All of ActiGraph's devices** will indicate that they are fully charged (the battery is at 100%) by displaying a solid LED (light) while attached to either a computer's USB port or powered USB hub. This light could be either white or green, depending of the device.
Please refer to the follow FAQ
here for more information on the LED.
** This excludes the ActiTrainer device, which displays a real-time battery indicator within its display.