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07-10-2015, 12:47 PM
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#1
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2 Rivet Member
2018 27' Tommy Bahama
2016 Interstate Grand Tour Ext
Clifton
, Virginia
Join Date: Aug 2014
Posts: 71
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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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07-10-2015, 12:59 PM
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#2
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Figment of My Imagination
2012 Interstate Coach
From All Over
, More Than Anywhere Else
Join Date: Dec 2011
Posts: 10,868
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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 battery— 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.
__________________
I thought getting old would take longer!
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07-10-2015, 01:10 PM
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#3
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2 Rivet Member
2018 27' Tommy Bahama
2016 Interstate Grand Tour Ext
Clifton
, Virginia
Join Date: Aug 2014
Posts: 71
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Thanks Mr P for your lightening-fast response! I guess the tech was partially right.
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07-10-2015, 08:04 PM
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#4
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Rivet Master
2011 Interstate Coach
Overland Park
, Kansas
Join Date: Aug 2011
Posts: 2,798
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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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Glass half full or half empty to an engineer is the glass is twice as big as it needs to be.
2011 Interstate SOLD! Upfitted 2017 Transit 350. SOLD!
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07-10-2015, 08:15 PM
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#5
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Figment of My Imagination
2012 Interstate Coach
From All Over
, More Than Anywhere Else
Join Date: Dec 2011
Posts: 10,868
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Quote:
Originally Posted by 73shark
Protag: Aren't the two batteries in parallel?
cjrdad: Is the Aux battery a flooded battery or an AGM?
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Yes, you're right. Senior moment there. Thanks for the correction.
__________________
I thought getting old would take longer!
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07-10-2015, 09:36 PM
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#6
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2 Rivet Member
2018 27' Tommy Bahama
2016 Interstate Grand Tour Ext
Clifton
, Virginia
Join Date: Aug 2014
Posts: 71
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Quote:
Originally Posted by 73shark
Protag: Aren't the two batteries in parallel?
cjrdad: Is the Aux battery a flooded battery or an AGM?
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Not sure. I wasn't paying attention. Why do you ask?
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07-10-2015, 09:53 PM
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#7
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Rivet Master
2011 Interstate Coach
Overland Park
, Kansas
Join Date: Aug 2011
Posts: 2,798
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Thought maybe they were using it as a third house battery if it was an AGM.
__________________
Glass half full or half empty to an engineer is the glass is twice as big as it needs to be.
2011 Interstate SOLD! Upfitted 2017 Transit 350. SOLD!
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07-10-2015, 10:03 PM
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#8
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Figment of My Imagination
2012 Interstate Coach
From All Over
, More Than Anywhere Else
Join Date: Dec 2011
Posts: 10,868
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Quote:
Originally Posted by 73shark
Thought maybe they were using it as a third house battery if it was an AGM.
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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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I thought getting old would take longer!
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07-11-2015, 09:53 AM
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#9
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Rivet Master
2024 Interstate 19
Fulton
, Maryland
Join Date: May 2010
Posts: 3,879
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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.
- - Mike
2013 Interstate Lounge EXT on 2012 Sprinter
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- - Mike
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2024 Airstream Interstate 19e AWD
Previous: 2013 Airstream Interstate 3500 Ext Lounge
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07-11-2015, 10:43 AM
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#10
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Rivet Master
2011 Interstate Coach
Overland Park
, Kansas
Join Date: Aug 2011
Posts: 2,798
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That seems like overkill unless you're planning on putting an inverter on it for something else.
__________________
Glass half full or half empty to an engineer is the glass is twice as big as it needs to be.
2011 Interstate SOLD! Upfitted 2017 Transit 350. SOLD!
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07-13-2015, 06:58 AM
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#11
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BikerBill
2014 Interstate Ext. Coach
Poconos of Pa
, Pennsylvania
Join Date: Sep 2013
Posts: 239
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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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07-13-2015, 09:59 AM
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#12
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2 Rivet Member
2018 27' Tommy Bahama
2016 Interstate Grand Tour Ext
Clifton
, Virginia
Join Date: Aug 2014
Posts: 71
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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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07-13-2015, 02:15 PM
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#13
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Rivet Master
2011 Interstate Coach
Overland Park
, Kansas
Join Date: Aug 2011
Posts: 2,798
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Quote:
Originally Posted by Bikerbill
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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Are you saying that the Aux battery was just sitting there and not connected?
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07-13-2015, 02:23 PM
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#14
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Figment of My Imagination
2012 Interstate Coach
From All Over
, More Than Anywhere Else
Join Date: Dec 2011
Posts: 10,868
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Quote:
Originally Posted by 73shark
Are you saying that the Aux battery was just sitting there and not connected?
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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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I thought getting old would take longer!
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07-13-2015, 06:23 PM
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#15
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Rivet Master
2011 Interstate Coach
Overland Park
, Kansas
Join Date: Aug 2011
Posts: 2,798
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Wouldn't the bad battery drag the good battery down if in parallel? Wouldn't a better solution be to put an isolator between them and a switchover relay for when the starter battery died?
__________________
Glass half full or half empty to an engineer is the glass is twice as big as it needs to be.
2011 Interstate SOLD! Upfitted 2017 Transit 350. SOLD!
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