With your setup I'd ditch the Ammeter in a nano second and just run a voltmeter.
The Ammeter was a good thing in the 30's 40's 50's 60's and 70's. But the generator was also good until the alternator came along.
Trying to run an ammeter on a system like your describing is opening the door for trouble with no real benefit.
HMMM how many vehicles made in the last 20 years have ammeters?????
There is a reason