[URL="https://www.apple.com/batteries/#mn_p]Apple - Batteries[/URL]
Most lithium-ion polymer batteries use a fast charge to charge your device to 80% battery capacity, then switch to trickle charging. That’s about two hours of charge time to power an iPod to 80% capacity, then another two hours to fully charge it, if you are not using the iPod while charging. You can charge all lithium-ion batteries a large but finite number of times, as defined by charge cycle.
Charge Cycle. Using and recharging 100% of battery capacity equals one full charge cycle.
A charge cycle means using all of the battery’s power, but that doesn’t necessarily mean a single charge. For instance, you could listen to your iPod for a few hours one day, using half its power, and then recharge it fully. If you did the same thing the next day, it would count as one charge cycle, not two, so you may take several days to complete a cycle. Each time you complete a charge cycle, it diminishes battery capacity slightly, but you can put notebook, iPod, and iPhone batteries through many charge cycles before they will only hold 80% of original battery capacity.
[URL="https://www.apple.com/batteries/maximizing-performance/#mn_p]Apple - Batteries - iPhone[/URL]
A properly maintained iPhone battery is designed to retain up to 80% of its original capacity at 500 full charge and discharge cycles. You may choose to [URL="https://www.apple.com/batteries/service-and-recycling/#mn_p]replace your battery[/URL] when it no longer holds sufficient charge to meet your needs.