...Help? I have a 2002 Gmc envoy, 4.2l 4WD. When I start the car after it has been sitting a while or overnight it says not charging so I rev it up and it will then begin charging. When It does begin charging and I put a big load on it, it will drop down to about 13 volts, lights will dim, etc. The battery is about 3 months old and the belt I just changed today and thought it could be slipping. I had the alternator tested by oreilys and by my local battery specialist and they said it tested good. So could it be the alternator diode going bad or something else? thank you everyone!!!!