A method for combined estimation of radial and axial thermal conductivity of cylindrical batteries

Abstract

The application provides a cylindrical battery radial and axial thermal conductivity combined estimation method, aiming at forming a temperature gradient through battery self-charging and discharging heat production and forced cooling, combining transient thermal simulation and optimization algorithm, and synchronously and high-precisely estimating the radial and axial thermal conductivity of the battery. The specific steps include: discharging the battery under natural cooling condition and placing, calculating the natural cooling heat transfer coefficient; applying forced cooling to one end of the battery and charging the battery with constant current, collecting the terminal voltage, temperature and environment data; establishing a transient thermal simulation model of the charging process, inputting time-varying heat production power and boundary conditions, simulating through the thermal conductivity combination, taking the minimum temperature error square sum as the optimization target, and determining the optimal thermal conductivity estimation value. The application does not need external heating device, is safe and convenient to operate, enhances the thermal conductivity decoupling identification ability through the axial temperature gradient difference, and realizes low-cost and high-precision nondestructive testing.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of battery, in particular to a combined estimation method for radial and axial thermal conductivity of cylindrical battery. BACKGROUND

[0002] Cylindrical batteries, represented by 18650, 26650, 2480 and other lithium-ion batteries, are widely used in many fields such as electric vehicles and power tools due to their high standardization and consistency, good heat dissipation performance, and flexible combination and expansion. In order to better design and optimize the cooling device and thermal management strategy of the battery pack, it is necessary to master the thermal characteristic parameters such as the specific heat capacity and thermal conductivity of the cylindrical battery. The specific heat capacity can be measured or calculated by a relatively simple and mature method, while the thermal conductivity of the cylindrical battery is difficult to obtain conveniently and accurately.

[0003] Due to the built-in winding type core of the cylindrical battery, there is a big difference in the radial and axial thermal conductivity, which needs to be measured and calculated respectively. For example, Chinese patents 201811610644.7 and 201910641508.2 disclose methods for obtaining the radial thermal conductivity of the cylindrical battery, and 202011180603.6 discloses a method for obtaining the axial thermal conductivity of the cylindrical battery. However, the current known technologies often have the following shortcomings:

[0004] (1) Some methods need to take destructive measures on the battery, such as setting heating rods and temperature sensors inside the battery, which not only damages the battery, but also affects the structure and thermal characteristics of the battery, and also has the problems of high cost, inconvenient operation and easy to cause battery leakage;

[0005] (2) Some methods need to take external heating means to heat the battery, but the cylindrical battery is generally small and irregular in shape, and the external heating device is difficult to form good adhesion with the surface of the battery to be heated, which has thermal resistance and is difficult to safely and uniformly heat according to needs.

[0006] Therefore, it is urgent to propose new technical means to complete the combined estimation of the radial and axial thermal conductivity of the cylindrical battery in a low-cost, simple, convenient and safe manner. SUMMARY

[0007] The present application provides a combined estimation method for radial and axial thermal conductivity of cylindrical battery with low cost, simple, convenient and safe, which comprises the following steps:

[0008] S1, testing, in a constant room temperature environment, sequentially executing the following test sequence:

[0009] Constant current discharge under natural cooling condition;

[0010] Rest under natural cooling condition;

[0011] constant current charging with forced cooling on the first end face;

[0012] During the test, the terminal voltage, ambient temperature, temperature of the center point of the two end faces, and the temperature of at least one monitoring point on the side face are synchronously collected at a fixed sampling frequency.

[0013] S2, natural convection coefficient calculation: based on the data in the resting phase, the natural convection coefficient is calculated by the formula h=(c·m·ΔT) / (a·S·Δt), wherein c, m, and S are the specific heat capacity, mass, and surface area of the battery, respectively, a is the resting time, ΔT is the change in the average temperature of the battery surface before and after resting, and Δt is the time series average of the difference between the surface temperature and the ambient temperature during the resting phase.

