I have a 98 150 4.6L. Going to work one day the battery went dead so i'm thinking alternator. I took the alternator into napa they tested it and I had a good friend test it both are telling me its good. I bought a new battery, new drive belt, and a new voltage regulator for the alternator. It is still happening the battery will drain while i'm driving my lights will get dim and the truck won't run right and all of the sudden it kicks in and charges up again than it will drain again. I don't know what the problem is. Experts out there please lend a hand. Thanks!!

 

Your battery has a 12vdc (volts dc) potential. Your alternator runs at about 14.4vdc. If your battery is dying and your lights dim while you drive there is DEFINITELY something wrong with your charging system.

Since you said that the alternator was tested and was good I would check your wiring. There could be a wiring problem that's inhibiting the flowpath (either an open/broken wire or short that bypasses).

That's all I can think of. Hope that helps.