Even under lab conditions, I doubt you'd get a good correlation between battery temperature and lab temperature if the phone were being used. Across an ensemble of phones and aggregating battery temperatures, you might expect to do better, but that would only prove this works in lab conditions.

These phones were sometimes indoors, sometimes outdoors, sometimes in bags, in conditions that are hard to replicate unless you have a very good model of average user behaviour.

We're working on ways to better detect the situation of the phone at the point of a reading, but even then it takes about 30minutes for battery temperature changes to take effect. If a phone was outside 15 minutes a go, the outdoors temperature will still have a large impact.

Modelling this or testing in lab conditions is not trivial!