Research On State Estimation Method For Escalated Utilization Of Retired Lithium Batteries For Electric Vehicles

After reaching the decommissioning standards,the lithium-ion batteries of electric vehicles can be used to scenarios with small power demand in stages according to different degradation levels of the batteries,which can not only make rational use of battery resources,but also have great significance for environmental protection,energy conservation and emission reduction.The State of Charge（SOC）,State of Health（SOH）and State of Energy（SOE）of the battery reflect the current aging degree and the maximum available energy of the battery.Accurate estimation of the SOH and SOE can accurately divide the working conditions of the battery.In this paper,the retired lithium batteries of electric vehicles are taken as the research object,and the state estimation method is explored to make use of the retired lithium batteries in appropriate scenarios.The specific research work is as follows:Firstly,three kinds of typical lithium batteries used in electric vehicles were taken as the research object,and their electrochemical mechanism was analyzed.The battery test equipment was used to conduct charging and discharging experiments on the batteries.The constant current and constant voltage characteristics,rate charging and discharging characteristics and cycle characteristics of the batteries were studied,and the parameter variation process of the batteries was observed.The Thevenin equivalent circuit model,the PNGV equivalent circuit model,the second-order and third-order RC equivalent circuit model were compared,and the equivalent circuit model with similar battery characteristics was selected to characterize the internal reaction process of the retired lithium battery.The state equation and observation equation of the battery were calculated,and the model parameters such as current and voltage were identified by the least square method.Then,a SOH estimation method based on Extended Kalman Filtering combined with regional concept algorithm（EKF-RC）was proposed to solve the problem of low SOH estimation efficiency and accuracy of retired lithium batteries.In this method,the SOH state equation and observation equation calculated by the third-order RC equivalent circuit model are realized by the EKF algorithm.The specific regional capacity and regional voltage are calculated by using the concept of region to reduce the data and time needed for battery parameter estimation.The SOC and R₀of the battery are estimated by EKF-RC algorithm,and then the SOH of the battery is estimated.The accuracy and rapidity of this method were verified by battery charging and discharging experiments.Finally,a three-order RC equivalent circuit model with high accuracy was established to estimate SOE of retired lithium batteries,and the state equation and observation equation of SOE were calculated by using the equivalent model.UKF algorithm is used to estimate SOE,and the influence of temperature,heat and other factors on the accuracy of UKF algorithm was analyzed.The battery was modeled in ANSYS,and the temperature and heat changes of the battery at different discharge rates were analyzed.The influence of temperature was processed by Square Root Unscented Kalmn Filter（SRUKF）algorithm,and the more accurate SOE was estimated.The estimation accuracy of SRUKF was verified by experiments.