When an iPad is "not charging" and is not sleeping, is it still drawing power from the USB port so it doesn't drain or at least drain slower?
Yes. It's just that the power it's drawing is not enough to both run the device and charge the battery at the same time.
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By trial with my own iPad, I have found that it also does charge while in this state just very very slowly. – cbbcbail Jun 13 '13 at 5:50