I recently was trying to figure out how to reprogram my garage door opener, so fob was in ignition on position 2. This must have taken like 15 minutes, after which I did not start car. Next day when I started car, I got message in picture below. So, I just ignored it and it didn't came back.

Today, I was resetting service C, during which time I saw voltage was 11.9

Is this voltage reading a problem, or is it within range? Hope nothing is wrong with my battery. Is this voltage for "trunk battery?"

 

Maybe...? What's the voltage at idle, and cruising speed?

Maybe have a look here...
https://mbworld.org/forums/w211-amg/...orth-cost.html

 

How do you pull up the voltage at idling or cruising? This morning after arriving at work, after car was off, I checked and it was 12.1 - 12.3

 

The only way I know of to get the voltage is to switch the display to temperature before starting and then hit the reset button three times. Then start the car, it will then display voltage while at idle and cruising speed. Don't know how else to get it once the car is started. Mine normally reads 11.9 volts, but my battery is about 7 years old at this stage. I think a fully charged battery is supposed to be in the 12.7v-12.8v range at 78 degrees.

 

Thanks Cetialpha5, I just came back from a test drive and here are the numbers:

Voltage before starting engine: 11.8
Voltage after starting engine (Idle): 13.8
Voltage while driving: 14.0 - 14.1
Voltage after engine turned off: 12.4

If I take car to Auto Zone to test, which battery should they test, trunk or engine bay?

 

I would go to either Advance Auto or PepBoys. Somewhat of a toss up for me. On the one hand, it's $189 at Advance Auto, but there's a $50 coupon code (WD48) which brings it down to $139. It's a 900 CCA AGM battery with a 3 year warranty. PepBoys is a Bosch for $200, but has a 25% off code (FALL25) so it's $150, but it's an 850 CCA battery with a 4 year warranty. I think OEM is 850 CCA. Have them check both, can't remember who had the best price on the front battery. It might have just been a one time thing because you were just sitting in the car running the battery down.

 

I concur. Those voltages seem acceptable. OP do you know how old the battery is?

 

I'm not sure how old battery is, as I've had car for only 3 years now. I only became concerned when I got initial message that battery was low and convenience functions were not available.

Just for my peace of mind, I'll have an auto parts store check it, but thanks a lot for your input.

 

I would recommend that. The link I shared is what could likely happen to you if the battery is old, and requires replacement.

 

FWIW, attached is a table comparing the amperage ratings of various standards to each other. Mercedes uses amperage (DIN), American batteries use SAE amperage (CCA), and the auxiliary Mercedes battery I just removed from my car was labeled 170A (EN). (JIS is a Japanese standard.)
Example, Mercedes training materials show the main battery being rated at 520A (DIN) which corresponds to a 900A (SAE) battery. Cold Cranking Amps (CCA) is the designation used by SAE.