The maximum output is a fully saturated winding. There is no more to be gained no matter how much current you provide to the excitation field. It is Ampere's Law: for a fixed number of windings around a fixed magnetic path length, current increases linearly to a saturation point where no more current is produced.
At least this is how my creaky brain remembers earlier physics and electronics classes
. I am open to be shown otherwise.
I don't agree with the voltage magnitude on the field wire as being so important. Once the current saturates the field coil, it is at maximum and no more current or voltage has any effect.
If voltage was important in the way you say it is, then an alternator could either not reach full output with anything less than a full battery
SOC, or the regulator
would have a dickens of a time trying to control a voltage that was in constant flux depending on how much it was being controlled by the regulator
- a constant chasing of its own tail. Neither of these happen.
Regulators work by switching the field current on and off at high speed with a duty cycle commensurate with the alternator output desired. The old mechanical ones actually used points clacking away to do this, and all new ones use transistors. Yes, the resultant average field current may be lower than full output, but that consists of packets of full field followed by no field. The alternator output would look the same if it behaved perfectly with no hysteresis, no battery connected to it, etc.