Research And Design On Battery Management System For Electric Vehicle

Electric vehicles recently have gained acceptance as a green transportation alternative.In electric vehicle battery applications,connecting many battery cells in series/parallel to meet voltage and power demand is required.Oprating these cells safely and efficiently is undoubtly essential,which is the main task of a battery management system(BMS).BMS is very important to maximize the battery pack performance,ensure the safe use of battery and prolong the battery lifetime,which becomes a founamental component of an electric vehicle.BMS has already drew a widely public attention.However,some technical problems of BMS haven't been solved yet.Here two key technologies State of Charge(SOC)estimation and cell balancing are studied.Firstly,Li-ion battery is modeled by a second-order RC equivalent circuit,and its parameters are obtained from Recursive Least Square method(RLS).Because this model can be described by a linear state equation and a non-linear measurement equation,an algorithm for SOC estimation based on mixed square high cubature Kalman filter is developed.The developed algorithm using the linear Kalman filter duing time update process,and adopting the square high cubature Kalman filter in measurement update process,can realize less SOC error and less computation difficulty.Secondly,an active equalizer based on a bidirectional forward converter is proposed.The equalizer allows energy transmission between the battery cell and battery pack.During charging period,it can processe peak-cut equalization to avoid overcharging.During discharging period,it can conduct valley-fill equalization to prevent over-discharging.Finally,hardware and software of the BMS for a battery pack composed of 12 Liion battery cells in series are designed.The system uses the controller MPC5634,voltage monitor chip LTC6811 and current monitor chip AS8510.Through bi-CAN bus communication,the system can realize data acquisition(battery pack voltage,cell voltage,current,temperature and insulation resistance),SOC estimation,cell balancing,protection and alarm function.The experimental results show the Li-ion battery pack can be efficiently monitored and managed in real time by the designed BMS,also validate that the proposed SOC algorithm and equalization method have good application value.