What's better, a Backup Starter Generator or Backup Battery
 

I was wondering - whats better to have - a backup starter generator or a backup battery on a turboprop?

It is a requirement to have a 30 minute emergency power source (at least for night/IFR), and a battery is needed for that.

There is no point having an extra generator on an engine because if that engine fails (or runs out of fuel) you lose your source of power. For reasons of control and performance you do not want to be driving a windmilling prop only to keep electricity running. If you had two or more engines to begin with, you will still have power being generated from your remaining engine(s) generators. If you lost your only engine you will be landing within 30 minutes anyway. If you lose your normal electrics on a multi engine, you will be back to your emergency power.

If you have long distances between fields, larger capacity batteries are not much heavier and can give more than the min 30 minutes.

As a rule, you need a battery for a generator to work properly (needs the reference voltage, so you may as well use one as big as feasible as backup power. It would be nice, to have a starter generator as well, as long as the extra weight wasnt a problem.

My theory of electricity is a bit oxidized now---but I believe that with an alternator you need a source of DC excitation in order to produce an AC current---i.e a generator or a battery----but not with say a DC generator/ inverter system --- then only a prime mover is required.


I'm sure there's an electrical engineer here to enlighten us more profoundly