I know for sure that for Li-ion batteries the life of the battery is measured in number of recharge cycles. So lets say a battery has a life of 100 charges. Now this 100 charges means 100 complete charges, ie. 1 charge = complete depletion to full charge. Therefore if you charge when the battery has discharged only half way, you will prolong the life of the battery because you have only consumed .5 charge, and you have 99.5 charges left, as opposed to if you had let it completely discharge and thefore would have consumed 1 full charge, leaving 99 charges.
Gandalf
