This circuit allows you to transform a mains adapter in to a charger for NiCd – NiMH batteries. With selectable charge current (50mA-400mA), the charger can be used to charge cellular phone, toys, portables gadget, video batteries, MP3 players etc. Supply voltage input starting from 6.5VDC to 21VDC (depending on used battery). The circuit features power supply polarity protection and LED indicator.
Determining the charge current:
Before building this battery charger circuit, you must determinate how much current will be used to charge the battery or battery pack. It is advisable to charge the battery with a current that is 10 times smaller then the battery capacity, and to charge it for about 15 hours. If you double the charge current, then you can charge the battery in half the time. Charge current selection chart is located in schematic diagram below.
A battery pack of 6V / 1000mAh can be charged with 100mA during 15 hours. If you want to charge faster, then a charge current of 200mA can be used for about 7 hours.
The higher charge current, the more critical the charge time must be checked. When faster charging is used, it is advisable to discharge the battery completely before charging. Using a charge current of 1/10 of the capacity will expand the lifetime of the battery. The charge time can easily be doubled without damaging the battery.
Determining the supply voltage:
To set minimum and maximum voltages to supply the charger, see supply voltage selection chart below.
To charge a 12V battery a minimum supply voltage of 18V is needed, the maximum voltage is then 21V.