Main battery and Aux Battery replacement write up

[McRuss]

So I finally got around to disconnecting the battery and then reconnecting. No more idiot message or bells!

Sponsored

 

[Mud Pie]

LOL
And just how would you do that since they are connected to each other in parallel at almost all times.
Be observant, there are often clues.........

If Jeep wanted to ? A wire run from both terminals of the aux battery into an Input block on the computer. Write the program to read the voltage across both inputs. Write the program to display the value on the dash with a push of a button. Write the program to throw an alarm if the value gets below X volts. Reading input values are the easiest thing in the world, programming wise.

One guess what I used to do for a living ? Yep. Write programs and create graphics that controlled millions of dollars worth of HVAC equipment to maintain a setpoint within .04 degrees.

 

[Labrat1970]

LOL
And just how would you do that since they are connected to each other in parallel at almost all times.
Be observant, there are often clues.........

Clue, auto start not available=aux battery going down

 

[Lost1wing]

Clue, auto start not available=aux battery going down

= either battery is weak or both batteries weak or some other things.

If you put a trickle charger on it overnight and ESS is ready, you will have an idea that one or both of the batteries was weak.

If that does not correct the issue, you can reset the IBS. However, you want to do this with fully charged batteries.

 

[Mr Miami]

From the start I used a Tazer to turn off the ESS and all was good until the AUX battery went bad on my Rubicon
The battery's where only 1.5 years old
When the AUX battery faild one morning I tried to start it 3 times and on the third time all went black and it fried my 8.4 radio (non repairable)
$750 for a used radio and $200 for a new main battery
From that point forward both my Willys and Rubicon I have done the AUX battery delete
Number 1 rule. Never never use dissimilarly battery's in a system

Hi, I have a question on your comment, "Never never use dissimilarly battery's in a system". What do you mean, the same brand or something?

 

[Lost1wing]

Hi, I have a question on your comment, "Never never use dissimilarly battery's in a system". What do you mean, the same brand or something?

The batteries should be at the same State of Charge. They can be different size batteries, as with our Gladiators. If one battery is 12.8vdc and another is 12.6vdc, there is a potential difference and the two batteries are trying to equalize. This is why it is important to fully charge batteries before installing. The last two or three batteries that I have purchased were at a 50% charge.

 

[Mud Pie]

The batteries should be at the same State of Charge. They can be different size batteries, as with our Gladiators. If one battery is 12.8vdc and another is 12.6vdc, there is a potential difference and the two batteries are trying to equalize. This is why it is important to fully charge batteries before installing. The last two or three batteries that I have purchased were at a 50% charge.

Excellent advice !! Agree 100%.

I bought both batteries, brand new Gold AC Delco AGM's. I metered them, they were at 12.1 vdc or so. Not fully charged. I put them on my AGM trickle charger, it took over 24 hours to bring each battery back up to full. 2 days of charging. Thankfully I was replacing mine as a maintenance item, not a "oops, it's dead, time for a battery" item. Put them in, zero issues.

Many years ago I learned to replace batteries on a schedule (mine is every 3 years, 4 if I'm forgetful), to not wait for the "whirr-whirr-whirr" groan of a dead battery or the dash display nagging me. Modern vehicles do not like that.

My Jeep is a 2023, I bought it in November 2025, new batts by December 2025. I now know the history of it.

 

[BalancedPete]

I've had the ESS problems twice now. Jeep is a 2020, 115,000 miles. I've always let the ESS do its thing. The first failure was in the early summer of 2025. My jeep would die while sitting at a stop light. Had to have it towed to the dealership. They found both the main and aux batteries were bad. Paid to replace them. This time, Spring of 2026, my ESS warning light kept coming on. I had planned on some regular service so waited until that happened, and asked the dealership to look into the warning light. They tested both batteries, found the Aux was bad, but the main was still good. They covered the Aux battery under warranty and away I went. The ESS wouldn't function from the moment I left the dealership. Brought it back a week later. They gave me a loaner.

They kept my jeep for 8 weeks, practically no communication except when I'd come in to exchange loaner trucks (7 times!!) before hitting their 600 mile restriction. The regional Jeep tech got involved, couldn't figure out the problem, then the upper level Jeep Mopar tech came in to look. His solution was to replace the AUX battery again. The dealership then kept it for another week to test it daily. This worked, but I and my dealership contact are not convinced this has solved the problem.

It's been 3 weeks, and the ESS has been working like normal. Keeping my fingers crossed.

 

[Mud Pie]

I've read posts where owners only replace one of the two batteries because the "other tested good". To be honest, that's totally against my train of thought.

When I was first discharged from the Corps, I got a job as an armed Security Officer. Started on nights. State law says we can't carry batons or Asps. Ironically, there are no State restrictions on pistol or shotgun, carry what you buy... So instead of batons, everyone carried 7 C cell Mag Lites; basically a lead pipe. THAT was legal and fit in issue baton holders. We had to supply our own batteries. I've seen MANY guys swap in single batteries until it lit. I said, "Replace them ALL !!" "Why ?The others are good, look, it's working." "For NOW, it's gonna come back and bite you !!".

So when I read, "I only replaced the Aux battery" or "I only replaced the Main battery.", it brings me back to my Security days in the early '80's.... I sit here and say to my laptop screen, "WHY ?!?! Do both, they have the same hours on them !!".

I'm sure there are points one can defend in replacing one and not both, I'm just a do both and be done kinda guy. I hate to revisit the same problem a second time.

Rant over, as you were.....

 

[Pismo61]

If a flash light with two batteries wouldn't light up,would you replace only one battery?

Sponsored