Auxiliary Battery - where is it and what's its function?

[Zman1403]

I am new to the Jeep world. Took possession of my 2020 Ribicon yesterday. Only 2,696 miles. It is the eco diesel model. I have seen numerous You Tube videos showing how to change the Auxiliary battery. I don’t need to do that now. However I cannot find even a mention of it in the owners manual or the eco diesel supplement. Can anyone advise where it is and what exactly it’s function is. I am planning to purchase a secondary power source like the Goal Zero to power my Alpicool refrigerator. However I still want to know about the auxiliary battery.

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[brewdad]

It is a small motorcycle size battery located beneath the main battery.

 

[Zman1403]

It is a small motorcycle size battery located beneath the main battery.

Is there somewhere to read up on it? What it services, etc?

 

[Heimkehr]

I am new to the Jeep world. Took possession of my 2020 Ribicon yesterday. Only 2,696 miles. It is the eco diesel model. I have seen numerous You Tube videos showing how to change the Auxiliary battery. I don’t need to do that now.

I still want to know about the auxiliary battery.

On my 2021 non-eTorque 2.0T, the auxiliary battery restarts the engine when the Engine Start Stop function is turned on.

 

[MEHillwalker80]

On my 2021 non-eTorque 2.0T, the auxiliary battery restarts the engine when the Engine Start Stop function is turned on.

You have it backwards. The Aux battery keeps keeps accessories and computer alive when the ESS stops the engine. The size of the Aux battery is insufficient to provide cranking current to the starter motor anyway. It only is there to keep systems alive when the alternator is not producing anything. Consider what happens to the systems when the Aux battery fails to keep the main computer happy. Lots of lights and warnings.

One of the best writeups on this is here: https://www.jeepfan.com/tech/how-electronic-start-stop-ess-works-on-the-jeep-wrangler-jl/

 

[Rhinebeck01]

You have it backwards. The Aux battery keeps keeps accessories and computer alive when the ESS stops the engine. The size of the Aux battery is insufficient to provide cranking current to the starter motor anyway. It only is there to keep systems alive when the alternator is not producing anything. Consider what happens to the systems when the Aux battery fails to keep the main computer happy. Lots of lights and warnings.

One of the best writeups on this is here: https://www.jeepfan.com/tech/how-electronic-start-stop-ess-works-on-the-jeep-wrangler-jl/

@MEHillwalker80

Actually that write-up that you linked to has lots of misinformation in it.... Suggest you research here on the forum for spot on info in regard to the dual battery system in the JL/JT.

By the way, the Aux/ESS battery has no problem at all starting the JL when the Main is depleted...

 

[Heimkehr]

You have it backwards. The Aux battery keeps keeps accessories and computer alive when the ESS stops the engine. The size of the Aux battery is insufficient to provide cranking current to the starter motor anyway. It only is there to keep systems alive when the alternator is not producing anything. Consider what happens to the systems when the Aux battery fails to keep the main computer happy. Lots of lights and warnings.

One of the best writeups on this is here: https://www.jeepfan.com/tech/how-electronic-start-stop-ess-works-on-the-jeep-wrangler-jl/

The linked article was written when the JL generation was new. Possibly, the author wasn't yet in possession of all the facts. The article mentions only the 3.6 Pentastar, not the 2.0T engine, and makes no mention at all of an auxiliary battery or the function(s) it serves.

The foregoing is for me just an academic discussion, anyway. I've disabled the ESS function with a proprietary harness.

 

[Rhinebeck01]

The linked article was written when the JL generation was new. Possibly, the author wasn't yet in possession of all the facts. The article mentions only the 3.6 Pentastar, not the 2.0T engine, and makes no mention at all of an auxiliary battery or the function(s) it serves.

The foregoing is for me just an academic discussion, anyway. I've disabled the ESS function with a proprietary harness.

@Heimkehr

You have not "disabled the ESS function"...

All that harness you bought does, is in essence, push the ESS button for you and no more. It basically remember's whether ESS was on or off when the JL was last used and has it doing the same same for you again, automatically.

 

[Gee-pah]

Can anyone advise where it is

Is the battery actually below the main battery on the OPs diesel? From what I understand, the main battery and Power Distribution Center (PDC): the intelligent fuse box switch places in the diesel JL as opposed to say the 3.6L JL.

I mean it would make sense if the Aux battery stays in the same place on the Eco Diesel model, and just the main battery and PDC switch places.

and what exactly it’s function is.

There's literal and interpretive answers to that question. The literal one is to keep your appliances running while the engine is turned off at a braked stop of some duration beyond a stop sign (e.g. traffic light) during an ESS evenet to spare the main battery so it can effect the ** bulk of the crank of the engine when the ESS event is terminated. At least that's how it works in the 3.6L.

** both batteries energize the crank, again, at least in a 3.6L engine.

A more interpretive answer is to improve reported fuel economy of the Jeep fleet so as to avoid EPA penalty, whose cost would have to be shared with the consumer.

