WHat is the major factor? heat?,size? consumption style like full discharge?
closed as off topic by Mokubai♦, ChrisF, soandos, DragonLord, Ƭᴇcʜιᴇ007 Jan 15 '12 at 23:32
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I have to replace the battery of my android phone every 18 months because it won't hold enough charge after being fully discharged on daily basis for 400+ days, so it's not really about what kind of device the battery is in.
The factor is mainly number of full discharge cycles and the operation condition. Lithium batteries can typically survive 400-600 full charge cycles and still hold a respectable amount of charge. However, if you constantly over-discharge it or use it under high temperatures its life will be drastically reduced. (Just 30 degrees Celsius is high enough !!)
Any temperature that is hot for humans is certainly too hot for li-ion batteries. That's places like the back or bottom of a laptop where the battery normally resides. If possible remove the battery when you're on a stable AC power and put it in the fridge. Never turn the phone back on if its automatically shutdown: you won't have enough time to do anything with it and you're killing the battery.