Study On The State Of Charge Of Lithium Ion Batteries

In recent years,with the development of economy and the aggravation of environmental pollution,the traditional fossil energy has not been able to meet the needs of human life.To this end,many vehicles such as trains,cars and so began to use electricity as energy.Electric energy can be stored in a variety of media,the daily life of the most widely used is the battery.Battery state of charge(SOC)estimation is a basic application of battery,battery charge and discharge depth in use,will greatly reduce the service life,accurate estimation of SOC can avoid this situation;accurately display the remaining battery power,but also help the auto control system can calculate the mileage,the driver can also be a better route planning.The above advantages indicate that the research of SOC is of great significance.The chemical structure of the lithium ion battery with the principle of lithium batteries is analyzed,the charge discharge voltage characteristics through the experiment on the ladder of charge and discharge of lithium batteries,to analyze the changes of the ambient temperature,the charge discharge rate of lithium battery performance.According to the four classic lithium ion battery circuit model,namely the existing internal resistance,Thevenin,deficiency of PNGV and GNL model,put forward two order RC equivalent circuit model used in this paper,and deduces the state space model of the two order model.Then,based on the experimental data of constant current discharge,the OCV-SOC relation of the lithium battery is approximated by the five polynomial.Based on the experimental data,the parameter fitting method and the RLS algorithm with forgetting factor are used to identify the parameters of the two order model of the lithium-ion battery off-line and on-line.MATLAB simulation results show that the two order equivalent circuit model established in this paper can well reflect the characteristics of the terminal voltage of the lithium battery.So as to lay the foundation for the SOC estimation of lithium ion batteries.Li ion battery system is a complex system,taking into account the extended Kalman filter(EKF)algorithm to solve the problem of nonlinear filtering system,this paper uses EKF to predict the SOC of lithium battery.Based on the two order equivalent circuit model of lithium batteries,deduces the linear discrete state space model,the lithium battery SOC,polarization voltage and noise variance matrices such as initial value,using EKF by constant current discharge,the periodic pulse discharge lithium battery state,FUDS state and BJDST state the SOC was estimated.And in the EKF under the condition of the SOC estimate of the results and the actual results of the implementation of the simulation experiment,the conclusion,for the FUDS and BJDST conditions,although larger in early stage of SOC error evaluation,but ultimately EKF can successfully predict the change of SOC lithium battery.The results show that EKF is feasible for SOC estimation of lithium battery,and EKF has good robustness in SOC evaluation of lithium battery.