As the mobile industry gathers for its annual phone-fest in Barcelona, the BBC takes a look at some of the key trends.

It is the curse of the modern age. After a day spent text messaging, emailing and browsing the web, your smartphone is dead. So now, after brushing your teeth and letting the dog out, there is another task: plugging in your phone.

But it never used to be like this. Simple handsets used to keep on going for days on end. So what changed? And more importantly, what is the mobile industry doing about it?

The big difference is of course the technology -we used to talk on 2G phones that ran on the old GSM networks. They could last for almost a week, partly because we asked so little of them. As detailed in the previous article in this series, smartphones became powerful computers because they needed to be to make strong, wireless connections to the network. And when they make those connections we ask them to do data-intensive tasks like stream video.

Every single step along the way involves information processing. That takes currents flowing around silicon chips; and currents running round silicon chips generate heat, wasting energy. PC and laptop users are constantly reminded about this by the whirr of the fans that keep their processors cool. We don’t notice it on phones, because everything they do seems to happen so silently, so effortlessly. But when you look at the detail, energy drips way like water from a leaky tap. And it all adds up.

Take the work of Dr Angela Nicoara, senior research scientist at the Deutsche Telekom Innovation Center in Silicon Valley. She has become something of a digital dietician, counting the joules that go into feeding our smartphones’ appetites. She recently laid out her findings in a report she enticingly called Who Killed My Battery?

In a collaboration with the group of Professor Dan Boneh at Stanford University, she connected a state of the art electronic meter to the battery of a smartphone, which typically store around 15-20,000 joules of energy, and then tracked the power for every millisecond it performed one of the most common applications for our handhelds: browsing the web.

What emerged when she broke down the actions millisecond by millisecond came as a surprise.

The first was that just hitting the “go” button and getting the phone to contact the network used 12 joules, as the phone exchanged details with the local base station, agreed transmission settings and so on. That’s before a single byte was downloaded. When that process begins, the real surprises come thick and fast.

It turns out that the design of the web page has a massive impact on the energy it takes to put it on your mobile screen. For example, the BBC News website, specifically designed for mobile users, took another 3 joules to download and process all the bits for a news story. For the worst performer of the websites she explored, Apple.com, it took another 34 joules – 11 times more than the BBC. Wikipedia came just behind.

Careless websites, she found, are at least contributors to killing her battery. For our PCs and laptops connected to the mains, these numbers don’t matter. With the constraints of a mobile battery they are critical.

The difference between these websites was in the way that they were built. For example, interactive elements that are used to customise a site experience and often run in the background of a site were a big drain. A half of those 34 joules needed to see the Apple page were down to these functions, without them even being touched; on Wikipedia, they alone cost 10 joules.