My 2015 35d has 2 batteries, one is larger than the second battery. The larger one is on the drivers side rear, the smaller one on the pass side rear. The larger one I think is the starting battery, not sure the second one I have no idea what it does. The larger one for replacement would be a BMW battery, or Exide MX-H8L549, the smaller 50 AH battery appears to be a european only and I have no idea what would be a replacement for it, also both of the BMW batteries have vent hoses connected to them, I think the Exide does also. The other thing is the smaller battery part number S:61 217 623 131 looks like it has been replaced with a new part number, this is very confusing, like just visit your local BMW Dealer, and that I care to avoid, so anybody know what works for the smaller battery??