A method for estimating the ageing of a battery powering an electrical system. A set of data about the voltage and current across the battery terminals, in response to the current perturbations provoked during the normal operation of the battery by the changes of consumption rate of the electrical system are stored. The impedance of the battery, within a predetermined frequency interval, from the voltage and current data is determined. The age of the battery as a function of the impedance is estimated by comparing the impedance with a previously stored model of evolution of this impedance with ageing.