I noticed back when I got the unit that my 2013 z6 started a little weak.  I recently replaced the battery but that didn't do anything to help so I ordered a new starter.  The replacement process wasn't too bad.  Reasonably easy to access the starter, only had to take off the center and back plastics inside and all it had were two 8mm hex bolts holding it down.  Much stronger starting now.

[2015 Z Force 800 53/Trail EPS]

Nice. Good info.