CLS w219 red battery visit workshop warning

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philnewmerc

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Dec 11, 2008
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957
Location
Gateshead, Tyne and Wear
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CLS55
Hello. Every once in a while i get this warning sign on start up. I press any button and it goes away. Can someone tell me where the (two i assume) batteries are located and what a switched off voltage should be? If replacing should they be done as a pair? Many thanks 👍
 

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So after reading around it seems this warning only appears when the small aux battery is low - as far as I understand it this is just a back up for the SBC brakes so I am not certain it would affect any other electrical items? Still worth changing though.
 
Put a multi meter on both batteries before starting the car and see what voltage reading you have.
Aux batteries tend to get forgotten about so if its more than 4 years old you may need another if its under 12v.
You'll find it under the pollen filter in the engine bay n/s against the bulkhead.
The main battery is on the right hand side in the spare wheel well.
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Put a multi meter on both batteries before starting the car and see what voltage reading you have.
Aux batteries tend to get forgotten about so if its more than 4 years old you may need another if its under 12v.
You'll find it under the pollen filter in the engine bay n/s against the bulkhead.
The main battery is on the right hand side in the spare wheel well.
View attachment 144049

Main boot battery was 12.5v and is new looking AGM (Varta) - seems to be ok according to the above
Aux under bonnet was 11.58v and I did not see any AGM writing, also looks to be original lead acid (is that possible?) as it is gen mercedes - according to the info above that puts it in the 0-25% range so I think I'll replace that. NOTE though that my ABC brake hold works fine and I would have thought that would be kaput if the battery was so weak... ? Anyway, many thanks
 
This is the one you need if you don't want the OE Fiamm battery that costs around £90+ from the dealers.

 
I have now got the red battery symbol on my E320 and have measured to main battery at 11.8v, but this was with the connections still attached. The above says open circuit so don't know if it's wrong or not.

I measured the voltages again whilst the car was running and there was no difference, shouldn't it be higher with the alternator running?
 
11.8v is low voltage. Check the pic in Post#4. If its more than 4 years old I'd just replace the main battery.
Yes, with the engine running you should be seeing 13-13.5+volts minimum.
Time for diagnosis. Alternator? BCM? Battery?
 
11.8v is low voltage. Check the pic in Post#4. If its more than 4 years old I'd just replace the main battery.
Yes, with the engine running you should be seeing 13-13.5+volts minimum.
Time for diagnosis. Alternator? BCM? Battery?

Thanks for the reply.

Yes, but it's low voltage after having been driven for 10 or more miles, at night, after the warning came on.

I suspect the alternator.

BCM?
 
Drive in the day. Park up and remove negative terminal. Check voltage. If less than 12.3 the battery is on last legs. If it looks old just change it (them). Mine has an auxiliary battery at the front that was the issue... not sure what yours has. My experience of alternaters is they either work or don't.
 
Drive in the day. Park up and remove negative terminal. Check voltage. If less than 12.3 the battery is on last legs. If it looks old just change it (them). Mine has an auxiliary battery at the front that was the issue... not sure what yours has. My experience of alternaters is they either work or don't.

If the alternator is knackered then surely the voltage on the battery would fall whether or not it was during the day?
 
If the alternator is knackered then surely the voltage on the battery would fall whether or not it was during the day?
Little load in day: if battery measures low, it's truly low and not due to load. If ok, but warning comes on at night: tends to point to charging.
 

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