Apple made the chargers in such a way that it shuts down when the Apple device is fully charged. That way there's no danger of overcharging your product.
I have read that to prolong battery use it should be discharged completely once a month.
although i knew this but i dont understand the concept of this...If it stops charging after fulll charge, should'nt the battery then go down? Or has it got some recognizer software that it remembers taht after 5 or 10 % charge it starts charging again? [Hope this was understandable ]