Ni-mh Battery Charger

This is a very simple Ni-mh and Ni-Cd battery charger which uses 5V USB to charge up to battery.

3x 56Ω resistors

1x diode 1N4148, IN4001-7 IN5819 or any diode which pass 100mA

1x battery holder (I've used AA holder which can also hold AAA batteries)

1x LED (I've used green led)

The design of the charger is very simple. It uses the technique called trickle charging to charge batteries. Advantage of trickle charge is that you don't need to terminate charging process the excess will converted to heat and battery will get little warm. At trickle charge you need to limit charge current to somewhere between 1/20C to 1/10C where C is battery capacity. I've used 2 56Ω resistors in parallel to to get 85-95mA of current. The diode is for helping to dissipate less in the resistors and also if USB supply got shutdown to prevent battery discharge through the USB power supply. When batteries get fully charge they get slightly warm which is a indication of end of charge. According to capacity of cells and charge current charge can take 10-15 hours in a 1000mAh battery (I've have 1100mAh AA batteries which takes about 12-13 hours to charge.)