The answer to the first bit is that it depends on the resistance of the circuit but as a rough guide if you have a generator that tracks the wind, if cut in is at 8mph the generator will be trying to produce 24v at 16 mph and 36v at 24 mph, but you will not see these volts, they are the internal emf of the alternator, the battery will stay at nominal 12v. On open circuit you would see a higher voltage than this because the blades will now run up to a speed limited by their losses .
The 48 v mill running at 12v will try to produce an excessive current because the internal voltage or emf is too high for the circuit resistance that it is working into, the generator will load the prop at a lower speed to try to produce this excessive current and the net result is that you produce less because there is not enough mechanical power from the prop to drive it.
If you fit a slower and more powerful prop you will get the higher currents but at the risk of burning the windings. A 48v alternator will only have 1/4 the current capacity of a 12v one of the same rating.
Hope this makes sense.
Flux
