Battery Status API was first introduced in HTML5 and shipped in browsers including Firefox, Chrome, and Opera by August last year. The API is intended to allow site owners to see the percentage of battery life left on a laptop, tablet, or smartphone in an effort to deliver an energy-efficient version of their sites. Last year, researchers from Stanford University demonstrated a way to track users’ locations with up to 90 percent accuracy by measuring the battery usage of the phone over a certain time.
Source: https://thehackernews.com/2016/08/advertiser-location-tracker.html