Jessica,
I have battery grips with two batteries in each of the Z7s. The camera automatically switches to the second battery when the first is exhausted. The only time I have had both batteries exhausted was when I shot over an hour of video of my ride on the flanger train. When shooting stills, I do not shoot both RAW and JPEG which uses more battery. I shoot only RAW and make JPEGs with Photoshop later.
Coldest I have experienced while shooting with the Z7 was 8F. I kept the spare batteries in a pocket next to my body. Didn't notice much slow down.