Ive had my SGS for 2 years now and I was always furious about the battery.
Would only last half a day on very light use (dictionary, wifi, and some kakao talk).
By the time I got home from work I would have to charge it before I went to bed so it would last through the night.
Then, at night it would drain about 30%.....while I was sleeping.
I am not too sure why it did this, but I am guessing it has something to do with the stock firmware and wifi (my phone would never sleep....battery stats said it was always awake).
Anyway, the other day, I was reading about Battery Calibration, so I gave it a go.
My phone lasts for 30+ hours now after using that app.....(I have also been using Nakyup ROM and Kernel).
Thing is, usually when I would charge my phone, it would be finished and I would unplug it, instantly the battery would drop down to about 92%.
What I do now is plug it back in and about 10 mins, unplug it and plug it back in again.
Keep doing this until when I unplug it, the battery stays at 100%.......