Battery Impedance Measurement with Step Current of Different Amplitude under Temperature and State of Charge Control

Electrochemical Impedance Spectroscopy (EIS) is often applied to analyze and describe the battery internal electrochemical processes. And the methods of battery state estimation including state of health diagnosis with electrochemical impedance have attracted a wide attention. In the paper, a novel fast impedance measuring method based on wavelet transform with a step excitation current is proposed and further studied. With the method, the current generated by the electric vehicle and the responding voltage of the battery can be utilized to calculate and provide the battery impedance for the battery management system. Taking into account the varying amplitude of the current and the battery states, the battery impedance was measured with step excitation signals of different amplitude at different temperature and state of charge (SOC). Then the battery impedance was calculated with the proposed method and compared to the electrochemical impedance measured with an electrochemical workstation. It is observed that the impedance in the mid high frequency range (>10 Hz) greatly related to changes of the internal processes and properties caused by the aging process is weakly affected by the varying signal amplitude. And the effect is limited and can be neglected at high temperature (>25 °C). SOC also has little effect on the impedance in the frequency range higher than 10 Hz. It benefits the state of health diagnosis with the impedance in the frequency range. Accordingly, a state of health diagnosis method based on the impedance calculated with the wavelet transform is proposed in the end.