Study On Hybrid Equalization Technology Of Lithium-ion Battery For Electric Vehicle

Electric vehicles are developing rapidly in China.Battery system,as the power source of electric vehicles,determines the performance and cost-effectiveness of the whole vehicle to a large extent.The equalization technology can extend the service life of the power battery and ensure the safe operation of the battery pack.But the efficiency of the common passive equalization method is low,and the active equilibrium method is complex and the cost is high.Therefore,this paper proposes a new hybrid equalization technology with high efficiency and fast equalization.The main work is as follows:Firstly,a class free active passive equalization circuit with switch array is proposed.The topology and working principle of the proposed equalization circuit are analyzed in detail.The theoretical analysis of the hierarchical active passive equalization circuit is verified by building the simulation model.Secondly,in view of the problem that the power representation of lithium-ion battery changes greatly and the accuracy is low,which leads to the long equalization time,a new active passive equalization control strategy is proposed by real-time segmentation.The strategy combines the relationship between open circuit voltage and charge state of lithium-ion battery,and carries out the transformation control of two threshold in charge and discharge process.The experiment shows that the charging accuracy of the battery pack is improved by about 2% and the equilibrium time is shortened by nearly 10% under the condition of the constant charging and discharging speed.The current and voltage are slowed down at the end of charging,which proves the feasibility of the control method.Thirdly,in the traditional charge transfer equalization method,there are repeated cycle charging and discharging phenomena among multiple single cells,which leads to the problem of long equalization time and low efficiency of lithium-ion battery group,a dynamic dual threshold active passive equalization control strategy is proposed.According to the boundary standard of charge state,the charging mode of each unit controller in different sections is adjusted in real time,the current size is changed,so as to reduce the number of invalid cycle charge and discharge,thus improving the equalization efficiency and delaying the reduction of the cycle life of lithium-ion battery group.The experimental results show that the proposed strategy based on dynamic dual threshold is suitable for active and passive equalization circuits,which can adjust the charge discharge rate in real time and reduce the number of useless cycle charge and discharge.The equilibrium time in charge state is about 15% shorter than that of single threshold voltage equalization method,and 19% in discharge state.Finally,the simulation model of active and passive equalization circuit is constructed,and the simulation experiments are carried out by using voltage or SOC single threshold equalization strategy and dynamic dual threshold equalization strategy.The control variable method is selected to compare and analyze the two strategies,and the performance of the dynamic dual threshold equalization strategy proposed in this paper is verified.In addition,the hardware and software design of the equalization system are carried out,and the hardware experimental platform is built.Combined with the software control,the experiment verification of the active passive hybrid equalization system proposed in this paper is verified.