W221 S-Class vehicles have two batteries, while the S400 has three if you count the hybrid battery.
It is important to note that 2009 and newer models no longer have this battery behind the rear seat. The main battery is located in the engine bay for 2009 and newer models (except the S400 hybrid). A smaller auxiliary battery is installed under the dashboard on the driver's side.
What you will need
- Open the trunk. If your car battery is dead, remove the metal blade from the keyfob and use it to unlock the trunk. There is a keyhole for the trunk right next to the license plate light bulb.
- Remove the panel by pulling it towards the back of the car. This will expose the battery behind the rear seat. Removing the panel requires patience. The panel is not held in place with any screws; it is pressed in there. Placing a hook at one of the corners and pulling back can help. This step is challenging and the most challenging part.
- Disconnect the negative battery terminal. The negative is on the left side of the battery.
- Disconnect the positive battery terminal.
- Remove the 13mm bolt to remove the bracket that holds the battery.
- Please remove the battery by sliding it towards the back.
- Install the new battery in reverse order. The negative battery terminal should be the last thing that you need to connect.
- Start the engine. If the Auxilary Battery malfunction light comes on, the small battery inside the dashboard has died as well. It is very common for the auxiliary battery to die after the main battery dies. For help on how to replace the auxiliary battery, follow this guide.
Saving the memory settings
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 engine bay's passenger side.
If you decide to use an external energy source to keep the car powered up, you need to be extremely careful not to short the positive terminal to the vehicle's body.
This guide applies to both CL and S class vehicles from 2006 to mid-2009.
S Class 221 Chassis
- S 280 S300
- S63 AMG
CL Class 216 Chassis