| With the increasing global energy problems,new energy vehicles represented by electric vehicles are favored by automobile manufacturers because of their advantages of zero-emission.As the main power source of electric vehicles,lithium-ion batteries have been developing rapidly.However,a lot of heat will be generated during the working process of the battery.The sensitivity of lithium-ion battery to temperature makes it necessary to dissipate the heat in time,so an effective and reliable battery thermal management system is essential to improve battery pack temperature and prolong the battery life.Compared with air cooling and liquid cooling methods,phase-change material(PCM)cooling has unique advantages in controlling the maximum temperature of the battery pack and regulating the temperature consistency of battery in battery thermal management,which has become a promising cooling method.Therefore,the structural design of battery pack thermal management based on PCM is studied.The main research contents include:1.Based on the Bernardi heat generation rate mathematical model,the heat generation mechanism and heat transfer mechanism of the battery were described,and the temperature rise experimental platform of Li-ion battery was designed and built.Through the surface temperature test and data analysis of lithium-ion battery under the same ambient temperature and different charging and discharging conditions,it is found that the temperature rise rate of the battery increases with the increase of charging and discharging ratio.The mixed pulse power characteristic internal resistance experiment was used to test the internal resistance data at different temperature and depth of discharge.The model of dynamic internal resistance was fitted by binary polynomial based on the least square method,and the results were compared with the experimental results.And the results show that the dynamic internal resistance model has high enough precision and can well express the dynamic internal resistance changes in the battery.It lays a solid foundation for the subsequent mathematical model with dynamic heat generation rate and the simulation analysis of the battery temperature field.2.Study on mathematical model with dynamic heat generation rate: the thermal effect model of Li-ion battery was established by computational fluid dynamics simulation analysis method.Then the dynamic internal resistance model was integrated into the Bernardi battery heat generation rate model to construct the mathematical model with dynamic heat generation rate.In the COMSOL Multiphysics multi-physical field simulation software,the temperature field distribution of the battery cell in the natural air flow field under the conditions of 1C constant current charging,3C constant discharge and high-rate cyclic charge and discharge(1C charge-3C discharge)was simulated.The results show that the maximum absolute error of the mathematical model with the dynamic heat generation rate is 2.5℃,and the maximum root-mean-square error is 1.2℃.A composite phase change material mixed with natural oil and graphene was selected,and the initial temperature of phase change was 43℃.To verify the validity of the phase change materials for battery cooling,the cell parcelled with 2 mm thickness of phase change materials was simulated for the calculation of the temperature field distribution,and compared with the cell in the natural airflow field,the results show that the temperature and temperature difference of a single cell parcelled with phase change materials have obvious improvement.3.Selection of battery pack arrangement mode and spacing determination:Based on the cooling method with composite phase change material and the comparative analysis of different battery arrangement methods(cross arrangement,line arrangement and dislocation arrangement),the research focus on the cross arrangement and line arrangement of the battery pack.Based on the structural design and geometric modelling of the PCM-based battery thermal management system,and the temperature field simulation of the battery pack was carried out under the conditions of 3C constant current discharge and twice repeated high-rate charge and discharge cycles.Compared with the temperature field of the cross-arrangement battery pack with the same battery spacing under3 C constant current discharge and repeated twice high-rate charge and discharge cycle,the simulation results show that the temperature control of the linearrangement battery pack is obviously better than that of the cross-arrangement battery pack with the same battery spacing.To ensure the minimum volume of the battery pack,the minimum spacing of the battery pack in two configurations is sought on the premise that the maximum temperature of the battery pack is less than 55℃ and the maximum temperature difference is less than 8℃.The results show that the battery spacing of 4.5 mm is required to meet the above requirements for cross-arrangement battery,while the battery spacing of 3.6 mm is only required for the line-arrangement battery.4.Battery pack optimization with variable battery spacing: In order to improve the battery pack temperature consistency and reduce the maximum temperature of the battery pack,the battery pack with 3.6 mm battery spacing is optimized with variable battery spacing.The cell spacing is arranged in a horizontal arithmetic sequence and a horizontal geometrical sequence,respectively.By comparing two different configuration simulation results of temperature field,it is found that the two kinds of variable spacing configuration such as the highest temperature of the battery pack compared with 3.6 mm spacing of battery has improved significantly,but the biggest difference in temperature increased.At the same time,the temperature difference mainly occurs in the longitudinal direction of the battery pack.It is found that the maximum temperature and minimum temperature of the two variable-spacing layouts are less than or equal to the maximum temperature and maximum temperature difference of the battery pack with equal-spacing of 3.6 mm.Besides,the results show that the optimized result of the arithmetic sequence is better than that of the geometrical sequences.The maximum temperature of the battery pack is 49.3℃,and the maximum temperature difference is 2.3℃. |
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