Mercedes W221 "Battery Not Charged"

Brandon Technical Support Specialist New York Posted   Latest   Edited  

W221 Battery Not Charged Warning Message 

Recently I have come across quite a few Mercedes-Benz w221 chassis (S-Klasse from 2007 through 2013) with a red battery symbol displayed in the centre of the instrument panel and the exact phrase “Battery not charged” beneath the symbol. 

All of the instances I have personally seen have been on the facelifted model vehicles (2010+) which is important to note (however, I am not stating it cannot occur on early years). 

The w221 chassis vehicles came with two different battery system configurations (excluding the somewhat rare w221 chassis with ECO start stop). Early w221 chassis (namely 2007 and 2008 model years) have a large H8 AGM mains battery in the trunk and a smaller starter battery under the bonnet (photographs depicting the locations are displayed below). A power supply controller (also called a battery control unit) is utilized in this system. The power supply controller is likewise depicted below in the top photograph (the black box directly above the mains battery with all of the power cables going to it). The second photograph, albeit geographically unrevealing, portrays the starter battery against the firewall under the right side of the bonnet.



Facelifted models (and I believe US specification 2009 models as well) no longer have a trunk mains battery or a power supply controller. A large battery is fitted in place of the starter battery which handles all functions of the predecessor’s dual batteries. Noteworthy, is the fact that a very small auxiliary battery has been installed on the facelift models which serves as a redundant safety battery for the electronic shift selector system. Below you will find photos illustrating the relevant locations on a facelifted w221 chassis. 




Additionally significant is the presence of a battery sensor fitted on w221 chassis vehicles that do not utilise a power supply control unit. Like many other recent model Mercedes the battery sensor is integral with the negative battery cable. The battery sensor reports via LIN to the front SAM (signal acquistion module), which contains the onboard power management software.

Now that system basics have been covered, we can shift our focus back to the complaint!


The car will likely have no relevant faults in any controllers, resting and running voltages will be good, and many spare parts may have already been fitted to the vehicle. Such parts may include the battery(s), alternator, & battery sensor all of which provided no rectification. Various control units were reset, a long capacitive discharge was performed, and several other service functions were attempted. Moreover, the LIN from the battery sensor was checked at the SAM with no issue found.

What is next, replace the SAM? After all it is the battery controller on the later models. 

Please first consider the instrument cluster as the source of trouble. Please check the instrument cluster to see whether the latest software is installed. If not update the software and SCN code as needed. Additionally, the front SAM should be checked for an available software revision. 

Sam Mobile Technician

Brandon, Thanks for posting. I don't want to sound like an idiot and do not do a lot of benz. I do not understand the facelift term. can you clarify?

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Steve Technical Support Specialist

Sam, MB has through out their history designated a basic chassis with a number. In this case the chassis 221 is the next to newest S Klasse model. The newest is the 222 and the one older than 221 was the 220. Don't let this string lead you to the conclusion that they will be in numerical sequence. The chassis before 220 was 140, before that was 126, and before that 116, then 108/109 (the 109 had

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Brandon Technical Support Specialist
New York

Good and thorough explanation Steve. The term facelift is used widely by Mercedes Benz within official documents on the MBUSA website.

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Bill Owner/Technician

Hello Steve I am so glad to see you here 😁 thanks for continuing to spread your vast MB knowledge with others!

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