You know what it's like. You're out and about with a smartphone whose battery was fully charged when you left home for the day - but next time you look you're down to less than 50%.

It might seem like a distinctly first world problem, but it's an issue nevertheless. After all when your phone dies and there's no recharger in sight it means you can't text, phone or catch Pokémon.
In fact, it's the sheer versatility of the modern smartphone, combined with its slim line design that creates the problem. With a number of apps running at the same time, not to mention data roaming plus the juice-draining calls you're making and texts you're sending, it uses more power than ever before.
Not surprisingly, phone manufacturers are not very keen about the obvious solution – to make the battery bigger and heavier – because you don't want to go back to the old days of having to carry a brick-shaped phone around with you. So they have worked with the operating systems designers to include some HTML5 script which tells any websites that are being browsed how much battery power you have left.
If the message shows that you're running low on power the website can then switch into a more economical mode to preserve your battery's power, for example by reducing or even eliminating images that need to be downloaded or stopping certain features of the site.
It sounds like an ingenious idea and a good solution to the problem – however researchers have discovered an unwanted side effect and have even published a research paper about it.

This is because it generates and transmits two key pieces of information. Firstly it tells the website the power left in the battery as a percentage and then the time left until that power runs out. When put together these provide enough unique identifiable information to identify a phone and, therefore, the person who is browsing the website. The concern is shared by

"As more and more technology and devices enter and become part of our lives, it becomes harder and harder to realise just how much of our privacy is compromised. Phones sharing battery charge is just the latest example of how information about us is being shared without our permission, and usually knowledge. Who knows what we will find out next, websites getting information about our photo library, or any app? This is the reason people need to become aware of how to control their privacy, as there are tools like VPNs who can really help with this."
As well as looking at the security issue the research team also came up with some suggestions about how to minimise the risk of data leaks. Firstly, they felt that user permission should be needed before the information can be shared, in common with other data protection procedures. Secondly, by making the information less precise by rounding up the percentage and time reported it would greatly reduce the number of possible combinations making individual users much harder to identify.