Research On Equalization Technology For Lithium Battery Pack In Electric Vehicle

With the development of society and technology,the resource consumption is growing and the energy crisis and environmental pollution problems are turning worse day by day.As an important part of national energy development strategy,Electric vehicles have received strong support from the government and companies.At present,the main power battery for electric vehicles is lithium battery.Because the voltage and capacity of single lithium battery are small,in order to meet the power requirement for practical applications,lithium batteries are generally used in parallel-series to form the battery pack.However,the imbalance caused by inconsistency of batteries in series and parallel may cause life reduction and safety problems.In this paper,lithium batteries are used as the research objects,and equalization technology for lithium battery pack is studied.In this paper,the existing lithium battery model is analyzed in order to understand the characteristics and inconsistency of the battery,and the appropriate second-order RC equivalent model is selected.Based on experimental data from the Hybrid Pulse Power Characterization(HPPC)cycle test,the identification of battery model parameters is completed and the battery equivalent model is build.What's more,the battery equalization variable is analyzed,and State of Charge(SOC)is selected as equalization variable.Then the existing methods of SOC estimation is analyzed and the Extended Kalman Filter(EKF)is selected to estimate the SOC,to solidify the foundation of control of balancing.The research on equalization system for lithium battery is divided into equalizer and the equalization circuit structure.In terms of equalizer,the advantages and disadvantages of the inductive equalizer,capacitive equalizer and traditional LC resonant equalizer are analyzed.Then a novel bridge LC resonant equalizer is proposed to solve the problem that the traditional LC resonant equalizer current is affected by the battery voltage difference and the equalization end speed is slow.In terms of equalization circuit structure,distributed structure and traditional centralized structure are analyzed.And a centralized structure based on relay array is presented,which has the advantages of low cost and control complexity.Considering that the battery balance current changes rapidly in the actual situation and the current sampling can increase the calculation load of controller,an EKF estimation method based on the predictive average current is proposed to estimate the battery SOC.According to the characteristics of equalizer and equalization circuit structure,a cell-to-cell balancing control strategy in the rest state is designed,to make SOC of battery consistent.Finally,the design of the experimental platform is introduced.And the experime nt is carried out to verify the feasibility.