According to a research paper published by four researchers, the websites you visit can easily track your location, and identify you by using your Android phone's battery level. The websites know about your Android phone's battery level with the help of the HTML 5 Battery Status API, which is used by Firefox, Chrome, and Opera as of now. The websites are provided with battery level information via the API so that they can disable the battery consuming elements as soon as the Android phone hits the low battery mark.
With this information, the website you are browsing gets to know the charge left till the phone discharges completely (in minutes), as well as the battery level left (in percentage) and these two values are updated every 30 seconds. After knowing these two numbers, websites can combine them to generate a potential ID number, which means your potential ID number will remain same for 30 seconds. If you visit any website in those 30 seconds, even via VPN or any other method, then still you may be identified because your potential ID number will still be same.
According to the researches, "Users who try to revisit a website with a new identity may use browsers’ private mode or clear cookies and other client side identifiers. When consecutive visits are made within a short interval, the website can link users’ new and old identities by exploiting battery level and charge/discharge times. The website can then reinstantiate users’ cookies and other client side identifiers, a method known as respawning."
However, the problem is not as serious as it might seems as not only is the time interval short, but the websites will have to go through a lot of combinations to determine your browsing habits and exploit it. We will keep you posted on this as further news come up, and in meanwhile, you can read the complete research paper here.