It may not apply since yours is a generator but a few years ago I had a Long 2360 that was boiling battery's dry after a few weeks.
With the tractor running at 1800 RPM's I would check the voltage at the batter with a digital multimeter and it was normal.
With the tractor running and at idle I unhooked one of the cables from the battery and connected the multimeter again to see near normal voltage at the end of the cables but at 1800 RPM's and the battery unhooked the voltage jumped to over 60 volts.
A new voltage regulator cured my problems but I have never fully understood what the battery was doing to keep the voltage from the alternator at normal levels until the battery was disconnected and the engine revved up. This was even when hooked to a boiled dry dead batter.
Be careful if you do this. When the voltage jumped to 60 volts I had all of the lights on. The 60 volts blew all the lights along with the electric tach.