Lithium-ion batteries are sensitive to temperature and to exposure to oxygen. They also degrade faster if stored with a full charge, but even under the best case scenario, they lose an average of 5% of their capacity per year; more if subject to the thermal stresses of use and proximity to a warm CPU… or the thermal stress involved in charging. Under normal use, 20–35% per year is more typical.
Most LiOn chargers are smart enough to cut off the charger when the battery is full, so it’s not a terrible issue except that you have the battery at 100% charge at all times and thus degrading at least twice as fast as it would at 40% charge. There isn’t a memory effect to worry about like there is with NiCads, but LiOn batteries have their own issues.
Personally, I leave my laptop unplugged most of the time, unless I have it set to do some unattended task that will last longer than 15 minutes. I don’t want it to shut down halfway through a file transfer of an anti-virus sweep, and my Power Options are set so that it shuts down after 15 minutes of idle time when on battery power but never shuts off when plugged in. Same goes for my iPod; when the battery meter reaches 100%, I unplug it unless I am syncing it.
Either way, I don’t plan on any of my LiOn batteries lasting long. My 4-year-old Sansa used to last about 14 hours of music playback and now is closer to 6. My 1-year-old netbook went from 2:53 to 2:25 on a charge (a loss of ~16%). Batteries wear out no matter what.
@gasman There is a reason why good power supplies and chargers don’t use actual transformers to step down voltages any more. While there is stil a few milliamps flowing through a charger that isn’t currently charging, it is nowhere near what it used to be with the old-school ones. That said, last time I calculated the effects based on the chargers in my house, the difference was pretty small. I saved a lot more by switching from incandescent to CFL lights.
@jaytkay Most laptops do indeed have such a charger. However, as laptops generally get warm, especially those that are not a CULV design (basically, anything in a desktop replacement or gaming laptop as opposed to something low-powered like mine which is designed to get 8 hours on a 6-cell battery) I am not surprised. The best you could’ve hoped for was losing about half your capacity in two years, and probably closer to 70–80%.