Tested with Modular Powersuits armor in a basic capacitor bank, the slots simply won't accept it.

Does the suit accept RF power or does it require EU?

It uses RF, last I checked Apparently it uses something called J or kJ, which RF used to auto-convert to. Also, it seems to be a bit more than this...using only Ender IO, there appears to be absolutely no way to charge the power armor. It used to work, something recent must have changed it.
MachineMuse/MachineMusePowersuits#688

see linked ticket

What do you mean by cell? I don't have Thermal anything so...

The armor pulls power from any source in your inventory right?
I usually just place a cell in my inventory and remove batteries from my PA.
The PA batteries just add weight anyways.