FADEC = complex

Yes, backup battery or alternator. The latter is a better choice I'd
think, since a battery only gives about an hour. If it's belt-driven
though, it seems risky to me.

You would get a dedicated battery for the FADEC, the ones I've seen are
smaller than a motorcycle battery and this would easily outlast your fuel
supply at any power setting that keeps you aloft. It draws very little
currrent.

In all fairness, you have to consider the drain of current to run a fuel pump,
if it is a low wing that has an electric fuel pump that runs all the time, and
also any other absolutely essential drains on the emergency battery. Still, it
would have plenty of endurance to get down for a precautionary landing, if
things were going wrong in a major way, like that.

The exception might be on a ferry flight accross the Atlantic, or something. A
"wise" pilot with a plane that had systems that had to have electricity to keep
the fan going would pack an extra battery along, and a way to connect it to the
emergency buss.

Are you reading this NW Pilot? g Seriously.
--
Jim in NC