Wondering why your phone keeps dying after a session or two of Angry Birds? One study says it's because you're too cheap to pay.

According to scientists at Purdue University who examined apps on Windows and Android-based phones, free applications only require about 10-30% of the energy they actually use. On the flip side, the scientists say that a whopping 65-75% of that energy goes toward powering third-party advertisements—the ones that keep those apps free. Buying the premium versions of games like Angry Birds could help alleviate this problem.

In other words, free apps aren't really all that free.

Free apps eat up your phone battery just sending ads [New Scientist, via Touch Arcade]