Equations that describe the voltage variations with time of a rechargeable battery during charging and discharging were used to determine the component values of the equivalent circuit of nickel–cadmium batteries under different states of charge (SOC). The equivalent circuit of the battery was described as an ideal voltage source in series with a resistor and the parallel combination of a resistor and a capacitor. The battery model used different values of resistance and capacitance, in the parallel combination, during the different phases of the discharge-rest–charge-rest sequence. The results show that the series resistance is approximately constant with variations in the SOC while the resistor in the parallel RC circuit increases as the SOC decreases. For the discharge and charge phases the capacitor value increased and decreased, respectively, as the SOC decreased. The value of the resistor or capacitor in the parallel RC circuit is an indicator of the battery SOC.