Start/Stop Problem

[Fudster]

I was totally struck by the cost of replacing the sensor and both batteries. Since my original post I have calmed down some. With your information I do feel like I can make a much more informed decision about what to do. I will probably suck it up and pay the bill. It is just a lot to pay after only having my Jeep for less than two months. Plus I really think this problem has been going on since day one but I just wasn’t informed enough to know to look for it. I thought the ESS just never kicked in because I was always running the A/C. I guess you live and learn…. This was an expensive lesson for me.

You should not adopt my or any one else's approach unless it serves your informed interest.

While I can't tell you your priorities I can sight the relevant factors without weighing their importance to you.

As someone who isn't a gear head you may prefer to keep the vehicle closer to stock and have the dealer service it. Neither will they bypass the Aux battery nor turn ESS off for you. That will likely be the cost of two batteries and labor to install them.

But as someone who doesn't want to spend for an Aux battery, that he not only doesn't need if he wants to bypass that battery and ESS, especially given that this aux battery's connection (even a new one) is known to be the source of problems, you may want to adopt one of the 2 methods I describe for bypassing this battery.

If you do adopt one of these methods, the first is cheaper but more obvious, turning on the ESS off light in the dash, while the other presents no "idiot lights" but requires you to turn off ESS, either manually or with purchased tech. That tech normally needs to be temporarily uninstalled (a trivial task) when getting your vehicle serviced by the dealer.

I run two batteries stock and ESS. I also trickle charge those batteries as Jerry @Jebiruph describes. I could easily adopt one of my one battery suggestions but choose not to. I do own and have connected the JL Tazer that could turn off ESS but I don't use it for that.

I've found that such trickle charging while the vehicle is parked, for me, is what keeps ESS working, much that nobody's fooling me that the savings in gasoline at stop lights may be somewhat counterbalanced by the house electric I use for the trickle charger.

Using some of Jerry's techniques I could flip switches to run on either battery or both batteries in parallel all the time but choose to leave the batteries and the connections stock.

But the important thing to consider is that my approach may or may not be right for you.

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[Reinen]

The battery system makes much more sense once you understand and accept certain fundamentals about it.

The Main and Aux batteries are linked 24/7, whether the vehicle is on or off, except during cold starts and warm ESS starts. You have to think of Main and Aux as a single battery because 99.9% of the time they are. Or more accurately, two parts of a single battery system.

Why does the dual battery exist?
So all the electronics of a modern vehicle and the starter motor can co-exist. Starter motors draw a LOT of power in a short period of time. They cause a significant voltage drop of the electrical system when in operation. The electronics of the vehicle can't handle that voltage drop and glitch out in strange random ways. Remember how in older vehicles if you started the engine with the headlights on they would dim? Same thing is happening except electronics don't dim, they glitch out. During starts the two batteries in the JL are isolated so that only one battery takes the heavy draw and experiences the voltage drop while the other maintains voltage to keep the electronics happy. When the starter motor has completed the start, the batteries are re-linked to balance out again and be recharged together.

What happens if I only replace one battery?
As AGM batteries age they progressively lose their ability to maintain their standing voltage and support heavy draw discharges. This isn't a problem until enough of its ability is lost to become a problem. Since the batteries are linked, the new battery will constantly try to balance its voltage level with the old battery 24/7. A voltage level the old battery can no longer hold. The end result is that the old battery will constantly stress the new battery, reducing its lifespan, until the new battery is at an equal level of wear as the old battery.
You will go through a lot more batteries and spend a lot more money in the long run if you replace them individually when each reaches the level of being declared bad. Batteries fail in a slow steady decline, not suddenly like a light switch. If you replace one, replace the other to maximize both of their lifespans.

AGM is a bad battery to use for modern vehicle electronics
The wheelhouse of a lead-acid (AGM) battery is applications with a large heavy load discharge followed by immediate recharge to 100%. That's how vehicles used to use their batteries and they would last a good 5-7+ years. Not anymore. Vehicles now have loads of always-on electronics constantly drawing from the batteries. The batteries are almost never at 100% charge. AGM batteries hate this, it slowly damages them. They want to be at 100% charge all the time. This is why you only see 2-4 years out of an AGM battery now. Modern AGM batteries are actually better but modern vehicle electronics are so much harder on the batteries that it completely offsets any battery improvements and results in a much shorter battery lifespan.

No, you can't just swap your AGM batteries for LiFePo batteries (like your phone has, which don't mind being at less than 100% charge). They have different charging requirements and they don't handle the large sudden draw of a starter motor well. You'd have to redesign the electrical system to do that.

