You know those free apps that you prefer over the paid ones because they are, umm, free? Well, not only do they force you to look at mobile ads every second or two, but, according to a team of researchers, they also take a high toll on your Android smartphone’s battery. The researchers analysed how Android apps use the battery and concluded that the ad-serving processes that run in the background are responsible for heavy battery drainage.

The research, a joint effort between Purdue University and Microsoft, was conducted in order to answer the elusive question of exactly how energy is spent by apps. An energy profiler named Eprof was developed to measure the energy consumption of applications running on Android smartphones.

Testing five popular Android apps, which include the Android browser, Angry Birds, Free Chess, NYTimes, and Mapquest, the team found that the apps continue draining the battery, even after they are closed. What’s more, a 30-second run of an ad-supported app can drain 0.35 – 0.75% of a fully-charged battery, which is enough to completely discharge the battery within a couple of hours if the process is repeated.

The researchers concluded that most free Android apps are not optimized and are too complex. Each tested app was found to invoke between 29 and 37 threads within half-minute of utilization, many of the threads belonging to third-party modules. For example, 70% of the energy that that was consumed on one level of Angry Birds went to the uploading of user information metrics and displaying adverts, with only 30% consumed by the game itself.

The team said that the Eprof energy monitor will be made available under an open-source license, in order to help developers create apps that are more efficient and consume less energy.

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