I have a F-350 with the 6.0 liter with two batteries. I've replaced the battery on the driver's side twice, and after a day or two of sitting, the truck won't start. It starts just fine after I've charged the battery back up,but something is draining it when it's sitting. The other battery on the passenger side is about 4 years old, and as I understand it, that battery works the trailer towing, etc...electrical. I can't find anything relating to a second battery in my owner's manual so I'm not sure what it's for. Could it be draining the new battery?? I can't believe I have two bad batteries in a row after only a couple of weeks of use....

Any information will be helpful, and I guess I'll invest in another new one to replace the passenger side battery.

Gene

 

you got a parasitic drain on the eletrical...it can be anywhere.
I'd start by checking the Alternator...make sure its putting out enough and not too much volts....over charging can ruin a battery too

 

......A short in the rectifier stack in the Alternator can still allow the unit to charge but will cause a discharge when the motor is shut down.
.....The other battery is connected so it can receive a charge other wise it would go flat.
.....The difference is an isolation diode between them to keep the battery circuits seperated.
.....You need to make some checks for charging and for drain or have the Alternator tested.
Good luck.