Start/Stop battery is STUPID!

[ShadowsPapa]

The Cherokee KL has ESS with only 1 AGM battery why did Jeep not do this for JL/JT too? It would be like pulling fuse 42 and removing the aux battery ground cable. Maybe get a larger main battery to be safe.

No aux battery at all - not even under the floor like some?

Likely due to very different electronics - if you connect a scope to things and watch during an ESS restart you can sort of see why there's another battery to handle the electronics of some vehicles (others must use some other system to prevent a quick drop and subsequent spike in power during an ESS restart)

What does that KL have for radio, and other systems?

That's been my thought since I've started reading all these horror stories.
If they're in parallel, not isolated from each other, what's the point?

The point is that during a start, when the starter is initialized, there's a fast drop in voltage and the electronics drain down quickly - then once the starter initialization is over, there's a surge in power returning.
It's not about capacity, it's about protecting systems from voltage drops when the starter is initially triggered. Someone did an analysis with a scope and showed what went on.
The aux battery is disconnected and fully responsible for the electronics during an ESS restart - starter initialization. This prevents a dip then surge in power for electronics that are already running. It's fine when you do a cold start - things aren't powered up until it starts. But during an ESS stop, things are still powered, like a full water tank. Hit a restart and with one battery, you take away power and drain the tank, so to speak.
It gets into a lot of electronic theory and even electronic fact -
electronics fail at power-up.
That's not theory - that comes from one of the world leaders in electronic control systems for turbo compressors. The QA guy had classes for the rest of us who wanted to learn more about the business and why they did things the way they did.

So, each time you go through an ESS restart, you are dumping power from the electronics and powering them back up without that battery.
It's to prolong life and prevent potential for issues during an ESS restart.

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[Lost1wing]

No aux battery at all - not even under the floor like some?

Likely due to very different electronics - if you connect a scope to things and watch during an ESS restart you can sort of see why there's another battery to handle the electronics of some vehicles (others must use some other system to prevent a quick drop and subsequent spike in power during an ESS restart)

What doe have for radio, and other systems?



The point is that during a start, when the starter is initialized, there's a fast drop in voltage and the electronics drain down quickly - then once the starter initialization is over, there's a surge in power returning.
It's not about capacity, it's about protecting systems from voltage drops when the starter is initially triggered. Someone did an analysis with a scope and showed what went on.
The aux battery is disconnected and fully responsible for the electronics during an ESS restart - starter initialization. This prevents a dip then surge in power for electronics that are already running. It's fine when you do a cold start - things aren't powered up until it starts. But during an ESS stop, things are still powered, like a full water tank. Hit a restart and with one battery, you take away power and drain the tank, so to speak.
It gets into a lot of electronic theory and even electronic fact -
electronics fail at power-up.
That's not theory - that comes from one of the world leaders in electronic control systems for turbo compressors. The QA guy had classes for the rest of us who wanted to learn more about the business and why they did things the way they did.

So, each time you go through an ESS restart, you are dumping power from the electronics and powering them back up without that battery.
It's to prolong life and prevent potential for issues during an ESS restart.

I belong to a Mahindra Roxor club. These Mahindra Roxors have a nice electronic cluster. Unfortunately, the Instrument cluster are prone to fail. The most common failure happen during a fail to start event when the battery is drained. It takes out the cluster. I think Mahindra could use a second battery just to save the electronics.

And this is why we have a second battery. It prevents a low voltage situation during an ESS restart.

 

[2terpsjeep]

this thread is so timely!

my aux battery literally started dying and throwing a warning on my dash yesterday. i had to jump it to get it started, but the main battery still reads as 14+ volts. was kind of worried about that aux battery slowly killing the main, so i did the delete (pulled fuse 42, and disconnected/zip tied the aux negative lead out of the way). i have a tazer, and have ESS disabled, so that's not a concern - i always hated stop/start anyway! worked like a charm - got rid of the dash warning, and it started right up.

the only issue i had was that disconnecting the battery reset my radio: the look of everything was off, sirius/xm disappeared, and climate control was wonky. i did some internet sleuthing and learned how to do a soft reboot (in ACC, press and hold the volume and tune buttons for 10-20 seconds), and when it came back on, it was back to normal with all my presets and everything.

so now i'm down to one battery, and the whole process cost me 20 minutes, 1 zip tie, and maybe a foot of electrical tape.

totally worth it!

