Wrangler Aux Battery advice

[Chrismayer]

Hi. I have a 2020 Wrangler JL and I'm a bit of a novice. I changed the main battery 2 months ago, but noticed that the stop/start still didn't work after the change. I read it may be the aux but stupidly didn't do anything about it. After a few days away over Eid, the car won't start. I've tired jumping but nothing. 

Question: Will I need to get both batteries replaced (I hope not)? and has anyone got any other idea why the jump wouldn't work? I'd prefer to get myself to a garage if I can,.

 

Edited April 2, 2025 by Chrismayer

[Alexanderrr]

I changed both recently, first start/stop stopped working, then the car wont start, checked the voltage of both, drained . Regarding the jump starter, have to idea, either not enough amp, or safety feature.Check alternator as well just in case.

[Chrismayer]

Thank you so much for the reply. All good in the end. Replaced aux and the main charged ok so less painful than I feared. 
 

and yes, i think youre right re jump start, probably just not enough amps. 

[Abhishek Chikara]

I just got the error for Aux Battery. Since I deactivate start/stop, I have no intention of changing it at all. However, I was under the impression that it will have no effect on car start. But reading the messages shows it does have an impact correct so I will have to get it changed as well.

[Chrismayer]

Yeah I hear that unless you bypass completely it will drain the main battery. May not be true of course but my experience suggests it probably is.  
as I couldn’t move the car I used 800Battery and it was 840 all inclusive.  A lot I’m sure but at least it is now done. 

[Aser]

 

[ChrisW]

This is something I ended up messing around with recently. 
 

Basically on the Wrangler the auxiliary battery is connected in parallel to the main battery almost all of the time, with the two being separated briefly during the engine start (where the voltages can be checked separately) and during an Engine-Start-Stop event (where the aux battery is used to power the cabin load, eg items like the air blower fan, radio/onboard entertainment etc) such that the main battery is preserved for cranking the engine. 
 

If the auxiliary battery or main battery goes bad (happens reasonably often), because they sit in parallel most of the time, they’ll tend to kill the other battery fairly quickly. That’s how you often end up having to change both at the same time. 
 

As an FYI, if anyone should wish to change the aux battery themselves, it’s easier to remove the fender (four bolts, one push pin, and replace a few clips on the reinstall) and drop the battery tray…looks extreme but it’s a fairly easy job and can be done in under an hour:

The access from below is much easier than pulling out the fusebox.

 

You could also isolate the aux battery and disable it…

 

There’s an argument to be made that the incorporation of a second battery in this way (dissimilar sized batteries running in parallel) is a really bad piece of design that was rushed into place to meet restrictive US automotive emissions regulations. 

There’s a negative cable that runs from the aux battery to the main. If you disconnect and isolate that, remove fuse F42, and persistently disable ESS (using eg a Tazer JL Mini), then your car will happily run on the main battery only - and probably have much longer battery life in that setup. 
 

However there’s also a good argument to be made to just leave it as it is and lean on the warranty!

Edited April 4, 2025 by ChrisW

[Chrismayer]

Well. I was so looking forward to my carnity drive today and despite the new aux battery the car was dead. 
 

I had the main battery replaced under warranty but the stop start still won’t work and I’m worried tomorrow the car will be dead again. Not sure what’s going on but think it’s time to take to a mechanic