Using a MacBook Pro, the battery charge shows 99% when it is plugged in all the time. Never 100%.
Prior to buying the MacBook Pro, I read and read about battery discharge. And discharging the battery was highly recommended once a month or so.
And then I remembered an old IBM laptop I used that was always plugged in, and that would never again charge a battery. Or at least no battery I ever bought would hold a charge long enough to power the laptop for more than 10 minutes.
So, I discharge the battery once a month or so, as Jack does. (Or I bring it home, and forget that the surge protector strip I use has "usb outlets" that don't power on unless the computer is powered on, and I don't turn on the computer because I have the MBP, and don't remember to look for the green light, and the MBP will shut down or go completely asleep when I get up for a coffee refill and the phone just rang for the 6th time, and I wonder "oh my goodness, has something happened to MY LAPTOP???" and then brain kicks in, and I think, oh, that's ok, I just did my monthly battery discharge...)