Research on Impedance and Nonlinear Characteristics of Lithium-Ion Batteries

Battery characteristics research and modeling are the key technologies for battery management system(BMS) of electric vehicles. Comprehensive understanding of the characteristics of the battery is critical for BMS to monitor and predict battery states accurately as well as to maximize battery energy utilization. At present, the researches on battery characteristics generally adopt the equivalent circuit model and mostly concentrate in the time domain. This paper proposed a method to establish a nonlinear model and identify model parameters in the frequency domain. Based on the electrochemical impedance spectroscopy(EIS) measured by experiment, three modified nonlinear models are built, the model accuracy and application feasibility are further analyzed according to the experimental results. The comprehensive comparisons between model data and experimental data show that two constant phase angle elements parallel model is more accurate, reflecting the internal dynamics of battery well. Moreover, each parameter of the optimal model is identified by dividing the impedance spectroscopy measured by the experiment into different frequency bans. In addition, the relationships between the optimal model parameters and the battery temperature as well as the state of charge(SOC) are further studied by polynomial fitting. The results show that two constant phase angle elements parallel model fits the impedance spectroscopy accurately in the frequency domain, and the parameters of the model have good regular relationships with battery temperature and SOC.