I don't understand why the JL isn't designed with two isolated 12v electrical systems, one exclusively for the starter with an AGM battery and another exclusively for the electronics (like an RV house battery) with a LiFePo battery. Then both would be using the right battery for the job. The only reasons I can come up with is auto designers are just more accustomed to AGM, redesign cost and/or LiFePo batteries have a narrower operating temperature range.

 

[Heimkehr]

I don't understand why the JL isn't designed with two isolated 12v electrical systems, one exclusively for the starter with an AGM battery and another exclusively for the electronics (like an RV house battery) with a LiFePo battery.

LiFePO4 (lithium iron) batteries have unique operating requirements. Stated simply, they are sensitive to temperatures below 45°F or so. This may predispose them for use in applications where the end user might not have the ability or the patience to accommodate a LiFePO4's idiosyncrasies, particularly in northern climes. This article discusses powersports applications; consider the difference when scaled up for automotive use.

Edited to repair link.

 

[Reinen]

LiFePO4 (lithium iron) batteries have unique operating requirements. Stated simply, they are sensitive to temperatures below 45°F or so. This may predispose them for use in applications where the end user might not have the ability or the patience to accommodate a LiFePO4's idiosyncrasies, particularly in northern climes. This article discusses powersports applications; consider the difference when scaled up for automotive use.

The link to the article you supplied is bad.
As I understand it, you wouldn't want to charge a LiFePO4 battery when it's cold but you can draw from it. Seems like that can be resolved with a battery heater and a relatively simple algorithm. The battery would just be sized large enough to have enough of a buffer until it's at acceptable temperature range.

I'm no battery expert but my phone works when it's much colder than 45 degrees. It does have a lower limit but that's sub-zero. IDK, it just seems like something that can be worked out. Tesla seems to be managing fairly well and it wouldn't be anywhere near that scale.

 

[Heimkehr]

The link to the article you supplied is bad.

Try this one.

 

[Reinen]

Try this one.

I guess the ultimate answer is that all this will be moot given the push to EVs. They won't have this problem. So this simplest solution is to suck it up and deal with it, buying new AGM batteries every 2-4 years. ?‍

 

[Fudster]

Why does the dual battery exist?
So all the electronics of a modern vehicle and the starter motor can co-exist.

IMHO ...not exactly. A better answer, and I assure you not for the purposes of being picky, but for understanding, is that "so all the electronics of a JL and the starter motor can co-exist."

A fair number of vehicles run ESS on one battery designs. And while I of course wasn't in on the design meetings at Stellantis regarding their two battery design, I have strong reason to believe that the engineers designed it with 2 batteries recognizing that Wranglers are a unique breed of vehicle where owner's customizations in particular can involve appliances, from light bars to other energy hungry appliances, that made isolating the main battery during ESS engine off events the best way to insure that the main battery has the power to quickly crank the engine once the ESS event is terminated.

Truth be told, at the end of an ESS event both batteries are joined in parallel and then the crank is effected. It's why you could in theory run a 3.6L on just the ESS battery if you wanted to.

Starter motors draw a LOT of power in a short period of time. They cause a significant voltage drop of the electrical system when in operation. The electronics of the vehicle can't handle that voltage drop and glitch out in strange random ways.

The electronics of a JL can normally handle that voltage drop. I'm not recommending it but if you ran your 3.6L with just the main battery and ESS on, most of the time you would be fine. ESS events will be terminated early when the Aux battery, or what the vehicle thinks through cable reassignments is the Aux battery's voltage drops below a threshold that the vast majority of time is ample enough to successfully crank the engine. And again, vehicles with one battery and ESS systems do this all the time.

Remember how in older vehicles if you started the engine with the headlights on they would dim? Same thing is happening except electronics don't dim, they glitch out. During starts the two batteries in the JL are isolated so that only one battery takes the heavy draw

True, the main battery is normally responsible for providing most of the post ESS cranking power, but again, both batteries are brought back into a parallel connection after the ESS event but before the crank to both energize that crank. If those batteries had different chemistries, such parallel connection could be problematic.

When the starter motor has completed the start, the batteries are re-linked to balance out again and be recharged together.

Again, the connection happens before, which is why you can run a 3.6L with just the Aux battery provided it has ample power. True, once cranked, the alternator attempts to charge both otherwise parallel connected batteries.

What happens if I only replace one battery?
As AGM batteries age they progressively lose their ability to maintain their standing voltage and support heavy draw discharges. This isn't a problem until enough of its ability is lost to become a problem. Since the batteries are linked, the new battery will constantly try to balance its voltage level with the old battery 24/7. A voltage level the old battery can no longer hold. The end result is that the old battery will constantly stress the new battery, reducing its lifespan, until the new battery is at an equal level of wear as the old battery.

