had the very same problem after upgrading. i fixed it without going thru the complicated fixes. go to your battery monitor, check whats chewing more than 60% of your battery. its most likely the screen or media. if its the screen reduce the number of home screens, put on auto brightness and reduce the time it auto switches. if its media, unsync all your accounts,samsung hub,google+,chat etc. open the task bar and clear your ram. my s3 was undownloading bloatware like fifa2012,golf games. and you wont see the downloads on your download program. i uninstalled it. also uncheck the auto update of the OS itself on settings, about device.