TLDR; It’s a bit of work, and you’d like to have some experience with electronics and Arduino. Basically I monitor charging current (or speed if you like) to determine when the charge on my iPhone battery has reached about 80%, and then cut power to the charger.
Note that on Android there are apps that will do this for you.
Also note that on iPhone you can use Automation in the Shortcuts app to send you a notification at a certain charge, and then take the phone off manually.
That said the method I use here could in theory be used to modify any device that uses lithium batteries. I must admit I did it mostly for the challenge, however I also find it useful.