Ammeter to Voltmeter...who does it?

I barely drove it, but it held steady for a day. Then parked itself at 12v instead of all the way left. The next day it was reading max 18v, but I checked with my voltmeter and it was 14.5 running. Then it parked at 18v. after awhile it just stayed at 18v and that's it.

The 2nd gauge (Bosch), worked fine when I tested it twice. Tested it a third time and wouldn't move from 8v. Not even a wiggle. Didn't even drive the car or start it.