One For The Whiz Kids

So, this week I've biked three times. On two of those three trips, my freshly-recharged iPod has run out of juice before I got home (therefore, after less than an hour of usage). On each of those occasions, when I got home I recharged the device fully, and then started it playing to see how long it would go before it ran dry. The first time, after it was still going an hour and a half later, I stopped the experiment and assumed that my sad biking experience earlier that day had just been a fluke. After it wimped out on me again today while biking home, though, I repeated the controlled, home version of the process... which is now halfway through its 3rd hour of continuous playing, with no signs of giving up the ghost yet.

So who has any ideas as to what the difference is between the two scenarios in question? In all cases, the iPod had been fully recharged not too much prior to me using it, although in the two bike examples I may have unplugged it a few hours before, whereas with my tests it's been just minutes before. Is that likely to be a significant difference? Also, the bike rides were both in very cold weather (close to freezing, although the iPod was inside a pocket and thus at least a little warmer than that) whereas my experiments have been at room temperature (22 C in our house). Does running in a cooler environment use up battery life more quickly? I can't think of any other differences, unless the somewhat jarring motion of being on a bicycle somehow affects it (which seems unlikely to me).

I may repeat the experiment tomorrow, but recharging it tonight and then running the test tomorrow (to see if the battery's losing strength during the time between charge and usage, even though the iPod isn't on). Can anyone suggest any other things to try, or provide insight into why this might be happening?

Update 1: I did just think of another, possibly significant difference in my scenarios, though it's purely unintentional. Since both of my experiments were done when I got home after the iPod had failed me, the battery had run out completely in those cases. The rest of the time I'm usually starting it charging when it - theoretically, at least - still has some charge left in it. Maybe that's the difference? Thoughts?

Update 2: It's still going, after 4 hours!! Where was that joie de vie when I needed it out of the stupid thing on the bike ride home today?!

Update 3: My repeat of the experiment the next day, with a longer waiting period after the recharge, yielded the same result: a little over 5 hours of running time. So I have no idea. We'll see what happens next time I bike somewhere.