Lithium ion battery thermal characteristic parameter in-situ test method based on isothermal calorimetry

Abstract

The invention discloses a lithium ion battery thermal characteristic parameter in-situ test method based on isothermal calorimetry. The invention aims to solve the problems of discrete measurement points, long consumed time and the like caused by offline measurement in the existing battery thermal characteristic parameter testing method. The method comprises the following steps: firstly, building a battery isothermal heat measuring device, and providing a thermal characteristic parameter in-situ test method according to an equivalent heat transfer model; secondly, carrying out a charge-discharge experiment on the battery by using a small-rate current, and successively superposing cosine modulation powers with different frequencies in the flexible heating sheets on the upper and lower sides to realize measurement of modulation heat flow in the charge-discharge process; and finally, performing fast Fourier transform, complex plane solution, model parameter identification and other means on the response voltage signal and the excitation current signal under the multi-frequency cosine modulation power to obtain thermal characteristic parameters such as thermal capacity and heat conductivity coefficient of the battery under the whole charge state distribution. According to the invention, the synchronous in-situ measurement of the charge-discharge heat production power, the heat capacity and the heat conductivity coefficient along with the change of the charge-discharge depth of the battery is realized.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery thermal management technology, and more specifically relates to an in-situ testing method for thermal characteristic parameters of lithium-ion batteries based on isothermal calorimetry. Background Technology

[0002] Lithium-ion batteries are widely used in consumer electronics, new energy electric vehicles, and aerospace due to their advantages such as high energy density, long cycle life, low self-discharge rate, good stability, and no memory effect. [1-2] In practical applications, operating temperature is a key factor affecting the lifespan, performance, and safety of lithium-ion batteries. High battery temperatures pose a risk of thermal runaway, while low temperatures reduce the battery's energy and power density. [3-4] To ensure battery safety and optimal performance, a well-designed battery thermal management system is crucial.

[0003] Heat capacity and thermal conductivity are key thermal characteristic parameters of battery thermal management systems. Existing technologies primarily employ mass-weighted summation, thermal impedance spectroscopy, and electrothermal impedance spectroscopy to measure battery heat capacity. [5-7] The mass-weighted sum method calculates the battery's specific heat capacity by summing the weighted values ​​of each component. However, this method requires disassembling the battery components in specialized equipment such as a glove box to obtain detailed information, making it impractical in real-world scenarios. Thermoelectric impedance spectroscopy (TIGS) and electrothermal impedance spectroscopy (ETIS) methods apply current pulses or low-frequency sinusoidal thermal excitation signals of different frequencies and measure the battery's surface temperature, respectively, fitting the battery's specific heat capacity from the impedance spectrum. However, recording the battery's thermal response is a very time-consuming process, and uncertainties are introduced because the battery dissipates heat to the surrounding environment during testing.

[0004] The main methods for measuring the thermal conductivity of batteries include the component weighted method, the steady-state method, and the unsteady-state method. [8-10] The component-weighted method calculates thermal conductivity based on the relevant parameters of each component. The steady-state method, based on Fourier's law, establishes a time-invariant temperature field within the battery under test, allowing heat to conduct in a one-dimensional manner. The temperature gradient and heat flow per unit area are then measured to determine the battery's thermal conductivity. However, establishing a stable temperature gradient using this method is difficult and time-consuming. The non-steady-state method typically operates in an adiabatic environment, utilizing transient laser methods to measure the battery's thermal conductivity. This method requires high-precision and high-sensitivity sensors and cannot perform in-situ measurements on the battery.

[0005] Existing research indicates that battery heat capacity and thermal conductivity change with the state of charge.

[11] The above measurement methods generally use offline measurement, and the measurement results are only discrete values ​​under a specific state of charge. Furthermore, the two parameters are usually measured using different methods and testing equipment, which inevitably increases the time and economic costs.

[0006] Meanwhile, isothermal calorimetry, as a technique for quantifying the heat generation characteristics of batteries, is often used to measure the heat generation characteristic parameters of batteries during charging and discharging at specific temperatures.

