I was just about to give this post my seal of approval with a like, but then came that last sentence. I’ve seen the battery heater come on whilst D.C. charging at battery temperature just below 10°C. The charging was quite slow until the battery heated up which took maybe 20 minutes or more. AFAIK, this is the only situation in which it actually comes on. It definitely does not need to be down to -10 for it to come on when rapid charging.Sorry to say that is not the case. The battery heater does not come on while driving and even if it did the heafing process would waste more energy than what you'd gain from the reduced resistance in the battery. Moreover, the heater comes on (only while DC charging) at temps below -10.
Peter