Estimation Of Lithium-ion Battery Internal Temperature And State Of Charge Based On Electrochemical Impedance Spectroscopy

To protect the environment and save energy,EV(electric vehicle)and HEV(hybrid electric vehicls)gradually cause concern.Lithium-ion battery is widely used as power sources for EV and HEV due to its excellent power density and current efficiency.To ensure the safety and prolong the service life of lithium-ion batteries,estimation of the internal state of lithium-ion batteries is essential.This paper estimates the internal temperature and SOC(state of charge)of lithium-ion batteries based on EIS(electrochemical impedance spectroscopy),and the following is done.First,the relationship between the transfer function and the electrochemical impedance spectroscopy is introduced,and it is proposed that the system must meet the three basic conditions of causality,linearity,and stability to obtain EIS.Then,the experiments of EIS are performed on two parallel batteries,and the images and data of the EIS at different temperatures and different SOC are collected.It is also proposed that the fractional-order impedance model can be constructed using constant phase elements and other equivalent electrochemical elements.The model is compared with the integer impedance model,which shows that the fractional-order impedance model can better simulate the battery impedance characteristics.To estimate the internal temperature of the lithium-ion battery without destroying the structure of the battery,the EIS data is analysed,and a method for measuring the internal temperature of the battery based on the battery impedance phase is proposed.And the impedance at a specific frequency for temperature estimation is selected reasonably.The reason why the single-point impedance of a specific frequency can be used to measure the internal temperature is explained from the principle of charge transfer process,and an internal temperature estimation model for the battery is established.By comparing the real data with the model fitting data,it is revealed that the model error is within a reasonable range,which proves the usability of the model.To estimate the SOC of a lithium-ion battery,the impedance model in the frequency domain is converted to the fractional-order equivalent circuit model in the time domain.Using the definition of Grünwald-Letnikov and the transfer function between the terminal voltage voltage and current,a fractional-order model is built in Matlab.The polynomial relationship between the open circuit voltage and SOC is established,and the the model parameters are identified by particle swarm optimization.Compared with the integer-order equivalent circuit model,the fractional-order model has advantages in accuracy.Combining the fractional order model,the fractional-order unscented Kalman filter(FUKF)algorithm is designed as the estimator of SOC.In the dynamic battery operating conditions,it is compared with the extended Kalman filter algorithm and the unscented Kalman filter algorithm based on the integer order model,which verifies the accuracy and robustness of FUKF.