Answer
Mar 29, 2026 - 04:21 PM
On the 2015 SL400 (R231), the auxiliary battery isn’t in the engine bay like older cars.
It’s usually located in the passenger side footwell area, behind a trim panel / under the dash, or sometimes tucked behind the carpeted panel on the side
You’ll need to:
- move the seat back
- pull off the side trim/cover in the footwell
- and you’ll see a small secondary battery, separate from the main one in the trunk
That aux battery handles things like start/stop system, electronics backup, etc.
If you don’t see it right away, it’s just hidden behind trim; not something you can spot without removing a panel.
