A power supply for electronic timepieces is constituted of a mechanical power generating section (1), a power storing section (30) composed of a large-capacity secondary battery which stores power generated by the section (1), a voltage detecting section (4) which detects the voltage at the section (30), an overcharging preventive section (5) which is controlled by the section (4) and permits the electric current generated by the section (1) to flow therethrough, a diode (2) connected between the sections (1 and 5), and a switching transistor (7) which is connected between the sections (5 and 30), and turned off when the section (5) starts to discharge so as to improve the voltage resistance and charging efficiency.