Cold weather battery power
Lithium-Ion batteries might be your best option for rechargeables...They have a low self-discharge rate, and apparently work better in cold weather.  Unfortunately, all of the popular types of batteries basically stop working at temperatures approaching freezing.

What you might consider doing is putting up a small solar cell for trickle charging, and insulating the batteries.  If you can get the insulation right, the trickle charge will warm the batteries during the day, and the insulation will keep them warm at night.

Energizer claims that their Ultimate Lithium works well all the way down to -40ºC