1.) You cannot overcharge. When the green light on the charger goes on, your battery is no longer charging.
2.) The eGo batt delivers a pretty consisitant level of voltage throughout its cycle. You will notice towards the end that your vapor production will taper off. The button on the eGo will blink when its done and needs charging again. I usually don't wait for the button to blink. I will start charging when I notice my vapor production tapering off.