"Cycle" is an important word here. Because batteries are designed to store a charge it's difficult to measure their usable life as a unit of time. A battery that lasts two years for you may only last six months for someone else because it's being used differently. So that we can have an estimate of how long to expect them to last, battery longevity is measured by charging cycles. A phone battery is typically designed to last around 500 to 600 cycles, and a cycle is defined as charging a completely dead battery to 100% then draining it to zero again. Charging a battery that has 50% charge left on it, then draining it back to 50% is a partial cycle, which is why you'll hear people telling you to charge your battery before it gets low and also hear people telling you the opposite as ways to game the system and stave off that 500th cycle. Of course, it doesn't work that way because the battery doesn't actually count the number of charge cycles. Five hundred is just an estimate.(Copied from a web source)
