although lithium ion batteries don't have a memory effect, they can be very temperamental if not looked after properly. When calibrating the battery (every 30 discharge cycles or so, the battery should be fully charged then allowed to drain at a constant rate as slowly as possible until completely empty then fully charged. After this, the battery should never be discharged below about 20% unless recalibrating it. Calibrating it can actually increase the battery life too. I've managed to increase the capacity by .2 ampere hours on my powerbook which represents about 20 minutes depending on the power usage at the time.