Discussion on CAE Application in Electric Vehicle Batteries-EEWORLD

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1 Introduction

CAE (Computer Aided Engineering) is a very effective method for battery development and design.

Examples of using CAE for battery development and design, including unpublished parts, are listed in Table 1. CAE (Computer Aided Engineering Design) is applicable to a variety of complex fields such as fluid, casting, and plastic processing. This article introduces an application example of studying the expansion of the scope of CAE and using it for the structural design of high-reliability fixed VRLA batteries. It is a case report on the use of CAE to study the shape of the battery tank in order to improve the performance of 36V-VRLA batteries.

2 Case (1): Stationary VRLA battery

2.1 Purpose of analysis

Fixed VRLA batteries are generally required to have a lifespan of more than 10 years, especially for batteries used for backup power. Battery performance must be ensured during use. Therefore, this type of battery requires higher reliability. As we all know, long-term trickle charging of backup power batteries causes the positive grid inside the battery to gradually oxidize and corrode. As the oxidation volume increases, the plate itself expands and deforms. Therefore, in order to maintain the performance of the battery during long-term use, it is necessary to absorb the expansion of the plate in some form to avoid deformation and breakage of the battery slot. When the battery is absorbed internally, it can also cause deformation and damage to the busbar, making it difficult to maintain battery performance.

When designing VRLA batteries, it is important to quantitatively predict and take countermeasures for problems that can be predicted during use. This study discusses the use of CAE (computer-aided engineering) design to analyze battery strength, trying to predict the phenomena that may occur during use, and requiring further improvement of battery reliability.

2.2 Analysis methods

The analyzed limited influence factor model of VRLA battery is shown in Figure 1. The positive electrode group was subjected to a three-level modeling analysis. After the modeling analysis, the slot was installed. The plate expansion is given an assumed temperature based on the expansion rate of the plate, and the expansion of the plate is expressed by thermal expansion. The analysis is based on the limited influence factor analysis program.

The physical property data used for the analysis were obtained by using various experiments shown in Figure 2 to measure the strength of the materials that make up each part of the battery.

2.3 Analysis results

The stress distribution of the battery and the electrode group in the battery when the electrode group expands is shown in Figure 3. The stress is concentrated in the resin sealing part of the electrode column and the battery slot and cover part close to it. The comparison of the deformation and stress distribution simulated by computer and the deformation and destruction test results using real batteries is shown in Figure 4.

The simulation model forced the pole group inside the battery to shift upward and observed the state when the battery was damaged. The deformation process of the two was consistent. It was confirmed that the battery slot whitened and damaged in the high stress area, thus confirming the validity of the computer simulation.

From the results of computer simulation and model verification, when the maximum expansion rate of the plate is set to exceed 5%, the battery compartment remains intact, thus confirming the safety performance of this battery.

2.4 Improving reliability

In the test of reducing the stress of the battery slot, the test effect after reducing the number of pole pins from 2 to 1 is shown in Figures 5 and 6. When comparing the analysis results, the maximum stress of the battery slot is reduced by 40% with only one pole pin, and the reliability performance is further improved.

The experiment of reducing the stress of the busbar inside the battery studied the form (shape) of the busbar. The stress distribution was calculated based on the traditional shape and the new shape with a wide width near the pole, and the results are shown in Figure 7. The comparison of the stress state in the long direction of the busbar is shown in Figure 8. The comparison of the two results shows that the arrow part that is prone to breakage can be reduced by up to 50%, which effectively improves reliability.

3 Example (2): 36V—VRLA battery

3.1 Purpose of analysis

36V-VRLA battery (hereinafter referred to as 36V battery) has very high requirements for battery performance in terms of its use conditions. It is well known that high-rate discharge performance is interdependent between the pressure of the plate, separator and pole group (hereinafter referred to as group pressure), but for batteries with multi-cell integral slot structure like 36V, the group pressure varies according to the position of the single cell, and the performance of the single cell varies. As a result, the single cell with poor performance will affect the performance of the entire battery. Here, by using CAE computer-aided engineering design to analyze the 36V battery structure, we can quantitatively grasp the impact of the battery structure on the performance, and at the same time, we can get a wider range of knowledge to improve battery performance.

3.2 Analysis methods

The limited influence factor model of the 36V battery used in the analysis is shown in Figure 9. In the figure, a three-level simulation test was performed on 1/8 of the entire battery based on the symmetry condition, and a pole group model consisting of a pole plate and a separator was inserted into the battery slot. In actual batteries, the pole group is inserted into the battery slot in a compressed state, and the pole group is usually in a group pressure state. In order to apply group pressure during the analysis model, the pole plate part is thinner than the actual one, so that the pole plate part expands (assuming thermal expansion) to reach the specified pole plate thickness, so the pole group in the battery slot is usually in a group pressure state.

The physical properties of the pole group used for analysis are obtained through compression tests of actual pole groups.

3.3 Analysis results

The distribution of battery slot stress in the initial state (when the pole group is inserted into the battery slot) is shown in Figure 10. Due to the expansion and deformation of the battery slot group, the stress is mostly concentrated in the corners. The change in the deformation of the battery slot from the initial state to the expansion of the pole group is shown in Figure 11. As the expansion rate increases, the deformation gradually increases. When the expansion rate exceeds 20%, the deformation increases sharply. This is because the expansion rate can be balanced by compressing the partition within a small range. However, when the expansion rate approaches 20%, the partition becomes inelastic, the rigidity of the pole plate increases, and the impact of the expansion on the side of the battery slot is fully manifested.

The deformation of cells at different positions is shown in Figure 12. The deformation of the 1st and 9th cells at both ends is larger than that of other cells. In particular, when the expansion rate is below 20%, more than 90% of the overall deformation of the battery is concentrated in the 1st and 9th cells. The pressure distribution between the pole group and the inner wall of the battery tank is shown in Figure 13, and the pressure values obtained for each cell are shown in Figure 14. It can be seen from the figure that the pressure of the side cells is smaller than that of other cells.

The results of the high-rate discharge test of a 36V battery and the discharge voltage of each cell are shown in Figure 15. The contents of Figures 14 and 15 are basically consistent, and the discharge performance is inseparable from the group voltage.

3.4 Research on battery slot structure

The deformation of the battery slot when the shape of the ribs is changed is shown in Figure 16. The deformation of the transverse ribs is small because the shape of the single cell is vertical. When the short side expands outward, the curvature of the transverse ribs is larger than that of the vertical ribs, so the transverse ribs have a significant effect.

4 Summary

It has been confirmed through practice that the use of CAE computer-aided engineering design to develop products is very effective in battery research and development. It can not only shorten the development cycle but also reduce costs.

Reference address：Discussion on CAE Application in Electric Vehicle Batteries

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