How the ESS / Battery system works - 3.6 engine

[andy29847]

We had a long running thread with a lot of contributors on the Gladiator forum trying to settle how the 2 Jeep batteries worked together and what actually happens when the "Start" process is initiated. Below is a synopsis of the discussion. I am also including the link to the thread in the case you are bored and you want all the details. https://www.jeepgladiatorforum.com/...teries-really-isolated-during-ess-stops.87524


Late model Jeeps with batteries have 2 starting regimes. Let's call the first one "Cold Start." This does not mean that the engine (or weather) is cold. For our purpose, a cold start is one where the vehicle ignition was in the "off" position when you start. When pressing the start button on a cold start, the first thing that happens is that the Power Control Relay separates the 2 batteries. This is for the purposes of testing the auxiliary battery to determine the suitably for a future ESS start. The relay rejoins the 2 batteries in a very short time (~40ms). Once rejoined, both batteries are used to start and run the Jeep.


Actual oscilloscope test for cold start:

Simplified illustration for a cold start:

The second starting regime is an ESS start. This is where you are driving, you stop for a light, sign, or traffic, and the Jeep turns off (when you reach a full stop with the brake depressed). When you are ready to resume your drive, you let off the brake and the Jeep automatically restarts. When you make a stop with the ESS system turned on, and the engine shuts down, the 2 batteries remain connected together until you release the brake. Both batteries are keeping the vehicle electronics powered. When the brake is released, the Power Control Relay separates the batteries. When the batteries are separated, the auxiliary battery powers the vehicle electronics, keeping things like the radio and AC on while you wait. Next the starter is energized solely on the power of the main battery. The batteries remain separated until the engine is started, and the starter turns off. Once the starter turns off, the Power Control Relay rejoins the batteries.

Actual test for ESS start:

Simplified illustration for ESS Start test:

Revelations?

For the purpose of a cold start, the 2 batteries function as one.

In an ESS Start, the auxiliary battery powers the vehicle electronics while the main battery starts the engine. The time that the batteries are separated on an ESS Start is decided by the time it takes to start the Jeep.

If you are running your Jeep as delivered (with 2 batteries), and you get an ESS Stop/Start message or the infamous Avengers warning light, this might be your first signal that your batteries are nearing end of life.

If you are running your Jeep with the auxiliary battery removed, then you should disable ESS Stop/Start. The reason for this is that when you experience a low battery condition (old or discharged battery drawn down by the power needed to run the starter) you might get a Powertrain Control Module (PCM) reset and error code. You could also damage some of the sensitive vehicle electronic.

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[Htfan]

Appreciate you sharing this. There numerous existing threads on this subject all over this forum dating back 8+ years.

 

[ScotM]

Nice write up, love the use of the oscilloscope to show what is happening

 

[dstevens]

This is a great analysis. I always thought that when ESS had stopped the engine then the vehicle will run off the aux battery, but seems like I was mistaken.

During the cold start PCR disconnect what is powering the dash / PCM etc? If it is the aux battery then a bad aux will crash the starting procedure. I believe it can detect a failed PCR test and skip that step for the next cold start, otherwise the vehicle will never start. As with the ESS start where it disconnects and relies on the aux to keep things going.

The overriding question is why? Many vehicles have ESS with a single battery and never have a problem with just one battery. It's like they hired a german over engineer for this part of the design. Or the person who replaced all the engine bay aluminum with plastic got moved into electrics.

In the marine world 40 years ago I learnt that when you have two batteries and one charging source they need to be the same chemistry, make, model and capacity. Otherwise one will charge preferentially over the other and eventually a cell in one battery will reverse and fail. Sound familiar?

With batteries for off grid solar mine not only recommend the same chemistry, make, model and capacity battery but also the same batch number. So having two different batteries in the Jeep is not a great idea.

 

[AndySpill]

@andy29847

Recognizing that different model year, different firmware and flashes, and different model implementations (JT/JL) of Stellantis' dual AGM battery ESS system may (or may not) work identically, your writeup leaves me with some questions that I'm not sure the thread you refer to, much as I did my best to read it all, answers.

I ask these questions as follows, that maybe you have knowledge of a consensus conclusion on:

* So unlike a post ESS event start, which is energized solely from the main battery, both batteries are believed to energize a cold crank start correct? This would jibe with the ability to cold start the vehicle just from the Aux battery as here:



* Does it remain the belief that if the two batteries are prevented from being disconnected, say with a fused jumper between N1 and N2, or by pulling Fuse 42, that after six (6) ESS events the ESS system will be disabled until the next cold crank. Is it a different number or will some particular conditions change this number?

