have you tried putting the device into Safe Mode? Safe Mode disables all 3rd party applications and helps us determine if the issue is caused by an app misbehaving or software. If you haven't, please follow the steps in this link to place the device in safe mode for 2-3 hours: Boot in Safe Mode and monitor the battery usage during this time frame for any changes.
I found this thread searching for why my battery started draining over night. I got an update about 3 days ago and like an above user said, after that length of time my battery drainage started. I thought the most annoying part of the update was the resetting of all my notifications and opting me in to new features. Nope, this is!
Went to bed with battery at 100, woke up it was at 64. Usually at 99/98. I have done the suggestions in this thread. Cleared caches, taken out battery, and went into safe mode for a couple hours. If I'm not connected to a charger I get rapid drainage. I checked what apps were using the battery and nothing was draining it, or using more than their normal smidgen. Even shutting them off, made no difference.
This is ridiculous. I cannot now rely on my cell's alarm to wake me up cuz the battery might die unless I have it plugged in. I can't live life tethered to a charger. What in the world did this update do to batteries??