alternator bad or gauge?

[Duncan563]

Yesterday it was cold so i cranked n cranked til my batteries were too low so i put it on a slow charge while i plugged it up. it sat for two hours on a slow automatic charge. when i cranked it it was ok for a while then while i was driving home my gauge shot up to 18 volts!! So i made it to advance and they tested it and said it was ok but wasn't positive about the regulator..