I replaced my battery 3 days ago the car was driving fine. When i have to turn the a/c on the car shuts off. When the a/c is off the car is running fine. What could this be.

2 things need to be tested 1. the battery. Even new batteries straight off the shelf can fail in 7 seconds. It happens. 2. The charging system. Make sure you getting at least 13.5 volts perferably 14.5 and that it stays up even with headlamps and all accessories are on. If at any point it drops below 13.5, the alternator should be changed and rechecked. I had 3 bum alternators one day in the shop so never take for granted because it was in a new box that it will work the first time. Electrical components are just that way. They can fail new, used, remanufactured, rebuilt, etc.. Let me know if you need anything else!