That's interesting about laptops with an 80% feature; IMHO, the thing to do with a laptop (not having such a feature) used as a desktop is just pull the battery and toss it in a desk drawer; when you want to move, slap it in, and go; when you get back, plug into mains, wait till the battery is charged, then pull it and toss back in the drawer. Of course, my present (ancient, NiMH powered) laptop has battery life about 30-40% original for the good battery, and about 20 minutes of the old one; I ordered some new cells, but when I get around to putting them in, it's becoming a primarily in-car machine... I'm not real sure what the best solution is for that.

For Li-ions, I think it's best to leave them at zero, from a battery-degradation perspective; but you can't do that, because if they drop below zero, you can't safely resurrect them at all. Since there are no internal electronics to drain power, 20-30% should be doable for N800 (and most cell phone) batteries, and is preferable to 40-50%, but one can always argue safe/sorry tradeoffs...