I use LiIon batteries with a voltage regulator.....it's all about the discharge curve NiCads discharge pretty quick then slowly discharge down some numbers as a visualization let's say NiCad starts at 5 volts goes to 4.6 pretty quick then slowly discharges down.
The lipo, LiIon, and life batts let's say start at 7.2 discharges to 7 then holds 7, 6.9 until it suddenly drops to unsafe level.
the mAh rating of the battery is the capacity of the battery the more it is the longer you can go. Hope this helps