Alarm/start stop - what battery?? W204

[BEETLE66]

C220 W204 2013 face lift

I have only recently purchased the above and love the car but have a few issues I need to iron out - Start stop not working and every day/other day have alarm activation (sounder and lights so belive not the sounder battery/unit its self)

I have 2 batteries circa 250mm length in the engine bay (rather than the 315mm one) and the small aux battery in the boot by the spare wheel.

Running the steering wheel battery diag it comes up 11.7v with IB -4ish

Even after a 150mile motorway round trip on friday this didnt do enough for the stop start and the alarm went off today.

Putting all this down to a battary ssue, but dont know which one I need to change or if both. dont want to change if not necessary so any advice or guidance welcome; hence the waffle

 

[KillerHERTZ]

The alarm issue is due to the Alarm battery needing replacing not the main battery.

 

[BEETLE66]

The alarm issue is due to the Alarm battery needing replacing not the main battery.

From what I could see the alarm battery in the sounder would cause a sounder only activation not sounder and indicators?

 

[BEETLE66]

But assume an alarm battery change whilst not necessarily easier (will do myself) will be cheaper.

 

[BEETLE66]

Sorry also for the stop start which battery ought i replace i assumd they are both agm. You this be linked to thd emerhancy brake niw always fully releasing? If I put my foot under the pedal and hrlp it back it goes off

 

[GeorgeR]

Sorry also for the stop start which battery ought i replace i assumd they are both agm. You this be linked to thd emerhancy brake niw always fully releasing? If I put my foot under the pedal and hrlp it back it goes off

11.x is way too low for the AUX battery. It will disable the Start Stop most possibly. Here how you can test and replace yours.

AUX battery - testing replacing | Parts, Maintenance & Servicing

I have spent some time testing and finally replacing the AUX battery. Let me share the knowledge here, it may help others. Nearly all MB cars manufactured in the last 10-15 have AUX battery. (So the hybrid models have 3 batteries in total, while the regular ones have two.) It is a small, 10-15...

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