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My s550 wouldn’t crank one day, tested the engine battery and it came back bad battery. Bought a new battery from Mercedes and got the test ran again, “good battery, good starter, bad charging system” Being that my car made a humming sound as well as the bad charging system, I assumed this was my alternator. I continued to drive the car and didn’t notice any electrical issues or failures. At least 3 months later, after continuously driving the vehicle daily, I cranked the car up and the dashboard lit up like a Christmas tree with all kinds of symbols even my check engine light. I know my check engine light shouldn’t be on so that immediately raised a red flag as well as remembering reading once your alternator goes out or is about to go out completely this will occur. Days following this i would try and crank the car just to try and move it and some days it would crank and others it wouldn’t. I bought a USED OEM Bosch alternator that was tested and good to replace my current alternator. After replacing the alternator that was in my car with the used one, the car cranked right up no problem but now there is a red battery icon that constantly pops up on the dash after the car is running for a minute or so. I even went and got both batteries tested in the car (good) but the scanner still reads “BAD CHARGING SYSTEM”. CAN SOMEONE PLEASE POINT ME IN A DIRECTION TO GO FROM HERE?
Last edited by W223motivation; May 17, 2026 at 12:01 PM.
Either your auxiliary battery in the dash behind an end cap is toast, or the starter battery. You may have a third battery in the boot. Normally when this goes flat, stuff stops working until the engine is running... A big clue is having to manually unlock the car.
Not starting points to the starter battery either being dead or the auxiliary battery not allowing the car to recognise the key fob.
Try using the physical fob rather than the push start if you have keyless go. If you have keyless go and this manual method works then the aux battery is probably done.
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i’ve taken both batteries out and tested them. They’ve come back good and I’ve went to the hidden menu on my dashboard while the car was off and the volt reader read 12v and then when the car was running, it stayed at 12v.
i’ve taken both batteries out and tested them. They’ve come back good and I’ve went to the hidden menu on my dashboard while the car was off and the volt reader read 12v and then when the car was running, it stayed at 12v.[/QUOTE]
You have 2 batteries in the car? I'm assuming you have a large 12v AGM H7 or H8 battery, what is the other battery?
With the main battery installed in the car use the multimeter and check voltage at the battery terminals, it should be 12v or above. With the car started and engine running the voltage at the battery terminals should jump up to 13.5v-15v depending on battery charge and RPM's.
If battery voltage at the terminals stays the same after starting the car then the charging system isnt working. Which you already know.
Inspect the drive belt, though you mention replacing the alternator so i'm assuming you checked the drive belt then and its fine.
You should scan the car with a Mercedes specific OBD scanner. If you dont have a scanner then buy one. An inexpensive Launch Creader should do fine. It will allow you to see any fault codes to lead you in the right direction.
I had the red battery warning popping up on the dash of my W204 awhile back. It would pop up shortly after starting the car and pulling out of my drive way. I would drive to the end of the street and shut the car off then start it up again and the warning did not pop up again that day. To diagnose what the issue could be I scanned with my OBD scanner and had one stored fault code related to alternator output but erased and it did not show again. I bought an inexpensive Ancel BM200 bluetooth battery monitor to connect to the battery and see battery voltage in real time from my phone thru the app. It is very convenient to have, it reads and stores battery voltage, state of charge, state of health and also battery temp. I can stand by the car and it tells me battery voltage without even unlocking the doors, I can go back and see battery voltage as the car was parked all night for any abnormalities. Most importantly for my issue i was able to see battery voltage jump up after starting the car which told me the alternator and charging system was fine. In the end the red battery warning stopped popping up so i didnt have to do anything.
I recently had starting issues and ended up getting a new battery as the old battery was bad. i have the battery monitor still hooked up just to observe how the car charges the battery. Its pretty interesting how it changes charging profiles depending on how the car is driven and battery health. For example on the same day I drove somewhere on the highway and noticed charging around 14.5v, then on the drive back same highway same speeds it appeared to charge at a lower voltage. I did the same trip the following weekend and observed the same thing.
From the photos below you can see how the battery voltage jumps after starting the car. You should see the same thing using a multimeter connected to the battery terminals in your car.
[/QUOTE] I went back out yesterday and retraced my steps & also noticed some questionable things I didn’t see before. First i tightened the bolt that holds the power cable to the alternator. I also noticed that the power cables going to my battery were showing some wiring slightly. I got some electrical tape (being that the wiring exposure was minor) I patched up my positive terminal in the trunk and in the engine bay battery. Since then I’ve been driving and not received the red battery symbol again.
Take it to an independent Mercedes shop and have them run the alternator load test. This will confirm whether or not your alternator’s voltage regulator is going bad.
Keep this up and you’ll be frying two more batteries.
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