12 Volt PP3 Battery Charger Circuit

This simple constant current PP3 battery charger was built as a side project to my Stereo Headphone Amplifier. It's essentially the same circuit as my USB Powered AA Battery Charger project except the resistor values have been changed to suit the slightly higher voltages involved. Running as it does on 12 volts (approx) allows it to be powered from a car battery or 12V solar panel

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 47 ohm emitter resistor will end up being approx 1.2V (1.8 - 0.6 = 1.2). The schematic is shown below

Now using good old ohms law, voltage (1.2) divided by resistance (47) gives a current of 0.025 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 battery will be about the same as that flowing through the 47 ohm emitter resistor, which is 0.025A (25mA)

I chose this amount of charge current because a lot of popular PP3 rechargeable batteries available at the moment are around 250mAh, 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 battery is rated at 250mAh (0.25 amp hours) we need to charge it at 0.25 divided by 10 = 0.025 amps (25mA). A veroboard layout is shown below

WARNING: There is a break in the track under the 1K8 resistor that must be there!

The circuit has the added bonus that the LED only lights up when the battery is charging. This is because the current flow through the base-emitter junction when the transistor is 'OFF' (no battery 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)

12 volts is fed in via a standard 2.1mm coaxial DC connector (the 'line' type with screw terminals). The output attaches to the battery via a PP3 snap fit connector and the whole thing slides neatly inside a 6cm length of white overflow pipe. This lets enough light through to be able to see the LED when it's on without having to drill a hole

The finished charger