Anyone taking possession of a shiny new iPad this weekend may have noticed an odd message when connecting their gadget to some computers or USB chargers: the words Not charging in the iPad's menu bar.
iPad has heftier charging requirements than iPods and Phones, and some USB ports--especially those on older computers and most USB hubs--don't provide enough power to charge the iPad during use. That doesn't mean ower-power USB ports can't charge the iPad at all; instead, whether they can charge the Pad's battery--and how quickly--depends on how the iPad is being used Confusing? A bit.
Here's a quick summary of your options, based on our experiences Apple's support site, and information provided to Macworld by Apple
* For the fastest charging, use the iPad's included 10-Watt USB power adapter. This wil fully charge the iPad in a few hours, even if you're using the iPad at the same time
* When connected to a high-power USB port--such as the ones on recent Macs and the Phone Power Adapter--the iPad will charge, even during use, but more slowly. (We haven't yet determined how much more slowly.) Some third-party powered USB hubs provide higher-power USB ports, but many don't; similarly, the USB ports on most Windows PCs don't provide this additional power.
* 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 Pad 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.)
If you're interested in the specific power differences between the USB ports on older and newer Macs, Apple has published a support article that includes this tidbit
The USB ports on Apple computers provide 5 V (Volts) and 500 mA (Milliamps) to each port, regardless of whether the port is USB 1.1 or USB 2.0. This is in compliance with USB specifications. On some newer Intel-based Macs, such as the MacBook (13-inch, Late 2007), when a device requiring more than 5V and 500mA is connected, the port with that device connected to it becomes a high-powered port capable of offering up to 1200 mA at 12 V. That port will continue to operate as a high-powered port until the device is removed
tested our iPad with a number of Macs, USB hubs, and USB chargers. We found that while Apple's latest laptops and desktops provided enough power to charge the iPad during use, an older MacBook Pro and an older Mac Pro didn't. Similarly, while the Phone Power Adapter provided enough power, several other USB chargers, as well as a number of USB hubs, did not Although we haven't done enough testing to be sure, it appears that even though the Pad's battery isn't charged during use when connected to a low-power USB port, the Pad at least gets enough juice that it doesn't have to draw from its battery. In other words, your battery level doesn't appear to go down.
In short, there's nothing wrong with your iPad; it's just hungry for power. In this respect the iPad is a lot like many USB hard drives and Apple's external optical drive for the MacBook Air: it needs more juice than the typical USB port provides