I get the battery icon to come up when I'm driving with my headlights on and then step on the brake or switch on the turn signal.
It's annoying, to say the least, but I haven't yet exhausted my possible problem areas- such as grounding issues (as mentioned above).
I also have a nasty 'alternator whine', which usually points directly to grounding issues.

It makes me wonder, though... would a capacitor in the power circuit smooth out such voltage fluctuations?