I have yet to test and record how long my camera batteries actually do last in extremely cold weather. My old Kaiser Baas X300 would last 30 minutes but 45 to 50 minutes in ideal warm temperatures.
My newer better cameras have batteries that last 1hr 45 minutes in warm weather. The thought of running charging cables all around my helmet and rear rack plus a method to secure the two power banks on both just doesn't appeal to me.
I have enough spare batteries for a six hour ride in warm weather. So if cold weather cuts that down to three hours, I should be ok too.