You may leave Sphero on your charger for as long as you wish. While charging, Sphero may be warm on the bottom (~100 degrees Fahrenheit), and this is considered normal. We encourage keeping Sphero fully charged so that he does not experience extreme battery drain, which may put him in deep sleep which will require 3 hours to achieve full charge.
IMPORTANT SAFETY TIP: DO NOT PLACE ANY OBJECTS IN THE SPHERO CHARGER OTHER THAN SPHERO. PLACING COINS OR METAL OBJECTS INTO THE CHARGER COULD CAUSE THE METAL OBJECTS TO HEAT AND BURN ON CONTACT.
EXAMINE SPHERO AND THE CHARGER FOR SIGNS OF DEFORMATION OR MELTING. IF ANY SIGNS ARE DETECTED - CEASE USE AND RETURN TO ORBOTIX FOR REPAIR OR REPLACEMENT. To do so you may email firstname.lastname@example.org and we'll address the situation immediately.