There's a lot of things that drain it; what refills it?

Electrical Succour: Just find a convenient power outlet and plug in.

An Explorator gets talents to start draining his Potential Coil at Rank 2. Electrical Succor isn't available until Rank 5.

That can't be the way to do it.

Its gradualy rebuilds its charge over time, most of the abilities that talk about using the Potencia Coil say they can only be used so many times a day, or they cause fatigue levels to use, at least they do in DH, and since its basicaly just a ramped up version of the Tech priest carreer, i would assume they are similar, not sure if there are specific rules for it though

Yes, the coil recharges over time. In most cases, if the priest can stomach another level of fatigue, the coil isn't empty.