The electric power source comprises a fuel cell and at least one flow channel the inlet and outlet whereof are respectively connected to a reactive fluid source and to a tank designed to contain the reactive fluid feeding the fuel cell and the water produced by the fuel cell. An inlet valve is arranged between the reactive fluid source and the inlet of the flow channel and is controlled by a control device. Once the tank has been filled with reactive fluid, the inlet valve is closed for a predetermined first time period. It is then opened for a predetermined second time period so as to evacuate the water accumulated in the fuel cell during the first time period to the tank, and to refill the tank with reactive fluid.