why does my nano discharge itself

Hi.

I have a 4th generation Nano ipod , i don't use it regularly , in fact it can be a couple of weeks sometimes between use.

The trouble is , that it seems to discharge itself whilst turned off , i always unplug the ear phones and always make doubly sure that the gadget is turned off.

Is this a common occurrence or is my battery trying to tell me something.

Thank you.

Ray.