20 ℃ along with time;

FIG. 4: the heat generation power of each heat source and the total heat generation power of 1.0C at 30 ℃ are shown in a time-dependent curve.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the embodiments of the present invention, and it is obvious that the described embodiments are some embodiments of the present invention, but not all embodiments.

Taking a 18650 ternary lithium ion battery charging process at a temperature of 20 ℃ and a rate of 0.5C as an example, a method for estimating a heat source generated by charging a lithium ion battery is introduced, and the method comprises the following steps:

first, experiment platform

Based on thermostated container and battery charge-discharge system: the constant temperature box comprises a temperature probe, a refrigeration compressor, an air heater and a box body with a door, and can keep the temperature in the constant temperature box constant. The battery charging and discharging test system can charge the battery according to a specified charging mode, and can record parameters such as terminal voltage, current and battery surface temperature in the charging and discharging process. A thermocouple for measuring the temperature of the battery was attached to the center of the surface of the battery with an adhesive tape. The experimental equipment is schematically connected as shown in FIG. 1.

Second, Experimental methods

Before the test starts, whether the circuit is correctly connected with each experimental device or not needs to be checked, and the working steps of the charge and discharge test need to be edited in a desktop computer.

1. Constant current and constant voltage charging test

Firstly, the battery is placed in a normal temperature environment for 2.5 hours, so that the battery and the environment reach thermal balance. And then, standard constant current discharge is carried out at the ambient temperature until the cut-off voltage is reached, namely, the battery is emptied and then stands for 1h, wherein the standard constant current discharge multiplying power is 0.2C, and the discharge cut-off voltage is 2.0V. And then placing the battery in an incubator, setting the temperature of the incubator to be 20 ℃, and after the temperature of the incubator is stable, similarly, standing the battery in the incubator for 2.5 hours to ensure that the battery and the environment reach thermal equilibrium. And starting a battery charging and discharging test system, performing constant-current and constant-voltage charging on the battery by using the charging current with the rate of 0.5C after the system reads the test step, recording the voltage of the battery end, the magnitude of the charging current and the change of the temperature parameter of the battery surface along with time by using a computer in the charging process, wherein the cut-off voltage of the constant-current charging is 4.2V, and the cut-off current of the constant-voltage charging is 0.13A.

2. Hybrid power pulse capability characteristic HPPC charging test

The preparation work of the HPPC charging test of 0.5C multiplying power hybrid power pulse capability characteristic at 20 ℃ is the same as that of the constant-current constant-voltage charging test, namely, the HPPC charging test is stood for 2.5 hours in a normal-temperature environment to ensure that the battery and the environment reach thermal balance, and then the battery capacity is emptied and stood for 1 hour by standard constant-current discharge. Then, the battery is placed in a thermostat, the temperature of the thermostat is set to be 20 ℃, after the temperature of the thermostat is stable, similarly, after the battery stands still in the thermostat for 2.5 hours to enable the battery and the environment to reach thermal balance, a battery charging and discharging test system is started to carry out HPPC charging test on the battery according to the hybrid power pulse capability characteristics, and the specific steps comprise:

1) charging for 10s at constant current with 0.5C multiplying power;

2) after the constant current charging is finished for 10s, the battery is placed for 1 min;

3) after the laying aside is finished, constant current discharge is carried out for 10s at 0.5C multiplying power;

4) after the constant current discharge is finished for 10s, the battery is placed for 1 min;

5) charging the battery at 0.5C multiplying power to a certain SOC point, and if the charging voltage reaches a cut-off voltage of 4.2V, performing constant voltage charging at 0.5C;

6) standing for 15 min;

the process steps 1 to 6 are circulated, and the circulation frequency is set to 10 times;

and the cycle is finished and the test is finished.

3. Temperature coefficient of variation of electromotive force dE/dT test

The preparation work before the electromotive force temperature change coefficient dE/dT test is the same as the above, after the battery stands still in the incubator for 2.5h to enable the battery and the environment to reach thermal balance, the battery charge-discharge test system is started to test the electromotive force temperature change coefficient dE/dT for the battery, and the specific steps comprise: charging the battery with 0.5C constant rate current, standing the battery for 5h at 20 ℃ ambient temperature after charging to a certain SOC point, simultaneously measuring the open-circuit voltage of the battery, reducing the ambient temperature by 10 ℃ after standing, standing for 5h, simultaneously measuring the open-circuit voltage of the battery, and obtaining the electromotive force temperature change coefficient at the SOC point by utilizing dE/dT. The above process is repeated, and the electromotive force temperature change coefficient of the battery at different SOC values is measured.

Third, parameter identification and simulation research

And (3) arranging data obtained by a 0.5C multiplying power constant current and constant voltage charging test and a hybrid power pulse capability characteristic HPPC charging test at 20 ℃. And identifying parameters such as battery terminal voltage, ohmic internal resistance, polarization internal resistance and the like based on a second-order equivalent circuit model. And simultaneously, establishing a battery geometric model in COMSOL software according to the geometric parameters of the diameter, the length, the mandrel radius and the shell thickness of the lithium ion battery in COMSOL software. Will ohm the internal resistance R₀And polarization internal resistance R_p1And R_p2The recognition result is fitted with the SOC of the battery and then input into a COMSOL model to serve as a source of ohmic heat and polarization heat of the battery. Similarly, the measured electromotive force temperature change coefficient dE/dT and SOC are fitted and input into a COMSOL model to serve as a battery reaction heat source. At this time, the heat generation power of the battery charging process can be expressed as:

wherein q is the thermal power of the battery in unit volume, V is the battery volume, I is the charge-discharge current of the battery, and T is the battery temperature. In addition, the specific heat capacity, density and heat conductivity of the battery active material, the mandrel and the shell material need to be input into the model. Wherein the specific heat capacity, the density and the heat conductivity coefficient of the active material area are obtained by adopting weighted average calculation, and the calculation formulas are respectively as follows:

wherein the specific heat capacity of each part of the material is C_piThe thickness of different layers in the single cell is L_i。

