What Mshawan says is spot on, with one exception: You should avoid keeping your phone battery fully charged as much as is practical.
The life of a lipo battery is a function of how many times they have been charged, at what voltage they are charged to, at what voltage they have been DIScharged to, what voltage they are stored at, exposure to heat and cold, mechanical damage, and the rate of charge and discharge (not usually an issue for phones, but, Samsung seems to have made an obvious exception).
Lipo battery life is significantly reduced when stored fully or largely charged. I have at least 20 active lipo batteries at any given time for my radio control helicopter hobby, and storing lipos overcharged has caused more battery failure than any other factor (except high speed crashes into solid objects). The damage from high charge storage is proportional to the duration and voltage of the charge. Unfortunately, many cell phone user habits keep batteries unnecessarily full, notably frequent "topping off" by constantly plugging it into the charger, storing the phone with a full charge, or storing it on the charger (which is essentially full time topping off).
Lipos HATE being fully or over charged. They also hate being undercharged. The least destructive charge is at about 45 to 50%.
Properly stored lipo batteries have a surprisingly consistent life span, differences in battery quality noted. You should assume that after 350 to 500 (max) charges, a lipo battery in a phone will be losing its "vitality" (my word, not an official technical term). I think of battery "vitality" in terms of how long the battery holds a charge and how much working time you get out of it before you need to plug it in. I count one charge on my helicopter batteries as any cycle from a "full" charge (which I define as 90%-100% on the battery gauge), to "empty" (which I define as any value below 30% +/-), and then back up to full to complete the cycle.
Lipos are very sensitive to existing at either the high end or low end of their narrow range of voltage tolerance.(3.0-3.2 volts is the absolute low end if you expect the battery to last - think of 3.0 volts as 0% full. At the top end, 4.2 volts at is the absolute max. Lipos love to live with a charge of 3.7 to 3.85 volts. I don't know how the battery gauges on apple devices correspond to battery voltages, but most lipos reading 3.8 to 3.9 volts are 50% charged. At 3.7 volts, most lipos are about 20% charged (obviously voltage and charge level are not linear, having to do with how amperage and voltage interact to produce a usable source of power). I assume if my battery is below 15% power, it is being hurt (I don't know what voltage that corresponds to, but intuitively, it seems like a prudent minimum). Unless Apple is very conservative in calibrating their battery charge indicator, I assume when I run my phone until the phone shuts down, that my battery is suffering mightily. I assume the phone turns off at about 3.0 to 3.1 volts, which is the voltage that many lipo controllers (say a speed controller in radio control applications) will shut down without special programming.
But charge cycles ain't all the same. Demanding applications, say, running high load motors, can degrade a lipo battery in 150 to 200 cycles. I have never heard of anyone claim to have a battery that remains healthy for 1,000 or more charges.
HEAT is very bad for lipos. Leaving a cell phone in a car that heats up to 120+ degrees in the summer will damage a cell phone battery fast. Charging a Lipo with a high amperage charger can also heat them up and damage them (most electronic devices have electronics that regulate how fast a battery charges, but you can find many unbranded chargers that are outside of Apples specs that will get a phone hot fast, especially if it is not open to the air). FREEZING lipos is also very bad.
A good rule of thumb: avoid leaving your phone in your car unless it is parked in a moderate ambient environment
