When at idle, my lights often flicker a lot. At low RPMs, my car shakes a lot like its struggling. My battery voltage is about 13.5 when at idle and its flickering. It doesn't always shake and doesn't always flicker. The alternator belt looks fine and has good tension. Could the alternator be the problem or could it be something else

It's a 08 Altima Coupe 3.5 btw.

Thanks

Sounds like an alternator getting ready to take a dump - Voltage fluctuations can cause all of the symptoms you describe.

You can grab the FSM for your car at nissanservicemanual.com and swap that bad boy out with a quickness!

If the engine is low enough below the rated rpm spec due to being out of tune for some reason then most engines will flicker the lights and shake car, the engine is low enough it is about to die.............