Why Nikon D40's Battery drains out very soon if it is inside the camera?

I was having a good time with my Nikon D40. Earlier days, if I charged the battery and put in the camera and left the camera unused (even the camera is on/off) it would not be completely drained even after a few days. But nowadays if do the same (charging the battery full, keeping it inside camera, leaving it) after one day the battery would be completely drained out! So now I'm keeping the battery separate. But I just need to know what caused the change.

The D40 is an old camera, and their batteries do tend to die with age. Time to at least buy a replacement battery.

I agree, pick up a couple of replacement batteries - you'll be back like you were.

D40 is a old camera. So its battery. Get it replaced.buy new.