... batteries degrade, but lithium-ion cells erode faster when highly charged and warm; an average notebook battery kept at full charge at 25 degrees Celsius will irreversibly lose about 20 percent of its capacity a year...So it's better to let them cycle completely before recharging. Leaving the device continuously plugged in (like our Nano) will shorten its lifespan.
Saturday, November 04, 2006
A SciAm review of LiON batteries mentions that leaving them fully charged shortens their lifespan:
Posted by JGF at 11/04/2006