From my experience: charge has been draining from my helmet before anything else, even when I have the full suit equipped and my power fist selected, and with batteries in all five pieces of the suit. So, charge first ran out in the helmet, causing it to then take durability damage as well, instead of being still "charged" (although, I was in the Twilight Forest - but I don't have Infernal Mobs installed). So I took out the batteries except for that in the helmet and fist, then maxed those out. That holds a total of 1M charge, which, along with my resonant flux capacitor, should be good for a while. It was just strange that all the charge went out from the helmet first, while the other pieces were still fully charged. It continues to do that no matter how I have the batteries installed...
I also had the kinetic generator installed, as well as all the possible "cooling" parts. Occasionally, heat would pop up for a minute, then disappear. WOuld that be due to the kinetic generator? Also, if I was using my jetpack/boots for an extended period, it would create a lot more heat buildup, but slowly, so that I didn't notice it until I tried to railgun something and subsequently overheated. I think I will not install that generator next time...
Thanks for the help, BTW. There doesn't seem to be an "official" mechanics wiki for MPS.