ipod battery

My 60G ipod was fully charged about 6 months ago.  Afterwards, I did not use it until yesterday.  Surprise, when I turned it on, the battery status remained fully charged, no leakage at all.  What kind of battery is Apple using for its ipod, ipad and iphone series.   What I know is that it is lithium-based battery with no memory effect.  I have no idea that the battery's charged capacity will not leak even for a certain  period of time.