Your battery's life is all about charge/discharge cycles. Say your battery has roughly ~1000 such cycles. Once you've exhausted the cycles, your battery is typically running on bonus power. Whatever performance you get after this is extra and cannot be relied upon.

Thus, the longer you can preserve your cycles, the longer your battery will survive.

You can also read this thread: http://www.reddit.com/r/askscience/comments/1snzg1/is_it_act...