I have an 01 f150 5.4l. This only happens when winter hits. While i'm driving to and from work at night my voltage will randomly drop, lights dim way down, blower slows way down, and voltage needle drops to below the battery pic(normally runs just above pic). Battery light is off. After a couple minutes voltage returns, needle for gauge comes up, lights brighten, etc. Once in a while the belt squeals when voltage returns. Alternator seems to be charging because idiot light never lights. Its like there is a massive load put on the battery when this happens that the alternator can't keep up with. Truck runs and drives fine while doing this. Any takers? Thank you.

 

Sure, replace the alternator.
It's output is what goes missing when the power to everything drops.
With Lighting loads of 10 to 15 amps, heater blower at 15 to 20 amps, the fuel pump at 7 amps plus all the other loads for any length of time drains the battery.
Is it any wonder that when the alternator comes back on line that something doesn't 'grunt'?
The belt squeal is telling you this.
Anything you don't understand about this common sense?
Repair the charging system. Go over all cables, connection and checks so you service the whole system to get out of return engagements with the problem.
It's wintertime.
Good luck.