Module structure reliability checking method for cell expansion

Abstract

The invention belongs to the technical field of lithium batteries, and particularly relates to a module structure reliability checking method for cell expansion. Comprising the following steps: S101, acquiring cyclic expansion force data of a battery cell group; s102, processing the cyclic expansion force data and extracting characteristic values in sections; s103, establishing a simulation model and calculating an initial state; s104, calculating the stress state of the module in stages; and S105, calculating and checking fatigue damage. According to the method, the expansion force change curve under the real working condition is obtained by implementing the multi-cell cyclic expansion experiment, the maximum expansion force and the minimum expansion force are extracted in a segmented mode based on the curve, and the fatigue damage accumulated value of the structural part under the alternating load is calculated in a segmented mode in combination with the simulation model. Therefore, accurate and efficient checking of the structural reliability of the battery module in the whole life cycle is realized.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery technology, specifically relating to a method for verifying the reliability of module structures in response to cell expansion. Background Technology

[0002] Lithium-ion batteries are the core power source for current new energy vehicles and energy storage systems. During normal charge-discharge cycles, the insertion and extraction of lithium ions into the electrode materials cause volume changes in the active materials, resulting in periodic expansion and contraction of the battery cells. When multiple cells are assembled into a battery module, this expansion force, which varies with the number of cycles, needs to be effectively constrained by the module's mechanical structure (such as end plates, steel strips, bolts, etc.). If the module structure design is inadequate, the long-term accumulated expansion stress may lead to fatigue damage to structural components, loose connections, or even cracking, thereby causing safety hazards such as battery performance degradation and thermal runaway.

[0003] The variation of expansion force is complex, and its magnitude is affected by a variety of factors, including but not limited to: initial preload of the cell, ambient temperature, state of charge / discharge (SOC), state of health (SOH), and the stacking effect between cells. Therefore, during the battery module design phase, these factors must be comprehensively considered, and the structural reliability of the module throughout its life cycle must be accurately assessed and verified.

[0004] Currently, the industry has relatively mature testing and simulation evaluation methods for the reliability of battery modules under mechanical loads such as vibration, impact, and extrusion. Relevant national standards (such as GB 38031) and enterprise standards also provide clear test specifications. However, for long-term, alternating loads caused by cell cyclic expansion, simulation methods are often used as an alternative analysis due to their long testing cycles and high costs. Existing simulation methods mostly adopt a single static loading approach, that is, applying a load representing the maximum or typical expansion force in the simulation model at one time, and then calculating the stress and deformation of the module structure. Although this method can reflect the strength reserve of the structure to a certain extent, it fails to reflect the dynamic changes of the expansion force during the actual cyclic process, and does not consider the fatigue accumulation effect of materials under cyclic loads. Therefore, it is difficult to accurately predict the structural reliability of the module throughout its entire service life.

[0005] Furthermore, while some existing patented technologies (such as publication numbers CN 116454440 A and CN 112036029 A) involve methods for predicting or simulating the expansion force of battery modules, their core focus remains on obtaining the expansion force or stress distribution under a specific state (such as a specific temperature or a specific number of cycles) through modeling. Essentially, they still fall within the scope of single-cycle or static loading analysis and do not propose a systematic method for assessing and verifying structural fatigue damage based on the full life-cycle cyclic load spectrum. Another patent (such as publication number CN202110467362.1) proposes calculating the module expansion force based on single-cell test data. While this method saves costs, it does not consider the potential synergistic effect and amplification of expansion force caused by factors such as the contact interface between cells and uneven temperature distribution when multiple cells are stacked together. Its prediction results may underestimate the actual expansion force level experienced by the module, leading to overly aggressive structural designs. Summary of the Invention

[0006] The purpose of this invention is to provide a systematic method that can accurately reflect the cyclic changes in cell expansion force and, based on this, perform cumulative fatigue damage calculation and reliability verification of the battery module structure throughout its entire life cycle.

