The only times I see the battery icon is when I'm starting the car when the the player has been playing in acc mode or when I restart the car immediately (as in the first 10 secs) after turning it off.
I have no explanation for this except maybe this one : I've noticed that when I play my music really loud (more than 3/4ths of the system's capacity), the lights of my car start "flashing" to the beat. I guess this is because my alternator probably won't last much longer, or because it isn't powerful enough. I'll look into in having it replaced one of these days.
I suspect this is related to the battery icon showing up.