Abstract: An electric power supply system for portable miniature size power consuming devices, comprising an AC generator, a rechargeable battery and a charging device to be fed from the AC generator and maintaining the battery charging level, the AC generator comprising a multi-polar rotor wheel having permanently magnetized poles, a stator having one or more windings providing the AC current and an eccentric mass driving the permanently magnetized rotor wheel; the rotor wheel, the stator and the eccentric mass being mounted coaxially, a transmission being arranged between the eccentric mass and the rotor wheel. According to the invention the transmission is a loose coupling means to be tightened by magnetic fields between the poles of the rotor wheel and the respective poles of the stator.