Most smartphone owners know, or at least have an inkling, that heat isn't good for battery life. High temperatures will cause lithium ion batteries to degrade and lose capacity more quickly than usual, and a really hot day makes a smartphone feel like it's about to spontaneously combust in your hand. Turns out, the relationship between battery temperature and the surrounding air is even closer than we thought.
Over the course of six months studying cellular signal strength and battery life--poor reception is another major battery drain--OpenSignal discovered that battery life acted as a weirdly accurate thermometer. The correlation is pretty obvious in the graph below.
OpenSignal writes: "The temperature of the battery depends on several factors: whether it is indoors/outdoors/in a pocket; how intensively it is used, and its efficiency. In producing the above graph we did not attempt to correct for any of these, yet through all this noise, there is a signal: the outside ambient air temperature."
Of course, there's not a simple 1:1 ratio between battery temperature and outdoor temperature. OpenSignal applied an equation to their 40 million battery temperature readings taken from 150,000 Android phones to eliminate out the temperature influence of the owner. They also had to factor in the barrier created by a phone sitting inside a pocket instead of being completely exposed to the air. And other factors, like one person spending a day indoors with air conditioning, don't have a large cumulative effect.
OpenSignal writes "Although for each phone and user the heat produced in the battery and body varies, as does the degree of insulation between phone and outside, when averaged over a lot of phones these values are roughly constant."
So they built an Android app called WeatherSignal to crowdsource temperature. The more users WeatherSignal has, the closer it will be able to measure localized temperature. With enough users, WeatherSignal could easily provide more accurate temperature readings than we can get today.