Girlfriends Galaxy S3 battery sucks balls!

[Greenman]

You could just pony up the $35 for a 7000mah battery. I just ordered one, they say it lasts for days.

 

[Binky]

Factory reset the phone. Try to use it for a day or two with ONLY the stock apps. Avoid re-installing lots of apps until you're happy with the battery life. The most likely cause (by far) is a bad app.

 

[StrangerGuy]

Constant autoswitching between 2G/3G due to poor signal kills battery life like crazy too.