The device is charged by linking the charger/docking station to your computer via a USB connection. When a unit is placed in the cradle and it is charging correctly the red light will flash (for 3 hours). If the unit is in the cradle and the green light is flashing it has finished charging. Any other light activity or inactivity indicates that the unit is not correctly in the cradle (or there is some other error) and the unit should be removed and then replaced.
If the unit remains in the cradle once charging has been completed the battery will start to be depleted. Once the charge level has dropped by about 10% the unit will automatically start charging again until completely full. During this top-up cycling only the green light will flash.