There must have been something running in the background. That just doesn't make sense unless the battery is completely defective which would be unlikely.
Is it a cellular model? Is the background app refresh feature turned OFF? Any chance you use photo streaming and perhaps a bunch of photos were being shared or uploaded? How about some social media app that could have been on and fetching new data or something?
Just thinking of some possibilities. That's quite a drastic amount of batter usage overnight....?
Thanks for your reply and suggestions.
All those items you listed are disabled on my machine (I do have cellular but keep it disabled at home). I don't use any social media apps and keep most programs shut down while not being used.
I checked the usage and nothing was in double digits (except the first on the list, home page or whatever it's called -- I don't recall off the top of my head, but it's normal).
I've been monitoring it all day today, and so far the percentage has dropped by 1% in the past four hours while on standby in the case. Seems better, but I will keep a close eye on it.