W223 Rear Battery Tray
 

To access the rear AGM 12V battery in the trunk you need not remove any fasteners. Simply open the hinged hatch. It can be suspended to the trunk lip by the plastic hook and lanyard. Then pull the felt covered molded plastic battery well cover rearwards and upwards. About halfway out, start to pull downwards on the cover where it pulls out from behind the bumper sill plate area. It's flexable enough in warmer weather as to not crack.
https://cimg4.ibsrv.net/gimg/www.mbw...244eab4653.jpg
Looks like the tray is made of carbon fiber or fiberglass.

I've done this a couple of times not easy to remove nor install. MB finally replaced by AGM battery under warranty. Think it
sat at the port too long. even though they may have charged it apparently it was not up to a full charge.

AGM Batteries are indeed expensive. The one in the photo says Made in Germany on top but there is a sticker on the the side that says Made in Czech Republic. I wanted to make sure the battery connections were good and see how much extra storage space would be available without the cover in place. Turns out, quite a bit more than the little recessed tub allows in the cover.