This error pops up and then my heated seats go out and the radio. I've read that there is a small battery under the hood that may need to be replaced, can anyone tell me where under the hood I can find this battery, and where do I get a replacement? If you feel this is a larger problem, please let me know.

 

That is a warning from the control computer from the ***main ****battery......the aux battery is a back up for systems incase the main battery fails or is failing.
Get your primary battery checked and the charging system output.5 year old battery it might be time for a new one.instructions for resetting everything after you are done is in the owners manual.
ohlord
heated seats,radio,rear window defroster all get cut out to preserve the important functions like the sbc brake system and then when the correct charge is attained the light warning consumers shut down goes off.So if the charging system is weak or most likely the battery(main one)is getting old and wont hold a charge that is why you have the symptoms you point out.
Hope this helps

 

How old is the main battery?
If its original its likely on the way out. 4years is the "normal" life and modern batteries fail very quickly. One day they work OK. The next they have gone for good.

However it could be the alternator output that is below par for some reason. If the vehicle has already had a replacement battery I would get the alternator checked out before replacing the battery again.

You are observing the "Power management System" doing just that. It shuts down less important "consumer features" when the power load balance reaches a set (low) threshold in order to protect the integrity of more important systems and of course the next "Start" demand.

Stuart

 

The car was purchased used, and if I had to guess, it still has the original battery. I will have the battery replaced, and post the results. Thank you both for your input. - RY

 

call roadside assist or if diy costco sams club interstate batteries.Have the battery tested first to be sure.
ohlord

 

Had the battery checked at NTB and it is not misfiring. 12 Volts are coming out and sustained after ignition. Next is a Benz shop to see what they say. I've read that this year and model was plagued with electrical problems, I just hope these issues aren't too costly.

 

12 volts is to low battery output should be around 13.2-13.5
ohlord

 

12 volts is not the correct voltage as measured at the battery. Sounds like your alternator is bad.

Voltage should be about 14v with engine running and most consumers eg headlights, seats, etc off. As consumers are added voltage will drop somewhat but if engine is speeded up to say about 1500rpm voltage should come back up to about 14v with everything on.

Sure you have fitted a NEW battery and haven't just had it recharged? If you have a NEW battery and it reads 12 volts it has already been damaged by dropping below 12.5v..

No need to go to the MB shop yet. get the alternator output checked. Straight forward garage job. MB's like many cars with high electrical loads (150 amp alternator) are prone to alternator problems.

All vehicles of this type are subject to electrical gremlins. MB is not the best nor the worse. Certainly no need to change the car!

Stuart.

 

The alternator was bad. I should have known because the heads up display was fading in and out while driving the vehicle. At first, I thought it was my eyes, but it was definitely fading in and out. The alternator is being replaced now (still under warranty).

Thanks to everyone for help and assistance!

 

W211s has a story of bad alternators. Ohlord and snanceki are both right, check the battery and alternator.