You are seeing excatly what Apple states can happen if you try to charge your iPad - any model/version of the iPad - using a USB port on your computer. This is copied from Apple's article about the iPad battery.
There is nothing wrong with the way that your iPad is charging.
The quickest way to charge your iPad is with the included USB Power Adapter. iPad will also charge, although more slowly, when attached to a computer with a high-power USB port. When attached to a computer via a standard USB port (most PCs or older Mac computers) iPad will charge, but only when it's in sleep mode. Make sure your computer is on while charging iPad via USB. If iPad is connected to a computer that’s turned off or is in sleep or standby mode, the iPad battery will continue to drain.
The quickest way (and really the only way) to charge your iPad is with the included 10W or 12W USB Power Adapter. iPad will also charge, although more slowly, when attached to a computer with a high-power USB port (many recent Mac computers) or with an iPhone Power Adapter (5W). When attached to a computer via a standard USB port (most PCs or older Mac computers) iPad will charge very slowly (but iPad indicates not charging). Make sure your computer is on while charging iPad via USB. If iPad is connected to a computer that’s turned off or is in sleep or standby mode, the iPad battery will continue to drain.
Apple recommends that once a month you let the iPad fully discharge & then recharge to 100%.
How to Calibrate Your Mac, iPhone, or iPad Battery
At this link http://www.tomshardware.com/reviews/galaxy-tab-android-tablet,3014-11.html , tests show that the iPad 2 battery (25 watt-hours) will charge to 90% in 3 hours 1 minute. It will charge to 100% in 4 hours 2 minutes. The new iPad has a larger capacity battery (42 watt-hours), so using the 10W charger will obviously take longer. If you are using your iPad while charging, it will take even longer. It's best to turn your new iPad OFF and charge over night. Also look at The iPad's charging challenge explained http://www.macworld.com/article/1150356/ipadcharging.html
Also, if you have a 3rd generation iPad, look at
Apple: iPad Battery Nothing to Get Charged Up About
Apple Explains New iPad's Continued Charging Beyond 100% Battery Level
New iPad Takes Much Longer to Charge Than iPad 2
Apple Batteries - iPad http://www.apple.com/batteries/ipad.html
Extend iPad Battery Life (Look at pjl123 comment)
New iPad Slow to Recharge, Barely Charges During Use
Tips About Charging for New iPad 3
How to Save and Prolong the battery life of your new ipad
Prolong battery lifespan for iPad / iPad 2 / iPad 3: charging tips
iPhone, iPod, Using the iPad Charger
Install and use Battery Doctor HD
In rare instances when using the Camera Connection Kit, you may notice that iPad does not charge after using the Camera Connection Kit. Disconnecting and reconnecting the iPad from the charger will resolve this issue.
Like Diavonex said it takes more power to charge the iPad even the mini. If you use the USB on a Mac it has enough power to charge an iPad. However every none Apple PC I have plugged any of my iPad's into they don't have enough power to show that it is charging and takes a very very long time to charge them.
I'm on my fourth iPad. 1,2,3 and mini. All have had same charging problems. Varying speeds of filling up. I've always used the correct "matching" apple cables for all. Have plugged into walls, USB"s and car. Completely irregular responses from one device to another, and even get differing responses from the same outlet at different times. All the nonsense apple puts out there about using a stronger charging USB, or not using wall outlets, or using certain outlets, I believe is a diversion from what they really know to be true: they can't figure out why devices charge at different speeds. Does anyone else get irritated that apple protocol is that they DO NOT answer us if they don't want to? All of these comment blogs are just that: civilians commenting on what they believe to be true. And we have to hunt through huge numbers of blogs and commentaries to even get near an answer! With all due respect to you heavy-hitting top notch responders out there, thank you! We'd be totally in the dark without you guys. But I've got so many dollars invested in apple product, as do so many users, you'd think they would offer the courtesy of answering users questions head-on, directly, rapidly and in a straightforward, easy to locate site. It's insulting that they do not take better care of us. Yes we can call into express support for many issues, but they would not be of any help on this charging thing. What will they do? Recommend that we plug it into a different outlet and wait with us on phone for results? Silly! Whenever there's an issue about potential bugs in devices, apple support just goes away leaving all of us to guess! Why do I keep giving them my money? !!!!! Because I'm in so Mac-deep, I can't imagine converting everything to non Mac environment without losing my mind! I really wish a bonafide apple support person would take on the challenge of answering why we cannot get effective consistent charging for iPads!
Does that Asus Ai charger tool work on non-Asus boards?
I am also very annoyed that I cannot charge my Mini on ANY of my computers. What a ridiculous notion that I must have separate charging and usage sessions (I use my Mini WITH my PC).
Perhaps I can find a way to wirelessly interface to my Mini?
BUT I can confirm that even though the Mini states, "Not Charging", next to the battery symbol... it will trickle charge (very slow) IF the Mini is asleep. Better than nothing, I suppose.
This is obviously a design flaw. Please AFB's stop making excuses for Apple.
Yes, Asus Ai Charger works on non-Asus PC brands!
I'm using it on 6 laptops (1x Compaq, 2x Toshiba, 2x Sony Vaio, 1x DELL).
I've been using it since released, and I've never experienced any issue with it.
Weeks ago, I plugged in my iPad mini, trying to sync it with iTunes, and I found out that the iPad was charging normally !
I don't know if Apple did something on the recent updates on this thing, but I'm really happy, finally I can charge my iPad mini from my laptop !