Well, they're either alkaline/NiMH in the standard AAAA, AAA, AA, C, or D at about 1.5 V, or they're Li-Ion 3.7 V batteries at 10440, 14500, or 18650 (and there's a few other sizes). IIRC, for the Li-Ion batteries, the first two digits are the diameter (in mm), and the last three are the length...