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 from the 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 not even on. So says a study presented at a meeting of the Association for Computing Machinery, in Portland, Ore., [Xiaomeng Chen et al, Smartphone Energy Drain in the Wild: Analysis and Implications]

Researchers eavesdropped on the activity of more than 1,500 Android phones—specifically, the Samsung 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 and weather, 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 other versions. 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 cellular paging: the pings from a cell tower every 1.28 seconds, that tell your phone if a call or message is on the way. Which is, ultimately, the price of being connected.

—Christopher Intagliata

[The above text is a transcript of this podcast.]