What's the correct voltage reading at the fuse?

[aquadj]

This is a follow up to my post yesterday..."charging problem / question." I've done a litlle more probing on my port motor and checked the voltage reading at the main fuse. Does anyone know if what I'm describing below is correct?

With the motor off, I have an equal reading at each fuse. When I start the motor, the reading at the fuse running to the battery increases, and the reading at the fuse running to the auxiliary circiut (my isolator) decreases. Does this happen because the battery is being charged by the alternator and does not need an additional charge from the auxiliary circuit, so it decreases?

[boats.net]

With the starter engaged the voltage will drop, when starts the alternator will cause voltage to increase.