The lamp L1 will be light brightly and the LED will be out when the battery is low and battery charging in progress, but the LED is very bright and the lamp L1 dim when the battery is almost ready to use (charge complete). L1 must be a bulb lamp rated for the current you want (usually the battery capacity divided by 10).

The diode D1 should be at least 1 A, and Z1 is a zener diode 1 W with a voltage determined by the fill-charge battery voltage minus 1.5 V. When the battery is fully charged, the circulatory flow around the battery capacity divided by 100 mA.

The supply voltage can be from 12V power supply or from 12V lead acid battery from your car via cigarette lighter socket.

Components List:

Resistor: 1 K Ohm, 68 Ohm, 2 K Ohm, Potensiometer 10K Ohm
Diode: IN4001, 2x Zener IN752
Transistor: 2N2219, 2N2222

This NiCAD battery charger circuit charges the battery at 75 mA until the battery is charged, then it reduces the current to a trickle rate. It will fully recharge a dead/unpowered battery in 4 hours and the battery can be left in the charger indefinitely. To set the shut off point, connect a 270 ohm / 2 Watt resistor across the charge terminals and adjust the potensiometer for 15.5V across the resistor.

The input voltage for this battery charger circuit is 24VDC, you may use a general purpose power supply or 24V lead acid battery from your car.