NiCd/NiCad battery charger circuit is still needed since some application demanding high current is still rely on NiCd type, since this type is still superior in term of high current output (low internal resistance) and low cost. This battery charger circuit is used to charge 12V NiCd battery at around 74 mA until battery is fully charged. This circuit need around 4 hours to fully recharge a totally empty/dead battery, depends on the battery capacity. Here is the schematic diagram of the circuit:
This circuit is basically a current source with auto cut-off. The current regulation is done by maintaining a fix voltage across a 68R at the emitter of 2N2219 transistor. This voltage is stabilized by a 5.6V zener diode 1N752, which keep the voltage at 68R resistor at around 5V, giving a constant current of 74 mA. The auto-off feature work by monitoring the output voltage (before the 1N4001 diode) relative to ground, as this voltage increases in accordance with the battery voltage which is being charged. After the battery voltage reach the fully-charged level, the lower 1N752 zener diode will pass the current to activate the 2N222 transistor, which short the upper transistor’s base, turning off the charging process. To calibrate the shut off point, connect a 270 ohm / 2 Watt resistor across the charge terminal and adjust the pot until the charging terminal voltage show 15.5V level.