Errrmmmm….how do I work out what the alternator itself drains,
from switching it on and off?
Even if every system is switched off after start in the aircraft I fly, the
battery will charge quite vigorously when the alternator is on. If I leave or
switch the alternator off after start, the battery won’t charge.
I was not talking about how much current an alternator may draw if left on but the additional load or strain on the starter by having to turn a loaded alternator in addition to the engine and other equipment. In a fixed pitch engine like on the piper warrior any increase in load on the engine will result in less rpm. Items like airconditioning will drain a substantial amount of power and therefor needs to be off during critical phases of flight. In turbine aircraft you can see very slight fluctuations on EGT/ITT when Gens are switched on/off.
I was only stating a test where you could see how little engine power the loaded alternator uses at low engine rpm.
The power required to excite the alternator is very minimal so even if it was drawing some charge during start it would be of absolutely no consequence. The reason it needs to be considered after an alternator failure in flight is that you are minimising to essential power only, cutting any non-essential systems no matter how small.