It might not be playing Angry Birds that causes your battery life to drop precipitously -- it could be the ads that the game serves. According to a recent study by Microsoft and Purdue University, Android apps that feature ads significantly affect your phone's battery life.

Using an HTC Passion running Android 2.3 "Gingerbread," the researchers looked at five popular apps -- Angry Birds, MapQuest, FreeChess, The New York Times and the Android browser -- and measured how much the ads affected each app's overall battery usage. They found that processes related to advertisements -- uploading user statistics, tracking the user's location with GPS and serving the ad itself -- accounted for up to 75 percent of an app's battery usage.

To conduct the study, the researchers developed a custom app called EProf, which measures both the overall amount of energy that an app uses as well as how much energy specific specific processes consume. Using this tool, the team discovered that in any given app, the processes related to ads drain the most power.

When they tested Angry Birds, for instance, the researchers found that the uploading user data, tracking the user's location, and downloading advertisements caused 70 percent of the app's battery drain -- only 30 percent of the battery usage was due to the game itself. Similarly, 65-75 percent of FreeChess's battery usage was due to ads. The New York Times and the native browser fared better, with only 15 percent of the apps' battery usage caused by tracking and advertisements.

According to the study, much of this battery drain is unnecessary -- poor coding of the ad modules results in inefficiencies such as the "3G tail," a period in which the ad continues using the phone's 3G connection when it no longer needs to. The team argues that reducing these inefficiencies can make apps that serve ads much less power-hungry.

The team plans on making their custom app, EProf, available as an open-source tool. According to the Verge, a version of the tool for Windows Phone is on its way as well.