Charging and Isolating the Aux battery

[AndySpill]

@Jebiruph

Is there a less than convoluted way to separate the Aux battery from the main battery in dual AGM battery JLs

...but, also charge it with the alternator when the engine is on, just as the alternator simultaneously but independently charges the main battery?

I ask because reconnecting the batteries prior to a service visit is advised, and an uncharged Aux battery that's been sitting there doing nothing for a time is that much more likely, when hooked up to the main battery in parallel as per factory prior to service visit, to cannabolize that main battery, and require a service visit.

TIA

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[Jebiruph]

@Jebiruph

Is there a less than convoluted way to separate the Aux battery from the main battery in dual AGM battery JLs

...but, also charge it with the alternator when the engine is on, just as the alternator simultaneously but independently charges the main battery?

I ask because reconnecting the batteries prior to a service visit is advised, and an uncharged Aux battery that's been sitting there doing nothing for a time is that much more likely, when hooked up to the main battery in parallel as per factory prior to service visit, to cannabolize that main battery, and require a service visit.

TIA

I'm not aware of an easy answer, any changes to the system to implement this would probably be more work than just occasionally charging the disconnected aux battery.

 

[jludave]

I ask because reconnecting the batteries prior to a service visit is advised

Advised, but not necessary depending on the dealership. I have fuse F42 pulled and my negative AUX battery cable disconnected and isolated. I had the vehicle in for an annual state inspection a few month ago and the dealership didn't notice or say a word about the bypassed AUX battery. Unless you tell them about it, they aren't going to look for it. The vehicle operates normally, just like any other single battery vehicle, so it should never be a problem.

 

[Rhinebeck01]

Advised, but not necessary depending on the dealership. I have fuse F42 pulled and my negative AUX battery cable disconnected and isolated. I had the vehicle in for an annual state inspection a few month ago and the dealership didn't notice or say a word about the bypassed AUX battery. Unless you tell them about it, they aren't going to look for it. The vehicle operates normally, just like any other single battery vehicle, so it should never be a problem.

@jludave You do realize for a NYS Inspection, the engine compartment hood is not opened

Yes, OBDIl port is accessed/used but that is it. Again, engine compartment is not opened by the gal/guy doing the State Inspection at a dealership or any NYS Inspection point.

To each us own, but especially seeing that it takes a minute to replace the F42 fuse and re-attach the Aux negative cable.... Geeez, it sure makes real sense to indeed do so when you bring into the Dealership Service Dept., a JL that is still covered under factory warranty.

YES, indeed, the JL running with just the Main battery is say just like a JK, or other single battery vehicle.I know I have been running my 2018 JL with just the /a Main battery since 2019... and 92,000+ miles. Never any issues doing so.

 

[jludave]

@jludave You do realize for a NYS Inspection, the engine compartment hood is not opened

Yes. My dad was a certified inspector.

I was making the point that unless told about the aux bypass, a dealership would never know it was performed in most cases.

 

[AndySpill]

After a bit of thought I've realized that this problem could be addressed with either battery isolation devices or DC to DC chargers but was hoping for a solution no more expensive that Jerry's @Jebiruph original N1 to N2 fused jumper (even less expensive pulling of Fuse 42).

Given your point Dave @jludave that Stellantis techs are far more problem solvers than policeman looking to deny service to Wranglers with mildly modified from factory and inperceptable to the vehicle's diagnostics battery changes, it's probably just easiest to--if an owner even feels it necessary to reconnect the Aux battery, just charge it prior to a service visit.

Then again, someone--I can't remember who--once said that "Anything in life worth doing is worth overdoing. And moderation is for cowards."

Ideas on who that might be (in addition to the Frogman) Dave?

 

[Mguy]

@Jebiruph

. . . uncharged Aux battery that's been sitting there doing nothing for a time is that much more likely, when hooked up to the main battery in parallel as per factory prior to service visit, to cannabolize that main battery, and require a service visit.

Any healthy, fully charged, AGM battery will have a monthly self discharge rate of about 3% of capacity. This is better than most other battery types. My isolated, healthy, aux battery is charged every three months by returning to stock wiring just before a day with 4 hours of engine charging. Before returning to stock I make sure the main is fully charged. Immediately after the day's engine operation the aux is disconnected. This procedure has successfully maintained the aux battery with no adverse effect on the main.

 

[AndySpill]

Any healthy, fully charged, AGM battery will have a monthly self discharge rate of about 3% of capacity. This is better than most other battery types. My isolated, healthy, aux battery is charged every three months by returning to stock wiring just before a day with 4 hours of engine charging. Before returning to stock I make sure the main is fully charged. Immediately after the day's engine operation the aux is disconnected. This procedure has successfully maintained the aux battery with no adverse effect on the main.

I imagine that you could (do) check that the Aux has accepted charge before reconnection, such that you could, when the time comes, choose to keep it isolated prior to a service visit if it ages out and refuses to accept charge.

 

[wanderer]

So random but related if the aux battery dies can you disconnect it and pull the fuse get ot to start and run?

 

[Ratbert]

So random but related if the aux battery dies can you disconnect it and pull the fuse get ot to start and run?

You have to remember that the two are wired in parallel. That is, if one is drained then the other most likely is too.

 

[AndySpill]

You have to remember that the two are wired in parallel. That is, if one is drained then the other most likely is too.

While true, @wanderer Ralph, the answer is it's not necessary on all dual AGM battery JLs but the early 2018's that haven't been flashed with TSB 18-092-19 as you're dual AGM battery JL will crank if the main battery has ample power to turn over the engine--which it may well lack for the cannibalization of it by a dead Aux battery in parallel reasons John @Ratbert cites.

If you take your steps all calls for power are directed to the main battery, the JL unaware that calls for power from the Aux battery are redirected to the main.

If you don't, the first time you try to crank it will fail. This is because the dual AGM battery JL tests that Aux battery in isolation prior to engine crank and won't proceed with the crank if it fails a threshold test for voltage.

But subsequent attempts to crank will proceed against the main battery even if the Aux battery is dead, and succeed if the main battery has ample power, turning the ESS off light in dash. Subsequent crank attempts will occur on the first try and the ESS off light will remain illuminated until the first cold crank, if any, after the Aux battery has been replaced/charged.

The 2018s I cite won't crank without either the steps you outline or replacement/charging of the Aux battery, or application of the flash at the dealer.

That's the full answer.

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