Auxillary battery malfunction

Tiny
BLESSING EWOBOR
  • MEMBER
  • 2012 MERCEDES BENZ GLK
  • 6 CYL
  • AUTOMATIC
  • 97,543 MILES
Hello, I am working on the vehicle listed above, it is displaying "auxillary battery malfunction" on its instrumental cluster. I decided to replace the auxillary battery with new one but the "auxillary battery malfunction" still persist on the instrumental cluster. Does that mean there is problem with the new battery or the display has nothing to do with replacement of battery. Is there any code to erase from its memory?
Wednesday, September 2nd, 2020 AT 12:56 PM

5 Replies

Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
More then likely you have a sensor on the main battery that measures the amount of amperage and voltage output. However, this is for the overall state of charge of both batteries. The display says the auxiliary battery is the issue but I suspect the main battery is drained at this point. What happens is the main battery gets drained down and as it drops it uses the smaller battery as a crutch. Then this light comes on and you replace the aux battery but the main battery is still drained. A lot of times this will make the light to go out for a short time then it will come back on. Basically this could happen with either battery but in essence one gets drained, pulls down the other, and then you replace one of them and the cycle continues.

I would go to the parts store and have them test the main battery and see what the state of charge is.

https://www.2carpros.com/articles/car-battery-load-test
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Wednesday, September 2nd, 2020 AT 4:16 PM
Tiny
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Thank you.
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Wednesday, September 2nd, 2020 AT 5:38 PM
Tiny
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You are welcome again :). I thought your name was looking very familiar.

Let us know what else you need. We are here to help. Thanks
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Wednesday, September 2nd, 2020 AT 7:23 PM
Tiny
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I have tested the main battery and and the alternator (generator), they are all working fine. I don't know if I could have the circuit diagram?
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Saturday, September 12th, 2020 AT 8:59 PM
Tiny
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  • MECHANIC
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Unfortunately they don't have a circuit diagram for this but if all that checks out then the body controller thinks the current is too low so it displays that message. The battery has what is called an intelligent battery sensor that basically just tells the computer the amount of current that is flowing from the battery system. Not just the aux battery but the entire system. Do you have a scan tool that can monitor the overall state of charge? I am not sure what they will call it on the tool but you are looking for something like battery state of charge and there will most likely be a percentage. Let us know what that is and we can go from there.
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Sunday, September 13th, 2020 AT 7:17 PM

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