Charging issues for Samsung Galaxy S2
I am using Samsung galaxy S2 and was facing the same issue. And, If you are using Internet package to your phone and not logged off from any of the social network apps and also gtalk, facebook, gmail, e-mail etc the battery will be low even after you have completely charged your phone. And, of course if your phone is almost 3 year old you will face such problems.

I would suggest you to kindly disable the data network mode when not in use and this will increase the battery life.... :-)

I know this is not the permanent solution, but you can avoid your phone turned off when not in use.