Yesterday I ran my iPad 2 out of power; to the state that it shut down. Plugged it into a wall socket to charge. After three hours it was sitting at 20% charged. Shut of the unit and restarted and left it over night. It now was sitting at 52% charged in the morning. Rebooted the iPad; and now after 3 hours it has moved to 61%. The wall socket and the charging cable is the one that I use to charge all my Apple devices. It is working as I used it to top of my iPhone this morning. What could the problem be.