I found this :
I often see many misconceptions about iPhone batteries, and Li-ion batteries in general, so I thought I'd write a mini-guide to address this issue.
iPhones have Lithium ion batteries. All Li-ion batteries have a limited life span, ie a finite number of charge cycles that they can undergo before becoming useless. The number of cycles you can achieve is determined in part by how you treat the battery.
The two biggest enemies of Li-ion batteries are heat and over-voltage charging. The latter is mainly taken care of by charger design (ie trickle charging and cutoffs).
Heat is a bigger problem. Batteries get hot when they are charging. Heat is also related to the other big reducer of Li-ion battery lifespan: Deep charge cycles.
Li-ion batteries function optimally and have the longest lifespan when discharged to around 60-80% then charged to full. Full (or deep) discharges* reduce the lifespan of the battery. Full discharges are bad due to intrinsic chemistry (small permanent changes inside the battery reduce its life) and also recharging from a low charge will inevitably heat (causing more permanent chemical changes) the battery more than shorter, more frequent charges.
Often people recommend infrequently fully discharging Li-ion batteries and sometimes even report better life after such treatment. This treatment of batteries is a legacy of times when Ni-Cad batteries were the predominant type and the subsequent conflation with Li-ion technology.
The apparent 'improvement' results only from the recalibration of the electronic gauge used to display the remaining battery percentage. Directly gauging a battery's current level of charge is not actually possible. The percentage shown is an average 'expected' value based on previous uses of the battery after being monitored by software for a period of time. If the calibrated values become inaccurate the battery gauge will also be inaccurate. A full discharge will recalibrate the software gauge, often resulting in more accurate battery-level readings. This in no way affects the battery at all. All that happens is the software is more accurate at reporting the estimate charge remaining. You will get no more performance from your batter by doing a full charge even if it appears to be so (actually, by doing a full discharge, you have reduced the lifespan of your battery by a tiny amount). Apple actually recommends infrequent full discharges. My guess is that they see this as a trade off; they know they are reducing the battery life by some tiny amount, but also receive far fewer customer support complaints due to poorly calibrated battery gauges being perceived as faulty devices.
* For the purposes of this guide a 'full discharge' is defined as reaching around 5-10%. For Li-ion batteries a 'full discharge' is not truly a complete discharge. Li-ion batteries have a cutoff within the battery to prevent complete discharge. If a Li-ion battery is completely discharged it can not be recharged. So when your device containing a Li-ion battery reports the battery is empty this is the cutoff coming into play and preventing permanent damage to the battery.
TL;DR
DO: Charge frequently. Small, frequent charges are best. Charge your battery first if you are going to leave it unplugged for an extended period. Charge your battery in a cool spot.
DO NOT: Let your battery get hot. Fully discharge your battery. Let your battery sit at low charge for extended periods. Believe the lie that full discharges improve battery lifespan.
