Using the battery level meter in this manner is comparable to using your car's fuel gauge to calculate miles per gallon. The only thing that matters is the total amount of operating time from full charge to auto-shutdown.
Use the wall-mount charger that came with the iPad and charge overnight. Admittedly the labeling is hard to read but, if you use a magnifier and squint your eyes, you'll see "Output 5.1V, 2.1A." The symbol between volts and amps is the symbol for DC. Do NOT use an iPhone charger. Do NOT use an iPod charger. Then, operate it normally until auto shut-down (ignore any low level alerts that may appear). An irony is that doing that test to determine the total operating time is also the exact procedure necessary to calibrate the battery level meter.
I'm not claiming that you do not have a problem. I am stating, however, that we don't yet know whether or not a battery problem exists.
Use Your iPad Regularly
For proper reporting of the battery’s state of charge, be sure to go through at least one charge cycle per month (charging the battery to 100% and then completely running it down).
Elsewhere, Apple elaborates and explains that two half-discharges (or four quarter-discharges, etc.) equals one full discharge.
Maybe I misspoke my issue. I Used the wall-mount charger that came with the iPad and charged overnight. . My Ipad showed 'Not chargeing' message when I woke up, after unpluged the wall charge, it showed only 80% . Replugged in only show 'not chargeing' message again after only couple minutes. So Ipad never make to charge to 100%. I guess I am not the only user who has this issue, but seems like few peple really reported this.
Try this first - Reset the iPad by holding down on the Sleep and Home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider - let go of the buttons. (This is equivalent to rebooting your computer.)
And this - Unplug the charger from the iPad and then replug. May resume charging.
Or - Recalibrate the battery. See link below.
The quickest way (and really the only way) to charge your iPad is with the included 10W or 12W (5W on Mini) 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 (2.5W, 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
Best Practices for iPad Battery Charging
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
To Extend a Device’s Battery Life, Get to Know It Better
iPad Battery Replacement
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.