A couple of weeks ago, I have rented a car (Citroen). Coupled my Polar Cell Jolla to this USB outlet in the car.

After a while I touched the Jolla and it was much hotter than with my regular charger (mix of Nokia, Jolla, Blackberry left overs).

I suggest to beware of using "any charger" for reasons of thermal protection being suspected to be not totally adequate in the PolarCell case.