Can’t start your Mercedes-Benz S-Class due to a dead battery?  Learn how to replace the 12-volt battery behind the rear seat on the 2007-2009 S550 S600 CL550 and 2010-2013 S400 hybrid. 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

  • H8 Battery
    • CCA 800 A or higher
  • 13mm AND 10mm 3/8″ socket
  • 3/8″ Ratchet
  • Battery terminal cleaner


  1. 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.
  2. 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 it back can help. This step is challenging and the most challenging part. MERCEDES BATTERY BEHIND REAR SEAT
  3. Disconnect the negative battery terminal. The negative is on the left side of the battery.
  4. Disconnect the positive battery terminal.
  5. Remove the 13mm bolt to remove the bracket that holds the battery. REMOVE 13MM BOLT
  6. Please remove the battery by sliding it towards the back.
  7. Install the new battery in reverse order. The negative battery terminal should be the last thing you need to connect.
  8. Start the engine. If the Auxilary Battery malfunction light comes on, the small battery inside the dashboard also dies. It is 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 on the engine bay’s passenger side.


If you decide to use an external energy source to keep the car powered up, you must 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
  • S350
  • S400
  • S450
  • S500
  • S550
  • S600
  • S63 AMG

CL-Class  216 Chassis

  • CL550
  • CL600
  • CL63 AMG

We hope you find the Replace Rear Battery Mercedes 2007-2009 W221 C216 S400 guide helpful. Check these troubleshooting and repair guides for more help on your Mercedes-Benz.

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