I've been using my ZTE Blade A7 Prime phone for home internet, recently, which necessitates leaving it plugged in all of the time.
I was surprised to pick up the phone and see that it was at 59% charge and there was a pop up that said that my phone was in "safe charging mode" since it had been plugged in for an extended length of time. It said that safe charging mode could be reset by unplugging and replugging the power source. (I chose not to.)
(Note: "safe charging mode" should not be confused with "safe mode". They're totally different from each other.)
A few hours later I checked it and it was down to 51%. Unhappy, I started Googling "safe charging mode" and found a dearth of information, other than owners of ZTE Blade A7 Prime phones and ZTE R2 phones who were similarly looking for information.
My first guess was that, after some predetermined time, charging simply shuts off and the phone gradually discharges, in some misguided attempt to protect the permanently installed battery.
This morning, however, I noticed that the phone was at 54%, without me doing anything. This actually made me happy. (Update 1: and now it's at 64%.)
I believe that, in situations where a phone is left plugged in for extended periods of time, Android keeps the charge level in the 50+% range to maximize the life of the battery. If that's true, then I'm a pretty happy camper.
I don't know if "safe charging mode" is something specific to ZTE phones or Android 9 (and higher?) but I'm initially optimistic about it.
Do any of you know any additional information?
(Update 2: while in safe charging mode I've watched it go all the way to 100%, which seems counterintuitive, since a maximum of 80% would probably be better for the battery. I don't know what the parameters of safe charging mode are. I'm just going to trust that they know what they're doing.)