What if I told you that the goal of limiting charging to 80% is not to very precisely hit that particular percentage, but to avoid the damage resulting from going all the way to 100%? If total battery capacity has degraded to the point where the voltage level that used to correspond to 80% now is 85%, then so be it. At least it's not a full charge.
An alternative is to reckon the percentage relative to the design capacity, so that a full charge can be less than 100%. That's what the i3 window manager's status bar shows by default, for example: https://i3wm.org/docs/i3status.html#_battery
An alternative is to reckon the percentage relative to the design capacity, so that a full charge can be less than 100%. That's what the i3 window manager's status bar shows by default, for example: https://i3wm.org/docs/i3status.html#_battery