True, but it's mostly the small Aux battery bastardizing the main one. This is why a better dual battery system from Stellantis, IMHO, would have involved two equally sized batteries, like the Genesis battery system has, where these two batteries periodically roll reversed playing main and aux battery. That also would have cost us consumers more base price. Stellantis has the luxury of designing systems in the JL that sometimes may not be top of the line, knowing that if the demands exists, MOPAR aftermarket OEM accessories, or those of 3rd party vendors will get produced.

To a battery purist, hooking up to dissimilarly sized batteries, even of the same AGM chemistry as here, in parallel, is less than ideal.

You will go through a lot more batteries and spend a lot more money in the long run if you replace them individually when each reaches the level of being declared bad.

IMHO that's close. Better stated, I agree that when changing the main battery that the aux should probably be changed as well. I'm not so sure that the converse is true. Load testing of the batteries can yield battery health insight here before $ is spent.

No, you can't just swap your AGM batteries for LiFePo batteries (like your phone has, which don't mind being at less than 100% charge).

Your phone has Lithium Ion batteries. LiFePO4 batteries are Lithium Iron Phosphate, an entirely different chemistry with different use cases, charge profiles, and even safety histories: LiFePO4 being much safer. This safety, allong with the off gassing in Lead Acid batteries (AGM batteries being a type) is why your apt to find a LiFePO4 inside the confines of a (reasonable climate controlled) motorhome, while Lead Acid batteries occupy space under the hood. (Yeah, the Prius battery in the trunk with its own dedicated off gas piping notwithstanding.)

Key to this discussion is one of the reasons LIFEPO4 batteries are safer: their battery monitoring systems or BMS', which paradoxically enough tend to make them poor under the hood cranking batteries: that BMS limiting the charge the battery (if not its chemistry alone) can deliver.

I don't understand why the JL isn't designed with two isolated 12v electrical systems, one exclusively for the starter with an AGM battery and another exclusively for the electronics (like an RV house battery) with a LiFePo battery.

My guess, it would have been costlier to manufacture. Now you need an alternator (or two) capable of charging two different battery chemistries, and there are high current drawing appliances that might bump heads with an Aux LIFEPO4 battery's BMS. Then there's engine heat and cold, as you and James @Heimkehr discussed.

Then both would be using the right battery for the job.

I believe that AGM batteries, like all battery chemistries have limitations, but are probably the best suited here. Again, my issue is with their dissimilar size in parallel and my desire to have them periodically switched off by the vehicle in their roles as main an aux.

 

[Heimkehr]

Thanks for providing the required clarity here, @Fudster

 

[Jammalynn]

There is a trade off between how much you are able/willing to do yourself and how much you are able/willing to pay for. A good compromise is to skip the charging/testing of the main battery and buy a new one from an auto parts store. They will install it for free and it should cost around $250.

Everyone will say you have to replace both batteries at the same time, but you don't. My main battery failed at 2.5 years and my aux battery still tested good at 3.5 years. It wouldn't be a bad idea to disconnect the aux battery which involves disconnecting a battery cable and pulling a fuse.

If you have access to an electrical outlet near where you park at night, it's a good idea to occasionally put a charger on the batteries anyway, so buying a charger first to charge and test the batteries might not be a bad place to start.

Whatever you do, don't pay for a new battery sensor, they almost never go bad.

I will be ready to argue that charge charge I pick it pick tomorrow!

 

[Jammalynn]

The dealer service manager who told you that gets a "Red Flag" I would NOT trust them! If the batteries are the original from the factory, they are simply warn out and should have been replaced by the dealer before it was sold 2 months ago.

As far as disabling the feature, many on this forum use one of the Tazer products. However, I have not tried it myself.

I bought the Jeep in KC but live in the Wichita area. So this is not the dealer I bought the Jeep from. The guy I dealt with in KC is out this week and won’t be in until Monday. I hope they will at least help cover part of the charges.

 

[Jammalynn]

You should not adopt my or any one else's approach unless it serves your informed interest.

While I can't tell you your priorities I can sight the relevant factors without weighing their importance to you.

As someone who isn't a gear head you may prefer to keep the vehicle closer to stock and have the dealer service it. Neither will they bypass the Aux battery nor turn ESS off for you. That will likely be the cost of two batteries and labor to install them.

