Easy test for alt/bat.

Start the vehicle. Remove the ground from the battery. If the vehicle dies, it is definitely your alternator.

If the vehicle continues to run, this is where you have issues. It could be a low output level where it is not fully charging the battery.
It could also be that the battery (even though it tested good) gave a false reading and not fully charging requiring a new battery.

Is the battery the OEM one, or aftermarket?
How are you testing the bat?
It can easily have enough juice to run the radio, etc but not to start the vehicle.

Ive had to change both, the alt was due to a mudhole. that I dropped into while out on a trail.


Good luck.