Charging an iPad’s battery can be a confusing endeavor. Macworld’s Dan Frakes breaks it all down:
Fastest charging – iPad charger: For the fastest charging of any iPad, use the iPad’s included 10-Watt USB Power Adapter or a third-party charger certified for fast-charging an iPad. This will fully charge a first- or second-generation iPad in a few hours, even if you’re using the iPad at the same time; the third-generation iPad will take a bit longer, as explained below.
Slower charging – high-power USB port: When connected to a high-power USB port—such as one on a recent Mac, Apple’s USB Power Adapter for iPhone, or third-party chargers certified for the iPhone—a first- or second-generation iPad will charge, even during use, but more slowly. (Some third-party USB hubs provide higher-power USB ports, but many don’t; similarly, the USB ports on many Windows PCs don’t provide this additional power.)
Slowest charging – lower-power USB port: When your iPad is connected to a lower-power USB port—those on older Macs, many Windows PCs, and many USB hubs (powered or unpowered)—the battery is charged only while the iPad is asleep. If a first- or second-generation iPad is awake and in use, its battery essentially holds its current charge level. 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 that once you put the iPad to sleep, the battery will charge.
The third-generation iPad: The newest iPad has even heftier charging requirements than its predecessors, for a couple reasons. First, it has considerably more battery capacity than the first two iPads—42.5 watt-hours, compared to 25 watt-hours—so even at the fastest rate of charging, the third-generation iPad takes noticeably longer to fully charge. (And, in fact, according to testing by DisplayMate, the new iPad’s battery doesn’t reach a full charge until about an hour after its battery meter displays 100%.) So no matter how you charge, it will take longer to fully charge a third-generation iPad than either previous iPad.