Free smart phone apps might seem like a deal. But they can have a hidden cost: your phone's battery life. That's because free apps often (serve) up ads, which can drain your battery more than using the app itself does.

Researchers built a tool called Energy Profiler, which breaks down how apps consume energy. Then they analyzed popular free Android apps like Angry Birds and Free Chess. Turns out both apps used only a quarter of their total (consumed) energy on game-play. The other three quarters went to advertising—things like user tracking, uploading user info and downloading ads. The findings were presented at the European Conference on Computer Systems. So, just as there’s no such thing as a free lunch, there are really no free apps.

The research team found a different (energy) leak when they did a Google search on the Android browser. The search happened in a blink. But the phone's 3G hardware stayed in high power mode for up to seven seconds afterward, eating up much more energy than the search itself. You may not be able to (avoid) that energy suck—the researchers say it's just how 3G networks behave.