Battery charging systems for road vehicles

A battery charging system for a road vehicle comprising in combination an alternator and associated rectifier providing power to first and second supply lines between which the vehicle battery is connected, the alternator also providing power to a third supply line which in use will be at substantially the same potential at the first supply line, an ignition coupling the third and first supply lines, and voltage sensitive means connected between the third and second supply lines for giving a warning if the potential on the third supply line exceeds a predetermined value, said voltage sensitive means comprising a p-n-p transistor having its emitter connected, (a) to the third supply line by a zener diode which has its anode connected to the emitter of said p-n-p transistor, and (b) to the second supply line by a resistor whereby a predetermined potential difference is established between the emitter of said transistor and the third supply line, a resistance chain connected between the second and third supply lines and providing a base voltage for the p-n-p transistor such that the latter does not conduct when the voltage between the second and third supply lines is normal, but conducts when the potential on said third supply line exceeds said predetermined valve, and means connected to the collector of said p-n-p transistor for energizing a warning device when said p-n-p transistor conducts.