My old iPod, which lived most of its life plugged in to an iPod adapter in the car, no longer had any battery capacity. Unplug it from the car and in an hour or less, without playing it, it would have lost its mind, requiring it to be restored from iTunes.
Gee, that was easy! Followed the guide, and it worked like a charm. Reinstalling the cable between the two case halves was the hardest part. No, I take that back - prying the battery loose was the hardest part. That glue is strong! And that, in case anybody is wondering, is why you DO have to separate the two case halves.
Put a towel or magazine under the iPod, especially when you're trying to pry the battery loose. It takes a bit of force.