AC impedance and state-of-charge analysis of a sealed lithium-ion rechargeable battery

A 7.2V, 1.25 Ah sealed lithium-ion rechargeable battery has been studied fo r estimating its state-of-charge (SOC) by AC impedance. The dispersion of i mpedance data over the frequency range between 100 kHz and 25 mHz comprises an inductive part and two capacitive parts. As the inductive behaviour of the battery is attributed to the porous nature of the electrodes, only the capacitive components have been examined. The data obtained at several SOC values of the battery have been analyzed by a non-linear least-squares fitt ing procedure. The presence of two depressed semicircles in the capacitive region of the Nyquist plots necessitated the use of an electrical equivalen t circuit containing constant phase elements instead of capacitances. The i mpedance parameters corresponding to the low-frequency semicircle have been found useful for predicting the SOC of the battery, mainly because the mag nitude of these parameters and their variations are more significant than t hose of the high-frequency semicircle. The frequency maximum (f(max)) of th e semicircle, the resistive component (Z') corresponding to f(max), the pha se angle (phi) in the 5.0 Hz-0.1 Hz frequency range, the equivalent series resistance (R-s) and the equivalent series capacitance (C-s) have been iden tified as suitable parameters for predicting the SOC values of the lithium- ion battery.