Lithium ion batteries are weird. If you charge them to 100% and then don't use them, you damage them; they start to degrade over time. If you charge them to below about 20% or so and leave them there, they degrade over time. If you run them down to 0% charge, you destroy them irrevocably. (The battery in your laptop has a computer in it that will stop it from ever reaching 0% charge. When it says it's completely empty, it actually has a small emergency reserve yet o keep it from being ruined.)
The best way to prolong the life of a lithium ion battery is to keep the electrons moving. There are all sorts of ways to d this, but generally speaking, they all involve running sometimes off the battery even if you have power available.
If you want to be totally anal, for maximum life, don't discharge them below 40% and don't charge them above 80%. That level of nitpickery isn't usually necessary, though.