SO, I surmise from your response that an at rest reading of 12.0 volts is NO indication of a good battery !
I guess the only test of a battery is to replace it, and if the problems go with it, then it was bad.
( A cheaper way would be to swap with a buddy, if he inherits your sins, then buy HIM/HER a battery )

Or, if you are mrmiata or reddog, just swap batteries among the dozen or so Crossfires you have all parked in a row.