Nickel Metal Hydride or NiMH batteries are considered better than NiCad batteries as they are less likely to exhibit the 'memory' effect, have a higher storage capacity (25%), and there is no toxic cadmium to dispose of when they finally wear out.
NiMH batteries last an average 6000 hours and can keep 80% of their charge at −20°. You can also recharge these AA and AAA batteries with the charger, available separately.