The Guardian reports that when HTML5 was introduced it it included a battery status API that allowed website, "owners to see the percentage of battery life left in a device, as well as the time it will take to discharge or the time it will take to charge, if connected to a power source." That enables tracking online, not just its intended use to let website owners switch users to low-power versions of their sites and web apps. Now security researchers from Princeton University have proven that the battery status indicator is being used to track users. "Steve Engelhard and Arvind Narayanan found two tracking scripts that used the API to “fingerprint” a specific device, allowing them to continuously identify it across multiple contexts," The Guardian reported. Lukasz Olejnik was one of four researchers who first warned about the abuses possible with the battery status API in 2015. “Some companies may be analysing the possibility of monetising the access to battery levels,” he writes. “When battery is running low, people might be prone to some – otherwise different – decisions. In such circumstances, users will agree to pay more for a service.” And, any app that can track users locations is dangerous for many reasons.