Research On Passive Balancing Strategy And SOC Estimation Of Electric Vehicle Battery Pack

With the rapid growth of car ownership,the energy crisis has become increasingly serious,and the development of electric vehicle is the general trend.The battery pack is an important part of the electric vehicle and the power source of the electric vehicle.The State of Charge(SOC)of power battery is equivalent to the fuel gauge of traditional fuel cars.Accurate SOC estimation reflects the correct amount of remaining power,indicating how far it can travel,and plays a crucial role in preventing overcharge and overdischarge of batteries and extending battery life and safety.Due to the inconsistencies in the production and use of battery monomers,there are differences in battery monomers,which affect the safety of battery usage.In this paper,the ternary lithium-ion battery pack is taken as the research object,and the passive equalization circuit is designed and the equalization strategy is used to reduce the inconsistencies between batteries,on this basis,the SOC of electric vehicle battery pack is estimated.First of all,under the conditions of ensuring accuracy and fast calculation,based on the Ternary lithium-ion battery,second-order RC equivalent circuit model is established.Genetic Algorithm(GA)is used to identify the model parameters that are verified under the conditions of HPPC and UDDS,respectively.The experimental results verified the validity of the model and the accuracy of the model parameter estimation.Secondly,in order to eliminate the influence of power cell monomer inconsistency on power battery pack,the causes and elimination methods of battery inconsistency are analyzed.A passive equalization method based on voltage equalization is proposed to reduce the inconsistencies among individual batteries.And a corresponding balanced control strategy is designed to realize the balanced opening and closing.Experimental results show that the designed circuit and algorithm make the voltage difference between individual batteries within the set equilibrium threshold,which achieves the design goal.Then,the Extended Kalman Filter(EKF)is used to compared with the Square Root Cubature Kalman Filter(SCKF)to estimate the SOC of the single cell.The comparison results show that under HPPC conditions,the root mean square error(RMSE)and average absolute error(MAE)of SCKF estimation are better than EKF.At the same time,the convergence rate of the proposed algorithm under different initial SOC is compared to prove the fast convergence of SCKF algorithm.In addition,the filtering algorithm is verified by Urban Dynamometer Driving Schedule(UDDS)and Dynamic Stress Test.The verification results show that the SCKF algorithm has better estimation accuracy.Finally,based on the determination of the SOC of the single cell,an adaptive smoothing filter for the SOC of the battery pack is proposed.According to the measured voltage of the single battery,the maximum and minimum voltage of the battery parallel are selected.By adjusting the weight factor and offset,the square root Cubature Kalman filter algorithm is used to find the maximum battery SOC and the minimum battery SOC in the battery pack.Finally,the SOC of the output battery pack is calculated,making the battery pack SOC between the maximum and minimum SOC smooth transition.Experiment results show that the proposed algorithm is reasonable and can avoid overcharge and overdischarge of battery pack.