Battery or aux battery or alternator issue?

pdale44

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I have a 2019 JL Sport S 3.6L manual 50,000 miles. I’m trying to identify a low voltage issue. This is my first vehicle with two batteries so I’m not sure if the aux battery is at fault or not. I have had the battery too low to start the Jeep once. It jumped just fine and I had thought we must have left a door open or something on. A week later now it is starting slowly. This morning the voltage was 11.5 before starting. It did start but not as fast as normal. Now at idle and revving the voltage readout on the cluster is 12.6-12.7. My experience says this should be higher if the battery is drained, the alternator should be working overtime to charge it and probably be in the 14.x range. But I’m wondering if it is that high coming out of the alternator but that second battery is pulling it down and causing the low reading on the cluster? Anyone have any insight or run into the same issue? I have seen that the aux batteries go bad and pull the main battery down with it. But it’s the low voltage reading throwing me for a loop here. Any help is appreciated, thanks.
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Yellow Cake Kid

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Keep in mind; if the aux battery is dead all bets are off regarding voltage readings. You will need to isolate the components from each other to make meaningful observations.

Everything you said would make sense if the batteries were still viable, but if one is dead then nothing may make sense until you cut the bad battery out of the circuit.

Good luck.




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pdale44

pdale44

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Just to throw in another oddity, I started it up tonight and the voltage before I started it on the cluster was 11.8, dropped to 10.5 while starting. It started and now the voltage reads 14.2, as I originally expected it to.
 

Suffolklou

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Mine reads 14.5 for a few minutes after start up at idle. then goes to 14.2 and eventually to 13.7 when the charge is done. I have a 3.6 auto with the 240 amp alternator.
 

WranglerMan

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Using the EVIC display for voltage is a oddity as it’s not a true reading of battery voltages, what you see is influenced by the alternator and don’t be surprised if it varies as this is dictated by the IBS.

The best and truest way to get the voltages of the batteries is directly at the battery.

The JL design on it using two batteries with a few hardware and software components in the mix along with two batteries that are not sized to compliment each other is a dumpster fire at best and then factor if these are FACTORY batteries and only last a few years before needing replaced or sooner if they are not maintained for a Jeep that is parked a lot.

Lots have had batteries and a few other things like the IBS replaced under warranty and others have added fused bypasses, digital gages directly at the battery, manual isolation disconnects and a host of other things that go up in price as the complexity goes up but in the end FCA handed out a crappy design that we have to make work.

This is how a properly functioning smart charging system is supposed to work on batteries that have good health


As you can see ( this is my Jeep ) the voltage floats, it floats up above 14 volts when slowing down or braking and then floats down during acceleration and once a a steady speed settles down.

The purpose of the smart charging system is to reduce load on vehicle during non demand use like slowing down or braking, it’s supposed to help fuel economy but it’s minimal at best.

In the end those with 2018’s and 2019’s that are having issues need to likely have batteries load tested and replaced especially if their batteries have not been parked a lot and not on a smart charger, their is a possibility that the IBS sensor is bad but I have not heard much info on alternators going bad.
 
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Jebiruph

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Just to throw in another oddity, I started it up tonight and the voltage before I started it on the cluster was 11.8, dropped to 10.5 while starting. It started and now the voltage reads 14.2, as I originally expected it to.
Check your ground cables, especially at the main battery negative terminal.
 
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