Research On Charging Characteristics And Control Strategies Of Lithium-ion Batteries For Electric Vehicles

In the past period of time,the world's major area for the development of social economy,the world's environment is further deteriorating.In order to reduce the impact of the car on the environment,the traditional fuel vehicles are gradually restricted.In addition,the rise in the price of crude oil has also caused a setback to the automobile industry and urges the necessity to develop new and alternative fuel vehicles.In order to address the problems,electric vehicle(EV)has gained broad application prospects in reducing greenhouse gas emissions,and its implementation is widely concerned by academic and automotive experts.The application of rechargeable batteries in EVs has become very popular in recent years.Among them,lithium-ion battery is widely used in EVs,mobile phones and various energy storage devices due to its high performance,little environmental pollution,high energy density and long lifespan.However,the power battery has been hindering the development of EVs due to the long charging time,high charging energy loss and high charging temperature increment.Therefore,the battery charging management is a key but challenge issue for the popularization and application of EVs.An excellent charging strategy can restrain the charging temperature rise,shorten the charging time,reduce the charging energy loss and ensure the safety and stability of battery charging.In this paper,the lithium-ion battery is taken as the research object,and the equivalent circuit model(ECM)is applied to establish the lithium-ion battery model.Two charging strategies with different targets are designed to improve the charging performance of the battery.Firstly,a lithium-ion battery test system is set up,the composition and test procedure of the test system are introduced in detail.The characteristics of the battery under different temperatures are obtained by the battery test procedure.On the bases of analyzing of battery characters,a first-order equivalent circuit model(ECM)of the battery is established.Moreover,the battery model's parameters are identified under various environment temperatures,and the database,which describe the relationship among battery parameters,temperatures and state of charge(SoC)is established,so that the parameters can adapt to all kinds of ambient temperatures.It can be seen from the identified results that the battery has the characteristics of lower available capacity,higher internal resistance and narrow open circuit voltage(OCV)curve platform area under low temperature environment.Inversely,the battery usable capacity is increased under higher ambient temperature,but the improvement is not obvious,thus the optimum battery charging temperature is gained.Secondly,the thermodynamic equations of lithium-ion battery during charging and discharging are analyzed,and the model of lithium-ion battery charging temperature rise is established.The model is validated under different charging rates and different initial SoC values condition.Aiming at the optimization target of charging temperature minimum and the charging time constraint,a multi-stage constant current(MSCC)charging strategy based on genetic algorithm(GA)is proposed.In order to verify the effectiveness of the proposed strategy,the proposed strategy is tested at different charging rates,different charging intervals and different initial SoC values.The results show that the proposed charging strategy can not only effectively reduce the battery charging temperature,but also shorten the charging time to some extent.Finally,considering the influence of the battery charging energy loss during the charging process,the principle of battery charging energy loss is analyzed,and the model of the charging energy loss of the lithium-ion battery is established.Combined with the model of charge temperature rise,a simplified mathematical equation of the battery charging temperature rise and charging energy loss model are deduced.A charging strategy with penalty factor considering the charging temperature rise and charging energy loss minimize as the goal and considering the optimal charging temperature of battery is proposed.The Lagrange multiplier method is used to solve the objective function.The experimental results show that the proposed strategy can effectively reduce the charging energy loss and shorten the charging time while increasing the battery temperature under lower ambient temperature.On the other hand,under the higher ambient temperature,the battery charging temperature,charging time,energy loss can be improved by the proposed charging strategy.Besides,the proposed charging strategy can adapt to different types of lithium-ion batteries.