Stuart, your logic is right on! A discharged battery will accept enough current flow to destroy a generator, so it must be current-limited to prevent it from burning itself out. An alternator is self-regulating on current, so the current regulator is not requirted.
To get an accurate voltage regulator adjustment, the charge current to the battery must be very low. That's accomplished by only adjusting the regulator when the battery is fully charged, or inserting a resistance into the charging circuit. The old Sun generator/regulator testers used to have a 1/4 ohm resistor we could put in series with the battery to simulate full charge. Then we could make an accurate adjustment of the voltage regulator. Someone who is checking battery terminal voltage while the generator is recharging a low battery will get a too-low reading that mkaes him believe the generator output is incorrect. The reading will be slightly above 6 volts, but not the 7.5 it's supposed to be, because it's only requiring that amount of voltage to refill the battery. it's like pumping water into a nearly-empty storage tank. The pump doesn't need much pressure to start the tank filling, but as the water level rises, the pressure goes up. The generator is the "pump", and tha battery is the "tank". It takes a higher voltage to force more charge into a well-charged battery.
Jerry
