I found this link from Apple...pretty informative!
Batteries - Why Lithium-ion? - Apple
"
Charge your Apple lithium-ion battery whenever you want. There’s
no need to let it discharge 100% before recharging. Apple lithium-ion batteries work in charge cycles.
You complete one charge cycle when you’ve used (discharged) an amount that equals 100% of your battery’s capacity — but not necessarily all from one charge. For instance, you might use 75% of your battery’s capacity one day, then recharge it fully overnight. If you use 25% the next day, you will have discharged a total of 100%, and the two days will add up to one charge cycle. It could take several days to complete a cycle. The capacity of any type of battery will diminish after a certain amount of recharging."