Congradulations, you've just discovered the horrible evil that is the Ipod battery. I've had 3 Ipod mini's replaced under one warranty, and every one has had it's battery die. I charge it for any amount of time, and within 2 seconds of playing something it'll stop with the "low battery" sign.

Apple is evil. The ipod interface is fantastic, but they'll give you one battery after another that'll crap out.