Power source

A power system for supplying a variable load, the system comprising:

• means for mounting a rechargeable battery;
• a mechanical generator for generating electrical power;
• a source of mechanical energy for driving the generator, output rails (17, 20) for connection to a load, and a control circuit for controlling the supply of electric power to the output rails, and characterized in that the mounting means and the generator are connected in parallel to one output rail, whereby output energy of said mechanical generator in excess of a load requirement is used to charge a battery mounted in said mounting means, said one output rail being connectable to said output terminal means by a mechanical switch which in its open position prevents conduction through the rail and by a switching transistor located between the switch and an output terminal of the output terminal means, the arrangement being such that closure of the switch enables a pulse to turn on the switching transistor to allow passage of electrical current to the output terminal, maintenance of the switching transistor in its on condition being dependent upon the output voltage supplied to the output terminal means being above a predetermined threshold.