edit : battery drain *MYSTERY* UPDATE 06/02/2015

i have a Galaxy Grand Duos and i constantly faced battery drain issues. after MANY ideas to what the problem could be, (even by samsung service), i decided to root the phone and flash custom ROM. Tried some, problem persisted. 
Finally i installed CM 11.0 without any apps, not even GAPPS just to narrow down what is wrong.. 

After some time i can tell you that there is something like a (maybe hardware related) BUG that for no apparent reason KICKS OFF and starts draining the battery (30degree slope drop in battery application) CONSTANTLY. I mean no matter what i do, if the phone is sleeping or w/e, that is the LEAST background battery consumption. Of course if i also use the device, the battery drops more vertical in the graph for that period of time. 

Then, at some point, for no reason that i can spot until now, [maybe turning on and off the airplane mode, WiFi, Screen Rotation, or somth else] the BUG stops! And from that point forward (until it starts again) the battery drop is totally HORIZONTAL when I'm not using the device, as it is supposed to be! Note that this normal behavior can last for even 2-3 days.

What will be useful : Find what triggers the bug. Find a way to stop it if has triggered.

(*edit 11/2/15- i think i've narrowed it down to how i can turn off this "bug" after it kicks in. The way to do it, might be turning on and off airplane mode and WiFi for some times. The way to see if this worked, is to leave the phone at standby for some time (1-2hr) and check the battery graph. If the graph has become HORIZONTAL for that period of time, then voila i have stopped the drain)

Note #1 : please help..!
It isn't airplane mode.... airplane mode only stops certain hardware from running, making it physically impossible for that to be the reason

You probably have a bunch of other applications running when you go to sleep. but it will always say that Android OS used the most battery, because the operating system handles everything...

try installing cleanmaster or some other application cleaner, it will save a lot of battery.