Websites can track people acros the internet by checking your device battery life - and may even be hoping to panic you into spending money.
This is according to researchers at Princeton University in the US, who crawled across the top million websites to see what type of tracking they used.
The idea of "fingerprinting" a user via battery status was first mooted last year, the report notes. This time around, researcher Steve Engelhard and Arvind Narayanan found websites are actually using the technique, with scripts combining charge level status, time to recharge and more with other identifying features including IP address.
The data is leaked via an API released as part of HTML5 in 2015, which was created to let developers serve a version of an app or site that is less of a drain on batteries for users whose charge is running low.
The research also uncovered the use of audio, WebRTC (real-time communications) and fonts for "fingerprinting", alongside the battery API.
The report notes that existing tracking-protection tools - such as Ghostery - are effective at battling standard tracking techniques, they "are not effective at detecting these newer and more obscure finterprinting techniques."
Lukasz Olejnik was one of the researchers who looked into the idea of tracking via the battery status API last year. He said in a blog post that the research highlights how security and privacy flaws can arise in "seemingly innocuous mechanisms".
He pointed out that companies may already be "monetising" the information that your battery is running out. "When battery is running low, people might be prone to some - otherwise different - decisions," he noted in the post. "In such circumstances, users will agree to pay more for a service."
Reports suggest some companies are already considering the idea, with Uber's head of economic research saying earlier this year that customers of the taxi-hailing service were more likely to accept higher "surge" prices if their battery was low.
Uber said at the time that it does not do that, however. "And we absolutely don't use that to kind of like push you a higher surge price, but it's an interesting kind of psychological fact of human behaviour," said Keith Chen at the time.
Olejnik said some browsers are considering removing support for the battery readout APIs, while the Princeton researchers said they were examining the use of machine learning to better detect and classify trackers.
