What you have is a self energizing, self regulating alternator. 

Replaced a lot of generators around the farm with those, for the most part work very well if you use a 2.5&quot; pulley.

One thing I have had is having a battery discharge and the alternator would test fine on the test stand. With the engine off, and in the shade or a darkened building disconnect the hot wire. If you see even the tiniest of sparks, replace the alternator.

My 1600D did this. Repair place tested the alternator and said it was fine, but had that teeny spark when disconnecting the hot wire, replaced alternator, end of problem.

The original generator worked on it, but on the rare occasions I could actually round bale past dark, the generator couldn't produce enough to keep up with the current draw from the lights, the Harvest Tec preservative applicator and the electronic monitors and electric motors on the round baler.