Retraining batteries

I've bought an Apple iPod (4G) three years ago (December 2004) and I've noticed its battery life decreasing over the years.

A few months ago (March 2008) I started on a new job, for which I spend about 2-3 hours per day commuting by train, during which I listen to music on my iPod.
When I first started, I could use my iPod for 2 days (approx. 5 hours of listening), without recharging the battery. But over the past months, the battery life increased. Last week I managed to use my iPod for 4 consecutive days (approx. 10 hours of listening), without recharging. It seems that battery memory can be 'retrained'.