iPod won't retain a charge
My iPod plays just fine when connected to charger, but it won't retain the charge once unplugged.
iPod nano (7th generation)
Does the iPod's screen indicate that the iPod's battery is charging, when connected?
Usually, this means the iPod's battery is faulty and not able to hold a charge. If you have not already, be sure to try a Reset (restart) of the iPod
http://support.apple.com/kb/ht1320
You can also try doing a Restore on the iPod. The Restore button is on the iPod's Summary screen in iTunes. This will erase the iPod, reinstall its software, and set it to default settings.