Not necessarily AA batteries. They could be any physical size and still produce 4.8 or 6 volts depending on the cell count but you had the basic idea. Larger cells, such a "A" or "C" will USUALLY have higher amperage capacity but not always. There are other variables. Check the labeling and just make sure all cells in the pack are the same. As far a a 6 v ESV is concerned it's not that big of a deal. The top of scale is over 5.5 volts and you aren't in danger until below 5 just like with a 4.8 v pack. If down to 5.2-5.3 recharge your 6 v pack if you want. All it's going to do is slow down your servos a little and reduce your torque perhaps.