I would recommend lithium-ion, which will lighten your load even more, and work better in the cold. They're a lot more reasonably priced than they used to be.

I had a homebrew system but got tired of connector and wire strain problems--truly the details that matter for a lighting system. Bought a DealExtreme P7-D flashlight. It takes one lithium 18650 battery, runtime about an hour on high (which is pretty crazy bright--just a smidge brighter than the 12v 20W halogen MR-16 it replaced powered by a 12V NiMH pack). I carry a spare battery just in case, but for breakdowns or something I can dim the light. When I lock up the whole setup comes off the bike. No wire routing0 no flaky connectors, only one place to mount something.