The iPod’s Sony Lithium polymer battery (model# UP325385 A4H in the June 2001 Catalog) is rated for more than 500 charging cycles – one charging cycle consisting of draining the battery, than recharging it to a full charge. Therefore, the life span you should expect from your iPod’s battery will depend on how often you have to fully recharge the battery. Worse case, assuming consistent, very heavy daily use, you might need to fully recharge the battery every 1 or 2 days. This would result in an expected battery life span of 2 – 3 years. A lighter, probably more typical usage pattern might result in a full recharge once a week on average. This should equate to an expected battery life span of 9 – 10 years.
Please bear in mind that these are estimates and are based on 500 recharge cycles – which is the minimum number of recharge cycles for which the battery is rated. Also, since the iPod has been around for less than a year, no one has any real world experience to back up these estimates.
Finally, please remember it is recommended to never allow your iPod’s battery to completely discharge since it is never turned totally off (it instead just sleeps – this is what provides iPod’s instant on capability). You should recharge it to full charge regularly, before it has an opportunity to fully discharge (Lithium polymer batteries do not have a “memory”). In fact, some experienced iPod users try to never let their iPod’s battery indicator get below 2 bars. Also, many experienced owners always recharge using the AC adapter as opposed to recharging from the FireWire port on their computers. In certain situations, the iPod’s HDD will spin continually when connected to a computer, causing excess wear on the drive & power drain. This can be avoided if you “eject” iPod in iTunes and Finder, making the FireWire port a “charge only” port. Or, you can opt to always recharge your iPod using the AC adapter (or a car recharger).
Find more information on the iPod battery at the iPod Battery FAQ.