3.6L ESS Dual Battery Consolidated Information

Jebiruph

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Due to the quantity and length of ESS battery and electrical related threads, researching issues can be time consuming and confusing. I am using this post to make information and related links easily available. This first post will serve as a quick reference and I'll use the second post to provide more details.

JL Wranglers with the 3.6L ESS have 2 batteries, the Main battery that you see in the engine compartment and an Aux battery that is hidden underneath the Power Distribution Center. Each battery powers separate systems during an ESS auto stop. During normal operation, the batteries are connected together in parallel by the Power Control Relay.

JL Wranglers are jump-started just like any other vehicle, except with two batteries it will take more time/power.

On a properly operating system, the batteries are always connected in parallel through the PCR except during an ESS auto stop.

The batteries can go bad independently, but they will run down (or drain) together. Except when a bad battery relay (PCR) or or a blown PCR (ESS) fuse separates them, which will also prevent the Aux battery from charging.

My battery is dead and I can't jump start my jeep
On a properly functioning system the batteries go dead together, so to jump-start a JL you have to overcome two dead batteries. Compared to a single battery system, it will take more time/power to over come two dead batteries. Disconnecting the main battery and jump-starting with just the aux battery should be easier.

My dash is flashing on and off and I can't jump start my jeep
This condition seems to be caused when the Aux battery can provide some, but not adequate power to system electronics, either because it is bad or partially drained. A bad aux battery should be bypassed using a jumper or by unplugging the PCR. Partially drained means either both batteries are drained, or the aux battery is isolated (bad PCR or PCR fuse) and not getting charged. If only the aux is drained, bypassing it with a jumper or unplugging the PCR may be enough to get started, but a jump-start may still be needed. If both batteries are drained, a normal jump-start (more time/power) should be enough.

Bypassing the Aux Battery
A bad or discharged aux battery can affect PCR operation, which can be disruptive to system electronics and it may need to be bypassed. Bypassing can be accomplished by using a (fused) jumper wire to connect the main battery directly to the system electronics at PDC terminal N1 or by unplugging the PCR, preventing it from disrupting the system electronics.

Using the Aux Battery as a Dedicated Reserve Battery
A good aux battery can be bypassed and disconnected from the system (by removing the ground cable) to be held in reserve in case the main battery is not able to power a start. A dead main battery should be disconnected from the system before attempting to start from the reconnected aux battery. The aux battery should also be load tested and fully charged before relying on it as a dedicated reserve battery.

Links to Related ESS Information
(added descriptions to links 05/12/19)

ESS Dual Battery Management (added 6/27/21)
This thread mainly discusses modifications needed to use the aux battery as an emergency spare. It also has some of the most current information on the ESS system in general.
https://www.jlwranglerforums.com/forum/threads/ess-dual-battery-management.60034/

3.6L ESS Battery Diagram
This thread started it all. Due to various conflicting information regarding the dual battery wiring, I made this basic diagram based on information available online.
https://www.jlwranglerforums.com/forum/threads/3-6l-ess-battery-diagram.14401/

3.6L ESS Aux Battery Bypass
An Aux battery bypass lets you start/run with just the Main battery. Handy if the Aux battery is bad or you want to disconnect it to use as a reserve battery.
https://www.jlwranglerforums.com/forum/threads/3-6l-ess-aux-battery-bypass.17293/

3.6L ESS Bad Aux Battery No Start Firmware Fix (added 6/27/21)
Due to a firmware change, 2019 and newer JL cold starts are less reliant on the aux battery. There is a firmware update to implement this for the 2018 Jls.
https://www.jlwranglerforums.com/forum/threads/3-6l-ess-bad-aux-battery-no-start-firmware-fix.53608/

3.6L ESS Battery Basics
Here is some basic electrical theory to explain the dual battery system and why it can be difficult to start with a portable battery booster.
https://www.jlwranglerforums.com/forum/threads/3-6l-ess-battery-basics.20794/

3.6L ESS IBS Information
This thread provides information about the Intelligent Battery Sensor that monitors the health of the Main battery. It's this type of sensor that makes ESS systems possible by analyzing the ability of the battery to restart from an auto stop.
https://www.jlwranglerforums.com/forum/threads/3-6l-ess-ibs-information.22585/

