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. By definition, other batteries are WORKING as designed.
Apple does not provide ANY support for a situation where weak a cell might cause the battery to lower its capacity in a burst, causing it to decline rapidly and shutdown suddenly at the low end of its capacity.
You are welcome to ask Apple support to provide you a new battery. I expect they refuse, and will remind you that Apple's policy is that a battery the holds 80 percent of its original charge is defined as WORKING.
Apple support representatives are trained to be Nice to you, so they will not tell you to stop using it that way. Since I am not an Apple employee, I am free to blurt out the Truth.
