Hi, 2 Lithium (LiPoly) cells 3.7 * 2 = 7.4 volts.
MANY sizes and mA-Hr ratings available.
Space? Weight? How easy to recharge? (Multicell LiPoly Batteries need a balancing charger. If you can pull two cells out and put them in a charger this takes care of that.
Where are you located?