Kleiss1
Well-Known Member
Dealer just replaced my Aux. under warranty. Said they couldn’t geT the main to fail. Now during cold start a red battery image shows on the dash.
Sponsored
Sounds like a trip back to the dealer is warrantedDealer just replaced my Aux. under warranty. Said they couldn’t geT the main to fail. Now during cold start a red battery image shows on the dash.
Same year and mileage as my JLUR. Lovely!Mine is JLURD 2021 22k miles.
It's *significantly* easier to remove the primary battery and simply unbolt and flip the PDC out of the way and hold it with a bungie cord to gain access to the aux battery below. I've replaced well north of 100 of them at this point. If you're lifting the vehicle, removing the wheel, pulling down the fender, to then balance the battery to reinstall it, then you're doing significantly more work then simply pulling the PDC out of the way. Jeep does state to go through the fender, but that's mostly because they can't verify a level 1 technician has the ability to remove the PDC efficiently.@speedymart
You say...
"The way most techs replace the aux battery at this point requires the primary battery to be removed. "
I'm unsure why you think this.. Techs more often then not do a fender liner peel which is the way Stellantis recommends they do the deed. This requires cutting a few plastic rivets and peeling back the fender liner a bit.. IF not a peel that cut two rivets and bungee cord the liner back a bit and do the deed.
NO, they do not dismantle the PDC even though the Aux on a 3.6 is under there... NO, the aux battery is not under the Main battery.
Also, Tech has to provide test result(s) to Stellantis if they want dealership to be reimbursed for bad battery(s).
I am not being confrontational in regard to my comments here... but rather just wanting to get correct info to other's.
.
You may be right, and there are certainly more demands on main batteries these days.The problem will reveal itself when the Main dies. Critical things will fail first increasing the likelihood of finding yourself stranded.
I'm not sure Jeep have implemented the Aux so that it works that way. It appears the way they wired the Aux and Main together allows one of them to drain the other when there is not enough charging available for both of them. What you describe sounds like a sensible approach and is sort of like the Genesis dual battery system. My understanding is the Aux was included to manage accessories-drain during ESS stops. It would be great if it were a more general backup support for the main but it doesn't appear to work that way.Aux is not exclusively used by ESS. It keeps the basic vehicle electronics happily juiced when the main battery is dumping power to the starter.
Jeeps are well known for having owner installed accessories. It's pretty much expected. These are often wired directly to the battery DIY style. So who knows what else the main battery needs to power besides the starter and how low volts/amps will drop in the process. We all know JL electronics do weird things at low voltage/amps and that's the last thing you want while you're trying to crank start your cold engine. The Aux battery eliminates that possibility by insuring the electronics has strong clean power....
No problem. Here's a deep dive into my thought process. (fair warning, I think a lot)Explain it to me like I’m 12 LOL.
I’m confused on the root problem here, I don’t really have knowledge on electrical, genuinely trying to understand the problem if there’s any info you or anyone else can give on how the AUX correlates to this and why bypassing it alleviates the issue.
Thank you for taking the time to write that out, greatly appreciated and awesome!No problem. Here's a deep dive into my thought process. (fair warning, I think a lot)
Lets start with some facts to keep in mind during the explanation:
Lets start with the OEM design.
- As others have already correctly said, Mopar OEM batteries aren't very good. This can lead people to blame design instead of the crappy batteries which is often the real root cause.
- The Jeep Wrangler is the most owner customized vehicle on the market. As a result, random owner electrical customizations are considered in the Jeep's design. Most other vehicles are designed with the assumption of no owner customization.
- The JL is a mostly electronically operated vehicle, especially the engine. The ECU (Engine Control Unit) and CANBUS (communications) require a steady minimum amount of power to operate reliably. When power dips too low, bizarre nonsensical things happen with the JL electronics.
- Unlike older non-electronic vehicles, the JL electrical system is NEVER turned off. Something is always drawing power. This is the cause of the parasitic drain on the batteries, not the splitting of the Main/Aux. Lead-Acid/AGM batteries like to be at 100% charge all the time and do not handle parasitic drain well. It reduces their lifespan significantly. Lead-Acid/AGM's wheelhouse is a heavy draw (i.e. engine start) followed by an immediate recharge to 100% from the alternator. That parasitic drain from the always-on electronics is why car batteries no longer last 7-10 years in spite of being improved since those days.
- Jeep design is tested over the long term. Owner observations tend to be very short term.
To accommodate a wide amount of owner customization, Jeep chose to split the battery into Main & Aux. They are linked in parallel 99.99% of the time (they only function independently while the starter motor is engaged which is typically the highest draw the batteries face without an alternator to back it up). So you need to consider Main and Aux as single battery. It would alleviate a lot of confusion if they were two batteries contained in a single battery housing, but that would be an expensive custom battery only for the JL. Jeep owners won't like that.
Why should you think of Main & Aux as a single battery?
Suppose for example that you have a TV remote control powered by two AA batteries. The remote control stops working. Do you test each AA battery, find the one that's dead (because only one of them will be dead, the other just weak) and only replace that one or do you replace both batteries? If you only replace the dead AA battery, do you think that remote will work for very long? No, it won't. The JL Main & Aux batteries are similar. (I know, they're not exactly the same as the remote batteries are in series while the JL batteries are in parallel but it's close enough to demonstrate how much the batteries work together and why only replacing one is a bad approach)
Splitting the battery into Main and Aux accomplishes the following:
Now for the Bypassed Aux configuration.
