There is a much better solution that plugs into your car ciggy socket or is happy to run of a standard 3S LiPo / 12V NiMh pack etc....
Does not use the DJI battery it is keeping warm .. I can get 3 P3 batterys into the 'bag' ... I can set the temperature I want ....
Turnigy Programmable Lipo Battery Warmer Bag (12v DC)
Why waste your DJI charge on doing something that can be sorted by other means AND cheaper ...
Another idea I use : I have Picnic Cooler bags ... one is designed for a 6 pack of beer ... and a larger one for food / pop bottles etc. I keep them open inside the house to warm up ... put batterys in ... close bag ... put in car and go fly. I open only to get battery out and close immediately - leaving in the car. A used battery being warm - is placed into a separate bag but in the cooler bag to help keep temperature up.
Simple. Works.
I fly in temps that freeze monkeys **** !
Nigel