https://www.theguardian.com/technology/2016/aug/02/battery-status-indicators-tracking-online
Read the reddit comments for how to disable the battery status API in your mobile browser
https://www.reddit.com/r/technology/comments/4vrlus/your_battery_status_is_being_used_to_track_you/
QuoteYou know your own battery life because your device tells you. There is absolutely no need to pass this info along to a third party.
In Firefox, pull up about:Config -- do a search on Battery and turn it to false. On Safari under the Mac and on iDevices, this API isn't even enabled: navigator.getBattery is not a function. (In 'navigator.getBattery()', 'navigator.getBattery' is undefined)
https://www.w3.org/TR/battery-status/ (https://www.w3.org/TR/battery-status/)
Even without knowing that this could be used for tracking, its a stupid idea to allow this information to be passed along to a third party. At least for web browsers being web browsers (I know Chrome likes to pretend it can be a desktop and a word processor and all that at the same time).