This circuit is an alternative to expensive battery chargers, the trick to lower the cost is to buy a cheap battery charger and expand it with an automatic charge breaker circuit such as the one presented here.
The heart of this automatic NiCd battery charger is the comparator which compares the NiCad battery’s voltage with a referance voltage. When the NiCd battery’s voltage exceeds a certain presettable maximum voltage level, the circuit breaks the charging via the relay and when the NiCad battery’s voltage sinks below the preset minimum level, the circuit closes the relay and the charging resumes.
When the battery voltage level rises, the voltage at the non-inverting input will also rise and by certain level (set with P1) the non-inverting input has a higher level than the inverting input.
The comparator is designed to have a hysteresis that prevents the circuit from “oscillating” (constantly switching on or off the relay) that could be cause by slight changes ti the battery voltage. This hysteresis is set with P2 which also sets the minimum battery voltage level at which the charging resumes.
Automatic battery charger circuit calibration
The best way to calibrate the circuit is to use a variable voltage, regulated power supply. This is connected to the circuit in the place of the NiCad battery. Set the regulated power supply to 14.5 volts and adjust the P1 to a point where the relay snaps open. Newt, set the regulated power supply to 12.4 volts and adjust P2 to a point the relay closes back. You might need to repeat this procedure severals time since P1 and P2 have an effect on each other.