Ni-CAD battery charger circuit schematic

The figure below is a schematic diagram of a Ni-CAD battery charger circuit that has current and voltage limiting to preserve battery life.

When the battery is low and battery charging is in progress, L1 will be bright and the LED will be off, but when the battery is almost ready for use (charging is complete), the LED will be very bright and L1 will be dim. L1 must be a bulb rated for the required current (usually the battery capacity divided by 10).

Diode D1 should be at least 1A, Z1 is a Zener diode 1W, and the voltage is determined by the fill-charge battery voltage - 1.5 V. When the battery is fully charged, the circulating flow around the battery capacity is divided by 100.

The supply voltage can come from a 12V power supply, or from the car's 12V lead-acid battery through the cigarette lighter socket.