when i am at an idole, my battery guage on my dash goes from in the middle (normal voltage) to about a quarter of the way... like half voltage or w/e...

now i have a newer battery, and im pretty sure my altinator is working... my truck does idole at a lower rpm than it used to, and it only goes down at an idle... its worse when i turn my cab fan on... (AC or no AC)... it hasnt caused any issues... its just anoying at the moment, but could anyone shed some light on this for me?

 

It could be your gauge or even a poor ground but why not go to one of the auto parts store and have them test the alternator and battery under load just to be on the safe side.