The method according to the invention is implemented in an alternator-starter system (1) coupled to an engine (2). The system is able to operate as a starter and as an alternator and the alternator-starter (3) comprises an excitation circuit (10) supplying excitation current through the brushes (90) to an excitation coil (8) housed in the rotor (5). In accordance with the invention, the method comprises the steps of calculating a mean duty ratio of the excitation current when the system is operating as an alternator, comparing this mean duty ratio with a threshold value and detecting an end of life (FV) of the brushes (90) when the mean duty ratio goes above the threshold value.