Saturday, January 20, 2018

Simple USB AA/AAA NiMH Charger

This post is about a simple USB powered Nickel Metal Hydride charger with an in depth analysis of how it works.

Parts Needed:


1 18Ω 1W
1 22Ω 1/2W
1 180Ω 1/4W


1 100μF 16V


1 1N4007

Battery Case:

For AA and AAA.


1 Micro USB 




How it works?

As the name suggests it is a very simple design so there will not be a lot of parts used. Here is the run down on what each of the parts are doing.

The circuit is powered by micro USB 5V. The first 100μF capacitor is there to just smooth things out. This capacitor might not be necessary. 

Next comes the Diode, which blocks any back flow from the cells and kind of isolate each cells.

The SPST switch is for selecting the Charging Current. Open for AAA about 80mA, Closed for AA about 175mA. If it is Open both the resistors will be used in series and total charging current will be around 80mA which is okay for AAA cell. When the switch is Closed it shorts the 22Ω resistor so only the 18Ω resistor is in the circuit now which causes the total current to rise to around 175mA which is okay for AA.

If you have a high capacity AAA however that can accept high amount of current you can charge AAA while the switch is in AA position. However if you have a low capacity AA and want to charge it slowly you can use the AAA position for that as well. (I'm talking about switch position). 

The LED is there to indicate the cell is connected and charging. If you disconnect the cell it will stop glowing. 

If you want to charge multiple cells just make the same circuit twice, you might use a common 220μF capacitor in that case.


This one is a dumb charger so won't do any fancy thing. It will keep on charging as long as the cell is in there. So you kind of have to measure the time yourself. For 700mAh cell, in AAA settings you will need around 12-14 hours to charge. For 1300mA AA in AA settings you will need around 8-10 hours. These time measurements are for fully drained cells.


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