About a month ago I saw the "Why can't they fix this?" video. I've had the same problem for years. I had actually just done a factory reset on my phone and had noticed that the battery usage was back to normal.
Fast to the past week, my battery is draining significantly faster than it was before. I think I have finally uncovered the culprit.
Gmail!
I know, I know, almost everyone with an android has their Gmail set up. How many accounts do they have?
I have slowly been adding more of my accounts to my phone as I have needed access to them. Last week I added 3 new ones... for a total of 5.
Before my last reset, I had at least 10 set up. At that point, the battery life was nearly unusable.
Prior to the three new accounts if I left the phone unplugged overnight while I slept in the morning I would have only lost about 5-10% battery.
Last night I went to bed with about 90% battery life and awoke at 17%.
While I cannot be certain that this alone is enough to cause the issue. I bet it is at least part of the problem.
Another reason I think this could be an issue is that as far as I can tell Gmail is invisible to Samsung's battery usage monitor.
This would also explain why putting my phone into power saving mode (where it globally prevents background data and apps) would provide ridiculous like 80-90% better battery life when the issue is at it's worst.
To be clear this is all just from my observations over the past month. I am going to remove some of my less important Gmail accounts again and see what impact that has.