 

[1550jrit]

I replaced both my aux battery and main battery in September 2023 prior to a Jeeping trip. I installed an Odyssey Extreme aux battery and a Batteries Plus X2 AGM H7 as my main battery (I was having a hard time finding an Odyssey Extreme main at that time). Both batteries were fully charged prior to installation.

In August 2025, my jeep started giving me the "service start/stop" error message though it was starting fine. My Jeep is my daily driver and it gets plenty of continuous run time to charge the batteries. I run a Tazer and the ESS is always turned off. the main battery was tested at Batteries Plus and tested as still good, though I typically take their test with a grain of salt. I debated long and hard about just bypassing the aux by disconnecting the negative, pulling the F42 fuse, and leaving the aux in place.

I had some time last Saturday and decided just to pull the aux battery since it was still under warranty. I'm sure happy that I did. the positive terminal was extremely corroded and the battery was extremely ballooned. My biggest concern about leaving the aux battery in place was always leaking, so I'm glad I just pulled the trigger and pulled it. I did go in through the fuse box, and since it's a pain, I made the decision just to fully pull and bypass the aux battery. I connected the negative cable to firewall ground (since there wasn't enough length to make it to the main negative terminal) and the positive auxiliary cable went to the main positive connection.

I got everything buttoned up and jumped in to fire up my jeep. It I got nothing but a few clicks before it barely turned over but started. I immediately got a check engine light that turned out to be an error for low voltage. I let my Jeep run for an hour and took it on a long drive. It ran fine for a few days but then barely started one morning and gave me the same low voltage error. I decided just to replace the main battery out of an abundance of caution.

I purchased a brand new Duralast Platinum Elite H7 AGM from Autozone (only cost me $80 since I warrantied out my Odyssey Aux battery). I did not go with another X2 from Batteries Plus because the sale rep said they have nothing but problems with them. (My original X2 got warrantied out for a battery for my son's Jeep). The new Duralast has a 4-year warranty and was fully charged prior to installation.

In the end, the new main battery solved all of the hard starting issues I was experiencing after removing the auxiliary battery. I guess the failing aux still provided enough juice for a weak main to do its job. My Jeep starts and runs great without any errors. ESS is still disabled and I'm happy to never have to pull that fuse box again. I'm also glad that I don't have to worry about the aux battery leaking, which could have been a real possibility with how badly the aux battery had ballooned.

 

[ShadowsPapa]

my aux battery literally started dying and throwing a warning on my dash yesterday. i had to jump it to get it started, but the main battery still reads as 14+ volts. was kind of worried about that aux battery slowly killing the main, so i did the delete (pulled fuse 42, and disconnected/zip tied the aux negative lead out of the way). i have a tazer, and have ESS disabled, so that's not a concern - i always hated stop/start anyway! worked like a charm - got rid of the dash warning, and it started right up.

To lurkers - some not exactly right info above -

That 14+ volts means the system says the batteries are low and need charging -
and that 14+ volts is the charging system voltage, NOT the battery voltage. Since the batteries are connected together, they get charged together.
In other words - it might be the main battery is low - or it might be the aux is low, since that 14+ applies to BOTH of them, not the main battery.

in his case, it was the aux - but those exact same things will be seen with a main battery going bad, too! Seeing constant cluster voltage display of over 14 volts means your batterIES are low. Not just one, not just the other, but a low voltage situation is seen and the system is trying, without success, to charge the batteries.

