W211 270 cdi battery warning

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r129-sl320

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May 14, 2016
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Car
sl320
In Decemmber 2016 when the temperature outside was very cold i began getting 'battery convenience functions disabled' message on instrument cluster but would go off after a whle of driving. More recently the red 'battery warning visit workshop' message came up and i left the car parked up for two days and the battery was completely drained- no charge left.

I bought a new battery and fitted and the same thing happening and battery completely drained over night.
I charged the battery and refitted and done a voltage check when running and also from instrument cluster which showed UB:11.8v so confirmed alternatir not charging.

I changed voltage regulator on the alternator which i bought from MB and checked the voltage when running which shows 13.8v now.

HOWEVER the red battery warning and the battery convenience warning still show.

The aux battery under the bonnet in showing 12.6v and a drop test shows its ok.

Any ideas on what to look at next?

Thanks
 
would be interested in any theories as I have the same problem and have also replaced both batteries, takes about 4/5 days of non use to flatten battery :dk:and drive for 5hrs then come to restart 1 hr latter and get the low battery warning
 
Have you check for any parasitic drains?


Yes, nothing more than 50mA when i have checked it.
I drive the car everymorning, the red battery sign with "visit workshop" always displays on the instrument cluster
 
STAR diagnosis and check the BCM is still ok?
 
Yes, nothing more than 50mA when i have checked it.
I drive the car everymorning, the red battery sign with "visit workshop" always displays on the instrument cluster

Hi,
My understanding of how the 211/219 twin battery system works is if you get the red warning light it is on because either the Aux battery is not receiving a charge or the Battery needs replacing, I don't believe the red warning comes up regarding the Main Battery condition unless the car is not charging at all.
The charge status of both the main battery and the Aux Battery are monitored by the Battery Control Module (BCM), if it senses the Aux Battery being low it triggers the Aux Battery relay to close and charge the Aux Battery.
I feel you either have a faulty relay or BCM, but the only way to tell definitively is with STAR.
There is also what is described as a Polyfuse 5A on the positive of the Aux Battery, but this is a thermistor type fuse so resets itself if it ever triggers, this is there to protect the voltage sensing circuit of the BCM.
 
You explained it so more eloquently :)
 
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