TLDR at the bottom

Evening everyone, my truck is acting kind of odd the past few days and want to get a sanity check on my thought process here.

A couple of weeks ago I started to notice that when I started the truck the screen in the cluster wasn't on the screen that I was previously on, it would go back to the main menu. Even though I thought it was odd there isn't really a whole lot you can do about that and I don't know what the dealer would be able to do for that, so I just left it alone. But then I was out of town for a week and when I got home the truck would try to start, as in the starter would engage and turn the engine over, but would stop before it actually fired. Since I don't have a jump box or another diesel to jump it with I tried turning off the climate control and the sync screen and it started. I thought that meant the batteries were really low but those two things gave it just enough juice to crank it. So initially I thought the batteries were dying, which would cause that screen to go to the main menu vs the last screen I was on, but my radio stations aren't reset, so I didn't think that was it.

Another week went by (I don't daily the truck) and the same thing happened, except this time it didn't start when I turned off the CC and the screen. I walked away from the truck to go back in the house to grab something and when I got back out to the truck it did start.

So at this point I thought for sure it was a battery issue. Today I took the truck to autozone and they tested the batteries and the left side battery was 100% good, the right side battery was only 13 amps below what it should be. Now since it is below the CCA rating it needs to be replaced, but I seriously doubt 13 amps low is enough to stop the truck from starting

UNLESS

Am I supposed to disconnect the batteries so they're not in series (or parallel? I'm not an electrician)? I don't expect someone at autozone to know this, and I'm not confident enough to argue with someone about needed to disconnect the batteries, but this crossed my mind after I left the parking lot.

TLDR: do I need to disconnect the batteries and test them isolated with each other to see if they're actually bad or not?

Thanks. its an 18 6.7

 

You should test the batteries with them disconnected. Otherwise you are testing the two batteries together, and most of the testers aren’t going to draw enough current to really load two batteries at the same time.

the batteries are wired in parallel, if they were wired in series you would get 24 volts.

 

Thanks for the reply!

 

My truck has been acting odd lately as well. Had first battery replaced 5/20. Next battery 2/21, after the bad battery swelled, overheated and spit acid everywhere. Both times, Ford refused to replace both batteries claiming one of the two was "ok."

Tried to start the truck Sunday morning in Sand Hollow UT, about 49 degrees. Had to jump it with the rock buggy. Replaced both batteries today with AGMs with 36 mo warranties. Engine spins so much more quickly while starting.

My experience with Ford batteries and Ford help has been disappointing.

 

You only need to disconnect one cable from one of the batteries to isolate them and allow a proper test. FWIW if you replace one of the batteries for anything other than a catastrophic failure you should replace both batteries. Since they are in parallel the alternator will charge them to the lowest common denominator, which means if you have a weak battery pulling the circuit down the alternator will try to charge it with increased voltage, which will likely damage your new battery.

On the trucks with two alternators I don't understand why Ford didn't design the system to have each battery charged by it's own alternator. Then you could replace them individually as they wear out / fail.

 

If they are original to your 18 just replace both of them. I barely got 3 years out of my originals.