We open the car door and pull up the floor covering to find the auxiliary battery. Once the floor cover is removed, we will find the battery under the cover plastics. Disassemble the locking pins of the battery terminals, then, remove it from the dedicated compartment and replace it with a new one (same voltage).
Replacing the capacitor/battery/converter is quickly done in your driveway. Located in the passenger's footwell, two screws, three nuts, and some clips are all that's blocking you from the part in question. However, you may also need to replace a fuse in the trunk once you observe the condition of the old part.
Twist the temp sensor 90° and pull it out of the panel. Then press in the tab on the footwell light and pull that out of the panel. The auxiliary battery sits behind that black plastic panel normally hidden by the carpet. Use a deep 10mm socket on an extension to reach the two lower nuts.
Your research is correct, the Aux Battery is indeed a large capacitor that Mercedes call a Voltage Converter. Your research is correct, the Aux Battery is indeed a large capacitor that Mercedes call a Voltage Converter. Just had mine replaced on my 2016 C 200. 10 months out of warranty but only 14000 miles. Mercedes offered a 75% goodwill.
The auxiliary battery provides power to the electrical consumers when the engine is not turning. This arrangement keeps the main battery free for starting the engine. Without the auxiliary battery, there's no power for electrical consumers and the engine won't switch off (i.e. Stop/Start won't work).
Without the auxiliary battery, there's no power for electrical consumers and the engine won't switch off (i.e. Stop/Start won't work). Once you have replaced the auxiliary battery, the Stop/Start functionality will work normally again (though you can obviously still switch it off if your don't like it).