"Android phones can be tracked without using their GPS or wi-fi data by studying their power use over time, a study has found." http://www.bbc.com/news/technology-31587621
I think battery power can also be used by website to track your browser via new battery status API. I disabled it from about:config as well as other realetively new APIs which may be used to track such as netinfo, resource timing, performance, idle observer etc. I can't stop sigh, maybe they think those craps make web faster and more rich, but I can live w/out them at all.