For FADECs on single engine piston planes, in addition to being powered by either alternator automatically, there's usually a dedicated FADEC emergency battery good enough for 1+ hours of engine runtime.
Understood :) As a fun note, i just remembered that the emergency FADEC power on the turbine engines is provided by a tiny hydro generator powered by the fuel flow itself, so it can run in case of total electrical failure. The throttle position sensors are energized by the same emergency circuit so they continue to work too :)