Hi! Correct, there is no cap for casts (or series, ie. Final Fantasy as a whole vs. by game, etc.).
On robots, pretty much yes. I'd say it's possible to jerry-rig a charging method via characters who can do lightning magic or something like that, but there's no tech to easily support it and no materials to build it, so anyone/thing that needs a charge would run out of juice eventually.
On Madoka, the grief seed/soul gem system can still work here! An item of similar use can be dropped by certain rare/boss monsters and used up to a point for purifying purposes—once the bestiary is up any special drops will be listed also so that'll be something to keep an eye out for. For added challenge, the incense is especially fond of taking someone Going Through It and making them worse, so that might be something they'll want to party up to deal with... juuuust in case.
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On robots, pretty much yes. I'd say it's possible to jerry-rig a charging method via characters who can do lightning magic or something like that, but there's no tech to easily support it and no materials to build it, so anyone/thing that needs a charge would run out of juice eventually.
On Madoka, the grief seed/soul gem system can still work here! An item of similar use can be dropped by certain rare/boss monsters and used up to a point for purifying purposes—once the bestiary is up any special drops will be listed also so that'll be something to keep an eye out for. For added challenge, the incense is especially fond of taking someone Going Through It and making them worse, so that might be something they'll want to party up to deal with... juuuust in case.