- It isolates the ECU & CANBUS power source from the high power draw and voltage drop of the starter motor, insuring the ECU has the steady power it needs to communicate and manage the engine start. This allows for a wider range of conditions in which the ECU can get the engine started.
- It isolates the ECU & CANBUS from any electrical owner customizations as owner electrical customizations are almost always connected to the Main. The only thing owner electrical customizations can impact on cold starts is the starter motor while the ECU still has steady power to manage it and is more capable of getting the engine started.
- It allows the best battery to be used for Cold and Warm Starts while keeping everything else operating. A Cold Start requires more power to the starter motor to get the engine running so the larger Main is used for the engine while the smaller Aux provides steady power to the ECU & CANBUS for management. Warm Starts (ESS) requires less power to the starter motor so the Aux is used for the start while the larger Main can continue to power everything else that has been turned on since the Cold Start without being impacted by the starter motor draw.
- ESS becomes a perfect "canary in the coal mine" for battery issues. Before every ESS engagement both Main and Aux are individually tested and it will not engage if either fails. This is excellent because nobody needs ESS and only crazy people will test their batteries as often as ESS engages.
This is an important thing. With a Main/Aux configuration the first failure that failing batteries causes is ESS failure. Something not critical to vehicle operation. This is the difference between "I'd better get new batteries soon" and "@#$%! The battery is dead and I'm stranded".
- A point of criticism here: A Check Batteries message should be displayed after a certain threshold of failed tests to make this more apparent. Many people don't realize what will inevitably happen after ESS stops working.
If you bypass Aux, yes it will work if you have a good battery. Many vehicles are designed like that. But it works until it doesn't, which will be both sooner and more fatal to vehicle operation than the Main/Aux configuration.
Given that Jeeps are far more likely to be out of range of Roadside Assistance and cell service than other single-battery passenger cars, this can go beyond inconvenience and into potentially life threatening. I'm sure this played a factor in Jeep choosing the Main/Aux design.
- The ECU & CANBUS are no longer isolated from the voltage drop of Cold and Warm Starts.
- The ECU & CANBUS are no longer isolated from owner electrical customizations that are drawing power during engine starts.
- The range of conditions in which the ECU can get the engine started is reduced because the ECU & CANBUS can be taken out by low voltage conditions during engine starts.
To clarify: At some point near the end of the battery's lifecycle, a Main/Aux configuration will be able to start the engine and operate the vehicle while a bypassed Aux configuration will not.- With Aux deleted and ESS disabled, the first failure that an end-of-life failing battery causes will be a critical function that will leave you stranded with little to no warning. You will have to manually test your battery yourself to get any advance warning and, lets face it, few of us will do that often enough.
So the real solution IMO is not to delete the Aux battery. It provides a lot of subtle benefits. Particularly at battery end-of-life, which happens much sooner and more often on modern electronic vehicles like the JL. Instead, replace Main and Aux with better batteries than Mopar OEM (which is almost any other battery) and always replace them as a pair. Use ESS as a battery failure early warning test. ESS failing to activate is about a 2-3 month warning before things you really care about start failing.
Really the only truly bad thing about the JL's Main/Aux design is where they decided to mount the Aux battery. What a PIA.
Another option is the Genesis Dual Battery approach. While it is subject to the same risks that a bypassed Aux has, you always have a mostly charged 2nd battery on standby to get you out of the situation and get yourself home. Having to engage that 2nd battery serves as your end-of-life failing battery warning. It won't be as much advance warning as Main-Aux w/ESS, but it's also a much better lifeboat out of the situation so I'd say they're at least equally good.
Yup. Assuming no other faults in the system, if the batteries drop below ~65% state of charge it'll stop engaging stop/start. If the issue remains after driving it a few days then the total capacity of your batteries has deteriorated to a point where you might just get stranded one day.Well said. My ESS stopped working a few months ago and at first I thought it was great not having to push the stop button every time I started the Jeep. Yesterday my car wouldn’t start… looks like I will be replace both batteries.
It happened before.... i had a dead aux, bypassed it. OEM main died and it was like any other vehicle.... it started showing sings before it completely went off.The problem will reveal itself when the Main dies. Critical things will fail first increasing the likelihood of finding yourself stranded.
With a JL/JT that has the Aux battery bypassed or deleted..... when the Main dies/becomes depleted.... it behaves just like your old TJ, JK, Chevy, etc.. The vehicle will just not start..It happened before.... i had a dead aux, bypassed it. OEM main died and it was like any other vehicle.... it started showing sings before it completely went off.
Do you use ESS with your JL running just the main?With a JL/JT that has the Aux battery bypassed or deleted..... when the Main dies/becomes depleted.... it behaves just like your old TJ, JK, Chevy, etc.. The vehicle will just not start..
Yes, with the JL/JT, the vehicle will before hand, give you hints lets call them that the 12v power it has, is not what it wants... Hints like a message saying Aux switches disabled, wipers activating on their own, etc..
For about 5 yrs. now, I have had my 2018 JL running with just the Main. It's just like my old TJ's and JK's when it comes to 12v power.. Monitor and maintain and replace the Main when needed...
.