WHEW
You could look at some heavy equipment regulators (catepillar, truck, etc). Or, the aircraft regulators. However, they may not do what you want as they would be designed for on-all-the-time operation which may not meet your needs.
Does your regulator have to regulate the field coils? Throttle?
However,
I wonder if you would want to design something specifically for intermittent duty power generation rather than what an auto/truck/tractor/aircraft regulator would do.
What is the max charging amps to push into a standard battery? Don't they reccomend between 2-8 Amps / battery? Do you have enough batteries to distribute the amps?
Are you planning an auto-start on the generator?
Consider designing a regulator:
10.5/21V --> Start the generator if not already running (or your desired low voltage). Push your field coils to max.
12.3/24.6 (or so) --> reduce power to field coils, let off of accelerator
12.6/25.2V (or high resistance) --> turn off the generator.
Note,
If you are pushing too much current to the batteries, it may be difficult to get an accurate reading of the voltage, although I assume the current drops as you approach full charge.
