I ran across this article recently and quickly realized this is probably the best article about battery technology as it pertains to mobile devices I have seen in a long time. The article explains the different battery technologies used in devices today and has some excellent tips on how to prolong the life of your batteries. Of particular note is the effect temperature has on lithium-ion battery life:
This means that the hotter your device runs normally the quicker your battery capacity will diminish. While this is no surprise the amount the capacity is reduced is eye opening.