[12] The isothermal calorimeter based on the power compensation principle uses an external circulation system to control the battery to a temperature below the target temperature while maintaining a constant power output from the external circulator. Simultaneously, it uses electric heating to heat the battery to the target temperature. If the battery absorbs or releases heat during charging or discharging, feedback control increases or decreases the heating element power to maintain a constant battery temperature.

[13] Transient thermal characteristic parameters can provide accurate data support for in-depth research on the heat generation mechanism of lithium-ion batteries.

[0007] In short, determining the transient heat capacity and thermal conductivity during battery charging and discharging is crucial for battery thermal characteristic research and thermal management system design. To accurately obtain the thermal characteristic parameters during battery charging and discharging, this invention, based on practical considerations, proposes an in-situ testing method for lithium-ion battery thermal characteristic parameters using isothermal calorimetry. This method significantly improves the testing efficiency of thermal characteristic parameters, enabling simultaneous measurement of heat generation during charging and discharging, as well as heat capacity and thermal resistance, greatly enriching the application of in-situ thermal parameter measurement in battery thermal management system design.

[0008] References

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[0022]

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[0024]

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[0025] 2020, 37(03): 103-106. Summary of the Invention

[0026] To address the shortcomings of existing battery thermal capacity and thermal conductivity testing methods mentioned in the background section, such as offline measurement, discrete measurement points, and long testing times, which lead to deficiencies in thermal characteristic research and thermal management system design, this invention designs an in-situ testing method for lithium-ion battery thermal characteristic parameters based on isothermal calorimetry.

[0027] The main technical concept of this invention:

[0028] First, an isothermal calorimeter for batteries was built. Based on the equivalent heat transfer model, an in-situ test and calculation method for thermal characteristic parameters was proposed.

[0029] Secondly, a charge-discharge experiment was conducted on the battery using a small-rate current, and cosine modulation power of different frequencies was superimposed on the flexible heating element to obtain measurement data of heat flow during the charge-discharge process.

[0030] Finally, by performing fast Fourier transform, complex plane solution, battery RC equivalent circuit fitting, and model parameter identification on the voltage and current signals in the time domain, the heat capacity and thermal conductivity of the battery under the entire state of charge distribution are extracted, realizing in-situ measurement of the changes in heat capacity and thermal conductivity with charge and discharge depth.

[0031] Compared with existing battery thermal characteristic parameter testing methods, the advantages of this invention are:

[0032] This invention, based on an isothermal calorimeter, utilizes signal modulation, fast Fourier transform, equivalent circuit fitting, and model parameter identification to achieve in-situ measurement of the changes in heat capacity and thermal conductivity with the depth of battery charge and discharge, accurately obtaining relevant thermal parameters. Compared to traditional measurement methods, this method can simultaneously measure the heat generation power, heat capacity, and thermal resistance during charge and discharge, significantly improving testing efficiency. The in-situ thermal characteristic parameters obtained improve the accuracy of thermal analysis and enrich the application of battery thermal management system design. Attached Figure Description

[0033] Figure 1 Schematic diagram of the battery isothermal calorimetry experimental apparatus of the present invention;

[0034] Figure 2 A schematic diagram of the installation of the core calorimetric components of the battery isothermal calorimeter described in this invention.

[0035] Figure 3 Hardware circuit design block diagram of the battery isothermal calorimeter device of the present invention;

[0036] Figure 4 A schematic diagram of the equivalent heat transfer model of the battery isothermal calorimeter device of the present invention;

[0037] Figure 5 A schematic diagram of the equivalent circuit for applying multi-frequency cosine modulation power to the upper flexible heating element of the present invention;

[0038] Figure 6 A schematic diagram of the equivalent circuit for applying multi-frequency cosine modulation power to the lower flexible heating element of the present invention.

