Effect of state of charge on impedance spectrum of sealed Ni-MH cells

Alternating current impedance spectroscopy was used to determine the state of charge of sealed Ni-MH cells in wide frequency range. A modified Randles' circuit was used to fit the impedance data. The effect of state of charge on the equivalent circuit parameters, e.g., phase angle, φ, impedance modulus, |Z|, equivalent series capacitance, Cs, ohmic resistance RΩ and charge transfer resistance Rr was determined. It has been found that the impedance parameters were sensitive to state of charge at low frequencies.