Why is keeping my Mac battery 40%-80% charged advantageous?

This article from Lifehacker notes that keeping my Mac battery 40%-80% charged is advantageous. http://www.lifehacker.com.au/2012/01/ask-lh-how-often-should-i-charge-my-gadgets-battery-to-prolong-lifespan/

Why? What happens when the battery is over 80% charged, and what happens when the battery is under 40% charged?

A study* claims that overcharging by 0.01V reduced the life of an Li-Ion accumulator by more than 50%. The conclusion was that by reducing the charging to 80% of the full capacity you can prolong the life of accumulator very significantly (click to enlarge):

If the battery is under 40% charged, higher charge current (when it finally is recharged) will result in more heat, which shortens the life of Li-Ion accumulator. Also, depleting the accumulator has the obvious drawback that you won't be able to use your device when you need it.

Some laptops (like my Lenovo T61) have an option to stop charging the battery when it is 95% full, in order to prevent this overcharging problem and prolong the life of the battery. I wish more devices had this option.

_{* K. Asakuraa, M. Shimomurab, Т. Shodai, "Study of life evaluation methods for Li-ion batteries for backup applications", Journal of Power Source, 2003, PDF: http://lib.gen.in/69cb086b10cb54303a7cf4af98754216.pdf}

Many MacBooks use lithium-ion batteries that feel better under certain conditions. That is chemistry, not computing. See Prolonging battery pack life

See the sources listed on Wikipedia for in-depth explanations: http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries

Some newer MacBooks use lithium-ion polymer batteries. For maintanence instructions for those, see Prolonging life in multiple cells through cell balancing.

Rip out your battery and read it's label if you want to know details about it.

Depletion on Wikipedia:

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.