[0039] Legend: 1 Host computer; 2 Switch; 3 Communication line; 4 Vacuum pump; 5 Exhaust valve; 6 Pressure relief valve; 7 Constant temperature oil bath equipment; 8 Calorimeter control box; 9 High airtight aviation plug; 10 Gas flow meter; 11 Inlet valve; 12 Nitrogen cylinder; 13 Battery charging and discharging equipment; 14 Oil bath pipeline; 15 Constant temperature heat sink; 16 Battery charging and discharging wire; 17 Uniform heating block; 18 Thermistor temperature sensor; 19 Flexible heating element; 20 Battery under test; 21 Calorimeter chamber. Detailed Implementation

[0040] To make the steps, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the implementation of the present invention will be described more clearly, in detail, and completely below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0041] Figure 1 and Figure 2 This invention provides an isothermal calorimeter for in-situ testing of the thermal characteristics of lithium-ion batteries. Based on the geometry of the selected battery 20, a flexible heating element 19 and a heat-regulating block 17 of appropriate size are chosen, and the battery's thickness and surface area are recorded. Charge and discharge parameters are determined according to the battery specifications, and a suitable diameter battery charge / discharge wire 16 is selected and connected to the external system via a four-wire connection using a high-airtightness aviation connector 9. During the battery charge / discharge and in-situ thermal parameter measurement experiments, the internal components of the calorimeter chamber 21 are arranged from top to bottom as follows: heat-regulating block 17, flexible heating element 19, battery 20, flexible heating element 19, and heat-regulating block 17. The positive and negative terminals of the battery are connected to the external battery charge / discharge device 13 via wires and electrical connectors on the calorimeter chamber. A thermistor temperature sensor 18 is installed in the grooves of the upper and lower heat-regulating blocks 17 close to the battery side, ensuring a good fit. The calorimeter chamber is closed after the battery installation is complete.

[0042] To maintain an isothermal state, the constant-temperature heat sink 15 is connected to an external constant-temperature oil bath device 7 via an oil bath pipe 14. During the experiment, the oil bath is circulated externally, allowing silicone oil at a certain temperature to be pumped into the heat sink, thereby controlling the heat sink at a constant temperature. If the required isothermal calorimetric temperature of the battery under test is lower than room temperature, the calorimetric chamber needs to be replaced with dry gas. During replacement, the calorimetric chamber is first closed, and the dry gas in the external nitrogen cylinder 12 is connected to the inlet valve 11 on the calorimetric chamber through a pipeline. Then, the inlet valve 11 and the outlet valve 5 (connected to a vacuum pump 4) on the calorimetric chamber wall are opened in sequence. The dry gas replacement time can be adjusted according to the gas flow rate displayed by the gas flow meter 10 on the calorimetric chamber wall. After replacement, the inlet valve 11 and the outlet valve 5 are closed. During the gas replacement process, if the gas pressure inside the chamber is too high, the pressure relief valve 6 installed on the calorimetric chamber is opened.

[0043] The isothermal calorimeter control box 8, the constant temperature oil bath equipment 7, the battery charging and discharging equipment 13, and the host computer 1 are networked via a switch 2. After verifying that the communication line 3 is connected correctly, the isothermal calorimeter is started, the oil bath temperature is set, and the oil bath is set to external circulation temperature control mode to control the heat sink temperature at the constant temperature point required for the experiment. Multi-frequency cosine modulation power is superimposed on the upper and lower flexible heating plates 19, and the battery charging and discharging equipment 7 is started. The charging and discharging parameters of the battery under different operating conditions are set, and the host computer 1 is used to collect and record parameters such as real-time heat flow of the battery under test during the charging and discharging process. Through further analysis and processing of the heat flow and other signals, the heat capacity and thermal conductivity of the battery as a function of charge and discharge depth can be obtained.

