The iPads charging challenge explained

When connected to lower-power USB ports—those on older Macs, most Windows PCs, and most USB hubs powered or unpowered—the iPad’s battery is not charged while the iPad is awake, but is charged again, slowly when the iPad is asleep. What’s 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. It’s the modern-day “Does the refrigerator light stay on when I close the door?” mystery.

via The iPads charging challenge explained | Accessories | iPhone Central | Macworld.