Why is my Galaxy S9 battery dying over night?

[Android Central Question]

AOD is always off. I don't use it. It died overnight, in roughly 7.5 hours.

I got the Galaxy S9 in June and it's December. This has never happened before. There has been no indication that the battery was failing.

I checked battery usage:
- fully charged 7 hr 53 ago (it is now charging)
- cell standby: 2%
- device idle 7 hr 35 min 2% (like I said, I was asleep)
- android system 1%
- google 1%
- screen 18 min 1%

What just happened and how can I prevent it? It's charging just fine, by the way. I'm typing this on my phone and it's up to 22% over the past 20-30mins of charging during active .

 

[Rukbat]

How low do you allow it to drop before charging, normally? Anything less than 40% will shorten the battery life. If you let it go until the phone shuts off, or you try to bring it as close to 0 as possible, you can kill the battery (yours isn't dead - yet) in a month or two.