[0044] (a) Before the experiment, determine the geometric dimensions, isothermal calorimetry temperature, and charge / discharge parameters of the battery to be tested, and select flexible heating elements, heat-equalizing blocks, and suitable charge / discharge wires of the same size; determine the components inside the calorimetry chamber, and install them in the following order from top to bottom: heat-equalizing block, flexible heating element, battery to be tested, flexible heating element, and heat-equalizing block; at the same time, install the thermistor temperature sensor in the heat-equalizing block on the upper and lower sides close to the battery side; use charge / discharge wires to connect the battery to be tested to the charge / discharge equipment outside the isothermal calorimetry chamber, and seal the calorimetry chamber after installation;

[0045] (b) During the experiment, the battery isothermal calorimeter was first started, and the oil bath temperature was set to control the heat sink temperature at a constant value below the target isothermal temperature. After the heat sink temperature stabilized, the target temperature was set to control the temperature of the uniformly heated block at a constant value. Next, the battery charging and discharging equipment was started, and the charging and discharging parameters under different operating conditions were set. At the same time, multi-frequency cosine modulation power was applied to the upper heating element. Then, the battery charging or discharging experiment was carried out, and parameters such as battery capacity and battery temperature rise were recorded during the process.

[0046] (c) After the above steps are completed, apply multi-frequency cosine modulation power to the lower heating element using the same method, and conduct battery charging or discharging experiments, recording parameters such as battery capacity and battery temperature rise during the process;

[0047] (d) After the experiment, the temperature-time signal obtained during the battery charging and discharging process was converted into a voltage-time signal. The response voltage signal and excitation current signal under multi-frequency cosine modulation power were subjected to fast Fourier transform. By decomposing on the complex plane, the real part, imaginary part and phase of the signal were obtained.

[0048] (e) By using the RC equivalent circuit model of the battery and fitting the model parameters in data analysis software such as Matlab, thermal parameters such as heat capacity and thermal conductivity that vary with the depth of battery charge and discharge can be obtained.

[0049] The specific data processing methods in the above steps are as follows:

[0050] 1. Temperature signal T(t) acquired by temperature sensor at any time can be converted into voltage signal V(t);

[0051] 2. Solve the excitation current signal I(t) and response voltage signal V(t) under cosine modulation in the complex plane to obtain the real part, imaginary part and phase of the excitation current I(jω) and response voltage V(jω) at different frequencies;

[0052] 3. The complex impedance Z of the battery at different frequency points is obtained by solving equation (7). b (jω);

[0053] 4. Using the RC equivalent circuit model of the battery, and based on the real and imaginary parts of the complex impedance at different frequencies, the heat capacity C of the battery is obtained by identifying the model parameters using data analysis software such as Matlab. b and thermal resistance R b ;

[0054] 5. Based on the relationship between thermal resistance and thermal conductivity, and parameters such as battery thickness and heat transfer area, the thermal conductivity λ of the battery is further obtained. b .

[0055] In the aforementioned isothermal calorimetry apparatus, during the charge-discharge experiment of the battery, heat transfer mainly occurs through flexible heating elements, a heat-regulating block, and a heat sink. To clarify the content of this invention, the mathematical expression will be derived in detail below using an equivalent circuit model to illustrate the in-situ testing method for the thermal characteristic parameters of lithium-ion batteries in isothermal calorimetry experiments.

[0056] Equivalent heat transfer model of isothermal calorimeter as follows Figure 4 As shown, where T s For heat sink temperature, R a1 The thermal resistance of the upper heat-dissipating block, Ra2 For the thermal resistance of the lower heat exchange block, C a1 The heat capacity of the upper heat-dissipating block, C a2 For the heat capacity of the lower heat-dissipating block, R h1 For the thermal resistance of the upper flexible heating element, R h2 For the thermal resistance of the lower flexible heating element, C h1 For the heat capacity of the upper flexible heating element, C h2 For the heat capacity of the lower flexible heating element, R b For battery thermal resistance, C b The battery heat capacity is represented by P1 and P2, which represent the multi-frequency cosine modulation power applied to the flexible heating element in the first and second cycles, respectively. T c1 and T c2 These represent the temperature values ​​measured by the thermistor.

