The BB Priv is often accused for heating issues. And indeed IF it is used intensively, it WILL heat.
The peak I have in recorded history was 48.5°C eight months ago on 2017-08-13 (on MarshMallow)
This was while using for GPS in a car (aircon out of order) around noon a warm summer day.
The above images are crops from Gsam battery-monitor.
Note that the Priv has stopped charging when the temperature reached about 40°C. And this is quite clever, as both charging and discharging a battery generates heat, and this will -of course- add on to the heat generated by the electronics. It is obvious that the climb in temperature is decreased as it stops charging, and after a while it even falls.
So the BEST thing you can do if you know you are going to do intensive work (like using for sat-nav on a car-trip), is to charge it fully BEFORE starting, and preferably keep it plugged in, so it will have the electronics as the ‘only’ heat source.
I do recall that it got (even) hotter on Lollipop, but have not got any measurements that far back.
I got several posts on how to get a better battery stamina – and avoid heating without a good cause. E.g this
The nice graphs are from 3C Battery Monitor