Yeah these cars have really sensitive electronics. The symptoms of our dying battery was the clock and trip odometer kept resetting and I think we lost some other settings when the car sat for a few hours. Never ever had issues starting, but did start getting excessive power consumption warnings.

I would get a scanner or one of those cigarette lighter adapters that allows you to monitor voltage while running and see if there's anything obvious going on.

__________________
'08 X5 4.8i - 196k
'15 i3 REX - 95k - Blown AC compressor

'98 323is - 170k - Garaged
'94 325i - 208k - Sold