[0057] The modulation power is a multi-frequency cosine modulated signal with a linear frequency interval Δf:

[0058]

[0059] In the formula: A i For a frequency of f i The amplitude corresponding to the cosine signal; x[n] is the discrete signal superimposed with cosine signals of length n; f s is the sampling frequency; N is the number of frequencies.

[0060] The equivalent circuit model can be obtained by applying multi-frequency cosine modulation power to the upper flexible heating element for the first time, as shown below. Figure 5 As shown, the following relationship can be established using Kirchhoff's voltage and current laws:

[0061]

[0062]

[0063] In the formula: I1(jω) is the current value at a single ω frequency after FFT processing under the first applied multi-frequency cosine modulation power; V1(jω) and V2(jω) are the converted voltage values ​​of the upper and lower temperature sensing signals at a single ω frequency after FFT processing under the first applied multi-frequency cosine modulation power; Z a1 (jω) and Z a2 (jω) represent the complex impedances of the upper and lower uniformly heated blocks at a single ω frequency after FFT processing; Z h1 (jω) and Z h2 (jω) represents the complex impedance of the upper and lower flexible heating elements at a single ω frequency after FFT processing.

[0064] The equivalent circuit model can be obtained by applying multi-frequency cosine modulation power to the lower flexible heating element for the second time, as shown below. Figure 6 As shown, the following relationship can be established using Kirchhoff's voltage and current laws:

[0065]

[0066]

[0067] In the formula: I2(jω) is the current value at a single ω frequency after FFT processing under the second application of multi-frequency cosine modulation power; V1'(jω) and V2'(jω) are the converted voltage values ​​of the upper and lower temperature sensing signals at a single ω frequency after FFT processing under the second application of multi-frequency cosine modulation power.

[0068] Since the flexible heating elements are from the same batch and of the same specifications, it can be approximately assumed that the upper and lower flexible heating elements have the same resistive-capacitive characteristics. Therefore:

[0069] Z h1 (jω)=Z h2 (jω) (6)

[0070] Solving equations (2) to (6) simultaneously, we can obtain the complex impedance Z of the battery at different frequencies. b (jω):

[0071]

[0072] Using the RC equivalent circuit model of a battery:

[0073]

[0074]

[0075]

[0076] In the formula: Z Re (ω) is the real impedance in the RC equivalent circuit model of the battery; Z Im (ω) is the imaginary impedance in the RC equivalent circuit model of the battery.

[0077] Complex impedance Z at different frequencies b The frequency, real part, and imaginary part of (jω) can be imported into analysis and fitting software such as Matlab for model parameter identification, and the battery thermal resistance R can then be obtained. b and battery heat capacity C b Furthermore, based on the relationship between thermal resistance and thermal conductivity, the thermal conductivity λ of the battery can be calculated. b .

[0078] Example:

[0079] This embodiment designs an isothermal calorimeter for in-situ testing of the thermal characteristic parameters of lithium-ion batteries, including a calorimetric chamber, a flexible heating element, a constant-temperature heat sink, and a heat homogenizing block, as shown in the specific structure below. Figure 1 , Figure 2 As shown.

[0080] Figure 1 The basic structure of the isothermal calorimeter includes an isothermal calorimeter chamber for mounting the isothermal calorimeter body. The isothermal calorimeter chamber is equipped with electrical connectors, oil bath pipelines, and gas replacement pipelines. The gas replacement pipelines include a pressure relief valve, an outlet valve, an inlet valve, and a gas flow meter. The calorimeter control box, constant temperature oil bath equipment, battery charging and discharging equipment, and host computer are networked through a switch.

[0081] Figure 2 The core structure of the isothermal calorimeter includes a heat sink for providing defined temperature boundary conditions. The constant temperature heat sink is connected to an external oil bath device through an oil bath pipeline. The heat equalization block is used to equalize the battery temperature and provide a defined heat conduction path. The thermistor temperature sensor is installed in the groove on the heat equalization block near the battery side to measure the battery temperature after passing through the heat equalization block. The flexible heating element is installed between the heat equalization block and the battery.

