Having a mobile phone is great when you need to talk on the go, but if you have a faulty battery, then you have to carry charger along with you throughout the day.
Many things can go wrong with a phone's battery, from the so-called "memory effect" to plain-old worn out batteries. Some problem can be corrected, but unfortunately, most of the batteries need to be replaced.
All batteries have a certain lifespan, and after a while you can expect them to degrade substantially. Most batteries will last between 2 and 3 years and can handle about 500 charge cycles. If you start noticing marked battery loss after a comparable amount of time, it is probably due to your battery being old.
Unfortunately, there is not much you can do about a battery getting old, and if enough time has passed, you will need to replace it to ensure your phone continues functioning as it should.