Does my NAO have a battery problem?
As long as the battery is inside the NAO, it is delivering a small amount of electricity to the robot, even while shut down. If the NAO sits for a long time unused, the battery can become so depleted that it can no longer be charged.
If you suspect this is the case for your battery, here are some of the telltale signs:
If you conclude that you need a new battery, head to https://www.robotlab.com/store/battery to purchase a new one.