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Old 07-10-2015, 01:47 PM   #1
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Aux Battery on 2016 AI GT

What purpose does the aux battery in the engine compartment serve, and how does it get charged? I ask because the tech at the dealer told me it has no purpose and is not connected to anything, which I think is not correct.
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Old 07-10-2015, 01:59 PM   #2
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It's connected to the main chassis battery through a relay. When the engine is running the auxiliary battery is connected and so can charge from the alternator. When the key is switched off, the auxiliary battery is disconnected.

Normally an upfitter will order the Sprinter vans with the auxiliary battery because they don't install house batteries. Examples would be TV news vans or ambulances. All electrical loads that need to be powered when the engine isn't running would be connected to the auxiliary battery, so the starting battery can't be run down while the engine is off.

Airstream SHOULD connect the awning and the retractable step to the auxiliary batteryŚ these items are not powered from the house batteries. But knowing Airstream, they probably didn't, and the step and awning are connected to the starting battery. In which case you really do have a spare battery that's not connected to any energy users.

You might consider removing the relay and making the connection permanent, so that you have two starting batteries in series. Otherwise that battery isn't serving much useful purpose.
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Old 07-10-2015, 02:10 PM   #3
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Thanks Mr P for your lightening-fast response! I guess the tech was partially right.
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Old 07-10-2015, 09:04 PM   #4
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Protag: Aren't the two batteries in parallel?

cjrdad: Is the Aux battery a flooded battery or an AGM?
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Old 07-10-2015, 09:15 PM   #5
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Quote:
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Protag: Aren't the two batteries in parallel?

cjrdad: Is the Aux battery a flooded battery or an AGM?
Yes, you're right. Senior moment there. Thanks for the correction.
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Old 07-10-2015, 10:36 PM   #6
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Quote:
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Protag: Aren't the two batteries in parallel?

cjrdad: Is the Aux battery a flooded battery or an AGM?

Not sure. I wasn't paying attention. Why do you ask?
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Old 07-10-2015, 10:53 PM   #7
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Thought maybe they were using it as a third house battery if it was an AGM.
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Old 07-10-2015, 11:03 PM   #8
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Thought maybe they were using it as a third house battery if it was an AGM.
It's a Group 49 AGM, same as the starter battery. Even Airstream wouldn't be dumb enough to place a Group 49 battery in parallel with two Group 24 AGM house batteries at the opposite end of the van.
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Old 07-11-2015, 10:53 AM   #9
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I'm fairly certain the Aux battery on a Sprinter powers the two 12v always on outlets in cab. One in the driver seat base and one in lower center console.


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Old 07-11-2015, 11:43 AM   #10
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That seems like overkill unless you're planning on putting an inverter on it for something else.
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Old 07-13-2015, 07:58 AM   #11
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On our previous one a 2014/13 the aux battery under the hood was not used for anything either MB or Airstream. I instead added a few 12v outlets in the RV and wired them to the aux battery. I carried a spare 12v fridge/freezer that was hooked up to that battery.
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Old 07-13-2015, 10:59 AM   #12
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It seems I should just think of the aux battery as a spare and not worry about it. Kind of a waste though if it's not serving any purpose.
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Old 07-13-2015, 03:15 PM   #13
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On our previous one a 2014/13 the aux battery under the hood was not used for anything either MB or Airstream. I instead added a few 12v outlets in the RV and wired them to the aux battery. I carried a spare 12v fridge/freezer that was hooked up to that battery.
Are you saying that the Aux battery was just sitting there and not connected?
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Old 07-13-2015, 03:23 PM   #14
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Are you saying that the Aux battery was just sitting there and not connected?
Connected through a relay to the alternator, but not connected to any loads.

I still advocate removing the relay and connecting the spare battery under the hood directly to the main battery in the driver's footwell. Doubles your available cold cranking amps, and it's easier than moving the retractable step and awning power circuits to the spare battery where they belong.

My 2012 Interstate doesn't have the spare battery under the hood, so it's a moot point for me, but considering that I had to replace my one-and-only chassis battery after just two years due to a bad cell, having another battery in parallel sure would have been handy.
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