[0014] S3, thermal simulation modeling: a transient thermal simulation model is constructed, which includes the following elements:

[0015] Three-dimensional battery geometry model and mesh division;

[0016] Input parameters: density, specific heat capacity, to-be-estimated axial thermal conductivity λa and radial thermal conductivity λr;

[0017] Time-varying heat source term: determined based on the difference between the open-circuit voltage and the terminal voltage during the charging phase multiplied by the charging current;

[0018] Boundary conditions: the first end face is set to the measured temperature value, and the remaining surfaces are set to the natural convection boundary, and the convection coefficient is taken as the value obtained in step S2;

[0019] S4, parameter identification: through multivariate parameter scanning, the combination of thermal conductivities that minimizes the objective function in the thermal simulation model is solved, and the objective function is the root mean square error sum of the measured values and the simulation values of each measuring point temperature.

[0020] Optionally, the discharge rate of the battery in step S1 is between 0.2C and 3C, the initial time of the battery is in a full charge state, and the end time of the battery is equal to the discharge cutoff voltage.

[0021] Optionally, the end condition of the battery resting in step S1 is that the real-time difference between the battery surface temperature and the room temperature is less than a preset threshold, and the battery surface temperature is the average of the measured point temperatures on the battery surface.

[0022] Optionally, the charging rate of the battery in step S1 is between 0.2C and 3C, and the end condition is that the terminal voltage of the battery reaches the charging cutoff voltage.

[0023] Optionally, the forced cooling on the first end face in step S1 is any one of directional blowing of a blower, refrigeration of a liquid cooling plate, and refrigeration of a thermoelectric refrigeration device.

[0024] Optionally, during the process of applying forced cooling to the first end face in step S1, the remaining surfaces of the battery are still maintained in natural cooling conditions in contact with indoor air.

[0025] Optionally, if the forced cooling to the first end face in step S1 is performed by using a hair dryer to direct air blowing, a partition is arranged between the first end face and the remaining surfaces of the battery to avoid cooling interference of the air flow on the remaining surfaces.

[0026] Optionally, the simulation model in step S3 further comprises the following initial condition: the surface temperature value of the battery at the end of the resting is assigned to the entire calculation domain, and the surface temperature of the battery is the measured average value of each measuring point.

[0027] Optionally, the open-circuit voltage of the battery at each time in step S3 is obtained in the following manner:

[0028] The state of charge at each time is obtained by integrating the state of charge of the battery at the start of charging and the test process current over time.

[0029] The open-circuit voltage of the battery at each time is obtained according to the preset battery state of charge-open-circuit voltage curve.

[0030] Optionally, in step S3, the root mean square error sum of the measured values and the simulation values of each measuring point is the sum of the error squares of all time steps, wherein the error square sum of the temperature collection values and the calculation values of all measuring points at a certain time step is calculated by the following formula:

[0031]

[0032] The above τ is the error square sum of the temperature collection values and the calculation values of all measuring points at a certain time step, n is the total number of temperature measuring points, i is the number of each temperature measuring point, and α i and β i are the temperature collection value and the calculation value of the temperature measuring point numbered i at the time step, respectively.

[0033] The beneficial effects of the technical scheme of the present application will be described below in combination with the principles of the technical scheme.

[0034] The technical scheme of the present application mainly comprises an experimental test part and a simulation calculation part.

[0035] In the experimental test part, instead of using the common external heat source heating method in the prior art, the battery is heated by the heat generated by the battery itself during the charging and discharging process, which is low in cost, convenient to operate, uniform in heating, and safe and reliable. First, the battery is discharged, and the battery is uniformly heated to a high temperature under the condition of natural convection by means of the heat generation effect of the battery itself during the discharging process; then, the battery is naturally cooled in a non-heat-generating state, and the convective heat transfer coefficient h under natural cooling is calculated according to the specific heat capacity, mass, surface area, standing time, temperature difference before and after standing, and time average of the surface temperature difference during standing of the battery; finally, the first end surface of the battery is subjected to forced cooling, and the natural cooling state of the other surfaces of the battery is still maintained, and constant current charging is carried out. During the whole experimental test process, the terminal voltage, room temperature, center temperature of the two end surfaces and temperature of at least one position on the side surface are synchronously collected at a fixed sampling frequency, which is used for data comparison in the simulation calculation part in addition to the calculation of the convective heat transfer coefficient h under natural cooling.

