Hello! first post hoping some of you Mercedes pros can help me out! Recently my car radio has started dying about 10-15 minutes into driving my 2012 C300, then shortly after all the lights on my cluster will come on including the red battery symbol and ill start to lose power until the car dies. ill let the car sit for alittle bit and the power will come back. i recently had my battery tested and it came back good, its only a year old, and i changed my alternator and it fixed the issue for about a day and now its doing it again. I looked for the axillary battery in all the locations youtube told me it could be, under the passenger dash, the trunk and the right side of the steering column and couldn't find it anywhere, I'm starting to think maybe it doesn't have one but i don't know if that's even possible. Thank you for reading this and your time! any help or direction would be highly appreciated!

 

In your car, do you have ECO Start/Stop? The secondary battery is only to support the Start/Stop function.

The problem definitely sounds like an alternator issue. Where did you get the replacement? On other cars, I have gotten a "remanufactured" alternator from an auto parts store chain and it died within a month. Doesn't matter that it had a lifetime warranty, it was still a pain to replace.

Other than that, you need to scan your car with a Mercedes specific scanner, such as XENTRY. I don't know if you are new to Mercedes or not, but if you are going to keep your car, I recommend investing in long-term service and joining @BenzNinja . You will get XENTRY, WIS/ASRA, EPC, and lifetime diagnostic support provided remotely.

 

^ This. If that's an Autozone alternator, they are quite literally, junk. As pointed out, lifetime warranty means nothing if you're on the hook for labor.

 

I would think if there was an issue with the aux battery (if your car even has one?) the message on the dash would say something like "Auxiliary Battery Malfunction". I'm speculating.

Have you scanned the car with a Mercedes specific obd scanner, that may lead you in the right direction.

If you have a multimeter see the battery voltage with the car off then again with the car running, it should jump to something like 14v with it running.

I think an autoparts store can test the alternator. Out of curiosity I was looking around at used alternators and wondered how I would know its still good, then came across something on Autozone I think it was saying they can bench test an alternator even out of the car. I never had it done so not sure if everyone does it.

When I was getting the red battery warning I bought an inexpensive battery tester which shows the charging voltage which is handy. From the readings I ruled out the alternator.

 

When the car detects battery failure, it'll be a battery symbol in the dash, traditionally white battery symbol when it's the main battery and red battery symbol when it's the auxiliary battery, but don't hold me to that - it could be the other way around.

 

It could be the other way around. After all, you guys drive on the other side of the road, winter is summer and summer is winter, etc.

 

Even worse, everything is upside down! Can't imagine doing an oil change.

 

Actually, it's quite easy. Instead of suctioning the oil out through the dipstick tube, just remove the dipstick and put something there to catch the oil.

 

Of course! Then just pop the drain plug and pour in the new stuff!