Research And Design Of Lithium-ion Battery Pack Equalization System

The design and application about Battery management system of electric vehicles is an important research direction nowadays.The battery pack inconsistency which is caused by the process,the using situation is an important factor affecting its performance and so on.Based on the different types of equalization structure,a battery active equalization system is realized for the six-unit series battery pack.The battery pack voltage is used to charge the cell with a minimum SOC(State of Charge)for reducing the differences among the battery SOCs and the purpose of balancing.The paper studies and analyzes different types of battery balancing structures.It summarizes the advantages and disadvantages of different structures and focuses on a kind of centralized balancing structure.It provides a series battery equalization system based on DC-DC isolation.Herein,we analyze the influence of different balancing variables on the equalization effect and the feasibility of its implementation.Finally,we choose SOC as the balancing variable with high feasibility and accuracy.The equivalent circuit model,based on the battery charge and discharge characteristics,was built in Matlab/Simulink.The experiment was used to identify the battery model and the remaining battery capacity was estimated through EKF(Extended Kalman filter).The battery model is used to build the simulation model of the balancing system.The main balancing strategy of the equalization system is to sort the SOC,to select the cell with the lowest SOC as the equalization object and to charge the cell through the battery voltage which is isolated by a step-down converter.The emulation of the battery pack is simulated under three working conditions:discharge,charge and rest,and the simulation results were analyzed.The balancing system experiment tests the rest state of the battery pack and the experimental results of the battery pack are similar to the simulation results which indicates that the balancing system has a good balancing effect on the battery pack.A high equalization speed and accuracy is achieved and the balancing system can improve the performance and the service life of the battery pack.Then,the characteristics of the battery were tested.The influence of different discharge current on the battery capacity was analyzed and the effect of current changes on battery capacity during the balancing process was eliminated.The terminal voltages of the batteries and the SOC curves verify the simulation and experimental results well.