Let's put this another way, to do what you want to do the charging circuit would need to support it, as well as your BIOS and a driver and software in Windows written to control the circuit. You could perhaps "roll your own" using a relay, and an Arduino. The Arduino would constantly be fed the battery percentage from Windows (you could write your own software in vb.net or C# to report this number to the Arduino via serial communications). Once the Arduino see's the number is 80 percent or greater it would trigger the relay to turn off the relay thus disabling the AC adaptors ability to charge the battery.
This is actually not a hard project at all, if I thought there was a mass need/want for such a product I'd make it, but I really think it's not that important. Batteries are around $15 anymore, and as long as you're not keeping your battery insanely hot, and calibrating it every 3 months or so I'm not sure the loss is such an issue as you might think. I've had the same battery now in my Lenovo Yoga 13 for years now and not noticed a major change (and I keep it charged 100% most of the time).
This is actually not a hard project at all, if I thought there was a mass need/want for such a product I'd make it, but I really think it's not that important. Batteries are around $15 anymore, and as long as you're not keeping your battery insanely hot, and calibrating it every 3 months or so I'm not sure the loss is such an issue as you might think. I've had the same battery now in my Lenovo Yoga 13 for years now and not noticed a major change (and I keep it charged 100% most of the time).