hey guys, there's something i wanna ask. i'm asking on behalf of my sister

Anyway, the problem is that her galaxy S2's battery somehow drains VERY VERY quickly.
For example, leaving the phone idle overnight for approx 8hours will cause the battery to drain from 100% to 70%.
(no wifi, 3g data, weather widgets set to update every 12hrs, and all that)
already hard restart the phone.
Even during normal daytime use, the battery drops like 1% every 5minutes.
This has been happening to her for a few weeks already.
On the battery graph, it doesn't show any weird wakelocks.
Instead it shows Android OS consuming the most amount of battery, at 80%.
Screen-on time is very minimal since it was left idle overnight.
I have BetterBatteryStats and it doesn't show anything out of the ordinairy.
Could it be an OS problem? or the battery is dead?
According to my sister, the problem only came up after the update to ICS.
She has had her phone since it came out last year

Has anyone faced this problem before?
Thanks in advance for your replies

(using everything stock. no custom ROM/kernels nor root either)
This post has been edited by YouDontKnowMyName: Oct 19 2012, 01:27 PM