But as someone who doesn't want to spend for an Aux battery, that he not only doesn't need if he wants to bypass that battery and ESS, especially given that this aux battery's connection (even a new one) is known to be the source of problems, you may want to adopt one of the 2 methods I describe for bypassing this battery.

If you do adopt one of these methods, the first is cheaper but more obvious, turning on the ESS off light in the dash, while the other presents no "idiot lights" but requires you to turn off ESS, either manually or with purchased tech. That tech normally needs to be temporarily uninstalled (a trivial task) when getting your vehicle serviced by the dealer.

I run two batteries stock and ESS. I also trickle charge those batteries as Jerry @Jebiruph describes. I could easily adopt one of my one battery suggestions but choose not to. I do own and have connected the JL Tazer that could turn off ESS but I don't use it for that.

I've found that such trickle charging while the vehicle is parked, for me, is what keeps ESS working, much that nobody's fooling me that the savings in gasoline at stop lights may be somewhat counterbalanced by the house electric I use for the trickle charger.

Using some of Jerry's techniques I could flip switches to run on either battery or both batteries in parallel all the time but choose to leave the batteries and the connections stock.

But the important thing to consider is that my approach may or may not be right for you.

I told them to go ahead and replace both batteries today. I have never had an ESS system before so I am really not sure if it is something I love or hate. I have a guy who usually does all my work on my cars and is amazing. I took it to the dealership because I thought it would be covered by warranty. But “electronic” issues are not covered. Oh well… in the scheme of life everything will be ok.

 

[Jammalynn]

The battery system makes much more sense once you understand and accept certain fundamentals about it.

The Main and Aux batteries are linked 24/7, whether the vehicle is on or off, except during cold starts and warm ESS starts. You have to think of Main and Aux as a single battery because 99.9% of the time they are. Or more accurately, two parts of a single battery system.

Why does the dual battery exist?
So all the electronics of a modern vehicle and the starter motor can co-exist. Starter motors draw a LOT of power in a short period of time. They cause a significant voltage drop of the electrical system when in operation. The electronics of the vehicle can't handle that voltage drop and glitch out in strange random ways. Remember how in older vehicles if you started the engine with the headlights on they would dim? Same thing is happening except electronics don't dim, they glitch out. During starts the two batteries in the JL are isolated so that only one battery takes the heavy draw and experiences the voltage drop while the other maintains voltage to keep the electronics happy. When the starter motor has completed the start, the batteries are re-linked to balance out again and be recharged together.

What happens if I only replace one battery?
As AGM batteries age they progressively lose their ability to maintain their standing voltage and support heavy draw discharges. This isn't a problem until enough of its ability is lost to become a problem. Since the batteries are linked, the new battery will constantly try to balance its voltage level with the old battery 24/7. A voltage level the old battery can no longer hold. The end result is that the old battery will constantly stress the new battery, reducing its lifespan, until the new battery is at an equal level of wear as the old battery.
You will go through a lot more batteries and spend a lot more money in the long run if you replace them individually when each reaches the level of being declared bad. Batteries fail in a slow steady decline, not suddenly like a light switch. If you replace one, replace the other to maximize both of their lifespans.

AGM is a bad battery to use for modern vehicle electronics
The wheelhouse of a lead-acid (AGM) battery is applications with a large heavy load discharge followed by immediate recharge to 100%. That's how vehicles used to use their batteries and they would last a good 5-7+ years. Not anymore. Vehicles now have loads of always-on electronics constantly drawing from the batteries. The batteries are almost never at 100% charge. AGM batteries hate this, it slowly damages them. They want to be at 100% charge all the time. This is why you only see 2-4 years out of an AGM battery now. Modern AGM batteries are actually better but modern vehicle electronics are so much harder on the batteries that it completely offsets any battery improvements and results in a much shorter battery lifespan.

No, you can't just swap your AGM batteries for LiFePo batteries (like your phone has, which don't mind being at less than 100% charge). They have different charging requirements and they don't handle the large sudden draw of a starter motor well. You'd have to redesign the electrical system to do that.

I don't understand why the JL isn't designed with two isolated 12v electrical systems, one exclusively for the starter with an AGM battery and another exclusively for the electronics (like an RV house battery) with a LiFePo battery. Then both would be using the right battery for the job. The only reasons I can come up with is auto designers are just more accustomed to AGM, redesign cost and/or LiFePo batteries have a narrower operating temperature range.

Thank you for all the info. That really helps. I feel like I am learning a new language. Your explanation makes a lot of sense. I did tell them to go ahead and replace both batteries. Next time I need the batteries changed, I won’t have it done at the dealership.

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