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...