Car starts fine and scan master shows 13.8-14.2... untill the car warms up. I talk'n like a good 40mins of driving then the scan master shows 12.8-13.4.. if the headlights, a/c blower, and radio are on and i hit the brake it drops to 11.8.. then bounces up to 12.8-13.2.. advance has checked the alternator on the car and i had an alternator shop check it off the car, it checked ok both times.. is this normal?