Study On The Optimization Of Battery Pack Based On Inconsistency Analysis

New energy vehicle is the key object of Made in China 2025 and The National key Research and Development Program of China.Power battery is one of the bottleneck technologies of electric vehicle,accurate state estimation of the power battery pack is essential to ensure the safety and reliability of the new energy vehicle.The following research for the inconsistency and the state estimation of the power battery pack are carried out in this paper.(1)Aim to the problem that the performance of battery pack will be restricted by the initial inconsistency,a novel battery screening method based on the extraction of curve feature is proposed.Based on factor analysis and K-means clustering,the battery samples were classified.This paper proposes a method for quantifying the consistency of battery groups,based on which,the performance of the selected batteries was studied under two working conditions,different SOC platform stages and two kinds of current excitations.The results showed that this method has a good screening effect for different operating conditions and the initial inconsistency of battery pack can be reduced effectively.(2)Aim to the problem about the failure of modeling caused by inconsistency,A complete battery model is established based on two different time scales including the average battery model and the difference model.The average battery model is helpful to solve the simulation of voltage characteristics in series,and the identification of the open circuit voltage inconsistency based on the difference model is helpful to optimize the state estimation of the series battery pack.The results show that the average battery model identified by the FF(forgetting factor algorithm)can fit the battery terminal voltage well in a long time,based on the proposed difference model,the identification method of the TLS(total least square)was used to quantify the inconsistency of the cells in the series group by the fixed window length.The results of quantitative identification are consistent with the capacity test.(3)Aim to the problem that the accuracy of SOC estimation for battery pack is seriously affected by the system inconsistency,in this paper,the definition and calculation method of SOC for battery pack based on series-parallel and balanced strategy are proposed.Based on EKF(extended Kalman filtering),the method of SOC calculation for pack is proposed.The calculation results showed that the method achieves high accuracy and fast convergence speed in battery SOC estimation.(4)Aim to the problem of verification for screening and the SOC estimation.a battery pack test platform has been built.According to the requirement of the screening method,the experimental testing process including charge-discharge characteristic test and multi-mode pulse discharge test,which make a data foundation for sample selection and result verification.A series pack with batteries in different cycles was tasted under two different operating conditions,which is used to verify the inconsistent tolerance and generality to working conditions.