3.6V is the max you can lower to, which is about 1/3 remaining bar on the batt display. Then you will need to charge them. Not only do you need to marry them, you usually label them like "snow wolf (1) and snow wolf (2). So if right now you have (1) in slot a and (2) in slot b, the next time after you charge these batts you have to switch their slots, i.e (1) in slot b and (2) in slot a. You reverse their positions after every charge. That's how series works and it's kinda troublesome.