Regulator gone bad?

First, you need to post exactly what you have. "one of those Mancini reproduction" doesn't tell me a thing. Is this alternator/ regulator wired like the original, or is it a one-wire conversion, or one with the regulator on the alternator??

First thing I'd do is eliminate the battery as a problem, it might be discharged, or have a bad cell. Quickest way out is if you have another vehicle with a known good battery--temporary for testing.

Next, suspect the voltmeter/ or it's wiring. Test right at the battery with a multimeter.

REAL nice lookin' ride, by the way. I like the wheels.