Since the AC pump is a belt-driven mechanical device, why do people think it is the source of electrical drain?

My WAG for current drain would be the electric cooling system fan that kicks in when the AC is turned on. Easy enough to see, just unplug it and cycle the AC; if there is still dimming, I'm wrong.

Also an idea for fixing the dimming -- a larger capacity battery.

 

It is the electrical cooling fan. On some older track cars we would wire the fan to always be on to combat underhood temps, especially between runs. I don't have to worry about cold weather driving, living in CA, but in cold if you leave the fan off it would warm up faster. Just wish there wass a temp gauge