How to store unused Macbook battery for long period of time?
Battery technology used by Apple is no different than other laptops. They're all Li-ion cells.

Apparently, the recommended practice for extended storage is to get the battery down to 40% and store it in a cool place, possibly the refrigerator.

Some relevant sources:


http://www.runpcrun.com/laptop-battery-use-and-storage-guidelines
http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries