2012 S550. 150K KM
The car's previous battery was dying every so often. Took it in and they confirmed that it had bad cells.
Replaced with Energizer AGM battery from Costco a year ago.
After almost exactly a year, it too died. Bad cells according to some tests the mechanic ran. The mechanic also said that since my car basically just sat there throughout extreme Canadian winters (I was only starting it for about 30 minutes every couple of weeks or so), it murdered the battery. ChatGPT and Google Gemini confirmed this comment.

Fast forward to today.
New Costco Energizer AGM battery is in only for a couple of weeks. And today the car started to show dying battery symptoms that I saw few weeks ago when the battery died. All sorts of lights came on. Warning messages etc. I hooked up the code reader and every DTC that could have been triggered had been triggered. Cleared them all. They didn't come back.
The secret menu built into Mercedes shows the voltage between 13.6 and 14.3 when the car is running.
Multi-meter at the battery terminals shows 13.5 v when the car is running.
This is most concerning: The code reader's live data shows the battery voltage at 12.2 V when the car is in accessory mode. When the car is started, the battery remains at around 12.2 to 12.3. Note how it's different than ~13.6 showing in car's built in secret menu. When the car is under load (running AC and fans at full power), the voltage in live data goes up to about 14.1 and then fluctuates between 13.8 and 14.1

I don't know if voltage readings in car's secret menu are to be trusted, or the readings in scan tool's "live data". If the brand-new battery starting to die (or lose charge again) is due to weak alternator related charging issues, then the readings in scan tool's live data make most sense.

Mechanic checked voltage at battery terminals when the car was running, and seeing it at 13.5+ V, he said the alternator is probably fine.
Kindly share any insight, suggestions, tips, guesses based on your experience. Is it time for new alternator? But how to conclusively establish that? If alternator is fine, what else could be responsible for battery not getting charged, or losing charge?

Thanks.

 

a joke reader won't give sensible data - if its via battery terminals you have a chance, if its via ODB2 then the volts here are not real battery volts mine is always 2 volts or more below real

you have another small battery under the dash to help the gearbox ISM know how to behave when the front battery is playing up - its beneath the ignition switch pop end cover of fuse box on dash and have a look - replace this might help the car behave

the car battery should be charged via these point NEVER via its terminals (start replacing small battery and charging the front one) you may have a small fault giving a constant drain

 

Thank you for always a very helpful reply.
I used to charge via terminals but i have learnt to use the remote grounding post Thanks for the reminder.

By the way, the code reader I have is a higher end XTool D8S. Not the cream of the crop, but still miles above the ordinary code readers. But I do agree, and your comment makes a lot of sense, that voltage through ODBII is not the real battery voltage. Multimeter and Vehicle Data in car's dashboard both showed voltage pointing to a healthy alternator.

I will arrange for a battery maintainer (trickle charger). Thanks again!

 

the dash voltage is very close to 100% accurate

13.9 idle 14.2 with revs up a bit - let it stabalise after a few mins it takes a while to finds its feet - but yours sounds a bit low - which might be the alternator getting old - the bit that wears is replaceable for vastly less money than the whole thing

don't forget the positive also goes on a post ina terminal box and not direct on the battery.... most modern cars work this way - it gives a better clue as to the load the cars is taking

 

I too think that alternator is a little on the low side. Not dead yet but getting there. Kindly let me know the part that wears out in these alternators, so I don't have to replace the whole alternator. I recently had the alternator in my Odyssey taken off that way. It only needed a new pulley, which the original part was like 15% of the cost of a rebuilt alternator.

I didn't know that positive (RED) terminal was also supposed to be connected somewhere else (i.e. not directly to the battery). Can you please point out where the red positive on my car (W221) should be plugged to when charging?

Thanks!!!

 

re battery - it should look like the picture above, the cover pops off ?


part 030 Genuine MERCEDES C216 W221 S-CLASS W221 Regulator Switch 0041540706 £141.19 from a main dealer - struggling to find Bosch direct part

 

Thank you very much!!!!

 

so mine just thew a wobbly

Key to position 1 Press and hold the OK and right phone button for 5 seconds - brings up volt meter

start the car takes a while for the alternator to kick in (4 to 5 seconds) then with revs at 2k hold 14.3V all day - coming to idle drops to 13.9V, but at time drops to 13V and takes a second or so to pick back up

I suspect a regulator and new brushes would be enough

WIS says pull the dipstick and do it in situ that looks like a barrel of laughs - and the part is https://www.autodoc.co.uk/spares-sea...rd=A0031546906

but Bosch scum list 5 versions that are compatible ranging from $50 to $120 - so what do we have

chocolate nasty
chocolate OEM
almost worth having
decent
OEM for Russian Market !!!

 

Let me know how the alternator removal goes. I plan to replace mine to an upgraded 300A alt for my stereo.

It looks almost impossible to remove. Step 1 is probably "disassemble car", Step 2 "remove alternator"