When connected to lower-power USB portsthose on older Macs, most Windows PCs, and most USB hubs powered or unpoweredthe iPads battery is not charged while the iPad is awake, but is charged again, slowly when the iPad is asleep. Whats confusing here is that the message Not charging appears in the menu bar when the iPad is awake, which might lead you to assume that the offending USB port can never charge your iPad. But rest assured, Apple says: once you put the iPad to sleep, the battery will indeed charge. If you could see the screen while the iPad was asleep, it might even display the charging icon. Its the modern-day Does the refrigerator light stay on when I close the door? mystery.