Pontiac: unknown, pontiac boneville, reading right

Question
I have a 1999 Pontiac Boneville SE with a little over 50,000 miles.  In January I had to replace the battery, because it was dead.  The other day while driving after the car had been drove about 20 minutes, when I would apply the brakes the battery hand would go down to the lower red zone.  Once I would drive off or put the car in park the hand would read around 12.  We went to the local parts store and had the battery and alternator checked and they said it was reading fine, that the belt may be too loose causing this problem.  I took it to a mechanic the following day and he said the belt was fine, that it could be the alternator going bad.  I had it changed.  Today it is still doing the same thing.  Should I take the battery out of the car and have it retested or is there something else I should look at?  Thank you for your time.

Answer
sounds like the volt gauge is not reading right(which they rarely ever do). You need to check the actual charging voltage at the battery and the alternator and compare the readings to the one on the dash. Chances are they will be different. If the battery is not going dead, you probably dont have a real problem.