Wherein the specific heat capacity of each part is rho_iThe thickness of different layers in the single cell is L_i. Since the temperature change of the entire battery is not large in the experiment, it is assumed here that the thermal conductivity of the battery internal material is not affected by the temperature change and is isotropic. The heat conductivity of the battery along the axial direction and the circumferential direction can be regarded as the parallel connection of the heat conductivity of different materials according to the principle of series-parallel connection heat resistance in the heat transfer science. Similarly, the heat flow of the cell in the radial direction can be viewed as a series of different materials in thermal conductivity.

Wherein the thickness of different layers in the single cell is L_i(ii) a The thermal conductivity coefficient of different layer materials is K_i. Setting the boundary condition of battery heat transfer as the third kind of boundary condition, that is, setting the heat transfer coefficient of the battery surface and the temperature of the outside air at any time, wherein the heat transfer coefficient is 18W/(m) considering the influence of the ambient temperature of the battery and the wind speed in the incubator²K) The temperature of the outside air is the temperature set by the incubator. Cell density rho, specific heat capacity C_pAnd the thermal conductivity lambda of the active material in the cell interior in the radial, circumferential and axial directions_r、

λ_zAfter the setting is completed in the modeling process, the COMSOL software can perform transient solution on the change of the internal temperature of the battery along with the time based on a built-in heat conduction differential equation.

And the value of the thermal power q of the unit volume battery in the thermal conductivity differential equation is the result obtained by the thermal power calculation formula. The transient solving result comprises the temperature change of each point in the battery, and the temperature change at the position of the thermocouple in the simulation process can be compared with the temperature change measured in the thermocouple experiment process.

To illustrate the applicability of the method, the results of two different temperature different rate constant current and constant voltage charging experiments at 0.5C at 20 ℃ and 1.0C at 30 ℃ are shown.

Fig. 1 and fig. 2 are graphs comparing the battery temperature measured by the thermocouple attached to the center of the battery surface in the above two sets of experimental tests with simulation results. As can be seen, the surface temperature of the battery during charging has three peaks as the charging progresses. Meanwhile, when the battery is charged at 0.5C rate at 20 ℃, the temperature measured at the center of the surface of the battery is increased from 293.95K to 295.55K at the maximum, and when the battery is charged at 1.0C rate at 30 ℃, the temperature measured at the center of the surface of the battery is increased from 303.75K to 305.35K at the maximum, and the simulation results of the two groups of experiments are basically consistent with the change of the temperature measured by the experiments along with the time, and the maximum error is not more than 0.4K. Because conditions are simplified during establishing the simulation model, the simulation result has certain errors, but the overall errors are small, and the effectiveness of the thermal model of the battery is demonstrated.

Based on the above research results, the graphs plotting the heat generation power of each heat source and the total heat generation power of the battery over time in the two sets of experiments are shown in fig. 3 and 4, and it can be seen from the graphs that the heat power generated by ohmic heat and polarized heat is positive, and the overall trend is gradually reduced along with the progress of charging. The heat power generated by ohmic internal resistance is higher than that generated by concentration polarization internal resistance and electrochemical polarization internal resistance. The thermal power generated by the reaction heat is negative, namely the reaction heat is absorbed in the charging process, and the thermal power generated by the reaction heat fluctuates up and down in the charging process, so that the fluctuation of the total thermal power of the battery in the charging process at 0.5C multiplying power at 20 ℃ and 1.0C multiplying power at 30 ℃ is greatly influenced.

In order to clarify the contribution of the thermal power generated by each heat source to the heat generation during the battery charging process, the thermal power generated by each heat source was integrated over the time period t1 to t2 by using the timeint (t1, t2, expr) operator in the COMSOL simulation results, and the integration results are shown in table 1. As can be seen from the table, the total heat generated by the ohmic internal resistance of the battery in the 0.5C rate charging process at 20 ℃ is 234.09J at most, and the proportion reaches 71.18%; the total heat generated by the electrochemical polarization and the concentration polarization internal resistance is 59.34J and 127.02J respectively, and the occupation ratio is 18.04 percent and 38.62 percent respectively; the total heat quantity generated by the reaction is-91.58J, and the proportion is-27.85%. The total heat generated by ohmic internal resistance is the most in the 1.0C multiplying power charging process at 30 ℃, which reaches 378.93J, and the proportion is 76.03%; the total heat generated by the electrochemical polarization and the concentration polarization internal resistance is 56.52J and 193.05J respectively, and the occupation ratio is 11.34 percent and 38.73 percent respectively; the total heat quantity generated by the reaction is-130.10J, and the proportion is-26.10%.

TABLE 1

The above-mentioned embodiments are only for describing the preferred embodiments of the present invention, and do not limit the scope of the present invention, and those skilled in the art should make various changes and modifications to the technical solution of the present invention without departing from the spirit of the present invention, and all such changes and modifications should fall within the protection scope defined by the claims of the present invention.