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Hello, my 2017 metris passenger van, 50k miles, zero problems with it-ok, I take that back, the rear washer fluid pump fitting broke, the dealer wanted $600 to repair and replace-I fixed myself with bonding the fitting that cracked haha. Anyway, got a light on instrument panel that comes and goes "AUXILIARY BATTERY MALFUNCTION"...not always on, but maybe half the time I drive. Any remedies for it? and please don't post "take it to a dealer", I am out of warranty and refuse to pay their prices for repair-I will assume its a aux battery is dead maybe? or some fuses?
 

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Metris Cargo 135 2020
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My 2020 135 with start/stop has an extra battery under the passenger seat. I don't know if that's what' meant with the aux battery?
I have a flap facing outwards in my cargo seat bases. I think in the passenger version it is flap to access from the rear.
Anyhow, in my metris the extra one under the seat is is pretty sizable.
There is room and a wire in my driver seat base for a third battery.

On my ML350, when it was maybe 7 years old (?), I got a battery fail / replace notification. The vehicle had under the passenger seat, the oversized main battery and a tiny 12v aux one - like kbcool showed - hidden behind it. One could not see it and certainly not access it unless the big one was pulled out. ( or maybe dealers can if they take the whole seat out )

I recall something about the sedans having these typically near a fuse panel in either the trunk or below the passenger glove box area ... but it can also be in the driver side outside facing dash panel ... that's where the one in my wife's car was hidden (despite the dealer insisting it was in the trunk).
 

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Got around to replacing this today. Original battery tested @ 13v today. But, it was warm overnight. So, will see next week if i still get the aux light malfunction as our temps drop into the 30's. Anyways super easy to replace. Popped two 10mm nuts holding the metal cover on. Unplugged the 15 amp automotive type fuse and pulled the spade connectors off the positive and negative terminals. They are impossible to mix up as each spade connector are sized to match the battery terminal. One small, one large. Also the battery just lifts straight up and out of it carrier. Re-assembled w/ no issues so far.
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Metris Cargo 135 2020
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Autohaus is great, purchased from them before.

So, where is this little bugger located?

Based on the black stamped metal, it looks like underneath the extended load seats. Passenger side ... behind the little 90 degree metal flap?
 

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That should be it. Main battery in the front half of passenger seat. Auxiliary battery in the rear half.

This is not the average compact $20 ups / alarm system backup battery, nor the $40 MB equivalent thereof as seen in typical MB sedans and SUVs ... but more like a custom size lawn mower battery with some amps behind it. Still quite expensive for what it is, I agree. Different brand of the same specs should be fine, but may last less long; or may with a little age cause the computer to complain about it sooner. I think in my limited experience, I have seen MB batteries last about 6 - 7 years before needing replacement.

I don't look forward to having to replace mine, with a partition wall and steel plate in the passenger seat base ...

PS: I guess it is more like a motorcycle battery.
Looks like the MB one is $125 online with free shipping ... vs $100 AutoZone

[Edit ... not sure what the motorcycle battery quality and specs are that come in at half that cost]
 
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