* Regardless of the number of ESS events in which the two batteries are hard wired together that it takes to turn ESS off during a driving session (i.e. between cold cranks) assuming some mechanism exists for to turn ESS off after a certain number of ESS events between cold cranks, what is that test and when does it happen? If it's one that compares the voltages of the two batteries independently, and those batteries have been connected during the ESS event, aren't those voltages apt to be very similar?

* If the batteries are connected during an ESS event how is it that during that event the "the auxiliary battery" rather than both batteries powers the cab electronics?"

* What is it that early terminates an ESS event? Before you answer that question please exclude for me any action that might have prevented the ESS event in the first place. What I mean to say by that, for example, is if I'm in an ESS event and I start turning the steering wheel, I think that early terminates the event just as sure as a turned steering is documented to prevent the ESS event in the first place. So we're on the same page, say I'm sitting at a traffic light, with ESS engaged, touching nothing on my vehicle, and the ESS event early terminates. What is most likely to have caused that? Was it a drop in the combined voltage of both batteries?

 

[AndySpill]

In the marine world 40 years ago I learnt that when you have two batteries and one charging source they need to be the same chemistry, make, model and capacity.

I too learned that...with "identical capacity" being the most forgiving of the rules, as is the case in our unequally sized factory dual AGM JL implementations....

 

[AndySpill]

@THAW really, I do not mean to put you on the spot here. I've recommended but not insisted that ESS events be avoided when bypassing the Aux battery, while you've said it's fine.

And I've only said if because of fear of being able to crank post ESS event, not the "you might get a Powertrain Control Module (PCM) reset and error code. You could also damage some of the sensitive electronic in the cab."

So I'm not going to say I told you so because in fairness, these weren't my ideas, and therefore I didn't tell you so.

But is was my idea that there may be more to the vehicle than we understand as to why Stellantis put in two batteries, beyond the "staticy" radio that one battery ESS sometimes produces, while you've pointed to how it works fine on one battery for other manufactures running ESS systems and is fine here?

What are sentiments in light of this? Do you think this guidance, for example, overly cautious or perhaps only relevant on an aged main battery solely running things?

 

[andy29847]

During the cold start PCR disconnect what is powering the dash / PCM etc? The 2 batteries act as one for a cold start. We did not test for your question, but the logical assumption would be the aux battery. I believe the test happens before the batteries are loaded.


The overriding question is why? Many vehicles have ESS with a single battery and never have a problem with just one battery. It's like they hired a german over engineer for this part of the design. Or the person who replaced all the engine bay aluminum with plastic got moved into electrics. - Only Jeep knows but I believe that the concern for a possible reset for Powertrain Control Module and the resulting waring light is the major driver. That's just my opinion - not tested.

In the marine world 40 years ago I learnt that when you have two batteries and one charging source they need to be the same chemistry, make, model and capacity. Otherwise one will charge preferentially over the other and eventually a cell in one battery will reverse and fail. Sound familiar? - A couple of years ago, I thought the same thing. A little study finds that chemistry (AGM in the case of Jeeps), Volt rating, and state of charge are the biggies.

 

[andy29847]

@andy29847

* So unlike a post ESS event start, which is energized solely from the main battery, both batteries are believed to energize a cold crank start correct? This would jibe with the ability to cold start the vehicle just from the Aux battery as here: YES



* Does it remain the belief that if the two batteries are prevented from being disconnected, say with a fused jumper between N1 and N2, or by pulling Fuse 42, that after six (6) ESS events the ESS system will be disabled until the next cold crank. Is it a different number or will some particular conditions change this number? Not tested.

* Regardless of the number of ESS events in which the two batteries are hard wired together that it takes to turn ESS off during a driving session (i.e. between cold cranks) assuming some mechanism exists for to turn ESS off after a certain number of ESS events between cold cranks, what is that test and when does it happen? If it's one that compares the voltages of the two batteries independently, and those batteries have been connected during the ESS event, aren't those voltages apt to be very similar? Not tested - not concerned.

* If the batteries are connected during an ESS event how is it that during that event the "the auxiliary battery" rather than both batteries powers the cab electronics?" Both batteries power the cab during an ESS Stop. The batteries are NOT connected while the starter is running. The aux battery is powering the cab.

