over the past several months I have charge my nook several times and unless it is plugged into the charger when it is turned off the battery drains. I used it last week on my vacation an it worked because I turned it on almost every day. I charged it less than a week ago and had it turned on for less than 4 hours and I went to use it today, without charging it and it was dead. If it is turned off the battery should not drain. What is causing this issue?