After tightening a loose alternator belt I noticed that when I started my car the battery power gauge (ampometer???) read well below the usual 13. When I rev the engine the gauge increases and as do the brightness of my headlights. Driving down the highway it shows well above 13, but when I am stopped at a stoplight and if I have my turn signal on I can literally watch the gauge decrease and my headlights go dim. Also with the gauge so low it seems that the car wants to die when trying to go in reverse. Any help is appreciated. Thanks

 

Take it to Advance or Autozone.
They can check alt and bat for free.

 

Already checked alt and it's fine.