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1)Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201999,China;2)State Key Laboratory of Development and Application Technology of Automotive Steels (Baosteel),Shanghai,201999,China

Abstract: As the only power source of pure electric vehicles,the performance of battery packs is easily affected by the temperature,and too high or too low temperature will make the performance of battery packs decline.In this study,the thermal analysis finite element modeling of a cast aluminum battery pack and steel battery pack of a pure electric vehicle is established to compare the thermal insulation performance of two kinds of battery packs under high- and low-temperature conditions.The simulation results show that the thermal insulation performance of the two kinds of battery packs meets the design requirements under high- and low-temperature conditions.The external environment of the cell and battery pack mainly transmits heat through heat conduction.Aiming at the problem that the uniform temperature performance of the steel battery pack is lower than that of the cast aluminum battery pack,several optimization solutions are put forward for the insulation design of the steel battery pack,and the optimal solution is obtained by comparing the simulation results.

Key words: battery pack; thermal management; insulation; finite element analysis

1 Introduction

In recent years,electric vehicles have become increasingly popular in the domestic market.However,due to the climate difference between the north and south in China and the temperature limitation of the power batteries of electric vehicles,electric vehicles still lack competitiveness compared with traditional vehicles.To improve the adaptability of power batteries to the environment,experts and scholars in China and abroad have conducted relevant research,so that the thermal management system can provide suitable working environments for power batteries under different working conditions.In addition to its own system design,an efficient thermal management system also needs to be matched with an excellent thermal insulation design.

The selection of materials and structures of battery packs has always been a hot issue for researchers.DONG et al.[1]chose the combination of aerogel materials and vacuum insulation technology.Compared with the simple aerogel insulation system,the decrease rate of the battery center temperature was reduced by 18.9%,and the effect was very obvious.Through the theoretical calculation of the heat transfer and computational fluid dynamics simulation analysis,WANG et al.[2]verified that a coated glass fiber three-dimensional fabric inside the battery pack had a very good thermal insulation effect.However,to meet the sealing and strength requirements,most battery packs normally use metal materials.However,in general,the thermal condu-ctivity of metals is high,and it is difficult to achieve long-term heat preservation under high- and low-temperature conditions.Therefore,it is necessary to study and improve the thermal insulation perfor-mance of metal material shells.In view of this pro-blem,WANG et al.[3]designed a battery pack insul-ation layer.After comparing different insulation materials,aerogel was used as the final insulation layer material.Through a temperature field simula-tion and test,the results show that under a low-tem-perature condition of -20 ℃,the cooling rate and cut-off maximum temperature difference of the battery pack after the insulation layer was arranged were relatively reduced,which proves that the insulation method has strong applicability in the battery pack and can improve the performance of the battery in a low-temperature environment.However,the method makes the structure of the battery case relatively complex.Therefore,based on the battery case shell material to study,many researchers have chosen different materials to enhance the thermal insulation performance of shells.JI et al.[4]used hollow fabric composite material as the battery case shell material.The results show that compared with the metal box,the time for the temperature of the hollow fabric composite box to drop from 20 ℃ to 0 ℃ is reduced by 2.5 h,which increases the operation guarantee of the power battery in the low-temperature environment[4].However,the model still has the defect of low strength of the key parts,so metal materials are the most suitable choice of raw materials for battery case shells so far.

In this study,the finite element analysis method was used to analyze and compare the thermal insulation performance of a cast aluminum battery pack and steel battery pack under high- and low-temperature conditions,and the thermal insulation performance of the steel battery pack was optimized.

2 Geometric model of the battery pack

In this work,the thermal insulation performance of two kinds of power battery packs based on a pure electric vehicle was examined.The power battery pack of the vehicle has two kinds:cast aluminum battery pack and steel battery pack.The upper covers of the two battery packs adopt a sheet molding compound material with the same thickness,as shown in Fig.1.The lower boxes are divided into a cast aluminum lower box and a steel lower box,as shown in Fig.2.With the same number and arrangement of modules,the battery pack has 32 battery modules,which are divided into two layers,18 in the lower lay-er and 14 in the upper layer.The size of a single module (length×width×height) is 355.0 mm×151.6 mm×108.5 mm,as shown in Fig.3.