[0036] In the simulation calculation part, a transient thermal simulation model of the charging process is established, in which the battery geometric model, density, specific heat capacity, time-varying heat generation power, boundary conditions and initial conditions are directly or indirectly known and determined information; and the axial and radial thermal conductivities of the battery are unknown information, so the estimated values of the axial and radial thermal conductivities are input into the material thermal properties of the model. A plurality of combinations of estimated values of the axial and radial thermal conductivities are set, and simulations are performed for different combinations of estimated values of the axial and radial thermal conductivities. According to the least square method, the error sum of squares of the temperature collection values and the calculated values of all measuring points at each time step is calculated, and the combination corresponding to the minimum total error sum of squares is taken as the final thermal conductivity estimation value.

[0037] Therefore, the cylindrical battery radial and axial thermal conductivity estimation method proposed by the application has the following advantages compared with the prior art:

[0038] (1) The heat generated by the battery itself during charging and discharging is used instead of an external heating device, which eliminates the temperature non-uniformity and safety hazards introduced by the traditional external heat source, greatly reduces the equipment cost and operation complexity, and realizes safe and economical non-destructive testing;

[0039] (2) Through the thermal boundary design of "natural cooling heat transfer coefficient calculation + single end surface forced cooling and determination of the first end surface temperature according to the collection results", the analysis process is simplified, the natural convection heat transfer coefficient is accurately calculated by using the data during the standing stage, and a clear axial temperature gradient difference is formed by combining forced cooling, which effectively enhances the decoupling identification ability of the radial and axial heat conduction characteristics;

[0040] (3) innovatively construct a transient thermal simulation model based on time-varying heat generation power and time-varying boundary, dynamically correlate the difference between open-circuit voltage and terminal voltage with internal heat generation, and traverse and screen the optimal thermal conductivity combination through a least square optimization algorithm, break through the limitation of traditional single-direction thermal conductivity measurement, and realize synchronous high-precision estimation of bidirectional thermal conductivity;

[0041] (4) adopt a partition to isolate forced cooling air flow interference, and rely on the mean value of multiple measuring points to correct local measurement error, and significantly improve data reliability.

[0042] In summary, the method does not require professional heating / detection equipment, so the test cost is low; only conventional charging / discharging devices are required, so the operation is convenient; the whole process is non-destructive testing, so the safety, reliability and precision are high, and the joint estimation of radial and axial thermal conductivities is realized scientifically and reasonably. BRIEF DESCRIPTION OF DRAWINGS

[0043] Figure 1 A flow chart of a joint estimation method of radial and axial thermal conductivities of a cylindrical battery in the embodiment of the application.

[0044] Figure 2 A grid chart of a transient thermal simulation model of a cylindrical battery in the embodiment of the application.

[0045] Figure 3 A surface distribution cloud chart at a certain time under a certain example of a transient thermal simulation model of a cylindrical battery in the embodiment of the application. DETAILED DESCRIPTION

[0046] The application will be further described below in combination with the drawings and embodiments.

[0047] As shown in the drawings, Figure 1 A joint estimation method of radial and axial thermal conductivities of a cylindrical battery, the method comprising the following steps:

[0048] S1, testing, sequentially performing the following test sequence in a constant room temperature environment:

[0049] Constant current discharge under natural cooling condition;

[0050] Rest under natural cooling condition;

[0051] Constant current charging with forced cooling on the first end face;

[0052] Synchronously collecting the terminal voltage, ambient temperature, temperature of the center points of the two end faces and temperature of at least one monitoring point on the side face at a fixed sampling frequency during the test;

[0053] S2, natural convection coefficient calculation: based on the data in the resting phase, the natural convection coefficient is calculated by the formula h = (c m AT) / (a S At), wherein c, m, S are the specific heat capacity, mass, and surface area of the battery, respectively, a is the resting time, AT is the average change of the battery surface temperature before and after the resting, and At is the time sequence average of the difference between the surface temperature and the ambient temperature during the resting;

[0054] S3, thermal simulation modeling: a transient thermal simulation model is constructed, which includes the following elements:

[0055] a three-dimensional battery geometry model and meshing;

[0056] input parameters: density, specific heat capacity, axial thermal conductivity la to be estimated, and radial thermal conductivity lr;

[0057] time-varying heat source term: determined based on the difference between the open-circuit voltage and the terminal voltage during charging multiplied by the charging current;

[0058] boundary conditions: the first end face is set to the measured temperature value, and the rest of the surface is set to the natural convection boundary, and the convection coefficient is taken as the value h obtained in step S2;

[0059] S4, parameter identification: through multivariate parameter scanning, the thermal conductivity combination that minimizes the objective function in the thermal simulation model is solved, and the objective function is the root mean square error sum of the measured values and the simulation values of each temperature measurement point.

[0060] Specifically, the surface of the cylindrical battery is composed of opposite first and second end faces and a side face, and the temperature measurement point positions on the battery surface during the test process include the center of the first end face, the center of the second end face, and at least one point on the side face. In addition, a temperature sensor is also arranged in the air at a certain position from the battery to measure the room temperature in real time.

[0061] In one embodiment, a temperature measurement point is arranged at the vertically central position of the side face of the cylindrical battery; in another embodiment, a temperature measurement point is arranged at each of the vertically upper, middle, and lower positions of the side face.

[0062] In order to collect the temperatures of each measurement point on the surface of the cylindrical battery in real time, contact and / or non-contact measurement methods can be adopted. In one embodiment, an infrared measurement method is adopted to collect the temperatures of multiple measurement points on the surface of the cylindrical battery in real time.

[0063] Specifically, the sampling of the terminal voltage and temperature is performed synchronously during the test process, and the sampling time interval can be between 0.1 s and 60 s.

[0064] Figure 2 A mesh diagram of the transient thermal simulation model of the cylindrical battery in one embodiment is shown.

[0065] Figure 3The temperature distribution cloud diagram of a certain time in a certain example corresponding to the transient thermal simulation model of the cylindrical battery in an embodiment is shown, Figure 3 The numerical value in the figure represents the temperature, with the unit of ℃.

[0066] Preferably, in the combined estimation method of the radial and axial thermal conductivities of the cylindrical battery, the discharge rate of the battery in step S1 is between 0.2C and 3C, the initial time of the battery is in a full charge state, and the end voltage of the battery is equal to the discharge cutoff voltage.

[0067] Specifically, before step S1 starts, the battery is charged to full capacity according to the charging method specified by the battery manufacturer and left for more than 1 hour.

[0068] Preferably, in the combined estimation method of the radial and axial thermal conductivities of the cylindrical battery, the end condition of the battery in step 1 is that the real-time difference between the battery surface temperature and the room temperature is less than a preset threshold value, and the battery surface temperature is the average of the temperatures of each measuring point on the battery surface.

[0069] Specifically, the preset threshold value can be between 0.5℃ and 2℃.

[0070] Preferably, in the combined estimation method of the radial and axial thermal conductivities of the cylindrical battery, the charge rate of the battery in step S1 is between 0.2C and 3C, and the end condition is that the battery end voltage reaches the charge cutoff voltage.

[0071] Preferably, in the combined estimation method of the radial and axial thermal conductivities of the cylindrical battery, in step S1, forced cooling is applied to the first end face, which is any one of directional blowing of a blower, refrigeration of a liquid cooling plate, and refrigeration of a thermoelectric refrigeration device.

[0072] Preferably, in the combined estimation method of the radial and axial thermal conductivities of the cylindrical battery, in the process of applying forced cooling to the first end face in step S1, the remaining other surfaces of the battery still maintain the natural cooling condition of being in contact with indoor air.

[0073] Preferably, in the combined estimation method of the radial and axial thermal conductivities of the cylindrical battery, if directional blowing of a blower is adopted to apply forced cooling to the first end face in step S1, the first end face and the remaining other surfaces of the battery are separated by a partition to avoid cooling interference of the remaining other surfaces caused by air flow.

[0074] Specifically, when directional blowing of a blower is adopted to apply forced cooling to the first end face, the blower sucks room temperature air from the natural environment without heating the air.