[0082] Based on the aforementioned isothermal calorimeter, a temperature measurement and power control constant current source circuit was designed, including a main control circuit, a power supply module, a temperature measurement module, a power control constant current source module, and a communication module. The specific hardware circuit design block diagram is shown below. Figure 3 As shown.

[0083] The main control module is the core component for overall system control; the normal and orderly operation of each unit depends on its regulation. The power supply module provides power to the entire main control module. The temperature measurement module measures the battery temperature. The power control constant current source circuit includes a flexible heating element control circuit to control the output of different levels of compensation power and superimposed multi-frequency cosine modulation power. The communication module establishes communication between the upper and lower level computers, enabling real-time sending of function commands and uploading of battery temperature data.

[0084] After the mechanical structure and hardware circuit design of the isothermal calorimeter are completed, a program control algorithm needs to be designed to realize the output of the modulated power. The modulated power is determined by... Figure 2 The power output of the flexible heating element is provided by a controllable constant current source. Then, a power control module is used to realize the output of constant current sources of different sizes. Specifically, the power control module outputs a voltage signal through a DAC8562, and then the voltage signal is converted into a current signal through a constant current source circuit, thereby realizing the output of the required power.

[0085] In this embodiment, the temperature control frequency of the calorimeter is 2Hz, and the temperature measurement frequency is 200Hz. That is, the voltage output of the DAC8562 is changed once every 0.5s, and the temperature of the sample battery is measured every 0.005s. The total electrical power output of the flexible heating element is the sum of the compensation power of the low-frequency closed-loop control and the modulation power of the open-loop control.

[0086] Because the lowest frequency of the applied superimposed cosine modulation signal is f min =6Hz, the linear frequency interval is Δf = 2Hz, and the number of measured frequency points is N = 18, therefore the maximum frequency is f. max = 40Hz. According to the Nyquist sampling theorem, the sampling frequency f s The following conditions must be met:

[0087] f s ≥2f max (11)

[0088] In this embodiment, f s =200Hz, satisfying the Nyquist sampling theorem. Simultaneously, to ensure the accuracy of the FFT analysis, each sinusoidal signal was applied for at least two cycles, thereby achieving power and temperature control every 0.5s. Table 1 shows the frequency components of the multi-frequency cosine modulated signal.

[0089] Table 1. Frequency components of multi-frequency cosine modulated signals

[0090]

[0091] The parameters of the experiment using a 53Ah NCM ternary lithium battery at the target isothermal temperature of 25℃ are shown in Table 2.

[0092] Table 2 Experimental parameters of NCM ternary lithium batteries

[0093]

[0094] By using the aforementioned isothermal calorimeter and experimental method to test the battery, its temperature-time signal can be obtained. Further conversion and FFT analysis of this signal yield I(jω) and V(jω) at different frequencies, and the corresponding Z value can then be calculated. b (jω).

[0095] Based on the complex impedance Z at 18 frequency points b Plot the Nyquist plot of the real and imaginary parts of (jω), and use the battery RC equivalent circuit model in Matlab software to identify the parameters and obtain the heat capacity C under different charging states. b and thermal resistance R b Simultaneously, the thermal conductivity λ of the battery is obtained by utilizing the relationship between thermal resistance and thermal conductivity, as well as the battery thickness d and heat transfer area S.b .

[0096] In summary, the in-situ testing method for the thermal characteristic parameters of lithium-ion batteries based on isothermal calorimetry proposed in this invention overcomes the shortcomings of existing testing methods, such as being only applicable to offline measurements, having discrete measurement points, and being time-consuming. This invention accurately obtains continuous values ​​of the battery's heat capacity and thermal conductivity as a function of charge and discharge depth through a single battery charge-discharge experiment, significantly improving testing efficiency and greatly enriching the application of in-situ measured thermal parameters in the field of battery thermal management systems.

