Bought an iPad air 2 three nights ago. Everything was fine. Last night I did not plug it in before bed. Battery was 80%. 24 hours later I tried to use it and the battery was dead. I've owned an iPad 3 for over two years and never had this problem. My iPad was in a new case last night but the cover was closed so it was on standby. All settings were restored from my iPad 3 so nothing should be different. Any insights?