I have a 13 GT.

Car started to be dead in the morning. I jumped it a few times.

Then, had battery checked and there was a cell bad. Looked at date on battery and it was less than a year old. So decided to check the alternator.

Took out alternator and had it tested. It was bad. Replaced it.

Driving today for the first time since both have been replaced and now I have a "Check Charging System" warning.

Tested battery voltage, its over 13.

Any ideas what could be going on?

 

I'd have the alternator checked again. It's possible that the voltage was fine when you checked it, but maybe too high or low at other times.

I had one go bad a while back, and only at a certain rpm the voltage would spike.