Sometimes when Sphero isn't settled properly in his charger, he'll get a false charge.
The charger light will blink blue and then turn solid, but Sphero won't get a full charge.
This can be avoided by making sure Sphero rests solidly with the heavy side down in the charger. A good way to do this is to put Sphero in the charger when he's awake. When Sphero is properly placed in the charger, he'll cycle through a fast rainbow of colors before going to sleep to charge. The rainbow confirms Sphero is sleeping and entering a charging cycle.