Try installing a battery app to check its health, just to make sure:Weird. My 10.5 had at least 85% charge when I put it down last night for bed. Around noon today I picked it up and it's dead.
Find my iPhone 1%
Siri 99%
https://itunes.apple.com/us/app/battery-life-check-devices-runtimes/id1080930585?mt=8
This one is free and can tell that. For example, in my case there's 8% degradation after a year.