I did some pretty extensive testing of Juice Defender on my Galaxy S3, and found it didn't make enough difference to be significant. You might get an extra hour or two using it. On the other hand, I have been testing BatteryGuru, also on the S3 (since I have weeks of usage data on it) and so far - no difference. That just makes sense. The biggest battery drain is always that display. There is nothing anything can do about that, except maybe dim the screen. Intermittent use stuff like mobile data does affect battery life, but to a far lesser extent. So reducing the use of those radios will make a slight difference, but far less than the people who make the battery saver utilities would like you to think. The effect of Juice Defender, for instance, was pretty much lost in the noise of my usage patterns.
So far with BatteryGuru on I am running about 3 1/2% an hour discharge rate. That's normal for me and is more or less in the middle of my data with and without Juice Defender. In the case of the JD test data, I had days where I used more than that both with and without JD running. I think the only way you will ever definitively find a difference with any software based solution would be under tightly controlled conditions - for instance, leave the phone on idle and in one place both with and without it and check battery after a specified interval to come up with discharge rates. Don't use the phone, don't even light it up. Just leave it idle.
Right now my S3 is running about 1% an hour without either Juice Defender or Battery Guru installed. That will change later in the day as I use it more. But I have come to think - based on data, not on "Hey, I think I'm getting better life" subjective feelings - battery saving utilities are a complete waste of money and machine cycles. If you want to make your battery last, don't play with your phone and hunt down and disable or find alternative apps for those which constantly use resources like radios to uselessly update themselves in the background - Facebook is a big resource hog like that, for example. Do you really, REALLY need to know what the weather is all the time? Or that your friend just went to a restaurant or to tell everyone you are "checked in" at the airport? It's OK if you do, but installing a battery saver isn't going to cure anything. Anything that lights up the phone is going to utterly swamp any savings you might get. And if you don't use your phone, what good is it? I just uninstalled the savers and wrote off the cost of JD as a lesson learned.
This is turning into a tirade, and that wasn't the intention. But just to continue for a minute - we have smartphones that we expect to do all these things, all the time. We wonder why our old flip phones and dumb phones lasted so long, and our smart phones don't. There is nothing inherent about smartphones that make them battery-poor. Rather it's our usage patterns. With a dumb phone you make a call or send a text. Done. With a smartphone you connect to mobile data, watch movies, facebook, call, text, look stuff up. Run web browsers. Listen to streaming music. ALL this stuff runs the radios and many run the display, and that burns electrons. What surprises ME is how long this little sliver of metal, glass and plastic runs on that teensy little battery! So, I am going to quit worrying about it. I want my sleek little smart phone and there is a price to pay for that. Until better battery technology comes along, we will have to live with it.
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