Instructions and video on this guide, show you how to replace the onboard electrical system battery on 2006, 2007, 2008, 2009 Mercedes-Benz S-Class (W221) and CL (W216) vehicles.
This is known as the onboard electrical system battery or G1 battery which is a Group Size 49 Battery. This is not the starter battery.
The W221 vehicles have two batteries. One is under the hood (known as the starter battery) and the one that we show in this guide, located behind the rear seat.
It is important to note that 2009 and newer models, no longer have this battery behind the rear seat. For 2009 and newer models, the main battery is located in the engine bay. While a smaller auxiliary battery is installed under the dashboard on driver's side.
It is possible to retain all the memory and settings, by connecting a secondary battery source. For this purpose, you will need to use a 12-volt trickle charger. The external source can be connected to the positive post located on the passenger side of the engine bay.
If you decide to use an external energy source to keep the car powered up, you need to be extremely careful to not short the positive terminal to the body of the vehicle.
This guide applies to both CL and S class vehicles from 2006 to mid of 2009.
S Class 221 Chassis
CL CLass 216 Chassis