USB Powered AA Battery Charger


This simple project will charge a couple of AA NiCad or NiMH batteries from a USB port. This is about as easy as it gets for a constant current charging circuit. The low 5 volt supply voltage does limit the number of cells that can be charged to just two, but there's nothing to stop you building several and running them off other available USB sockets. The schematic is shown below

The circuit relies on the fact that the current flow through a transistor is virtually the same in the collector as it is in the emitter. There is a small current flow through the base-emitter junction but it can be ignored for the sake of this project. The base of the transistor is fed from the voltage that is present across the LED. Red LEDs have a voltage drop across them of around 1.8V which is ideal as a voltage reference in this application (Green LEDs are slightly higher at around 2.1V). The base-emitter junction will drop about 0.6V, so the voltage across the 5.6 ohm emitter resistor will end up being approx 1.2V (1.8 - 0.6 = 1.2)

Now using good old ohms law, voltage (1.2) divided by resistance (5.6) gives a current of 0.21 amps flowing through the resistor. This can vary slightly due to tolerances of the components used but it should be close enough. As mentioned earlier, the current flow through the collector can be considered equal to the current flow through the emitter, so here the current flowing through the batteries will be about the same as that flowing through the 5.6 ohm emitter resistor, which is 0.21A (210mA)

I chose this amount of charge current because a lot of popular AA cells available at the moment are around 2100mAh, and the simplest and safest way to charge NiCad or NiMH cells is to charge them at a tenth of their Ah (Amp hour) rating for about 15 to 16 hours. So if our batteries are rated at 2100mAh (2.1 amp hours) we need to charge them at 2.1 divided by 10 = 0.21 amps (210mA)

The circuit has the added bonus that the LED only lights up when the batteries are charging. This is because the current flow through the base-emitter junction when the transistor is 'OFF' (no batteries connected) is enough to pull the voltage across the LED down below its switch on threshold, which is very useful. The component values shown are not set in stone and can be varied to suit other battery capacities (resistors are 0.6 watt metal film). A veroboard layout is shown below

The whole thing is housed in a small plastic box with a 2 compartment AA battery holder mounted on top. For the connecting lead I used a USB cable that I had to hand. The socket end was removed and the +5V and 0V (ground) connections verified with a volt meter, though generally these will be colour coded red and black respectively, with the two unused data lines being green and white. These unused data lines should be terminated safely so they cannot short out anywhere

The finished charger