Research On Fast Charging Strategy Of Electric Vehicle Based On SOC Online Estimation

With the increasing severity of energy crisis,environment pollution and greenhouse effect,the concept of green energy consumption has been promoted and deepened around the world.Electric vehicles which have the most energy-saving potential and market prospects of new energy vehicles,have become the focus of the automotive industry in various countries.It has been put on the agenda of the alternative to the traditional automobile industry.However,the driving range of the electric vehicle battery has not satisfied the requirement of customers,which seriously restricts the popularization and application of the electric vehicle.Therefore,it's important to research on the related technical problems of the fast charging strategy of electric vehicle,to provide theoretical and practical support for the design of the charging system.Firstly,the research status and development trend of electric vehicle charging system,charging method and SOC estimation is introduced.The interleaved parallel bidirectional Buck/Boost converter based on auxiliary inductor is chosen to be the main circuit topology.The comparison of working principle is made over independent conduction and complementary conduction.The auxiliary inductor is designed to realize soft switching.Based on the comprehensive consideration of the model accuracy and the algorithm rapidity,the Thevenin model is chosen to be the battery equivalent circuit model.The parameters of the model are identified by using the HPPC test method.By means of the least square method and piecewise linear function method,the values of each parameter in different state of charge are obtained.The online estimation algorithm of SOC based on sliding mode observer is designed.The sliding mode observer is designed by parameter identification and stability analysis.The saturation function is improved by using the sine function to speed up the convergence rate and reduce the system chattering.Then according to the characteristics of the battery,combined with the advantages of the topology which can work bidirectionally,a fast charging algorithm based on SOC online estimation is proposed.Taking SOC as the switching sign rather than the battery terminal voltage is more accurate.Finally,a 2kW prototype with digital control is designed and built.The simulation and experimental results show that,the fast charging strategy is effective.