Auxiliary battery.

[DougMb]

Can anybody tell me what the auxiliary battery is for on my Cls 320 2006.
Some sites have said that it's purely for the SBC breaking system others say its a back up for the main battery. I'm slightly confused now, so any help would be much appreciated.

 

[AMGeed]

The aux battery provides system voltage when the main battery's voltage drops below a certain level.
This is usually accompanied by a dashboard message saying "Convenience functions disabled"

The car will draw power from the aux battery until the main battery has sufficient voltage.
This article describes the battery system on the W219 and W211 very well
https://www.dropbox.com/s/doiwpgwubkw1sff/319 HO Dual Battery (ACB - ICC) 11-29-02.pdf?dl=0

Its wise to check the voltage on the aux battery from time to time and if it is over 4/5 years old, replace it.
Tanya sell an identical auxiliary battery to the MB branded one for half the price should you need one.
https://www.tayna.co.uk/car-batteries/varta/mercedes-aux/

 

[DougMb]

Many thanks for your reply
I possibly need a new auxiliary battery because I'm getting the red battery and visit workshop sign. My main battery seems fine also the alternator and v poly belt.

 

[LostKiwi]

Red battery warnings are a bit of a giveaway!

 

[AMGeed]

Have you tested the main battery for voltage when at rest? If its over 4 years old it may well be time for a new one as well as the aux battery..

You should have a reading of at least 12.4V standing without the engine running.
With the engine running this should go up to 13+V showing the alternator is charging.

 

[DougMb]

Have you tested the main battery for voltage when at rest? If its over 4 years old it may well be time for a new one as well as the aux battery..

You should have a reading of at least 12.4V standing without the engine running.
With the engine running this should go up to 13+V showing the alternator is charging.

 

[DougMb]

Yea tested the battery in the boot and its running at 12.5 v engine off and 14.1v with engine on and running the lights air con and windscreen wipers plus radio.
Just had the auxiliary battery tested at halfords and the fella said its only running at 7v so it's time to order the one that you recommended. Many thanks for your help