Hi Guys. I drive a 2006 Z4M and have the same problem.
Here are my voltage test results. With Engine off, the battery is measuring at 12.5V. With the car on it's shows a reading of 14.2 at idle and jumps to 15.5 (max) if rev-ed up to 3K. I believe this to be within normal limits. Not sure what to do next to figure out if it's the battery or the alternator.
Also, am thinking of getting
this, if the alternator is the issue, unless you can suggest something else.