I've been noticing that when I sit at idle, the headlights dim. When I get the car revving (moving or standing) the headlights get brighter. Also when I have the fan running (not on A/C), the fan slows down as well. I installed a Red Top battery a few months ago and am not sure if its the battery or my alternator. I don't have any power drawing equipment in the car (i.e. subwoofer, undated stereo or lights. Pretty much stock.)

 

Probably alternator if that's the case.