Battery Replacement

[ResqFE]

Does it make sense to replace both batteries when one starts to go out? Haven't researched prices on both yet. I have a early model 2018.

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[Gee-pah]

Ron, for clarity sake, you refer to the 3.6L JL, correct?

(following)...

 

[ResqFE]

Yep, 3.6L

 

[Krondor]

IMHO

I'd only replace the one that tests bad. Most batteries will be good for 5-8 years, have you had yours tested?

I have noticed that while I no longer commute to the office, the Jeep sits a lot. To help the battery I keep it on a battery tender. This kept my motorcycle batteries good for years. Might be a better alternative.

Battery Tender 3 Amp Charger and Maintainer

 

[Gee-pah]

Hey Ron:

Others may disagree but I'm not sure that there is a categorical yes or no answer to your question.

At the risk of pointing out things you and other readers may know, in the 3.6L, the two batteries are connected in parallel 99% of the time. In fact the only time they are not connected this way are during ESS events and for an instant when the engine cranks; more on the latter in a moment.

And as a result of this parallel connection the voltages of the batteries will have a tendency to equalize, much as two independent water columns of different heights, once connected near their bottoms, will cause water to flow from the higher water column to the lower until they are both of equal height--the so called "water seeks its own level" adage realized.

So without question a sub par 3.6L JL battery will tax the one in superior condition. But the extent that such taxing permanently damages the better one's ability to hold, delivery and acquire charge I imagine are a function of how bad the subpar battery is, how long it's been taxing the good one, and sheer luck, etc.

Particularly in the case of ESS battery failures, I would think that independent testing of the main battery's ability to acquire, hold, and deliver charge remain the gold standard of whether it too needs replacement.

Another thing. 2018 3.6L JLs, out of the factory appear to go through an engine cranking algorithm that, prior to attempting a crank, isolates the ESS battery, testing it for sufficient power. This pre-crank ESS battery threshold appears to be much less than that necessary in the ESS battery to engage ESS, but if this pre-crank threshold isn't met, no crank will be attempted, despite the fact that were this threshold passed, both batteries are then connected in parallel to crank the engine, and your main battery could be "a nuclear power plant," more tan capable on its own of cranking your engine.

It appears that later M.Y. 3.6L JL's instead, upon ESS battery pre-crank threshold testing will, rather than strand the vehicle, will switch over the the main battery to crank the vehicle if this crank fails. Additionally, this flash is reported to make 2018 3.6L behave in this new way.

https://www.jlwranglerforums.com/forum/threads/3-6l-ess-bad-aux-battery-no-start-firmware-fix.53608/

You see, this 2018 algorithm works just fine when starting the engine to end an ESS event. The event never would have occurred had numerous conditions not first be met, not the least of which is a topped off ESS battery, just as sure as the ESS event will end prematurely if this ESS battery's charge dips below a certain threshold. So the issue of adequate ESS battery charge prior to an ESS ending engine crank is not an issue.

It's the adapting of this logic to cold cranks that was stupid IMHO; as it's not as if, by analogy, the 3.6L prevents you from turning off the vehicle (not entering ESS by analogy) if the ESS battery is running low.

With all this said, if one of my batteries died of old age I would definitely contemplate replacing the other. Sure, they're different type of batteries and all--but I suppose I'd adopt the "when in Rome" replacing one battery...why not do the other.

Funny though, the "when in Rome" adage, in terms of work involved, anything but doesn't apply. The ESS battery's replacement procedure is blessed by FCA to be done through the passenger's front wheel/fender; the main battery removed the conventional way.

I hope some of this helps.

P.S. I too am a trickling charging advocate as @Krondor above is

 

[motor12]

My gut says yes but your pocket book may say no for replacing both at the same time. Living in Texas its hot and heat is a battery killer. My JL is a 2018 also and I am replacing both very soon. Its true a bad battery draws down a good one, if you do any off road exploring the last thing you need is a vehicle that wont start. The down side to replacing batteries is the difficulty in getting to the second battery. Just my 2 cents.

 

[DanFelix]

In today's car market, I know of no battery that will last 5 to 8 years. What Gee-Pah said about the peculiar behavior of the two batteries is, frustratingly, correct. I have used a trickle charger and observed that the batteries "share" their charges - mostly behind by back. In my case, I changed the Aux battery and two days later changed the Main battery.

 

[Gee-pah]

and @ResqFE Ron, as a side note, said trickle charger need only attach to the terminals of the main battery, the far easier one to gain access to than the ESS battery, because of this parallel connection....

both batteries will benefit from the trickle charger in such a "main battery connected only" configuration....

 

[ResqFE]

The aux battery wasnt holding a charge. Replaced it tonight, all good now.

 

[WranglerMan]

The aux battery wasnt holding a charge. Replaced it tonight, all good now.

Any words of wisdom on replacing the aux battery.... like how tough was it to replace, I know replacing a battery is pretty easy but getting to the aux battery looks like it takes some time

 

[Htfan]

Any words of wisdom on replacing the aux battery.... like how tough was it to replace, I know replacing a battery is pretty easy but getting to the aux battery looks like it takes some time

Here's a good thread about replacing the aux battery through the passenger fender -

https://www.jlwranglerforums.com/fo...ent-the-easy-way-pull-the-fender-flare.56807/

 

[ResqFE]

Time consuming but easy enough. Bungy plugs and other BS out of way....... 7 mm and 10mm. Flat head to pry snaps.

 

[Htfan]

Time consuming but easy enough. Bungy plugs and other BS out of way....... 7 mm and 10mm. Flat head to pry snaps.

Wow! A lot of work. Great job!

However, it's easier and quicker to remove the front passenger fender.

 

[Gee-pah]

Time consuming but easy enough. Bungy plugs and other BS out of way....... 7 mm and 10mm. Flat head to pry snaps.

You raise an interesting point for me Ron. Thanks for sharing your "through the engine bay" replacement of the ESS battery alternative to FCA's blessed through the front passenger fender way.

Sure, there's a lot of bolts to loosen your way, but my knees have seen better days that to do it FCA's way.

Here though, in interest of full disclosure, is what in seems to involve. Perhaps you can concur:

 

[Htfan]

Here's a video link on getting access through the fender

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