Apparatus and method for determination of the state-of-charge of a battery when the battery is not in equilibrium

The invention relates to a method and an apparatus, like a charger for determining the state-of-charge of a battery which has been charged or discharged and which has not reached its equilibrium state, the method comprising the steps of determining the EMF of the battery by extrapolation of the battery voltage sampled during relaxation after the charge or the discharge process, wherein the extrapolation is based on a model using only variables sampled during the relaxation process and deriving the state-of-charge from the EMF of the battery by using a predetermined relation between the EMF and the state-of- charge. This method is a voltage-prediction method without the need to store parameters beforehand.; Instead, the voltage relaxation end value is determined based on the measured first part of a voltage relaxation curve and mathematical optimisation/fitting of a function to this measured part of the relaxation curve.