The rough schematic for the battery charger I am playing with is attached. It's not the cleanest schematic ever but I trust it is sufficient to get the idea across.
D1 indicates that power is connected to the device. D2 and D3 are controlled by the PIC and I have them shown as being used to indicate that a charge is active and when it is complete.
VR1 provides the regulated +5V for the PIC. VR2 is set up to provide a constant current of ~80 mA when GP1 on the PIC is low and about 220 mA when GP1 is high. The relay switches the resistance between 16 ohms when open and 6 ohms (16 and 9 in parallel) when closed.
R7 and R8 create a voltage divider so that roughly 30V at the top of the batteries will show up as 5V at AN0.
The PIC, a 12F675, will on startup begin a "fast" charge cycle. Once it detects that (a) the voltage is too high on the cells, (b) the voltage has stopped increasing (or, for NiMH, started decreasing), or (c) too much time has elapsed it will open the relay to put it in trickle charge mode.
There are any number of items that could be changed but I think this is a decent start...
Best Regards,
Paul
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