I have notice when my iPad gets run down it takes forever to fully charge back up. A couple weeks after I got it I ran it down till it shut off. I plugged it in that night and the next morning it was at only like 66% after 6+ hrs. The next time I ran it down it did the same thing. This last time it only went down to 10% before I could charge it and 7 hours later it was at 77%. Normally I plug in before I go to bed 20-35% and by miring it is at 100%. Is there some reason for this?