In this paper a new method for the determination of the double layer capacitance and the internal inductance of a cell or battery is described. The resonance frequency of the double layer capacity with the internal inductance is determined by means of a phase measurement. The method can be used during operation of the battery. During a constant current discharge it is possible to predict the available discharge time from the resonance frequency at the start of the discharge and the actual resonance frequency. The method is tested with success on lead-acid batteries (VRLA, Plante) and nickel-cadmium batteries and it shows that the active surface is proportional to the state-of-charge (SOC). For primary zinc-MnO2 cells the measured electrical capacitance is not simply related to SOC.