What causes it and car shuts off battery check is good and alternator check good

 

The battery light comes on any way when the engine shuts down, do you have the batt light on when it is running?

If the Alt and Batt are good then unless the batt ran flat but charges up ok, you are left with a wiring issue shutting the car down.

 

I've seen issues where the power cable corrodes where it bolts to the alternator. Causes charging issues.