When the light is green, the charger thinks the battery is fully charged. Almost all the time, that means the battery is fully charged.
I can tell you that on some USB ports, my eGo Fast charger would go green when the batt was not fully charged. I dunno why it doesn't have some indication, like a blinking red light, to tell that it isn't getting enough amperage from the USB port, but that's what I conclude was the problem.