I don't know where this myth started, but it's a common idea that you shouldn't leave your phone plugged in after it's reached full charge. It'll further depreciate the battery, that thinking goes. I talked to Daniel Steingart, an associate professor in the Department of Mechanical and Aerospace Engineering at Princeton University, about whether this is true. No, he says. In fact, "the easy thing to do is to keep [your phone] plugged in all the time, as much as you can." Modern smartphones do a pretty good job of battery management, he says. (Android O, for example, is supposed to ship with better battery management in regards to background apps.) Once a phone battery reaches 100 percent, extra energy isn't flowing into it. The battery remains constant.
I've been telling this to people for YEARS and somehow the myth still is out there and living strong. Leaving modern electronics plugged in IS NOT BAD FOR THE DEVICE.