Q: What's wrong when the alternator is not charging the battery, both parts are new, car Will go dead after driving 20 mins

Your vehicle has a fusible link in the wire harness that goes from the main wire on the alternator to the starter. If you had the starter replaced then maybe the wire was not put on the starter.

The fusible link acts like a fuse, but also looks like a wire in the harness. Check if this wire is connected to the starter and that it’s not burnt in two. Sometimes the fusible link wire will burn in two but the wire outside cover stays intact. You have to pull on the wire to tell if it is burnt. I’d recommend having a mechanic diagnose your vehicle in order to fix your batter/alternator issue accurately.