Batteries are a wear item, like the soles of your shoes and the tires on your car. Their expected lifetime depends on use (as reflected in Charge cycles) and also in the passage of time, used or not. After a while, you should expect to have to replace the battery.
Battery life also depends on two factors you may be able to control a bit better. Lithium polymer batteries deteriorate when they get too hot, and when they are subject to very deep discharge. These should be avoided when possible.
Despite using these batteries in millions of Apple devices, there is still a lot of of randomness in exactly which battery will fail when.
Apple SUGGESTS (but does not Warrant) that your battery MAY last as long as 1000 charge cycles, provided all other factors are well-controlled.
Apple uses the criterion that a battery that doesn't to hold 80 percent of its original charge capacity should be serviced, and likely replaced.
When designing systems, that 80 percent rule is intended so that, over a Very large sample of devices, almost all will be able to achieve their 'mission' -- still have at least 80 percent capacity at the three-year mark.
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When you present your Mac at the Genius Bar with a battery complaint, the technician will perform some diagnostic tests on the battery, looking for hidden faults. If your battery passes all those, the question the technician will ask is "is this battery still able to deliver 80 percent of its original capacity TODAY."
If it is, that battery is working as intended and will not be replaced.
When your battery can no longer can perform as expected, you will need to replace it. Batteries wear out. Their levels decrease with time.
Batteries - Maximizing Performance - Apple
