i have a 2001 ford F150 with a 4.6.
Around a month ago it would not start so i changed the tested and not so good battery, It worked fine for a few days and same thing, discovered i had a parasitic drain through the alternator so i replaced it. Again after working fine or a couple days now its over charging, +18V , My next step is to remove and have alternator checked or is there something else I should be looking at?

 

I would recommend getting your alternator tested.

I've had bad luck with alternators lately.

 

Check all your grounds.