Many cheap battery chargers usually assume that charging the battery slowly is OK although the battery is already full of charges. All they do is usually charging the battery with constant current no matter if the battery is almost completely discharged or has been fully charged. Off course the battery wouldn’t be damaged immediately after few charging cycles, but actually they’re damaged slowly and the life time can’t be maximized as stated by its data sheet.

automatic nicad chager circuit schematic

This circuit is automated that you don’t have to manually set the alarm timer to wake you up when your battery has been fully charged so you can unplug the charger to prevent overcharging. This battery sense the voltage while charging the battery, and automatically stop the charging when it’s fully charged.

To set the R8, turn the variable resistor R8 to its maximum value (highest resistance), connect a fully charged Nicad battery (about 1.44V) to the battery terminal. Press and release the start push button, and make sure the LED D1 is turned on after releasing the start push button. Now turn the R8 slowly toward its minimum value and stop turning exactly when the LED D1 turned off. Now your automatic Nicad battery charger is ready. Place a discharged battery and press the start button, the D1 LED will turn on  to indicate that the charging is in progress. LED D1 will turn off after the battery has been fully charged.

To provide multiple battery charging, make few more similar circuits shown in the dashed line box, and you can charge up to ten Nicad battery (50 mA charging current per battery) using 1 Ampere 9-12 volts transformer.