[0075] Further, the above-mentioned combined estimation method of the radial and axial thermal conductivities of the cylindrical battery, the initial conditions in the simulation model of step S3 further include: assigning the battery surface temperature values at the end of the resting to the entire calculation domain, and the battery surface temperature takes the measured average of each measuring point.

[0076] Preferably, the above-mentioned combined estimation method of the radial and axial thermal conductivities of the cylindrical battery, the total length of the transient thermal simulation model in step S3 is equal to the length of the constant current charging in step S1, and the time step is equal to the sampling time interval in step S1.

[0077] Preferably, the above-mentioned combined estimation method of the radial and axial thermal conductivities of the cylindrical battery, the method for obtaining the open-circuit voltage of the battery at each time in step S3 is:

[0078] The state of charge at each time is obtained by integrating the test process current over time and the state of charge of the battery at the start of charging;

[0079] The open-circuit voltage of the battery at each time is obtained according to the preset battery state of charge-open-circuit voltage curve.

[0080] Specifically, the above-mentioned combined estimation method of the radial and axial thermal conductivities of the cylindrical battery, in step S3, the root mean square error sum of the measured values and the simulation values of each measuring point is the sum of the error squares of all time steps, and the error square sum of the temperature collection values and the calculation values of all measuring points at a certain time step is calculated by the following formula:

[0081]

[0082] where τ is the error square sum of the temperature collection values and the calculation values of all measuring points at a certain time step, n is the total number of temperature measuring points, i is the number of each temperature measuring point, and α i and β i are the temperature collection value and the calculation value of the temperature measuring point numbered i at the time step, respectively.

[0083] Preferably, in step S4, first, the upper and lower limits of the axial and radial thermal conductivities are determined according to experience, thereby forming a feasible region of the combination of the axial and radial thermal conductivity estimation values, then setting uniformly distributed combination points in the feasible region, one combination point corresponding to one combination of the axial and radial thermal conductivity estimation values, and finally traversing different combinations of the axial and radial thermal conductivity estimation values to perform simulation, calculate the error square sum of the temperature collection values and the calculation values of all measuring points at each time step, and take the combination corresponding to the minimum total error square sum as the final thermal conductivity estimation value.

[0084] Specifically, the charging process transient thermal simulation model can be a three-dimensional model, or can be simplified into a two-dimensional axisymmetric model according to the geometric symmetry characteristics of the cylindrical battery. If a three-dimensional model is adopted, a cylindrical coordinate system containing the axial, radial and circumferential directions is established, and the circumferential thermal conductivity is set to be equal to the axial thermal conductivity according to the structural characteristics of the cylindrical battery; if a two-dimensional axisymmetric model is adopted, a coordinate system containing the axial and radial directions is established.

[0085] Embodiment

[0086] Please refer to Figures 1 to 3 A more specific embodiment will be described. In this embodiment, the cylindrical battery is of LFP18650 type, with a battery diameter and height of 18 mm and 65 mm, respectively, a positive electrode material of lithium iron phosphate, a negative electrode material of graphite, a rated capacity of 1.5 Ah, a charging cut-off voltage of 3.65 V, and a discharging cut-off voltage of 2 V. The test process is carried out in a room temperature environment of 20±1℃, and the battery is not blocked around to maintain the natural air flow around. In this embodiment, the battery surface has a first end face center, a second end face center, and three temperature measurement points on the side surface, wherein the distance between the temperature measurement points on the side surface and the two end faces is equal. In this embodiment, the battery constant current charging and discharging rate is 1C, i.e. the current size is 1.5 A, and the sampling interval is 1 s.

[0087] During the test process, the battery in a full charge state is first discharged at 1C constant current to the discharging cut-off voltage, and is left until the real-time difference between the battery surface temperature and the room temperature is less than 0.5℃, and then forced cooling is applied to the first end face and the battery is charged at 1C constant current to the charging cut-off voltage.

[0088] In this embodiment, the specific heat capacity of the battery c=860 J / (kg·K), the mass m=0.048 kg, the surface area S=4.2×10 -3 m 2 , the standing time a=2500 s, the difference between the battery surface temperature before and after standing ΔT=8.2 K, and the time average of the difference between the surface temperature and the room temperature during standing Δt=3.3 K, so the natural cooling heat transfer coefficient h=cmΔT / (aSΔt)=9.8 W / (m 2 ·K).