3.6L ESS Battery Basic Load Testing (Battery Health Check)
If you're concerned about the condition of your batteries, here's how to check them with a cheap load tester.
https://www.jlwranglerforums.com/forum/threads/3-6l-ess-battery-basic-load-testing.22031/

3.6L ESS Cold Start (added 4/26/19)
Why is the Aux battery so critical to the cold start process? (Critical, but unnecessarily so?)
https://www.jlwranglerforums.com/forum/threads/3-6l-ess-cold-start.28297/





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Jebiruph

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Physical Appearance Diagram
(updated 10/14/19) (added PDF 10/13/19 - jumper diagram 2)

This diagram shows the physical locations of the cable connection and helps to understand what's being done in the photograph's that follow.
underhood jumper 3a.PNG


Jump Starting with a Battery Pack using just the Aux battery
(updated 04/16/2019)

With both batteries drained, disconnecting the main battery and jump-starting with just the aux battery should increase your chances of starting with a portable battery booster, as long as the Aux battery is a good battery (only drained). Disconnect the Main battery negative terminal and connect the booster to the Main battery positive and the disconnected negative terminal. This will boost just the Aux battery and not split the charge between both batteries. This is discussed later in this thread.

Here's the disconnected Main battery negative terminal.

charging aux.jpg


(updated 10/03/2019)
Depending on how depleted the Aux battery is, it may jump start quicker using a temporary jumper to provide power to PDC terminal N1 as shown in the picture. This effectively bypasses the pre-start Aux battery power test.
jumpstart aux boost.jpg



Aux Battery Bypass (PCR Bypass)
(updated 10/14/2019)

To accommodate the dual battery ESS system, the JL electronics have been separated into Aux powered systems and Main powered systems. The PCR (power control relay) is used to connect and disconnect the systems as needed. A bad or discharged aux battery can be disruptive to system electronics, especially during the starting procedure.

(4/26/19 An explanation of the cold start starting procedure is here https://www.jlwranglerforums.com/forum/threads/3-6l-ess-cold-start.28297/)

(updated 10/13/2019)
Using a jumper to bypass the PCR or unplugging it may restore the ability to start. For safety in case of an accidental short circuit, the jumper should have a fuse (30 amps worked for me). The jumper is installed between PDC terminals N1 (aux systems) and N2 (main systems) and directly connects the two systems, effectively bypassing the PCR. Unplugging the PCR (disabling it) keeps the main and aux systems always connected, but this will not work with a bad PCR or a blown PCR fuse.

ess bypass simple 4.PNG



Here's what using a jumper looks like, and it's discussed in this htread.
https://www.jlwranglerforums.com/forum/threads/3-6l-ess-aux-battery-bypass.17293/

bypass installed.jpg


(updated text and pics 10/10/19)
The PCR is located behind the front passenger side tire, and it is discussed in this thread.
https://www.jlwranglerforums.com/forum/threads/3-6l-ess-battery-diagram.14401/ (See posts 75, 92, 93)
Unplugging the connector (red arrows) will disable the PCR, keeping the batteries connected full time.

pcr location b.jpg

pcr location 2b.jpg


Reading Individual Battery Voltage
(updated 04/13/2019)

Since the batteries are usually connected in parallel, following is how to electrically isolated them to check the individual battery voltage. Keep in mind that the best way to check the condition of a battery is load testing it, which is covered in this thread.
https://www.jlwranglerforums.com/forum/threads/3-6l-ess-battery-basic-load-testing.22031/

There are two cables connected to the Main battery negative terminal, one is from the Aux battery negative terminal and the other is to the body ground. Disconnecting the Aux battery negative cable from the Main battery negative terminal will isolate the batteries to facilitate reading the voltage of the individual batteries.

Here's reading the aux battery voltage - Main positive terminal to Aux battery ground.
aux bat gnd.jpg



Here's reading the main battery voltage - Main positive terminal to Main battery ground.

main bat gnd.jpg
 

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tablerock

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Thank YOU SIR !!! Your summary of the ESS [ I hate it] is Great .Will save for future reference.Bob
 

allieboy

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What is proper way to isolate and jumpstart aux?
 
