A Electricity based Backtank,can be charge via Tesla Coil.
if you got damaged by Tesla Coil,it will charge instantly,so you dont need to put down your backtank anymore.

and maybe make Tesla Coil also charge the creeper it damaged.

I know this is an old suggestion but I second Tesla Coils charging Creepers! I was a bit disappointed when I tried that and it didn't work since Charged Creepers have utility in both Vanilla and several mods.

As for the backtank, it could be styled as a Portable Air Compressor, as that would explain both why it uses electricity, and how it's still able to power the pneumatic devices that the backtank normally powers (aside from the Diving Helmet, unless it was also an Electrolyser to generate Oxygen when underwater, but Create doesn't pretend to be terribly realistic so I think that can be handwaved away)