2015 s550 aux battery location

There are different W222 models too, if you don't want to remove the trunk panels, post your VIN and we can double check. I did not read the pdf from konigstiger but it probably states which models it is valid for. Only some hybrids likely have a different configuration.
Its the rear panel in the boot/trunk on the right hand side - remove the cover (had a look - looks like the picture) remove the panel with the 3 screws and swap out the battery behind it.
Mines a W222 2014 350d but assume its the same.
From what i read seems fairly simple and if I can save £160 for 10 minutes with a screwdriver than I'll give it a go!
Thanks



