With the last battery operated device I worked on developing we reported the user 0% battery level when it was still at 15% actually, and we indicated 100% char

With the last battery operated device I worked on developing we reported the user 0% battery level when it was still at 15% actually, and we indicated 100% charge level when the battery was only at 85% (and the charger was also turned off at that level) - so we let the user charge and discharge anyway they want, and cheat a bit with the reported battery level, for protecting the battery itself, instead of relying on the user. I imagine that most modern battery operated devices do something similar by default, unless they have a dedicated option to turn it on/off.