If your 8.4 is running like a 7.2, it probably IS running at 7.2. Using shitty chargers kills batteries and their voltage. I've had at least 3 batteries do that because I was a cheap ass and kept using chargers that came with guns.
From what I have seen the size of the cells shouldn't matter.
Guardians of Asgaard - KF32
VFC H&K 416C
VFC KAC SR16 JL Billet SBR
TM FN Five-seveN