12.5V with the engine off is spot on, but 15+ with it running is bad and will eventually damage the battery. Sounds like the regulator is bad on the alternator. We were just having a discussion on this in the Audio forum, and I posted this link:

http://www.contour.org/ubbthreads/showthreaded.php?Cat=&Board=trouble&Number=426586&page=&view=&sb=5&o=&vc=1

Beer is my Gatorade. Hooray Beer. '98 "Sport" Pacific Green '98 E0 SVT Silver Frost Pictures