alternator Help?

[doa8907]

I'm new to cars and my dad went offshore for 14 days so i need some help here. My car is a 05 and i just had the battery changed cause the car stopped running u could say cause it was old. Now the car runs fine but it started doing what the car did before the battery died, while sitting at idle the car turns off but it comes back on fine. I did the voltage test for the alternator and when the car is idling it is 14.4 but with all 4 lights on and radio and air on full blast it drops to 12.5 and goes back and forth between 12.5 and 12.0. I think its bad, but wanted to make sure.