Are you trying to charge the iPad using a USB port on your computer? If so, it may not supply enough power to charge the battery.
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.
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