AC impedance technique for dynamic and static state of charge analysis for Li-ion battery

An AC impedance-based technique for active online measurements of Li-ion battery systems is developed in this paper. An AC impedance technique is proposed to estimate state of charge (SOC) for Li-ion battery. The technique includes a set of static and dynamic measurements that are required for state of charge estimation. In this paper, potentiostatic measurements of Li-ion batteries and AC signal amplitude of 10 mV are used. The impedance spectra are measured by sweeping frequencies between 1 Hz and 10 kHz with 6 points/decade. The measurement results are presented in a Nyquist plot, a plot of real impedance against imaginary impedance. The test results show that Nyquist plot of dynamic and static measurement of a battery cell are different, due to the internal electrochemical response. This is caused by the AC impedance that is different at the same range frequencies. In discharge process, when SOC of cell is dropped, its AC impedance is also changed. Therefore, through active online detection, the proposed AC impedance technique can be utilized to measure the SOC of Li-ion battery.