Fig.1 Battery pack upper cover

Fig.2 Lower boxes of the battery packs

Fig.3 Battery modules

3 Establishment of the finite element simula-tion model of the battery pack

3.1 Modeling of the battery pack box

The cast aluminum battery pack box includes the upper cover,cast aluminum lower box,and model bracket.The steel battery pack box includes the upper cover,border,ear,bottom plate,transverse and longitudinal beams,and model bracket.HyperMesh has an extremely powerful pre-process-ing meshing function.The pre-processing module was used to reasonably simplify the model and divide the grid.Choosing the appropriate grid type and grid size has a great influence on the efficiency of modeling,the accuracy of analysis and the speed of solution.The shell under the battery pack is mainly composed of quadrilateral elements,supple-mented by triangular elements.The unit size is controlled at approximately 5 mm×5 mm.The finite element models of the cast aluminum battery pack and steel battery pack box are shown in Fig.4.

3.2 Battery module modeling

The battery module includes a battery cell,heat insulation pad,module end plate,module side plate,and module upper cover.The heat insulation pad,module side plate,and module upper cover use a 5 mm×5 mm quadrilateral element.The module end plate uses a tetrahedral solid element with a base size of 5 mm,and the battery cell uses a hexahedral solid element with a base size of 15 mm.The finite element model of the battery modules is shown in Fig.5.

3.3 Modeling of other accessories

Other accessories include the thermal manage-ment system,electrical system,and plug-in inter-face.The thermal management system mainly includes a heating film,which is modeled by a 5 mm×5 mm quadrilateral element.The electrical system includes a copper plate,cable,battery disconnect unit,and battery management system.Considering the influence of the heat transfer and the solution efficiency,the electrical system only retains the connection between the copper plate and module and uses the quadrilateral element of 5 mm×5 mm to model.The finite element model of other accessories is shown in Fig.6.

Fig.6 Finite element model of the thermal analysis of other accessories of the battery pack

3.4 Thermophysical parameters setting of the battery pack parts

The finite element models of the thermal analysis of the cast aluminum battery pack and the steel battery pack were imported into the thermal analysis software to set the boundary conditions after the grid drawing was completed.The physical pro-perties of the parts,initial conditions,environmental conditions,and thermal connection were set.The thermal physical parameters of the finite element model of the main parts are shown in Table 1.

3.5 Thermal insulation analysis of the battery pack simulation conditions

According to the performance of the pure electric vehicle and the weather characteristics of China,the ambient temperature was set to 40 ℃ under high-temperature conditions,and the initial temperature of the battery pack was set to 20 ℃.The ambient temperature was set to -15 ℃ at the low-temperature condition,and the initial temperature of the battery pack was set to be 20 ℃.

Table 1 Thermal physical parameters of the battery pack components

4 Analysis of the simulation results

After the thermal simulation models of the cast aluminum battery pack and the steel battery pack were built,the finite element solution software was used to perform the calculation,and the post-processing module of the software was used to view and analyze the calculation results.According to the requirements of enterprise standards,the average temperature change rate of the cell within 8 h should be less than 3 K/h,and the maximum difference of the high and low temperatures of the cell should be less than 5 K.

4.1 Simulation results of the thermal insulation performance of the cast aluminum battery pack

4.1.1 Simulation results of the high-temperature conditions

The simulation results show that the average temperature of the aluminum battery pack cell increases from 20.00 ℃ to 32.45 ℃ after 8 h,and the average temperature change rate of the battery pack cell is 1.56 K/h,as shown in Fig.7.The temperature difference between the high and low tem-peratures of the cell increases first and then decreases within 8 h,and the maximum value is 2.32 K,as shown in Fig.8.The average temperature change rate of the cell and the maximum difference between the high and low temperatures meet the standard requirements.

By using the post-processing function of the software,the heat transfer of the aluminum battery pack cell was counted.The heat entering the cell by the heat conduction mode is 3 242.94 kJ,which mainly occurs in the end plate of the battery module and the bottom of the cell.The heat entering the cell by convective heat transfer mode is 637.20 kJ,which mainly occurs in the upper part and side plate of the cell.The heat entering the cell by radiation heat transfer mode is 604.56 kJ,which mainly occurs in the upper part and side plate of the cell,as shown in Fig.9.

