Originally Posted by Booxtor
Please avoid too deep discharging of a Li-Ion battery.
The best way to extend the lifetime of such a battery is just to charge it occasionaly. It is also not neccesary to charge the battery to 100% either.
Some tipps from Wiki:
Li-Ion batteries last longer if the user avoids deep discharge (depletion) and instead charge more often between uses. The smaller the depth of discharge, the longer the battery will last.
Lithium Ion batteries definitely do not need the "conditioning" of being fully discharged and charged the way NiCd and NiMH batteries do.
Li-Ion batteries may last longer if users avoid storing the battery in full discharged state. As the battery will self-discharge over time, its voltage will gradually lower, and when it is depleted below the low-voltage threshold (2.4 to 2.9 V/cell, depending on chemistry) it cannot be charged anymore because the protection circuit (a type of electronic fuse) disables it. It is frequently recommended to store batteries at 40% charge level.