Lithium ion batteries DO NOT develop a memory so people thinking you should drain your battery till it is almost dead and then charge it because of the "memory" of the battery are wrong. Second
The charge cycle for a iPhone battery is from dead to fully charged. So charging your battery often means many less full cycles. A lithium-ion battery provides 300-500 discharge/charge cycles. The battery prefers a partial rather than a full discharge. Frequent full discharges should be avoided when possible. Instead, charge the battery more often. There is no concern of memory when applying unscheduled charges.
A lithium-ion battery in use typically lasts between 2-3 years. The worst condition is keeping a fully charged battery at elevated temperatures, which is more a problem with running laptop batteries. If used on main power, the battery inside a laptop will only last for 12-18 months.
So again: Avoid frequent full discharges
because this puts additional strain on the battery. Several partial discharges with frequent recharges are better for lithium-ion than one deep one. Recharging a partially charged lithium-ion does not cause harm because there is no memory.
(In this respect, lithium-ion differs from nickel-based batteries.)
Keep the lithium-ion battery cool. Avoid a hot car. For prolonged storage, keep the battery at a 40% charge level.
I Hope this sheds some light on the subject. If you do not believe me please feel free to research it yourself.