Claims

1. An in-situ testing method for thermal characteristic parameters of lithium-ion batteries based on isothermal calorimetry, characterized in that: Based on the principle of isothermal calorimetry, when the battery is charged and discharged, cosine modulation power of different frequencies is superimposed under the compensation power of the flexible heating element to realize the in-situ measurement of the modulated heat flow during the charging and discharging process. The heat flow of cosine modulation power at different frequencies is analyzed and processed. By using fast Fourier transform, complex plane solution, RC equivalent circuit fitting and model parameter identification, the battery heat capacity and thermal conductivity can be obtained in situ and accurately. Specifically, the following steps are included: a) Measure the dimensions of the battery under test, including surface area and thickness; b) Start the battery isothermal calorimeter and charging / discharging equipment, and set the temperature control parameters and charging / discharging parameters; c) Apply multi-frequency cosine modulation power to the upper heating element and simultaneously conduct a charge-discharge experiment to measure the battery capacity and temperature rise; d) Apply multi-frequency cosine modulation power to the lower heating element and conduct a charge-discharge experiment to measure the battery capacity and temperature rise; e) Analyze and process the temperature rise data; f). Solve for the heat capacity and thermal conductivity of the battery.

2. The in-situ testing method for thermal characteristic parameters of lithium-ion batteries based on isothermal calorimetry according to claim 1, characterized in that: In step b), the components inside the calorimeter cavity, from top to bottom, are a uniform heating block, a flexible heating plate, the battery under test, the flexible heating plate, and the uniform heating block; at the same time, the temperature sensor is installed on the uniform heating blocks close to the battery side on both the top and bottom sides.

3. The in-situ testing method for thermal characteristic parameters of lithium-ion batteries based on isothermal calorimetry according to claim 1, characterized in that: The multi-frequency cosine modulation signals in steps c) and d) are: In the formula: A i For a frequency of f i The amplitude corresponding to the cosine wave; x[n] is the discrete signal of the superposition of cosine waves of length n; f s is the sampling frequency; N is the number of frequencies.

4. The in-situ testing method for thermal characteristic parameters of lithium-ion batteries based on isothermal calorimetry according to claim 1, characterized in that: In step e), the temperature signal is converted into a voltage signal.

5. The in-situ testing method for thermal characteristic parameters of lithium-ion batteries based on isothermal calorimetry according to claim 2, characterized in that: The specific method for step f) is as follows: 1) Perform Fast Fourier Transform and complex plane solution on the excitation current and voltage signals at different frequencies to obtain the real part, imaginary part and phase; 2) Calculate the complex impedance Z of the battery at different frequency points. b (jω); In the formula: I1(jω) is the current value at a single ω frequency after the first application of multi-frequency cosine modulation power and fast Fourier transform processing; I2(jω) is the current value at a single ω frequency after the second application of multi-frequency cosine modulation power and fast Fourier transform processing; V1(jω) and V2(jω) are the converted voltage values ​​of the upper and lower temperature sensing signals at a single ω frequency after the first application of multi-frequency cosine modulation power and fast Fourier transform processing; V1'(jω) and V2'(jω) are the converted voltage values ​​of the upper and lower temperature sensing signals at a single ω frequency after the second application of multi-frequency cosine modulation power and fast Fourier transform processing; Z a1 (jω) and Z a2 (jω) represent the complex impedances of the upper and lower uniformly heated blocks at a single ω frequency after fast Fourier transform processing; Z h1 (jω) and Z h2 (jω) represents the complex impedance of the upper and lower flexible heating elements at a single ω frequency after fast Fourier transform processing; 3) Based on the RC equivalent circuit model of the battery, the heat capacity C of the battery is obtained using data fitting software by utilizing the real and imaginary parts at different frequencies. b and thermal resistance R b ; 4) Based on the relationship between thermal resistance and thermal conductivity, the thermal conductivity λ of the battery can be calculated. b .

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