Was the battery holding a normal charge just yesterday? If so, I guess there's nothing wrong with the battery.
The Nook HD+/HD models do have a few charging quirks. Once I could not get my HD+ to charge. Whenever I plugged it into a USB charger, the light would just be green no matter what. I got it out of that mode and into regular charging mode by connecting it to my PC with the USB cable. Windows somehow sends back a message to the Nooks when it recognizes a connection via USB, and that was enough to "wake up" my HD+ and fix the problem. So you might want to try that.
--Pat
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