The charger

The charger uses a MAX713 fast-charge controller. Maxim IC were awesome and sent me two free samples. I promptly blew one up.

The charger is based on the reference design for the MAX713. The same design will work for between one and 16 cells if you modify the PGM pin straps. The MAX712 is preferred for NiMH cells - it'll cut off the charge a bit earlier and stop the cells heating up so much. The MAX713 cuts off the charge a little later than is preferred for NiMH cells.

I powered the charger from a 24V switchmode supply, originally designed as a laptop power supply replacement. It needs to be a pretty beefy power supply - if the voltage drops below a certain threshold, the MAX713 will lose track of where in the charge process it is, and you could overcharge your cells.

You also can't charge the cells too slowly with the MAX713. C/2 is the recommended minimum rate for NiMH cells. I presume that at lower rates, the cell voltage changes too slowly for the chip to detect.

I couldn't get the 2N6109 transistor specified in the reference design, so I used an MJE2955 instead. It worked fine.

The transistor needs a reasonable heatsink. The datasheet outlines the worst-case conditions, and my heatsink is very much not capable of handling them. So long as you don't run the cells completely flat, you can get away with something smaller. My one gets a bit hot, but at the same time, the transistor is pretty heat-tolerant. Cover the back with thermal grease, bolt it down solidly, and you should be fine. You must use insulating hardware on the transistor casing, as it is live.

My charger was built on the night before the 2005 Sydney 24 Hour, and as such looks quite nasty. I covered it with duct tape during the race to make it look a little less dodgy, and so far, no-one has complained.

My nasty charger