Study On Modeling And Performance Analysis Of Series-Parallel Connected Battery Pack For Electric Vehicles

To satisfy the voltage and capacity requirements of electric vehicles,large numbers of lithium-ion battery cells are always connected with series-parallel forms to construct battery packs.However,the inconsistencies between cells are inevitable,because of the difference in internal physical parameters and the difficulty in controlling the chemical reaction process.It will affect the performance of the battery packs,and the degree of influence on the battery packs with different series-parallel forms is different.The paper studies the improvement of the performance of series-parallel connected battery packs under the condition of inconsistent cells,aiming at guiding the selection of the series-parallel form of a battery pack and the distribution of the cells within it.Firstly,the six common equivalent circuit models of a battery cell have been established and the corresponding parameters of the models have also been identified by using the experimental data and the genetic algorithm.Considering the accuracy and complexity of the models,the first-order RC equivalent circuit model is selected for subsequent research.Secondly,the initial capacity models of the series-parallel battery packs have been established to analyze the influence of the different arrangement ways of cells and the different standard deviation of capacities of cells on the initial capacity of the battery packs.And a method that makes the battery packs achieve their maximum initial capacity has been proposed.After that,a dynamic modeling and analysis method for the battery packs has also been proposed.And the accuracy of the simulation model and the correctness of the relevant conclusions are verified by the performance analysis and evaluation platform of the battery packs.Finally,the charge/discharge performance of the battery packs under different working conditions,different discharge C-rates and different ambient temperatures have been compared by analyzing the relevant experimental data.The results show that the battery pack in parallel and then in series has a better performance on the initial capacity.Moreover,in the case of constant-current charge/discharge,it also has better terminal voltage performance and higher charge/discharge capacity and utilization rate of cells.Besides,the battery pack has stronger adaptability to different working conditions,and has better charge/discharge related performance under different discharge C-rates and different ambient temperatures.Therefore,to improve the performance of a series-parallel battery pack,the grouping method of in parallel and then in series and the arrangement of cells that makes the battery pack obtain the maximum initial capacity should be selected.