I've been using a Galaxy S3 the last several months and it's had pretty good battery life, until a few weeks ago. Right after the last update it started running it's battery down everyday, even when I wasn't using it all day. It's turned out to be a common problem for anyone who received that update, and it causes the phone to get pretty noticeably hot. I've seen recommendations for JuiceDefender on some Android forums so I thought it might be a good solution until a fix comes along. It's free in the Android Store, they have a paid version too with extra features but the free version works well enough for me. I was amazed by how well it worked, my battery went from being critical or dead within 8 hours of zero to very minimal use before installing it, to 95% power remaining after 12 hours under the same conditions with JD running. I don't notice any difference in my usage of the phone, everything still works as it's supposed to. But I no longer have to recharge my phone throughout the day to keep it running and it doesn't get hot to the touch just sitting there idle.
That from their website. And here's a Cnet article on it.JuiceDefender extends your battery life by intelligently and transparently managing the most battery-draining components of your Android device, like 3G/4G connectivity and WiFi.
It is ready to go right after installation: the preset modes are the perfect way to easily gain precious hours of battery life - it's literally as easy as one tap!
And if you want to go under the hood, you can tweak everything to your unique needs, because JuiceDefender is also fully customizable through a clean and intuitive user interface.
Once configured, JuiceDefender runs by itself, improving your battery life in a fully automated manner.