Batteries don't last forever, and have only so many charge cycles, it is also not a Good idea to store a device that uses Lithium polmr batteries below 30% charge for long periods , rather charge then store.
Charging Tips
The quickest way to charge your iPad is with the included 10W 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. 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.
Maximum Battery Life1
To maximize battery life, Apple engineers took the same lithium-polymer battery technology they developed for Mac notebook computers and applied it to the iPad. As a result, you can use iPad for up to 10 hours while surfing the web on Wi-Fi, watching videos, or listening to music. While surfing the web on a 3G data network (Wi-Fi + 3G model), you can get up to 9 hours of battery life.2
Charge Cycles3
A properly maintained iPad battery is designed to retain up to 80% of its original capacity at 1000 full charge and discharge cycles. You may choose to replace your battery when it no longer holds sufficient charge to meet your needs.
Let It Breathe
Charging your iPad while in certain carrying cases may generate excess heat, which can affect battery capacity. If you notice that your iPad gets hot when you charge it, take it out of its case first.
