When I got my '09 M50 it would occasionally go dead for no apparent reason. I finally figured out the nut on one of the battery terminals was cross-threaded. A screw goes through a hole in a box-shaped lug on top of the battery and into a square nut that's inside that 'box.' It felt tight, but it was not clamping the connection together tightly. You may have the same type of connection, and it may have the same problem.