iPod Battery Fail?

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I was using my iPod today (30GB 5.5th generation) not too heavily. I played maybe a half hour of uninterrupted music. Then I went through some songs and did some seeking (basically changing songs, seeking to the middle, listening for a bit, repeat) for about 4 minutes. It gave me the out-of-battery-life message and died when the life meter still read green. I tried turning it back on but it shut down as soon as it was finished booting (so I assume the battery was really dead at that point; it couldn't sustain the operating system). Then about 3 hours later - I left it off for that period - I turned it on and it played music for 30 minutes straight just fine. The battery meter read something in the green (don't specifically remember)... My friend once said he left his iPod Classic on playing music all night once and the battery was just half dead. Mine wouldn't survive half a night. The iPod is about a year and a half old, and its battery life is usually 5 hours of music. So is my battery screwed up? What's your average battery life on your iPods (iPod Classic, iPod Video)?