A cell phone battery is an incredibly expensive little piece of equipment that will go bad with no warning and will often cost more to replace than the phone is worth.
That's one of the few drawbacks to the convenience of a cell phone. They are portable and so very handy, but when a phone with $50 of technology goes bad, it will cost less to replace it with a new one than it will to fork over $80 for a new battery.
The key is to make the battery last as long as possible, and one way to do that is to let it run down completely every so often before recharging it. This helps maintain the battery's capacity and health.