[0089] According to experience, the upper and lower limits of the axial thermal conductivity are 25 W / (m·K) and 10 W / (m·K), respectively, and the upper and lower limits of the radial thermal conductivity are 2.5 W / (m·K) and 1.0 W / (m·K), respectively. The feasible values of the axial thermal conductivity are 10 W / (m·K), 11 W / (m·K), …, 25 W / (m·K), and the feasible values of the radial thermal conductivity are 1.0 W / (m·K), 1.1 W / (m·K), …, 2.5 W / (m·K), so the feasible region of the axial and radial thermal conductivity estimation value combinations is formed by 16×16, i.e. 256 axial and radial thermal conductivity estimation value combinations.

[0090] The 256 combinations of axial and radial thermal conductivity estimation values are simulated respectively, the error sum of squares of the temperature collection values and the calculated values of all measuring points at each time step is calculated, and the combination corresponding to the minimum total error sum is taken as the final thermal conductivity estimation value. The optimal estimation values of the radial and axial thermal conductivities are 1.2 W / (m·K) and 21 W / (m·K) respectively.

[0091] It should be noted that the specific heat capacity of the cylindrical battery is easy to obtain compared with the thermal conductivity. For example, it can be measured by a known technical solution, or the specific heat capacity of the cylindrical battery can be calculated by mass-weighted average of the specific heat capacities of various substances constituting the cylindrical battery according to the material composition of the cylindrical battery. However, there is no good estimation method for the thermal conductivity of the cylindrical battery in the current known technology, and the radial and axial thermal conductivities of the cylindrical battery are closely related to the process control in the generation and manufacturing process, and cannot be accurately estimated by the design scheme.

[0092] In the embodiment, the first end surface is the bottom surface of the cylindrical battery, and the second end surface is the top surface of the cylindrical battery. By applying forced cooling to the first end surface to form a temperature that is significantly lower than the side surface, an axial temperature gradient difference of the battery can be formed, which helps to enhance the decoupling identification capability of the radial and axial thermal conduction characteristics. As shown in FIG. 6, the numerical value in the temperature distribution cloud diagram of the cylindrical battery transient thermal simulation model at a certain time in a certain example represents the temperature in ℃. Figure 3 Figure 3 As can be seen from FIG. 6, there is a significant axial temperature gradient from the bottom to the top. Figure 3

[0093] The technical solution in the embodiment only needs a conventional charging and discharging device and a forced cooling device. The heat generated by the battery itself during charging and discharging replaces the external heating device in the known scheme, eliminates the temperature unevenness and safety hazards of the traditional external heat source, reduces the cost and operation difficulty, combines natural cooling heat transfer coefficient calculation and single-end surface forced cooling design, uses the data during the standby stage to calculate the natural convection heat transfer coefficient, and enhances the decoupling identification capability of the bidirectional thermal conductivity through the axial temperature gradient difference. The transient thermal simulation model of the time-varying heat generation power and the boundary is innovatively constructed, the voltage difference and the internal heat generation are dynamically associated, the least square algorithm is used to screen the optimal thermal conductivity combination, the radial and axial thermal conductivities are simultaneously and highly accurately estimated, and the synchronous estimation of the radial and axial thermal conductivities of the cylindrical battery is scientifically realized.​​