[0007] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for verifying the reliability of a module structure in response to cell expansion, comprising the following steps: S101. Obtain the cyclic expansion force data of the cell pack: Based on the cell model and quantity used in the target battery module, stack the cells of the same model and in quantities equal to or greater than those of the target module into a cell pack according to the module structure, conduct a cyclic charge and discharge experiment under preset pre-tightening force and preset temperature conditions, and collect the cyclic expansion force data throughout the experiment. S102. Processing cyclic expansion force data and extracting feature values ​​in segments: Based on the cyclic expansion force data, fit the expansion force change curve of the cell pack within the target life cycle; divide the target life cycle into N stages, where N is an integer from 8 to 16, and extract the maximum and minimum expansion force values ​​corresponding to each stage. S103. Establish a simulation model and calculate the initial state: Establish a simulation model based on the actual structure of the target battery module, apply the initial preload force of the module and the bolt preload force corresponding to the preset preload force in the simulation model, and calculate the initial stress state of the target battery module before cyclic expansion. S104. Calculate the stress state of the module in stages: Based on the initial stress state, simulate the expansion of the battery cell in the simulation model, and calculate the stress state of each structural component in the target battery module when the expansion force on the module end plate reaches the maximum expansion force value and minimum expansion force value extracted in S102 for each stage. S105. Calculate and verify fatigue damage: Based on the stress state of each structural component at each stage, the number of cycles in each stage, and the fatigue performance curve of the material of each structural component obtained in S104, calculate the fatigue damage value of each structural component at each stage; sum up the fatigue damage values ​​of each stage to obtain the total lifetime damage value of each structural component; if the total lifetime damage value is less than 1, it is determined that the structural design of the target battery module meets the reliability requirements.

[0008] Preferably, in step S101, the preset preload force is equal to or slightly higher than the initial preload force applied by the target battery module design.

[0009] Preferably, in step S101, the preset temperature is equal to or slightly higher than the ambient temperature when the target battery module is operating normally.

[0010] Preferably, in step S101, the number of cycles in the cyclic charge-discharge experiment is not less than 500.

[0011] Preferably, in step S102, fitting the expansion force variation curve specifically includes: extracting the maximum and minimum expansion force for each charge-discharge cycle from the cyclic expansion force data; generating a data point set with the cycle number as the abscissa and the maximum and minimum expansion force values ​​as the ordinates; performing curve fitting on the data point set and extrapolating it to the target life cycle end point to obtain a fitting curve characterizing the change of the maximum and minimum expansion force with the cycle number.

[0012] Preferably, in step S103, the simulation model includes at least a battery cell, an end plate, fasteners for constraining the end plate, and a module support structure.

[0013] Preferably, in S103, the initial preload of the module is applied by simulating the shortening of the constraint steel strip length and establishing a contact relationship; the bolt preload is applied by simulating the shortening of the bolt length and establishing a contact relationship.

[0014] Preferably, in step S104, by assigning an equivalent coefficient of thermal expansion to the cell material and applying a temperature load, the increase in cell thickness is simulated in the simulation model until the expansion force on the module end plate reaches the set target value.

[0015] Preferably, in step S105, when calculating the fatigue damage value of a single stage, the stress state of the structural component corresponding to the start time of the stage is set to the stress state under the minimum expansion force value, and the stress state of the structural component corresponding to the end time of the stage is set to the stress state under the maximum expansion force value, and the calculation is performed in combination with the number of cycles in the stage and the material fatigue curve.

[0016] Preferably, the method is applicable to the reliability verification of battery modules using the same type of cells but with different structures, or to the reliability verification of module structures within a CTP battery pack.

[0017] The beneficial effects of this invention are as follows: Compared with the existing method of evaluating the expansion resistance of module structures through single-load simulation, this invention obtains the expansion force variation curve under real working conditions by conducting multi-cell cyclic expansion experiments. Based on this curve, the maximum and minimum expansion forces are extracted in segments, and the cumulative fatigue damage value of structural components under alternating loads is calculated in stages using a simulation model. This achieves accurate and efficient verification of the structural reliability of the battery module throughout its entire life cycle. This method not only significantly improves the consistency between the verification results and actual working conditions, avoiding evaluation deviations caused by load simplification, but also greatly shortens the traditional long-term test cycle and reduces testing costs through a strategy combining experiments and simulations. Furthermore, it can be widely applied to the reliability assessment of different structural modules and CTP enclosures, possessing outstanding advantages such as high verification accuracy, wide applicability, and strong engineering practicality. Attached Figure Description

[0018] Figure 1 This is a flowchart of the module structure reliability verification method provided in the embodiments of the present invention; Figure 2 This is a schematic diagram of a cell assembly cyclic test device for obtaining cyclic expansion force data in an embodiment of the present invention; Figure 3 This is a schematic diagram of the curves showing the changes in maximum and minimum expansion forces with the number of cycles, obtained by fitting experimental data according to an embodiment of the present invention. Figure 4 This is a schematic diagram illustrating how the expansion force curve throughout the entire life cycle is divided into multiple stages in an embodiment of the present invention. Figure 5 This is a schematic diagram of the finite element model of the battery module structure used for simulation calculation in an embodiment of the present invention. Detailed Implementation

[0019] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings and embodiments. It should be noted that the following embodiments are only used to explain the present invention and do not constitute a limitation on the scope of protection of the present invention.

