What causes the alternator to go bad

What is your question?

Replaced 2 alternators with new and not rebuilt alternator and it's fine but after a couple days it seems like something is killing out the alternator. The battery icon comes on and when I remove and test the alternator it fails the tests so I have to replace it.

How long has your 2002 Lincoln LS had this problem?

The car was fine but sat for 10 months. The alternator was bad that was in it when I bought it