Claims

1. A method for jointly estimating the radial and axial thermal conductivity of a cylindrical battery, the battery having a first end face, a second end face, and a side face, characterized in that... include: S1. Testing: Perform the following test sequence sequentially in a constant room temperature environment: Constant current discharge under natural cooling conditions; Shelving under natural cooling conditions; Constant current charging that forces cooling of the first end face; During the test, the terminal voltage, ambient temperature, temperature at the center point of both end faces, and temperature at at least one monitoring point on the side are collected synchronously at a fixed sampling frequency. S2. Calculation of natural convection coefficient: Based on the data during the resting period, the natural convection coefficient is calculated using the formula h=(c·m·ΔT) / (a·S·Δt), where: c, m, and S are the battery specific heat capacity, mass, and surface area, respectively; a is the resting time; ΔT is the average change in battery surface temperature before and after resting; and Δt is the time-series average of the difference between surface temperature and ambient temperature during the resting period. S3. Thermal Simulation Modeling: Construct a transient thermal simulation model that includes the following elements: 3D battery geometry model and mesh generation; Input parameters: density, specific heat capacity, axial thermal conductivity λa and radial thermal conductivity λr to be estimated; Time-varying heat source term: determined by multiplying the difference between the open-circuit voltage and the terminal voltage during the charging phase by the charging current; Boundary conditions: The temperature value of the first end face is set according to the measured temperature, and the other surfaces are set according to the natural convection boundary. The convection coefficient is the h value obtained in step S2. S4. Parameter Identification: Solve the combination of thermal conductivity that minimizes the objective function in the thermal simulation model by multivariate parameter scanning. The objective function is the sum of the root mean square error between the measured and simulated temperature values ​​at each measuring point.

2. The method for jointly estimating the radial and axial thermal conductivity of a cylindrical battery according to claim 1, characterized in that, In step S1, the constant current discharge rate of the battery is between 0.2C and 3C. Initially, the battery is fully charged, and at the end, the battery terminal voltage is equal to the discharge cutoff voltage.

3. The method for jointly estimating the radial and axial thermal conductivity of a cylindrical battery according to claim 1, characterized in that, The end condition for battery placement in step S1 is that the real-time difference between the battery surface temperature and the room temperature is less than a preset threshold, and the battery surface temperature is the average temperature of each measuring point on the battery surface.

4. The method for jointly estimating the radial and axial thermal conductivity of a cylindrical battery according to claim 1, characterized in that, In step S1, the constant current charging rate of the battery is between 0.2C and 3C, and the termination condition is that the battery terminal voltage reaches the charging cutoff voltage.

5. The method for jointly estimating the radial and axial thermal conductivity of a cylindrical battery according to claim 1, characterized in that, In step S1, forced cooling is applied to the first end face, which can be any one of the following: directional air blowing by a blower, liquid cooling plate cooling, or thermoelectric cooling device cooling.

6. The method for jointly estimating the radial and axial thermal conductivity of a cylindrical battery according to claim 1 or 5, characterized in that, During the forced cooling process of the first end face in step S1, the remaining surfaces of the battery still maintain natural cooling conditions in contact with the indoor air.

7. The method for jointly estimating the radial and axial thermal conductivity of a cylindrical battery according to claim 5, characterized in that, If a blower is used to apply forced cooling to the first end face in step S1, the first end face and the other remaining surfaces of the battery are separated by a partition to avoid airflow interfering with the cooling of the other remaining surfaces.

8. The method for jointly estimating the radial and axial thermal conductivity of a cylindrical battery according to claim 1, characterized in that, The simulation model in step S3 also includes the following initial conditions: the battery surface temperature value at the end of the resting time is assigned to the entire calculation domain, and the battery surface temperature is taken as the average value of the measurements at each measuring point.

9. The method for jointly estimating the radial and axial thermal conductivity of a cylindrical battery according to claim 1, characterized in that, The method for obtaining the battery open-circuit voltage at each moment in step S3 is as follows: The state of charge at each moment is obtained by integrating the battery state of charge at the start of charging and the current over time during the test. The battery open-circuit voltage at each moment is obtained based on the preset battery state-of-charge-open-circuit voltage curve.

10. The method for jointly estimating the radial and axial thermal conductivity of a cylindrical battery according to claim 1, characterized in that, In step S3, the sum of the root mean square errors between the measured and simulated temperature values ​​at each measuring point is the sum of the sums of squared errors corresponding to all time steps. The sum of squared errors between the collected and calculated temperature values ​​at a given time step is calculated using the following formula: The above τ represents the sum of squared errors between the collected and calculated temperature values ​​at a certain time step, n is the total number of temperature measurement points, i is the number of each temperature measurement point, and α... i and β i These are the temperature acquisition value and calculated value of temperature measuring point i at this time step, respectively.

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