In the third of a five part series on the future of mobile phones, Roland Pease explores what drains a smartphone battery so quickly... and what can be done about it.

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.

Another drain are the so-called CSS files that tell a browser how to layout the page. Again, on the Apple site, these required 12 joules to download and render onto the screen. Simple improvements reduce this by 5 joules.

Each joule might seem a trivial amount, but like a slimmer on a diet lots of small savings soon add up – especially when some websites take eleven times as much energy to visit as others.

Electronic warfare

But website design is not the only drain on a phone’s batteries. The way we access these websites – and in particular the settings our service providers use - also has a huge effect, according to Hari Balakrishnan, Fujitsu Professor of Computer Science at the Massachusetts Institute of Technology (MIT).

Surprisingly, it is the network that decides what power setting our handsets use. When we need to transmit, the network tells our handsets to power up to maximum.  But the network’s interests are different from ours. We would like to power down again as soon possible to save our battery.

But only a slender portion of an already crowded radio spectrum is assigned for these so-called control messages, says Dr Balakrishnan. Because of that, the network tries to minimise the number of control signals it sends, even if that leaves our handset powering away while doing nothing.

 “A classic example,” says Dr Balakrishnan, “is when you’re running an application that wakes up every 20 or 30 seconds and sends one byte of information. That would, in many cases, keep your radio in the high-power, active mode all the time. But it doesn’t have to. That kind of trivial workload should consume next to no energy, but ends up using huge amounts of energy because of the sub-optimal controls implemented by the networks.”

Like Dr Nicoara, Dr Balakrishnan decided to test his suspicion by wiring his battery to a meter and watching the battery drain. The results showed that by letting the phone control its own settings, he could halve the power consumed by the wireless transmissions with minimal effect on the network performance (although he stresses the details change from device to device, and from network to network).

Again, these might seem small improvements overall – but a network that can give our phones a few more hours of working battery life is highly likely to attract more customers these days.

Of course, it wouldn't matter how long your phone was fired up to exchange bits with the base station, if it could do so with minimal energy. But it turns out the mobile industry has been fighting a war with its electronics for years because they are so inefficient.

Specifically, it is the purity of the radio wave that they use to connect to the base station that requires so much power.

With radio spectrum restricted (see the two previous articles in this series) transmission frequencies are squeezed in close together, and any distortion in the signal would lead to catastrophic interference. And it turns out that the basic physics of the amplifiers that drive the antennae can only generate those pure waves when drawing a lot of electrical current.

When the 3G standards were introduced, manufacturers turned to a special design of amplifier first concocted back in the 1930s, because of the stringent demands of the networks. But even then, for every watt of transmitted radio power, there's more than another watt simply warming your handset to create this pure wave. Basically, simple physics is doubling the drain rate on your battery.

“The industry is racing furiously to mitigate this problem,” says Joel Dawson, Professor of Power Engineering at MIT, who has himself come forward with a smart fix that involves electronically compensating for this distortion, something that could apparently halve the losses. But how soon any of those fixes will make it to the market remains to be seen.

Energy harvest

Of course these aren’t the only drains on a battery. Modern smartphones pack everything from small accelerometers to energy hungry global positioning systems. And then there is the display - the whole selling point of the smart phone is the interface that lets you access the world wherever you are. Until researchers can develop electronic-inks similar to these used in the Kindle e-book or other pixelated passive systems that refresh as fast as lit screens, simply viewing the phone comes at a substantial cost.

The obvious solution would be to pack in a bigger battery, but of all the components in your handset, the battery is the one that represents seriously mature technology. Most of the electrochemistry was laid down a hundred years ago and, as one battery manufacturer confessed to me, there's little they can do about. The introduction of the rechargeable lithium-ion battery in the 1990s was something of a step change. But since then … well, while virtually every other phone parameter doubles in a year or two, it takes a decade to double battery performance.

There are ongoing efforts to improve battery technology (See: Charging Tomorrow’s smartphones) but there may also be another route: harvesting energy from the environment.

Solar power is long-established, and has been tried many times with mobile phones, but has failed largely because of inconvenience. As the front of phones is crowded out with buttons, photovoltaic cells have largely been put on the back – but that has required turning a phone upside down to keep the battery charged.

But the form of the modern smart phone, freeing up the top face, has changed all that. And one potential solution is on display at this week’s Mobile World Congress in Barcelona this.

WYSIPS (“What You See is the Photovoltaic Surface”), as it is known, is a joke, according to Francis Robcis, sales director for the new product. “Because you can’t see it,” he explains.

And it needs to be like that, because WYSIPS has silicon solar cells embedded within the large smartphone display.

“Go into the office, and most people will leave their phones lying around with the face up,” says Robcis. So that when the set is idle, the tiny silicon strips (too small to see) are busy recharging the battery.

The challenge has been to integrate the photovoltaics into the display without dimming the image by more than 10 per cent – a reduction users should find acceptable. And to allow it to sit alongside the touchscreen function. If the system takes off, the future mobile screen could become a busy place.

The company hopes to have a pilot line producing screens by the end of the year, to prove the technology is viable.

Powering by sunlight, of course, is conventional, even if unfamiliar in mobile phones. At the University of Washington Seattle, ex-Intel researcher Joshua Smith is planning something much more radical: powering mobile phones - with gratifying circularity - by radio waves.

His lab, funded by tech giants Intel and Google, has already made great strides. “The largest TV towers in the US put out around a megawatt of energy,” Smith explains. “We’re able to operate our energy harvester as much as 30 km from one and get small but usable amounts of energy.”

In essence, harvesting radio energy requires rewiring the phone antenna to extract not the signal but the power of the carrier wave. And it’s not only powerful TV transmissions that you can tap into. Recently, Smith’s lab has been able to harvest energy from mobile phone masts.

The quantities available are down in the microwatt range, so not enough to power voice transmissions. But enough energy can be accumulated, Smith says, to send short texts, or just to stretch the battery life.

“It would be enough to have perpetual standby. My favourite example is driving along in my car. My son is playing Angry Birds on the cell phone, and completely drains the battery. And then I get a flat tyre. It would be nice if I could send an emergency SMS using this ambient energy. And of course, compared to solar power, you could even do it at night.”

All of which means that the daily ritual of recharging your phone could soon be a thing of the past.   

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