the apple storewill tell you, at least this is what they told me, discharge it. Then charge it up again... then when it says FULL chrage unplug the cable for like 30 seconds, then plug it back in so it says charging again. Do the unplugging/charging thing like 4-5 times.. the iPod will probably take a LONG time to say charged again.
This worked for me, they said don't trust the battery meter, even when it says fully charged it's not,