 

[2terpsjeep]

To lurkers - some not exactly right info above -

That 14+ volts means the system says the batteries are low and need charging -
and that 14+ volts is the charging system voltage, NOT the battery voltage. Since the batteries are connected together, they get charged together.
In other words - it might be the main battery is low - or it might be the aux is low, since that 14+ applies to BOTH of them, not the main battery.

in his case, it was the aux - but those exact same things will be seen with a main battery going bad, too! Seeing constant cluster voltage display of over 14 volts means your batterIES are low. Not just one, not just the other, but a low voltage situation is seen and the system is trying, without success, to charge the batteries.

Appreciate that... so is that voltage reading still taking a reading of both batteries together, and not just the main, even though I've now done the aux delete/bypass? If so, I take it I can't rely on what's displayed in that dash menu and would only know the main battery's true health by having it tested. That sound right?

 

[ShadowsPapa]

Appreciate that... so is that voltage reading still taking a reading of both batteries together, and not just the main, even though I've now done the aux delete/bypass? If so, I take it I can't rely on what's displayed in that dash menu and would only know the main battery's true health by having it tested. That sound right?

The way these work - they have a sensor, the IBS, that monitors the amperage going out of the main battery and into the main battery. It watches the voltage, figures state of charge and so on.
The condition of the aux battery is "calculated".

If one battery starts to go - the combined voltage of the two (in parallel) can drop.
The battery losing capacity or ability will drop, pulling down the voltage of the pair.
The lower state of charge the system figures for the batteries, the more the voltage is boosted to try to top them off. A voltage of over 14 at all times means that either the batteries are ok but in need of charging, or, they aren't taking and keeping a charge and it can't keep up.

Normally with healthy batteries in a vehicle that is driven often and long enough to keep them charged, you will see voltages in the 13s, even upper 12s.

Since it senses off the main and "calculates" for the aux battery, it's going to be the same rule -
if you see constant 14+ voltage, it means the main battery is either losing health, or not being driven enough to keep it charged.
Before it meant one battery of the pair was dragging the pair down - could be main, could be aux.

If you disconnected the aux, and have a FULLY charged, healthy main battery, you will see lower voltages when driving any distance.

So if you keep seeing 14 volts - either the main battery isn't taking a full charge, or, you aren't driving it enough to keep it charged fully.

 

[Caspien]

The way these work - they have a sensor, the IBS, that monitors the amperage going out of the main battery and into the main battery. It watches the voltage, figures state of charge and so on.
The condition of the aux battery is "calculated".

If one battery starts to go - the combined voltage of the two (in parallel) can drop.
The battery losing capacity or ability will drop, pulling down the voltage of the pair.
The lower state of charge the system figures for the batteries, the more the voltage is boosted to try to top them off. A voltage of over 14 at all times means that either the batteries are ok but in need of charging, or, they aren't taking and keeping a charge and it can't keep up.

Normally with healthy batteries in a vehicle that is driven often and long enough to keep them charged, you will see voltages in the 13s, even upper 12s.

Since it senses off the main and "calculates" for the aux battery, it's going to be the same rule -
if you see constant 14+ voltage, it means the main battery is either losing health, or not being driven enough to keep it charged.
Before it meant one battery of the pair was dragging the pair down - could be main, could be aux.

If you disconnected the aux, and have a FULLY charged, healthy main battery, you will see lower voltages when driving any distance.

So if you keep seeing 14 volts - either the main battery isn't taking a full charge, or, you aren't driving it enough to keep it charged fully.

So, in your opinion.. would you say there's benefit to doing the aux delete (temporarily) to see which of the two batteries is the problematic one, since testing the aux is kind of a pain, without removing it? (Or.. hmm.. I guess I could pull the aux negative, and test from there to the positive, which should remove the main from the circuit, giving me the current status of the aux, itself)

Mainly concerned, because none of my local parts stores have an H7, apparently. Can order one, but it'll take 2+ weeks to show up. (Which, as mentioned prior, is partially why I'm reluctant to even consider a dual system, price notwithstanding.. the availability of battery sizes)

If I can determine the Aux is the problem, at least I could pull it and limp for however long it takes for the H7 to arrive. (showing 12.2 at the cluster, while not running, and pushing 14.2-14.4 while running. Aka: keeps trying to charge the batteries.

