Downloading free mobile apps such as mega-popular game Angry Birds could come at a price – by draining your handset's battery life.
New research carried out on Android and Windows phones showed free apps were more likely to zap energy from your mobile battery in order to power the advertising they run alongside the content.
Boffins at Purdue University in Indiana, US, found 65-75% of the energy used by each app could be eaten up through third-party ads alone.
Displaying offers and adverts within the free software is key for developers to ensure they make enough money to offer them to the public for nothing.
The researchers used a special energy profiler, nicknamed eprof, on six popular smartphone apps, including Angry Birds, which has itself been downloaded more than 50 million times on Android. They also looked at the Facebook app.
The report calls for developers to look much more carefully at optimising apps, stating: "Despite the incredible market penetration of smartphones and exponential growth of the app market, their utility has been and will remain severely limited by the battery life.
"As such, optimising the energy consumption of millions of smartphone apps is of critical importance."
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The findings showed that using the trialled apps over 3G for around 30 seconds drained up to 0.75% of a full battery - enough to discharge it entirely within a couple of hours if the app wasn't properly exited.
Nick Barnett, CEO of Mippin - apps developer for a range of big brands - believes smartphone owners are now used to the perils of battery drain.
He said: "Typically, 80-90% of a smartphone’s battery life is consumed by the display, the OS and the radio kits. Internet connected apps - with or without ads - such as social networking, news readers or maps, typically hold a constant or intermittent data connection and therefore will use more of the battery."
The research also found that a handset's camera and GPS takes a big toll on battery life when they are used within an app. Because these items aren't given a specific order to switch off they continue to drain energy in the background.
Apps expert Stuart Dredge, of TheAppside.com, said: "I don't think the new research will make many people change their app habits - it's just another incentive to always have a charger to hand.
"In some app categories like games, there are often two versions of the app: paid without ads, and free with ads. The latter is a good way for people to sample games, but if they upgrade, it's likely to be driven by the desire not to see ads, rather than concerns about battery life.
"If there's a challenge for mobile advertising companies in 2012, it's more around privacy than battery concerns. The question is how are apps tracking users and harvesting their data, and how is this being used to target ads? That should be more of a concern for smartphone owners, and thankfully it looks like they'll be getting more transparency from apps companies on this score."