Fig.7 Change in the cell temperature of the cast aluminum battery pack at high temperatures

Fig.8 Cell temperature difference curve of the cast aluminum battery pack at high temperatures

4.1.2 Simulation results of the low-temperature conditions

The simulation results show that the average tem-perature of the cast aluminum battery pack cell decreases from 20.00 ℃ to -0.27 ℃ after 8 h,and the average temperature change rate of the cell is 2.53 K/h,as shown in Fig.10.Within 8 h,the temperature difference between the high and low tem-peratures of the cell showed a trend of increasing first and then decreasing,with a maximum value of 3.58 K,as shown in Fig.11.The average temperature change rate of the cell and the maximum difference between the high and low temperatures meet the standard requirements.

Fig.9 Heat transfer of the cast aluminum battery pack by different heat transfer modes at high temperatures

By using the post-processing function of the software,the heat transfer of the aluminum battery pack cell was counted.The heat transferred out of the cell by the heat conduct mode is 5 342.49 kJ,which mainly occurs at the end plate of the battery module and the bottom of the cell.The heat by the convection heat transfer mode is 1 109.16 kJ,which mainly occurs in the upper part and the side plate of the cell.The heat by the radiation heat transfer mode is 856.26 kJ,which mainly occurs in the upper part and the side plate of the cell,as shown in Fig.12.

Fig.10 Cell temperature change of the cast aluminum battery pack at low temperatures

Fig.11 Cell temperature difference curve of the cast aluminum battery pack at low temperatures

Fig.12 Heat transfer of the cast aluminum battery pack by different heat transfer modes at low temperatures

4.2 Simulation results of the thermal insulation performance of the steel battery pack

4.2.1 Simulation results of the high-temperature conditions

The simulation results show that the average temperature of the steel battery pack cell increases from 20.00 ℃ to 32.13 ℃ after 8 h,and the average temperature change rate of the battery pack cell is 1.52 K/h,as shown in Fig.13.The tem-perature difference between the high and low tem-peratures of the cell increases first and then dec-reases within 8 h,and the maximum value is 2.45 K,as shown in Fig.14.The average temperature change rate of the cell and the maximum difference between the high and low temperatures meet the standard requirements.

Fig.13 Change in the cell temperature of the steel battery pack at high temperatures

Fig.14 Cell temperature difference curve of the steel battery pack at high temperatures

The heat transfer of the steel battery pack cell was counted using the post-processing function of the software.The heat entering the cell by the heat conduction mode was 3 169.56 kJ,which mainly occurred in the end plate of the battery module and the bottom of the cell.The heat entering the cell by convection heat transfer mode was 634.86 kJ,which mainly occurred in the upper part and the side plate of the cell.The heat entering the cell by radiation heat transfer mode was 601.74 kJ,which mainly occurred in the upper part and the side plate of the cell,as shown in Fig.15.

4.2.2 Low-temperature simulation results

The simulation results show that the average temperature of the steel battery pack cell decreases from 20.00 ℃ to 0.32 ℃ after 8 h,and the average temperature change rate of the cell is calculated to be 2.46 K/h,as shown in Fig.16.Within 8 h,the temperature difference between the high and low temperatures of the cell showed a trend of increasing first and then decreasing,with a maximum value of 4.01 K,as shown in Fig.17.The average temperature change rate of the cell and the maximum difference between the high and low temperatures meet the standard requirements.

Fig.15 Heat transfer of the steel battery pack by different heat transfer modes at high temperatures

Fig.16 Change in the cell temperature of the steel battery pack at low temperatures

Fig.17 Cell temperature difference curve of the steel battery pack at low temperatures

By using the post-processing function of the software,the heat transfer of the steel battery pack cell was counted.The heat transferred out of the cell by the heat conduct mode was 5 262.84 kJ,which mainly occurred at the end plate of the battery module and the bottom of the cell.The heat by the convection heat transfer mode was 1 103 kJ,which mainly occurred in the upper part and the side plate of the cell.The heat by the radiation heat transfer mode was 854.7 kJ,which mainly occurred in the upper part and the side plate of the cell,as shown in Fig.18.

Fig.18 Heat transfer of the steel battery pack by different heat transfer modes at low temperatures

4.3 Cast aluminum battery pack and steel battery pack insulation performance comparison

4.3.1 Comparison of the cell average temperature change rate

Under high-temperature conditions,the average temperature change rate of the cast aluminum battery pack cell is 1.56 K/h,and the average temperature change rate of the steel battery pack cell is 1.52 K/h,as shown in Fig.19.Under low-temperature conditions,the average temperature change rate of the cast aluminum battery pack cell is 2.53 K/h,and the average temperature change rate of the steel battery pack cell is 2.46 K/h,as shown in Fig.20.

