I have been under the passenger footwell again and the stop start battery cannot be seen. The plastic cover is for the fuse box and no other battery is visible apart from the tiny auxillary battery and it is very small with push on connectors. This battery is definitely not the stop/start battery as the wires are too small to start the car. Visible is a brown (positive) thick cable which disappears into the central console and looks like it may be the size of a starter lead. As the car is still under warranty I do not want to strip all the carpets out looking for it. MB must have made it accessible if its life is only 4-5 years.
If you google MB B Class 246 stop start battery you will see a motorcycle size battery which is 150x90x45mm and this is what I am looking for. The MB instructions you sent me (I have seen already) mention removing the bolts in the additional battery terminal and the tiny battery has push on terminals.
I may have confused you with the model. It is a 2016 W246 B 180 SE D Auto, I took the description from the last MB service sheet which was wrong. If I could work out how to post a photo on here I would send you a clear photo. Different MB models with Stop start seem to hide the battery(ies) under seats, Floors, trunk, side panels, I just cannot find mine!
Having spotted that all recent MB cars have a QR Code (Marked Rescue Card) on the Drivers door pillar, I scanned it. It did indeed show a diagram which listed Fuel tank, airbag and other major components. Both Batteries were listed as: On-board electrical system Battery and Additional battery for ECO Start-stop.
Though the diagram (shown looking down on the car) shows the steering wheel on the Left (on my car it is on the right) it shows the position of the large main battery to be under the hood on the left (looking forward) which is correct for my model. However, it shows Battery number2 (additional Battery for ECO Start/Stop as being on the opposite side to the main battery just to the right and below the centre video screen. A side view shows this second battery as being in line (at 2 O'clock) to the wheel hubcap, under the footwell. I have found the tiny 1.2 AHC battery in the passenger footwell (above the fuse panel in photo) but no joy with the other battery which measures 150x90x45mm.
Despite having the "treasure" map, I still cannot find the battery location. Interpreting the diagram, it would be where the foot pedals are located. I have removed the only cover in that area and pulled up the carpets as best I can. On the passenger side, unless it is buried under the fuse box panel I cannot see it or any cover that might be hiding it. I have seen other photos that show a stout red wire leading to it but I cannot see any such wire leading to it.
Trying not to get fixated with this but having spent 4 years as a vehicle mechanic and rebuilding 6 classic motorcycles from the ground up, it should not be impossible for me to find this da**ned battery. Photos attached.