house battery issue e350 7.3

[surferdave]

Hey Guys
So here is my set up. I have a e350 7.3 diesel. So it already has two starter batteries. I added a 3rd house battery. Very standard set up. I connected the second starter battery that is under the barn doors. to a 100amp fuse followed by an isolator then the house battery. I connected my dometic cfx50 to the house battery. Pretty straight forward, right.

Battery is a deep cycle 85ah (not agm)
dfx 50 ambient@90F, Internal@41F) 0.77 Ah/h

The issue I am having is when I start the van I am not getting the voltage to the house battery that I would expect. After starting if I test the voltage on the main battery in the engine compartment it gives me 14.11-+,. If I test the second starter battery it only reads 12v. Shouldn't that be 14v too?

Am I not setting this up correctly?

[arctictraveller]

It sounds like somewhere there is excessive resistance, causing a voltage drop. Measure the voltage starting at the start battery under the hood, then follow the circuit (cable) and continue checking at every possible point. It's best to use the same ground point when checking too, that's to say, use a long jumper wire from the under hood battery to measure along the circuit since grounding to the frame can be problematic. When you see the lower voltage, you know the drop is forward of that point. Also check your ground connections, I discovered one battery ground on my 7.3 was corroded at the frame, and since it was bolted through a large hole in the frame, the connector had a minimal amount of contact. Lastly, the steel frame is a relatively crappy conductor compared to copper, so running a dedicated ground conductor will help.

[86Scotty]

Yep, or isolator isn't closing. Test the power on both sides of the isolator with a multimeter, with van running of course.

[daveb]

Originally Posted by surferdave

Hey Guys
After starting if I test the voltage on the main battery in the engine compartment it gives me 14.11-+,. If I test the second starter battery it only reads 12v. Shouldn't that be 14v too?

Yes it should or about that.



Your second starting battery should be tied to the first starting battery with large gauge wire. I'd check connections as suggested. Just because a connection looks good doesn't mean it is good. Look for any green/white corrosion. Start at the source and work your way back. If you're getting low voltage at the second battery something is causing a voltage drop. In some cases you can check if there is any voltage difference between the lug on the battery and the connector before you pull it off. At least on my 6.0 there is a jump start lug close to the barn doors and you can check voltage there as well but I just don't know if the 7.3 has that. Lastly if the second starting battery is going south on ya, it might draw down the charge voltage. If that's the case I doubt the house battery would be getting a charge as well because the separator (if you have one) would never close. Good luck on your search.

[surferdave]

figured it out. It was a bad ground. I had grounded to the chassi. Once I grounded direct to the battery the voltage was correct. Now I have to pull the whole thing apart and rewire with a ground to the battery

[arctictraveller]

You will get the best results if you run a cable directly from the under hood start battery to the other batteries. Using clean, tightly connected frame grounds works ok if all ground straps are low resistance, but a direct, copper ground wire is better.