Battery

The Consumer battery as referred to in the MB owners manual is located in the bulkhead between the rear seat and trunk. The S Class and CLS have the consumer battery in the trunk next to the spare tire. The CL is similar however the location under the package tray is covered with several trunk lining panels which can be challenging for any mechanic or DIY guy to remove. The plastic retainer buttons on the inside front of the scuff guard panel must be first removed. Then the two torx head screws and plastic retainers holding the spare tire cover must be removed. Then remove the right side panel rubber gasket retainer from the top of the panel starting at the closest point to you and then move your thumb or non scaring tool until the entire top of the panel is free of the rubber retainer seal. Then remove plastic retainer from right side trunk hinge, spin off retainer lip and turn counterclockwise until it is completely free. Pull up bottom of rear trunk liner panel and pull gently from one side until panel is free. there are no fasteners in the side or rear panels . The left side panel can be left in place. The battery is located on a shelf on the right in front of you. Follow Mercedes Benz Manual for turning consumers on and off before battery removal and procedure for removing and charging. The Starter Battery is located under the hood on the left, in front of the firewall.

You can download the owners manual here http://pdfservicemanual.com/automotive/2008-mercedes-benz-cls550-cls63-amg-w219-owners-manual

The battery for that vehicle is located in the trunk. The vehicle owner's manual should give you the exact location and may have instructions on how to access it.