Re: my battery drains even when I am not using my phone
Even if the battery is removed from the phone, it will drain just a bit. Lithuim batteries (and most batteries) have a small bit of self-discharge. If the phone is going to be put away for a long time (months), check it every month and top it up if it falls below 50%.
If the battery is in the phone and the phone is turned off, it's still drawing a little bit of current, so that it can recognize the power button press to turn on. (It's done by electronics in the phone, it's not an on-off switch.)
And, as belodion sayd, if the phone is just sitting on standby, it's going to be running Linux, Android OS and a few system apps. That's why phones have battery standby ratings. The phone doesn't draw much when it's sleeping, but it does draw something.