[0020] refer to Figure 1 The specific steps of this method include: Step 101: Complete the cell pack cyclic expansion force test and collect expansion force data, including the following steps: S1: Prepare battery cells of the same model as the target module, in equal or greater quantities, stack them into groups, weld the necessary aluminum busbars for circulation onto the battery cell tabs, and connect the battery cell groups using... Figure 2 The baffles 01 and 02 shown constrain it; S2: After calibrating the pressure sensor, install it between baffle 02 and baffle 03, as follows: Figure 2 Move baffle 03 toward the cell assembly to clamp the cell assembly and pressure sensor, and apply an initial preload force to the cell that matches the target module or is slightly higher. The specific value of the preload force is read from the measurement value of the pressure sensor. S3: Place the battery cell assembly into the temperature chamber and adjust the temperature chamber temperature to be equal to or slightly higher than the ambient temperature of the target module. Connect the charging and discharging equipment and start the cyclic experiment using the current used by the target module when it is working normally. The number of cycles should not be less than 500. During the experiment, use a pressure sensor to collect the expansion force value of the battery cell assembly during the cycle, and collect it once every 10 seconds.

[0021] Step 102: Using the expansion force data obtained in Step 101, extract the maximum and minimum expansion force values ​​for each stage, including the following steps: S1: From the expansion force data obtained in steps 101-S3, extract the maximum and minimum expansion force values ​​for each cycle. Plot the maximum and minimum expansion force as a function of the number of cycles on the x-axis and the expansion force value on the y-axis, as shown below. Figure 3 ; S2: Based on the slope of the curve, the expansion force curve of the battery pack throughout its entire life cycle is fitted. The solid line represents the measured part, and the dashed line represents the fitted part, such as... Figure 3 According to the fitted curve, when the module is cycled 2000 times, the maximum expansion force that the module frame can withstand is 21kN and the minimum expansion force is 14kN. S3: The entire lifecycle consists of 2000 cycles, divided into 10 equal stages, each stage containing 200 cycles. The maximum expansion forces F1max, F2max, ..., F10max and the minimum expansion forces F1min, F2min, ..., F10min for each stage are extracted. Figure 4 .

[0022] Step 103: Establish a simulation model and calculate the initial force state of the target module before the cycle, including the following steps: S1: Based on the structure of the target module, establish a simulation model, which includes battery cell 1, aluminum busbar 2, steel strip 3, end plate 4, bolts for fixing the end plate of the module 6, and a local box structure 5 that is bonded to the bottom of the module. S2: Input the material parameters, simulation algorithm, and boundary conditions of the module structure into the simulation software. The material parameters include... Figure 5The density, elastic modulus, yield strength, tensile strength, and elongation at break of the materials corresponding to structures 1-6 in the middle structure are set; the simulation algorithm is selected as statics and matched with the expansion process; in the boundary conditions, the lower end of the fixing bolt of the module end plate and the position of the connection between the box frame and the local box structure bonded to the bottom of the module are constrained; the steel strip 3 is set to contact the end plate 4 and the side of the battery cell 1; the aluminum busbar 2 and the battery cell 1 are bound at the electrode welding point; the end plate 4 is in contact with the bolt 6; and the adhesive area between the bottom of the battery cell and the local box 5 is bound. S3: Establish a surface at the midpoint of the module's length direction. Using this as a reference, shorten the length of the steel strip 3. Apply the same initial preload force to the battery cells in the module as in step 1 using the contact between the end plate 4 and the steel strip. Establish a surface in the screw. Using this as a reference, shorten the bolt length. Apply a bolt preload force matching the torque to the bolt using the contact between the bolt 6 and the end plate 4. After adding both preload forces, select the statics algorithm to complete the simulation calculation and obtain the initial stress state of the target module.

[0023] Step 104: Based on the initial stress state in Step 103, increase the cell thickness by thermal expansion, extract the expansion force borne by the module end plate, and calculate the stress state of each structural component in the module when the expansion force borne by the module end plate is equal to F1max, F2max, ..., F10max, F1min, F2min, ..., F10min.

[0024] Step 105: Calculate the fatigue damage value of the module structure over its entire lifespan to determine the reliability of the module structure. This includes the following steps: S1: Using the stress conditions of each structural component in each stage module obtained in step 104, in the first stage, the initial state of the module structure is set to the stress condition corresponding to F1min, and the final state is set to the stress condition corresponding to F1max. S2: Input the SN curves of each structural material in the module, set the fatigue repetition count to 200, and you can calculate the fatigue damage value of each structure in the 0-200 cycle stage, denoted as D1. Similarly, calculate the fatigue damage value D2-D10 of each structure in the second to tenth stages to obtain: D1-D5=0, D6=0.01, D7=0.04, D8=0.11, D9=0.23, D10=0.40; S3: Calculate the sum of D1-D10, which is the total damage value Dsum of the structural component during the module's life cycle. The calculated value is Dsum=0.79. Since Dsum<1, it can be determined that the module's structural design is sufficiently reliable.

