full charge battery icon is lit up when battery is not charged

Hi my 1st generation Ipod touch has been showing the full chaarged symbol after a minute of so of regular charging. my battery can be at twenty percent life and i will plug it in to be charged and after a minute or so the fully charged "plug" icon appears and battery is not charging anymore. happens with multiple cables and on two different computers using different usb ports. the only way i can get it to charge with relative consistency is by using my aftermarket car audio unit cord. any help would be appreciated!