Hooked up a NOCO 10, today, and let it go for an hour before it went slow pulse green... so it THINKS everything is charged... *shrug*

 

[Hootbro]

Mainly concerned, because none of my local parts stores have an H7, apparently.

Can you define "local parts store"?

H7 size is one of the most common out there and have a hard time believing any chain type retailer not having one in stock or getting one the next day. Any Walmart should also have a H7/94R AGM in stock most times.

 

[Caspien]

Can you define "local parts store"?

H7 size is one of the most common out there and have a hard time believing any chain type retailer not having one in stock or getting one the next day. Any Walmart should also have a H7/94R AGM in stock most times.

Local Walmart only carries flooded batteries for us. AutoZone and O'Reilly carries 35s in agm, but h7 are flooded.

We have a local Napa, too, but they have VERY few batteries in stock, with everything being ship/order.


'local' is within about 20 miles. Anything more than that, and I'd have to head south to the main STL area.

 

[Hootbro]

Local Walmart only carries flooded batteries for us. AutoZone and O'Reilly carries 35s in agm, but h7 are flooded.

We have a local Napa, too, but they have VERY few batteries in stock, with everything being ship/order.


'local' is within about 20 miles. Anything more than that, and I'd have to head south to the main STL area.

I have had good luck with Amazon for H7 AGM batteries and two day delivery. Downside is very limited to no brick and mortar store to deal with if you have a warranty issue.

 

[Supazuk]

Yea…. Net time the Aux battery starts to fail, I’ll just permanently delete it..

left click save thanks... i have been startstop free since i put my tazer in

 

[ShadowsPapa]

Local Walmart only carries flooded batteries for us. AutoZone and O'Reilly carries 35s in agm, but h7 are flooded.

We have a local Napa, too, but they have VERY few batteries in stock, with everything being ship/order.


'local' is within about 20 miles. Anything more than that, and I'd have to head south to the main STL area.

Sounds about like Des Moines. Capital city, but hard to get anything here.
Even the NAPA distribution center on the east side often doesn't have stuff I need.

 

[jmr]

No aux battery at all - not even under the floor like some?

Likely due to very different electronics - if you connect a scope to things and watch during an ESS restart you can sort of see why there's another battery to handle the electronics of some vehicles (others must use some other system to prevent a quick drop and subsequent spike in power during an ESS restart)

What does that KL have for radio, and other systems?

Mine was a 2019 Cherokee Altitude with 2.0L turbo and 7" uconnect head unit. The electrical architecture is somewhat similar to the WL Grand Cherokee. I changed a setting to enable the factory security alarm with alfaobd which worked but every light on the instrument panel was triggered. I took it to the dealer and they did a proxy realignment and my security alarm change still worked afterwards.

 

[ShadowsPapa]

If I can determine the Aux is the problem, at least I could pull it and limp for however long it takes for the H7 to arrive. (showing 12.2 at the cluster, while not running, and pushing 14.2-14.4 while running. Aka: keeps trying to charge the batteries.

Hooked up a NOCO 10, today, and let it go for an hour before it went slow pulse green... so it THINKS everything is charged... *shrug*

I've found not all chargers seem to deal with really low batteries, or they react as if the battery is charged, when it's not but isn't in great health, either.
Some it takes something with some real push - like 20 amps - to make things happen.

Drove my JT to church and back tonight (guess the destination won't matter LOL) - 26 miles each way. It was running in the low 14s for a while, then dropped into the low 13s for most of the trip. When letting off the throttle, going down hills and so on, it ramped back up to 14.1-14.2, then when I was cruising or needing power, it dropped back again to 13.1 for example.
In other words - working perfectly.
Battery voltage (with things fully connected, but truck off) at about 12.6. If I removed the parasitic load or the thing went to sleep and no fob was nearby, it would probably read closer to 12.7 with NO load at all.

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