Introduction to CAE simulation of power battery box

The main body of CAE software is Finite Element Analysis software. The basic idea of ​​the finite element method is to discretize the structure. The finite number of easily analyzable units represent complex objects. The units are connected to each other through a finite number of nodes. Coordination condition comprehensive solution. Since the number of cells is limited and the number of nodes is also limited, it is called the finite element method. This method is more flexible, and changing the number of units can change the accuracy of the solution and obtain a solution that is infinitely close to the real situation. Some people have estimated the time spent in each stage of CAE: the establishment of the model and the data input is about ~45%, the result analysis and evaluation is about 50%, and the real calculation time is about 5%.

CAD tools are used to establish the geometric and physical models of CAE, and the input of analytical data is completed, which is called CAE pre-processing. The pre-processing module is mainly used for modeling and parametric modeling of solids, Boolean operations, automatic segmentation, node numbering and node parameter generation, direct input of load and material parameters, parameterized import of nodes, automatic generation of node loads, Model information is automatically generated and so on.

This is followed by finite element analysis, unit characteristic analysis, finite element unit assembly, finite element system solution and finite element result generation for the finite element model. The finite element analysis module includes a finite element library, a material library and related algorithms, a constraint processing algorithm, a finite element system assembly module, a static, dynamic, vibration, linear, and nonlinear solution library. By decomposing into sub-problems, the analysis is done with different finite element analysis subsystems. The subsystem includes a linear static analysis subsystem, a dynamic analysis subsystem, a vibration modal analysis subsystem, a thermal analysis subsystem, and the like.

Of course, CAE results also require CAD technology to generate graphical outputs, such as displacement maps, contour maps of stress, temperature, and pressure distribution, color shades representing stress, temperature, and pressure distribution, and generations as a function of mechanical and temperature loads. Dynamic display of displacement, stress, temperature, and pressure distribution. The post-processing module includes data smoothing, processing and reality of physical layers, data verification and specification checking for engineering or product design requirements, design optimization and model modification, etc., which can be regarded as CAE post-processing.

The following figure is a general CAE analysis process

The CAE analysis of the battery pack structure generally includes modal analysis, static analysis, dynamic analysis, fatigue analysis, and the like. The CAE analysis of the battery box generally includes:

1. Establish a simulation model

a) Define material properties. Relevant material parameters are determined based on the material of the battery box: density, modulus of elasticity, Poisson's ratio, strength limit, and the like. Static strength, stiffness and modal analysis are carried out. When using linear elastic material simulation, the elastic modulus, density and Poisson's ratio of the material are generally input. When performing axial vibration shock simulation, the material is generally nonlinear and stress- Strain curve definition material model

b) Divide the grid. For example, Hypermesh is used to perform digital-to-analog processing and meshing of battery cases. It is generally necessary to divide the grid according to the quality requirements so that the unit aspect ratio is as close as possible to 1. At the same time, the mesh distortion is processed. For example, the quadrilateral is as close as possible to the square. If the distortion is avoided, the refinement mesh can be used to reduce the distortion. Warpage problems can be eliminated with warpage conditions and mapped meshes, and inevitably, fine meshes are used to reduce warpage. The grid of each component of the battery box needs to be cross-penetrated, and the weight is adjusted until it is qualified.

c) Define interactions, boundary conditions, and applied loads. Defines the module's contact with the battery compartment during soldering, bolting, and multi-axial vibration shock simulation between components. The boundary conditions mainly simulate the connection between the battery box and the vehicle body, and all the degrees of freedom of the bolt connection holes are constrained.

2, simulation analysis

a) Static strength analysis. The battery box is not directly subjected to the working load. When the car is driving on uneven road surface, the inertial impact force generated by the battery module swaying acts on the inner wall of the battery box. When performing finite element simulation, the impact load of the module is equivalent to the static load through the dynamic load coefficient. , evenly applied to the inner wall of the battery box. The load conditions are divided into: bumpy driving and turning: the load acting on the side wall is determined according to the acceleration common to the turning of the car and the acceleration generated by the unevenness of the road surface. The load loading of the bumpy ride and the brake is similar, but the load acts on the other side wall at this time.

b) Stiffness analysis. The main considerations are to reverse and play. When the structure of the battery box is large, deformation is likely to occur. Through the stress cloud diagram, it can be found that under the action of twisting or playing the load, the battery box structure can be easily damaged.

c) Modal analysis. The mass distribution of the battery module is simulated by adding a mass point at the bottom. The purpose of modal analysis is to obtain the natural frequency of the battery box structure. If the natural frequency of the structure is close to the frequency of the fixed-frequency vibration test, it is necessary to improve the structure, change its natural frequency, and avoid resonance.

d) Multi-axial vibration shock simulation. For fixed-frequency vibration: including up, down, left and right, and front and rear, it is necessary to apply three directions of load to simulate the excitation of the battery box by the test bench. The vibration load is applied by converting the acceleration with time into the corresponding displacement, and the vibration load is applied to the tow, that is, a forced displacement is applied to the tow portion according to time; when the frequency is vibrated: the false load according to the test condition .

e) Fatigue analysis. This requires the vehicle CAE analysis results and experimental data, the S-N (stress-life curve) of the battery box material, the fatigue limit diagram of the material, the effective stress concentration factor, the size factor and the surface quality factor. If there is not enough experimental parameter support, the fatigue analysis of the battery box structure under virtual conditions can be performed, that is, the battery box is subjected to the sinusoidal alternating load under the real constraint condition, and the fatigue life of the battery box at this time is analyzed.

f) Collision performance. Battery box crash performance also requires vehicle collision data support or analysis based on empirical values.

First introduced here, the latter needs to be specifically developed for each aspect.

references

Optimization techniques and applications in the CAE method. Ni Zhengshun

NASTRAN battery box finite element analysis and actual measurement research. Li Xiliang

The application of CAD/CAE technology in the design of automobile frames. Zhang Tieshan

Virtual test study on fatigue life of auto parts . Long Jianyun

HyperWorks analyzes application examples. Li Chulin

CAE analysis optimization study of vehicle power battery pack structure. Wang Lijuan

Experimental study on vibration characteristics of electric vehicle battery box. Sanglin

Mechanical structure optimization method based on CAE technology. Zhang Hongqi

Author: 129Lab

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