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.
The memory effect is a common issue in older cell phones that use a nickel-cadmium (NiCd) battery. This issue arise when the user charges the battery before it has drained repeatedly, and the battery "remembers" the point at which it was prematurely drained and then only drains from that point onward (thus losing a portion of its full battery potential.)
The memory effect is only present in older cell phones, so if you have one that was made in 2005 or before, you may know of it.
To prevent the memory effect from occurring, make sure to deplete your battery as much as possible before charging it.