A new ac impedance spectroscopy method is proposed to follow the charge–discharge processes of nickel oxo-hydroxide electrode (NOHE) in KOH media and thereafter to determine their state of charge (SOC), involving in situ measurements of the impedance between two electronic contacts soldered on opposite edges of the electrode. A transmission line (TL) model is developed and fitted to experimental results. The physical significance of its parameters is discussed, taking into account one- and two-phase domains.