[0025] Compared with the prior art, the method of this application has at least the following improvements: This invention proposes a method for verifying the structural reliability of a battery module that conforms to the variation law of cell expansion force in the battery module. This method is used to evaluate the safety status of a battery module structure after normal operation throughout its entire life cycle. The method includes a complete process from experimental design and data processing to simulation verification. The calculation method conforms to actual laws and can be used to verify the structural reliability of modules or CTP enclosures using different structures with the same or fewer cell types, thus having a wide range of applications.

[0026] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for verifying the reliability of a module structure in response to cell expansion, characterized in that, Includes the following steps: S101. Obtain the cyclic expansion force data of the cell pack: Based on the cell model and quantity used in the target battery module, stack the cells of the same model and in quantities equal to or greater than those of the target module into a cell pack according to the module structure, conduct a cyclic charge and discharge experiment under preset pre-tightening force and preset temperature conditions, and collect the cyclic expansion force data throughout the experiment. S102. Processing cyclic expansion force data and extracting feature values ​​in segments: Based on the cyclic expansion force data, fit the expansion force change curve of the cell pack within the target life cycle; divide the target life cycle into N stages, where N is an integer from 8 to 16, and extract the maximum and minimum expansion force values ​​corresponding to each stage. S103. Establish a simulation model and calculate the initial state: Establish a simulation model based on the actual structure of the target battery module, apply the initial preload force of the module and the bolt preload force corresponding to the preset preload force in the simulation model, and calculate the initial stress state of the target battery module before cyclic expansion. S104. Calculate the stress state of the module in stages: Based on the initial stress state, simulate the expansion of the battery cell in the simulation model, and calculate the stress state of each structural component in the target battery module when the expansion force on the module end plate reaches the maximum expansion force value and minimum expansion force value extracted in S102 for each stage. S105. Calculate and verify fatigue damage: Based on the stress state of each structural component at each stage obtained in S104, the number of cycles included in each stage, and the fatigue performance curve of the material of each structural component, calculate the fatigue damage value of each structural component at each stage. The total lifecycle damage value of each structural component is obtained by summing the fatigue damage values ​​at each stage; if the total lifecycle damage value is less than 1, the structural design of the target battery module is determined to meet the reliability requirements.

2. The verification method according to claim 1, characterized in that, In step S101, the preset preload force is equal to or slightly higher than the initial preload force applied by the target battery module design.

3. The verification method according to claim 1, characterized in that, In step S101, the preset temperature is equal to or slightly higher than the ambient temperature when the target battery module is operating normally.

4. The verification method according to claim 1, characterized in that, In step S101, the number of cycles in the cyclic charge-discharge experiment is not less than 500.

5. The verification method according to claim 1, characterized in that, In step S102, fitting the expansion force change curve specifically includes: extracting the maximum and minimum expansion force for each charge-discharge cycle from the cyclic expansion force data; generating a data point set with the cycle number as the abscissa and the maximum and minimum expansion force values ​​as the ordinates; performing curve fitting on the data point set and extrapolating it to the target life cycle end point to obtain a fitting curve characterizing the change of the maximum and minimum expansion force with the cycle number.

6. The verification method according to claim 1, characterized in that, In step S103, the simulation model includes at least a battery cell, an end plate, fasteners for constraining the end plate, and a module support structure.

7. The verification method according to claim 1, characterized in that, In S103, the initial preload of the module is applied by simulating the shortening of the constraint steel strip length and establishing a contact relationship; the preload of the bolt is applied by simulating the shortening of the bolt length and establishing a contact relationship.

8. The verification method according to claim 1, characterized in that, In step S104, by assigning an equivalent coefficient of thermal expansion to the cell material and applying a temperature load, the increase in cell thickness is simulated in the simulation model until the expansion force on the module end plate reaches the set target value.

9. The verification method according to claim 1, characterized in that, In step S105, when calculating the fatigue damage value of a single stage, the stress state of the structural component corresponding to the start time of the stage is set to the stress state under the minimum expansion force value, and the stress state of the structural component corresponding to the end time of the stage is set to the stress state under the maximum expansion force value. The calculation is performed by combining the number of cycles in the stage with the material fatigue curve.

10. The verification method according to any one of claims 1 to 9, characterized in that, The method is applicable to the reliability verification of battery modules using the same type of cells but with different structures, or to the reliability verification of module structures within CTP battery packs.

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