When the engine runs, the battery voltage varies from 12.8 to 15 volts. All lights are off, a/c is on. Is it normal for the voltage to get down to 12.8 volts?
When the engine fist starts the voltage is always above 14 volts and it may drop to below 13.5 volts within a few minutes or stay above 14 volts for an hour or more. I can't identify anything different that would make the voltage drop sometimes and not at othertimes.
The Battery is 1 month old and this same thing happened with the previous battery which was the exact same battery.
Is my alternator bad/weak?

Dennis