What can cause my alternator to stop charging the battery while driving?

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12.7V on battery with car off. !4.5V with car running and accessories on. When driving the alternator stops charging and the battery begins to drain until car dies. I've replaced and tested the alternator and battery. Also added a new belt, tensioner and pulley. No codes show on diagnostic. The starter now seems to have died. Could this be a loose ground under the front of the block? A bad positive connection at the starter? Any help would be great.