When Flying electric in the cold, you need to keep the batteries warm.
In the photo, on the back of the starter bench I had a red cooler. No Beer, sorry [].
Thats what my batteries are in. I preheat the cooler with the batteries in it by placeing it on the floor of the van as I drive to the field.
I have hot air blowing under the dash directly into the cooler. The internal of the cooler ( or warming box ) for now is warmed up as well as the batteries. Then when I arrive at the field I drop in a couple of those hand warmer packs you can use inside your gloves when you do stuff outside in the winter.
One of the guys I fly with buys them in bulk because he raises snakes. Uses them when he transports them.
Anyway I get them from him for about .10 cents a piece. They last up to 7 hours. Home depot sells the same thing in pairs or boxes of 12 pairs.
A couple of them maintain the internal temp of the cooler. The batteries go into the plane at just above room temperature.