While I am driving my car, the battery dies.. But, if I restart it, it still works.?

I really have no idea what is going on with my car and I am confused. When I'm driving, my car will just slow down and I look down and then the battery light is blinking. I pull over, park it, turn it off, then turn it back on and it drives just fine afterwards. I don't keep anything plugged into it and if the battery dies it shouldn't turn back on, right? If it helps any, I have a 2001 Volkswagen Beetle.