I have a 2005 6.0 PSD F350 4X4, double battery pick up. For a long while now the truck loses battery power over time but charges right back up again with a good run. Batteries test fine at 12.5 volts, and the alternator charges at about 13.5 at 2000 rpm. Everything works. Step 1. I disconnected both grounds and used a multimeter between one - post and the battery - cable. There was a 12.5 volt reading so something must be allowing a flow, either a proper flow or a short. Step 2. I pulled every fuse, fusible link,and relay out of three fuse boxes, one by the parking brake, one near the firewall on the drivers side, and one beside the left battery with two relays in it.
This should eliminate all power to everything except any unfused wires like the huge red wire to the starter. The meter still showed 12.5 v. When I disconnected the two secondary cables at the + terminals the meter showed no voltage. If one was reconnected then the 12.5 reading came back. Both disconnected, no loss of juice. The one secondary cable from the left side seems to power the fuse box inside BUT what does the other one run to? I tried to follow it but it goes into a larger covered and wrapped cable and is untraceable. By thunderin' jeepers I hate this kind of auto electrical problem! The Haynes book shows a starter solenoid relay mounted on the right side fender wall but there is no such animal in my 2005, or its somewhere else and I can't find it. So, forum guys, Can you help me ?

 

How did you test the batteries? Hooking a voltmeter to them tells you absolutely nothing about their capacity or their ability to hold a charge. They must be load tested or hooked to a dedicated battery testing machine.

You absolutely cannot check for power draws using a voltmeter. You are using the wrong tool and the 'test' you performed is completely invalid. You must use an ammeter and see that the power draw is under 50 milliamps after waiting for the battery saver to time out. If your meter has the capability to measure amps, be sure to read the manual very carefully as how to use it for these measurements because using it incorrectly will fry the meter in milliseconds. There are many writeups on the net on how to use an ammeter or multimeter to check for battery draws. Make sure you understand the process very well before beginning.