Thursday, January 25, 2018

Single Cell NiMH Battery Charger

With the advent of lithium ion batteries Nickel Metal Hydride batteries are uses less and less yet they can be used in many scenarios and to use them you have to charge them. A smart charger would be perfect fit for that purpose but those chargers are often very pricey so if you don't have that many cells to charge you can make a simple charger and just it a day. Here is how you can make such a device.

Parts List:

Resistor:

1 10Ω 1W
1 1KΩ

Capacitor:

2 470μF 25V

Transistor:

1 TIP 127 PNP
Heat sink

Diode:

4 1N4007
1 Green LED

Transformer:

220V - 9V 1A

Battery Holder:

1 For Size AA.

Diagram:


How does it work?

From the left the transformer steps down the AC line voltage to 9V which is then rectified by the four diodes and smoothed by the capacitor next to them.

The Green LED plays a very crucial role and keeps the base of the Transistor roughly at 2.6V. After the 1.2V drop we get around 1.4V to charge the NiMH cell.

The charging current is around 140mA.


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