OK, here is the situation...
I have a 2000 F150, 97k miles, 4x2, V6. The battery light started flickering on the highway about three weeks ago, the voltage indicator dropped into the negative, radio flickered then turned off, brake light came on, ABS light came on and I started losing power. I shut off the air and headlights and the voltage started to climb, warning lights turned off and I made it home. It started fine after this.
I had the battery replaced and the alternator checked, everything was good to go. One of the terminals was corroded and was replaced as well. Since the battery was replaced I've taken it on a 2hr highway drive with no problems except the battery light stays on now. Yesterday I was driving around town at dusk, with the headlights on and everything started to happen again, same symptoms as the beginning of this rant. It seems that the headlights are a big factor in the power loss. When I turn them off the power slowly comes back, warning lights start to turn off, the radio comes back and I can turn the dome lights on. Any thoughts or suggestions would be greatly appreciated. Thanks!!

 

Sure, a simple case of the battery is not being charged and the alternator is not supplying much power.
Your trying to run on the battery and it going dead.
Have or check out the charging system and find the problem.
You have a battery, the cables and the alternator.
Nothing else to work with.
The issue or issues are there somewhere.
Simple to check out with either a voltmeter or an LED stick with it's LED level indicators.
After cranking, the voltage should be at 15 volts +/- then slowly come down to 14 volts as the battery recovers it's charge.
If the LED lamp goes toward 11 volts at cranking the battery is not charged or if poor for capacity.
Battery light; it is powered through the ignition switch to the alternator.
A ground from the alternator regulator lights the lamp.
As soon as the alternator voltage come up the lamp ground is removed by the alternator regulator indicating the alternator is putting something out even if too low. Measure it!
If you find the cables are all good and cleanly terminated the alternator is suspect. There is also a small fuse in the alternator circuit.
Be where of store checks of the alternator. What happens for real is the most important. Worn brushes will get you every time.
Good luck.

 

If it were mine at this point i would remove the alternator and pull off the voltage regulator/ brush assembly, chances are your slip rings are getting worn thru or poor brushes. They seem to go about 125,000 miles, but some do not.

 

just change the entire alternator and get good parts through out.

 

Wow, thanks for the responses! I'm going to do a little troubleshooting today if the rain holds. I did try and start it yesterday and the brand new battery was dead, got the familiar clicking sounds but no turn over. I'll post as soon I find something out. Thanks again!

 

After running some tests it ended up being the alternator. Runs and charges fine, but now I think the serpentine belt needs to be replaced. Pretty loud squealing going on. The belt looks fine and the tensioner looks to be in good shape as well.