Fig.19 Average cell temperature curves of the cast alumi-num battery pack and steel battery pack at high temper-atures

Fig.20 Average cell temperature curves of the cast alumi-num battery pack and steel battery pack at low tempera-tures

4.3.2 Comparison of the cell temperature difference

Under the high-temperature conditions,the maxi-mum temperature difference between the high and low temperatures of the cast aluminum battery pack cell is 2.32 K,and that of the steel battery pack cell is 2.45 K,as shown in Fig.21.At low tempera-tures,the temperature difference between the high and low temperatures of the cast aluminum battery pack cell is 3.58 K,and that of the steel battery pack cell is 4.01 K,as shown in Fig.22.

5 Thermal insulation performance analysis of the steel battery pack

The simulation results of the previous section

Fig.21 Cell temperature difference curves of the cast alumi-num battery pack and steel battery pack at high temperatures

Fig.22 Cell temperature difference curves of the cast alumi-num battery pack and steel battery pack at low temperatures

show that the average temperature change rate of the steel battery pack cell is lower than that of the cast aluminum battery pack cell under the high-temperature condition,but the maximum difference between the high and low temperatures of the steel battery pack cell is greater than that of the cast aluminum battery pack cell.In view of the above analysis,to improve the thermal insulation performance of the steel battery pack,it is necessary to optimize it and reduce the maximum difference between the high and low temperatures of its cell.

Based on the analysis of the simulation results of the steel battery pack,the highest temperature of the steel battery pack cell appears in the second module at the front end of the first layer,and the highest temperature always appears there throughout the simulation process,as shown in Fig.23.The reason for this situation is that the front beam at the bottom of the steel battery pack is set at the front end of the battery pack to ensure the structural strength of the battery pack.The front beam is located at the bottom of the second module at the front end of the first layer,which increases the heat transfer between the module and the external environment,resulting in a high temperature in the module,thus affecting the global temperature difference of the steel battery pack cell,as shown in Fig.24.

Fig.23 Position diagram of the high-temperature module

Fig.24 Installation position of the high-temperature module

Based on the above analysis,it is necessary to block the heat transfer path between the high-tem-perature module and the external environment.Four optimization solutions are proposed for this pur-pose.The solution methods are shown in Table 2,and the schematic diagrams are shown in Fig.25.

After calculation,the calculation results of each solution were counted and compared with the original cast aluminum battery pack and steel battery pack.The detailed data are shown in Table 3.

Table 2 Optimization solutions of the thermal insulation performance of the steel battery pack

Fig.25 Schematic diagrams of the optimization solutions

Table 3 Optimization solution result statistics

Based on the analysis of the simulation results of the optimization solution,the average temperature rise rate of the original steel battery pack is close to that of the four solutions and is lower than that of the cast aluminum battery pack.The maximum cell temperature difference of solution 2 is lower than that of the original steel battery pack and cast aluminum battery pack.The maximum cell temperature difference of solution 4 is lower than that of the original steel battery pack and cast aluminum battery pack,too.The maximum cell temperature difference of solution 1 is similar with the original steel battery pack.The maximum cell temperature difference of solution 3 is higher than that of the original steel battery pack.

6 Conclusions

In this study,the thermal analysis finite element modeling of the cast aluminum battery pack and steel battery pack of a pure electric vehicle is established,and the thermal insulation performance of the two kinds of battery packs under high- and low-temperature conditions is compared.The simulation results show that under the high- and low-temperature conditions,the average temperature change rate of the cells of the cast aluminum battery pack and steel battery pack meets the requirement that the average temperature change rate should be less than 3 K/h within 8 h,and the average temperature change rate of the cells of the steel battery pack is less than that of the cast alumi-num battery pack.The maximum difference between the high and low temperatures of the cells of the cast aluminum battery pack and steel battery pack meets the requirement of being less than 5 K,and the tem-perature uniformity of the cells of the steel battery pack is lower than that of the cast aluminum battery pack.The heat transfer between the cell and the outer environment of the battery pack is mainly carried out by heat conduction mode.Aiming at the problem that the temperature uniformity of the steel battery pack is lower than that of the cast aluminum battery pack,four optimization solutions for improving the temperature uniformity of the steel battery pack are proposed,and the optimal solution is determined according to the simulation results,which provides a certain refer-ence value for the subsequent battery pack insul-ation design.