Main and AUX Battery Charging Sequence

[Jsmith1529]

What is the in-vehicle logic for charging the Main and Aux batteries? I bought a used 2021 that had been on a dealer lot for a few months and likely hadn't been driven much. The Start/Stop Warning Light came on the day I got it home. Based on this forum, I knew it was related to the AUX Battery. When I took it back to the dealer, they took both batteries out, charged them, and put them back in. They tested "Good" with no bad cells. I'm wondering why driving the vehicle didn't have the same result. If they're directly connected and a bad AUX Battery can kill the Main Battery, then why wouldn't it charge the AUX off the Main?

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[ShadowsPapa]

What is the in-vehicle logic for charging the Main and Aux batteries? I bought a used 2021 that had been on a dealer lot for a few months and likely hadn't been driven much. The Start/Stop Warning Light came on the day I got it home. Based on this forum, I knew it was related to the AUX Battery. When I took it back to the dealer, they took both batteries out, charged them, and put them back in. They tested "Good" with no bad cells. I'm wondering why driving the vehicle didn't have the same result. If they're directly connected and a bad AUX Battery can kill the Main Battery, then why wouldn't it charge the AUX off the Main?

It gets a lot more complicated when batteries age, internal resistance changes and so on.

The "Stop/Start not ready" generally has nothing to do with batteries unless it says "stop/start not ready, battery charging".
Otherwise, any number of things can cause it to not work.
A bad main can kill the aux.
Be careful - a bad aux doesn't always kill the main, and a bad main doesn't always kill the aux. It just gets yelled loudly on the internet.

Yes, the batteries are connected together, but if one battery is going weak or bad, it may not charge the same at the same voltage as the other battery.
The aux battery is charged directly from the alternator or electric system through high current fuse N3.
the main battery is similarly charged, but the regulation for voltage and charging is all taken off the main battery.
So if the aux is weak, going bad, has internal resistance that's gotten high, then 14 volts may not charge it while 14 volts does charge the main battery.

But again - check your message - if it doesn't say "battery charging" after stop/start not ready, then it could be a lot of other factors involved.

Check the voltage on the dash - using the left steering wheel buttons, scroll up or down until you get where the oil pressure, engine temp, tire pressure and so on, and then go left or right on the left steering wheel buttons to see the voltage (it will say battery voltage, but that's sort of misleading)

 

[sharpsicle]

What is the in-vehicle logic for charging the Main and Aux batteries?

They're in parallel when they charge, so there's not really any logic in there.

I bought a used 2021 that had been on a dealer lot for a few months and likely hadn't been driven much. The Start/Stop Warning Light came on the day I got it home. Based on this forum, I knew it was related to the AUX Battery.

Don't believe internet stories. The batteries were just low, likely both. Nothing more than that unless one is going out. And in that case, you need to investigate both batteries anyway. It's not always the AUX, it could just as likely be the main, no matter how much people will want you to think it's only the AUX that craps out.

When I took it back to the dealer, they took both batteries out, charged them, and put them back in. They tested "Good" with no bad cells. I'm wondering why driving the vehicle didn't have the same result.

Driving will take significantly longer to charge the batteries than doing it on the bench. The dealer did it the right way, and there's no surprises here.

If they're directly connected and a bad AUX Battery can kill the Main Battery, then why wouldn't it charge the AUX off the Main?

It does. Back to the top, they're in parallel in all situations except active ESS event, which means in those situations charging one is effectively charging the other.

 

[ShadowsPapa]

To add to that, dealership probably did it right - batteries out, each charged independently. How long did it take? Did they cut the charging short and charge just enough to run their FCA specified tests? Did they fully charge each?

If you were to do it, yourself, you can charge both in the truck together, like the electric system does, but I like to separate the two ground cable and charge them independently, one then the other, when I have a chance or if I see something creeping up on me by watching the systems.

 

[Hootbro]

Did they give you a print out of what each battery tested at? Testing "Good" can still mean it tested below it's rated capacity. Generally if it test 75% or higher of rated capacity, they will call it good but you may be on the margins.

 

[ShadowsPapa]

Did they give you a print out of what each battery tested at? Testing "Good" can still mean it tested below it's rated capacity. Generally if it test 75% or higher of rated capacity, they will call it good but you may be on the margins.

Excellent point (as always)
They have criteria for warranty, etc.

 

[Irikumi]

I just bought a 2023 that been sitting on the lot since around December of last year. First thing I did when I got it home, was put it on a trickle charger and let it do it's thing until it indicated a full charge.

I noticed when I had drove it home for the first time, that the voltage was showing around 14.7 volts according the to dash. After fully charging it, now it shows around 12.8 volts. Must have been down quite a bit, glad I threw it on the charger when I got it home.

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