Biltit: there's one other thing that happens. if an alt is supplying an output at a given speed and that load is lower than its rating at that speed, the output voltage is regulated at about 14.0 and life is good.
To what you said, the alt is maxed when the field current and voltage is at the max. Alternator field voltage is derived from the system voltage; once you overload the alt, system voltage drops towards battery voltage and the max available field voltage drops, further reducing the alt output.
This whole scenario could be several things, but bottom line, adding electrical loads require more energy. The only way the engine can provide the increased energy is to open the throttle, otherwise the speed drops, voltage drops, and the lower voltage reduces the electrical power to the loads until the engine reaches equilibrium again.
That's what it looks like to me.
George
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