I've got a BatterySpace 14.4v NiMH bottle battery and charger. I used it with a HID light head down to about -20*F and it worked OK. It probably had seriously reduced runtime, but at those temps I was taking it inside to charge and charging it daily so I never came close to its runtime.

I don't use the thing anymore, if you want to buy it for a seriously reduced amount, PM me. If you're interested I'll have to check and see if the battery is still good.

In general I agree with Paul though; for the money you'd pay for those batteries and chargers, you could just trash the whole setup and buy a Magicshine. It's not a super durable system, but it's hackable and if you are willing to hack on the system you already have, you can make the magicshine last as long as you like. The only serious problem I have had with mine is the connectors are a little fragile, so I just bought 1m extension cables from DealExtreme for $3 each and replace connectors as necessary (so far only one has needed replacement).

Or just get a P7 flashlight, it's brighter than a 20w overvoltaged halogen, in my experience.