Need some help!! I bought a 2015 F-250 1 1/2 yrs ago....battery was dead on test drive & days later when I went to pick it up. A mechanic friend had it for 3 days, no luck! Battery, alternator & starter have been tested and are fine.

 

Any aftermarket equipment on the truck?
You have had it a year and a half, how often does it cause trouble and is it only when it has been sitting?
You say battery has been tested, you mean that it has been load tested? Batteries can be intermittent and hard to diagnose sometimes.
If you have a multi-meter you can do a parasitic draw test yourself and see what the draw is.

 

Worked at a shop that had an alternator test bench. Some times known bad units would pass, other times brand new alternators (not rebuilds) would fail out of the box. Some times it was accurate, though.

— Dave

 

As far as i know, there are no aftermarket parts on/in it. It only happens after the truck has been sitting for a day not being run. My first trip with our camper last Sept...the battery that came in the truck crapped out that night. I replaced it. It's a deep cell gel marine battery. I have been having this issue ever since. This past weekend with all the snow we had, it sat on day & I couldn't get it started! Once I did, i've been using a trickle charger to keep it charged over night as a percaution.

 

Personally I wouldn't use a deep cell in a vehicle. The internet is full of opinions but my take is that a deep cell marine battery is intended to be continuous use, (i.e. trolling motor) and not the momentary cranking draw that you get in a vehicle. It does depend on the exact battery, some are rated for cranking and continuous.
Are you in a cold climate?
Diesel or gas engine? I assume gas since we are talking about 1 battery...
Do you know the cold cranking amp rating on the battery?
When it dies have you checked the voltage reading on the battery? Or does the truck just not have enough to turn over?

 

Disconnect the battery ground, and hook an amp meter between the battery and the ground cable. The parasitic drain should show up on the amp meter. Start pulling fuses until the amperage disappears, that should give you a good idea of where to look.

 

This, but hook the amp meter up and then wait several minutes to get a reading before you start pulling fuses. It takes a few minutes for all the modules to go to sleep.

Around 30 milliamps or less is normal once everything is asleep. If you remove and reconnect the ground cable, you may "wake up" the security module or other components that will read more. So use a jumper wire or connect your amp meter and wait for it to all shut down.

 

It takes about 40 minutes for the battery saver to time out after ANYTHING is done (lights, open door, break.make connections). Anything measured before then is invalid.

See a lot of guys waste a lot of time chasing ghosts because they don't allow for that timeout to expire.