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Jebiruph

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I would remove the positive terminal from the Main battery and connect the jumper to the disconnected terminal and a good ground. The disconnected terminal is still connected to the Aux battery. Just be careful to not let the disconnected terminal short to ground.
 

allieboy

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I would remove the positive terminal from the Main battery and connect the jumper to the disconnected terminal and a good ground. The disconnected terminal is still connected to the Aux battery. Just be careful to not let the disconnected terminal short to ground.
Thanks so very much.
 

allieboy

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I would remove the positive terminal from the Main battery and connect the jumper to the disconnected terminal and a good ground. The disconnected terminal is still connected to the Aux battery. Just be careful to not let the disconnected terminal short to ground.
If aux battery starts Jeep after jump...can i reconnect positive terminal while engine running or will electronics crap out?
 
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Jebiruph

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If aux battery starts Jeep after jump...can i reconnect positive terminal while engine running or will electronics crap out?
I don't know why electronics would be affected. I've only heard warnings about exploding batteries being caused by sparks from re-connecting discharged batteries. It's what I would do unless I was aware of explosive vapors.
 

allieboy

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I don't know why electronics would be affected. I've only heard warnings about exploding batteries being caused by sparks from re-connecting discharged batteries. It's what I would do unless I was aware of explosive vapors.
Thank you!
 

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Thanks for putting all this (good info) in one thread.
 

Rahneld

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It's refreshing to see people appreciating Jerry's (and other poster's) work, as do I. I've seen people share information here that respondents (who often offer little informative content of their own) seek critique of, having done little research, rather than remain hush or say thanks for.

Jerry: I'd like to add something to your thoughts.

When you say that "On a properly operating system, the batteries are always connected in parallel through the PCR except during an ESS auto stop," while by no means do I anything but basically agree, I'd also like to add that the batteries, I believe, are also isolated just prior to an engine cranking event: be that crank on a cold Wrangler you're first starting for the day, or one sitting in an ESS event at a traffic light, where the light's turned green and the operator has taken their foot off the brake.

In short I'm suggesting that there's this exception to your "except." Fire away if I'm wrong. I'll take the flames so we all get this 411 correct.

At these brief instants, or so I understand it: again correct me if I am wrong, the ESS battery is on its own, isolated, to energize the starter relay to tell the starter to crank the engine, but before doing so mind you, the two batteries are reconnected in parallel so both can provide current to the crank process.

This point--which I could be wrong about--isn't being made to split hairs but reveal practical points about the 3.6L's operation. And that being: if your ESS battery is dead, your 3.6L JL isn't going to cold crank even if you've taken the battery leads connected to your main battery, and hooked them up to a "nuclear power plant."

While this could happen during an ESS event it's unlikely. ESS isn't going to engage in the first place with a less than well charged ESS battery, and ESS will stop, and the engine will crank--even with foot still on brake--if the ESS battery's power drops below a threshold still ample for it to signal the starter relay and get the rig and alternator going.

The of course exception to this is your well documented (again thank you) jumpering of the first and second terminals on the higher order fuses on the right side of the Power Distribution Center (upper left corner under the hood as you stand in front of the engine bay) the "first" terminal being the one closest to you.

Along these lines Jerry, and recognizing their are 1000 critiques for every innovator (you) you talk of the main battery starting the vehicle in your last paragraph. Again, not to split hairs, as it's true, the main battery (along with the ESS battery from the factory I think) does energize the starter, that starter won't fire unless the starter relay tells it too. And that starter relay relies on the ESS battery from the factory, or both batteries using your jumpering.

As I see it, your jumpering changes the 3.6L JL from having the batteries in parallel 99.9% of the time (baring pre-cranks and ESS events) to 100% of the time. And I believe you may share my belief in suggesting people disable ESS if using your jumpering.

I say this because with this jumpering, ESS events can tax the main battery which may need its juice to run the starter. And how this main battery is left alone during ESS events (without your jumpering) by design (the 3.6L running on the ESS battery during these times) to devote this main battery to the engine start, where after the engine's alternator resupplies both batteries with current.

Again, thanks.
 
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Jebiruph

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I added the first section to the "details" post (post #2). Aux Battery Bypass (PCR Bypass)
Let me know if anything needs clarification.
 

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