Thanks. I was trying to figure out how the charge level relates to the START STOP feature actually working, and some things I've noticed.. (using the system menu power supply status)
When I start my engine, my voltage goes up to 14.75 approx and stays that way for about 25-30 minutes until the stop start feature starts to work.
When my engine stops due to the stop start at traffic lights, the voltage drops to 12.79 and steadily drops almost every second 12.78, 12.77, ... until after 30 seconds or so it reaches 12.4 and then the engine springs into life.
Once it's restarted after a stop start, it stays at 13.5 rather than going back up to 14.7.
So it seems that this system of keeping it undercharged means, if you just regularly do short journeys...
a) stop start never kicks in until after 25-30 mins of driving
b) Once it does, it only stops the engine for 30 seconds or so before it comes back on.
I say short journeys because I have also noticed if I've been on a long motorway drive, the stop start seems a bit quicker to come on (10 mins rather than 30 mins) for the next couple of days.