Auxiliary Battery

Bay Leaf

Senior Member
Joined
Aug 19, 2019
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Location
Stockport
Your Mercedes
SL 350 R231 2013
Hi everyone. please forgive my ignorance.as this is the first MB I have owned and I am slightly confused as to what the auxiliary battery does. I've read posts saying that it can lose its charge so is it on a seperate system than the main battery But mainly as said before what does it power and how long should it last before it needs replacing:confused:
 

LostKiwi

Senior Member
Joined
Aug 25, 2006
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Location
Midlands / Charente-Maritime
Your Mercedes
'93 500SL-32, '01 W210 Estate E240 (RIP), 02 R230 SL500, 04 Smart Roadster Coupe, 11 R350CDi
Depends on the car.
On the r230 the large battery in the boot is called the consumer battery and runs all functions on the car outside of those required to start the engine.
The smaller battery under the bonnet is used for all requirements relating to starting the engine.
You can have a completely flat consumer battery but the car will still start.

On the 231 it's different. The main battery provides power for pretty much everything. The aux battery is there to put the car into and out of park. It will automatically provide power to put the transmission into park when leaving the vehicle and will also allow the car to be taken out of park even if the battery is flat.

On start stop equipped cars one battery is purely to support start stop.
 

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