Abstract: A method for protection of an energy storage source (2), in particular a battery of a motor vehicle, against possible overloading caused by prolonged application of an electric voltage to the energy storage source, this voltage being generated at least partially by a rotary electric machine (1) such as an alternator or an alternator-starter, the method comprising the following steps: permitting measurement of a charge level of the energy storage source, this charge level being represented for example by a voltage measured at the terminals of the energy storage source or a PWM signal, whilst the rotary electric machine is functioning in a predetermined functioning mode, and electric excitation is being applied to it; comparing a value (SENSE) of this measurement with a reference value (Max_SP); if the value measured for the energy storage source is greater than the reference value, interrupting the excitation applied to the rotary electric machine.