Another interpretive answer is a poorly designed system that recognized the need to spare the cranking battery from the draw of all the potential appliances Wrangler owners run, that would have been better designed by making not only both batteries of the same size, but alternating which one was the cranking, and which one was the Aux battery every few engine cranks IMHO. Battery purists talk about the disadvantages of hooking to dissimilar sized batteries up in parallel, as is the case here. A single battery ESS would probably be okay for a minivan (as some are designed) but not a modded Wrangler with energy hungry (lights, etc.) gear.

I am planning to purchase a secondary power source like the Goal Zero to power my Alpicool refrigerator. However I still want to know about the auxiliary battery.

There have been some excellent writeups here, I don't know if they apply to the Eco diesel as much as they do the 3.6L model, that allow the two batteries to run as factory, or allow the JL to run on just one of them, or both batteries constantly in parallel, even in ESS events when the batteries are suppose to be isolated.

Such functionality can allow appliances at the campsite to run off just one of the batteries, why the other is preserved for cranking the rig, where after the depleted battery can be rejoined to the alternator post crank to recharge.

Despite this I'd recommend a dedicated battery to run appliances like a refrigerator. But then discussion turns to how and when this dedicated battery is recharged, from shore power, to solar (like your Goal Zero can,) to alternator/starter battery only when the engine is cranked, etc.

 

[RJNitke]

Does anyone here know if a depleted aux battery can cause my head unit to no longer power up? In my 2018 JLU Sport, I have already replaced the batteries once, when returning from a trip, the Jeep wouldn't start in the airport parking lot about 3 years ago. No, right after picking the Jeep up from the shop for a steering gearbox replacement, the head unit is dead. The mechanic couldn't figure it out. He checked the codes and there were a bunch of elctrical issues (but the turn signals and rear sensors have shown alerts since I switched out the rear bumper and the fenders (which contain LED turn signals), so he was confused. I don't think he was aware of the aux battery. Once I thought of this, I payed attention to whether the EES was kicking in while stopped, and it did not, so I'm thinking the aux battery may be depleted and now causing issues with the head unit. BTW, one of the codes said something about having no communication with the radio.

 

[bd100]

An old AUX battery can cause many unrelated random error codes, and can also drain the main battery. Change both batteries. Find instructions on this forum. Be sure to disconnect and wrap both ground cables where they connect to the main battery. You want both cables isolated from each other, and also from the chassis. Failure to insulate both ground cables from each other and from the chassis can result in a blown fuse.

For the 3.6L I recommend changing through the fender instead of through the fuse box. Less to go wrong.

 

[bd100]

And by the way, according to testing others have done, the AUX battery is disconnected from the main only for a fraction of a second during an ESS restart, to protect the electronics and accessories from the momentary power loss due to the inrush of current into the starter motor. The batteries are connected together almost always, during regular engine run, and also during the entire time the engine is shut down for ESS, except for this fraction of a second for an engine restart.

 

[Heimkehr]

Does anyone here know if a depleted aux battery can cause my head unit to no longer power up?

Possibly, if the ESS battery is still connected to the vehicle's electrical system. An ESS with an insufficient resting voltage can parasitize the main battery, which in turn can cause any number of seemingly unrelated electrical issues.

Mine has been isolated for some time now, meaning only the primary 12V battery is connected to the vehicle's electrical system, and I've not noticed any malfunctioning or anything else untoward that could be walked back to either battery. As the ESS battery has been isolated, it's nothing but ballast now anyway.

Since the current problem seems to have started since a mechanic laid hands on the vehicle, additional investigation may be warranted. Did he disturb the battery, or anything else electrical on the vehicle, before or during the course of replacing the steering gearbox?

 

[RJNitke]

He claimed that he did nothing with anything electrical. There are a lot of wires behind the inner fender liners from rock lights and LED turn signals, but the turn signals are working. Half of the rock lights were disconnected (the crimp had been pulled apart) and he reconnected those, but said I should get them recrimped and blamed the installer for the poor splice.

 

[Heimkehr]

He claimed that he did nothing with anything electrical. There are a lot of wires behind the inner fender liners from rock lights and LED turn signals, but the turn signals are working. Half of the rock lights were disconnected (the crimp had been pulled apart) and he reconnected those, but said I should get them recrimped and blamed the installer for the poor splice.

The only other thing that comes to mind at the moment is to confirm that replacing the steering gearbox is a purely analog operation. Said differently, ensure that there aren't any wires or anything else with electrons that are associated with the gearbox's functioning, nor is any vehicle reprogramming required after steering gearbox replacement...as there sometimes is when, for example, replacing halogen headlights with LED versions.

Cross-walking wires to the steering hardware might sound odd, but our Wranglers have evolved into rather complex machines, with electricity and electronics now evident where the owner might not expect to see either.

Keep us posted here.

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