* What is it that early terminates an ESS event? Before you answer that question please exclude for me any action that might have prevented the ESS event in the first place. What I mean to say by that, for example, is if I'm in an ESS event and I start turning the steering wheel, I think that early terminates the event just as sure as a turned steering is documented to prevent the ESS event in the first place. So we're on the same page, say I'm sitting at a traffic light, with ESS engaged, touching nothing on my vehicle, and the ESS event early terminates. What is most likely to have caused that? Was it a drop in the combined voltage of both batteries? - Not tested. The batteries are together during an ESS Stop. The batteries are separated when the ESS start begins. If the cause for your question is the batteries, it would be made on the value of the 2 batteries together.

 

[AndySpill]

During the cold start PCR disconnect what is powering the dash / PCM etc? The 2 batteries act as one for a cold start. We did not test for your question, but the logical assumption would be the aux battery. I believe the test happens before the batteries are loaded.

The video clip I included above, where only the Aux battery is connected, and everything, including cold cranking the engine works would support this.

 

[andy29847]

@THAW

So I'm not going to say I told you so because in fairness, these weren't my ideas, and therefore I didn't tell you so.But is was my idea that there may be more to the vehicle than we understand as to why Stellantis put in two batteries, beyond the "staticy" radio that one battery ESS sometimes produces, while you've pointed to how it works fine on one battery for other manufactures running ESS systems and is fine here? Everybody that participated in the original thread had to overcome preconceived ideas.

What are sentiments in light of this? Do you think this guidance, for example, overly cautious or perhaps only relevant on an aged main battery solely running things? The Jeep systems work regardless of the number of batteries you have. For me, that means that even when you are operating with just one battery, and when you have ESS turned off, the alarm/warning systems work. A battery in a poor state of charge will generate a warning light. Since I believe that the threshold for batteries to make an ESS stop/start is ~12.4 volts, and that 12.4 volts has been a good battery for years, I would be unconcerned about running one battery and using ESS. Important though is that the warning signs (Avenger light, ESS stop/start messages) should not be ignored.

 

[58Willys]

OP - Thank you for a wealth of tested information on this!

 

[andy29847]

OP - Thank you for a wealth of tested information on this!

I mostly compiled the info. There were some real smart guys in on this.

 

[AndySpill]

So I'm not going to say I told you so because in fairness, these weren't my ideas, and therefore I didn't tell you so.But is was my idea that there may be more to the vehicle than we understand as to why Stellantis put in two batteries, beyond the "staticy" radio that one battery ESS sometimes produces, while you've pointed to how it works fine on one battery for other manufactures running ESS systems and is fine here? Everybody that participated in the original thread had to overcome preconceived ideas.

What are sentiments in light of this? Do you think this guidance, for example, overly cautious or perhaps only relevant on an aged main battery solely running things? The Jeep systems work regardless of the number of batteries you have. For me, that means that even when you are operating with just one battery, and when you have ESS turned off, the alarm/warning systems work. A battery in a poor state of charge will generate a warning light. Since I believe that the threshold for batteries to make an ESS stop/start is ~12.4 volts, and that 12.4 volts has been a good battery for years, I would be unconcerned about running one battery and using ESS. Important though is that the warning signs (Avenger light, ESS stop/start messages) should not be ignored.

I think you get this @andy29847 but my thoughts were directed at someone who has said that running ESS events on one main battery is fine. I honestly don't know, prior to these findings where you stood on this.

And maybe it is fine to do ESS on one main battery.

You've pointed out that all the diagnostics are still very much there to preempt ESS events and report problems if the one battery proves troublesome. I get that. My concern was in your summary of the thread, where running such events on one battery could damage some of the sensitive electronic in the cab, and wanted to know what a "one battery ESS is fine on the dual AGM battery JL/JT" proponent thought of your summary.

Again, this is not to my mind a "told you so, ha ha, I was right and you weren't moment directed at anyone."

 

[andy29847]

I think you get this @andy29847

And maybe it is fine to do ESS on one main battery. I ran my Gladiator on one battery using ESS for a year with no problem. I have a thread that has been up for a year for Jeep owners who are using one battery to report any trouble attributed to using just one battery. No trouble has been reported. I follow the Wrangler Forum and the Gladiator forum daily with a focus on the battery section. I've never seen a post where someone with one battery had a problem with their Jeep because they had just one battery. I think it is OK. I believe the bigger concern is the possible PCM reset and the associated check engine light that come with it. Lots of Jeep owners do not have the ability or equipment to clear that code.

You've pointed out that all the diagnostics are still very much there to preempt ESS events and report problems if the one battery proves troublesome. I get that. My concern was in your summary of the thread, where running such events on one battery could damage some of the sensitive electronic in the cab, and wanted to know what a "one battery ESS is fine on the dual AGM battery JL/JT" proponent thought of your summary. See above. I'm willing to change my mind if the information changes.

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