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　　Smartphone battery down to 10 percent?

　　Time to turn down the brightness on the screen,right?

　　It's a classic strategy to squeeze more juice fromthe battery.

　　And it works.

　　But that trick might be less effective than you think.

　　Because it turns out nearly half the battery drain on a phone happens when the screen's noteven on.

　　So says a study presented at a meeting of the Association for Computing Machinery, inPortland, Ore.,

　　Researchers eavesdropped on the activity of more than 1,500 Android phones—specifically, theSamsung Galaxy S3 and S4.

　　They used an app they developed called eStar Energy Saver.

　　The app logs the energy drain of the phone's apps and activities as they happen.

　　(And you can try out eStar Energy Saver yourself, if you have an Android.)

　　They found that some 45 percent of the battery drain happened while the screen was off.

　　A lot of that energy suck was apps updating in the background—downloading headlines andweather, while you're not actively using them.

　　And different versions of the same app require different energy supplies.

　　For instance, some versions of the Facebook app were twice as energy thirsty as otherversions.

　　So app version choice could slow your battery drain.

　　But one of the most hidden energy draws, zapping 12 percent of the battery, was just cellularpaging:

　　the pings from a cell tower every 1.28 seconds, that tell your phone if a call or message is onthe way.

　　Which is, ultimately, the price of being connected.