D800e and cold temperatures

​I'll be travelling down to the Ross Ice shelf in Antartica next month and the latest temp' forecast is -20 to -25 C.Last time I went batteries were kept warm inside my clothing, but the major problem was misting of the optics when taking camera out, Nikon D90, worked ok though but battery life was well reduced.
Do any of you guys have any tips to share with me?
Is there a battery pack that I can keep inside my clothing with a trailing lead to plug into the camera?
At Vostok Station previously if I remember correctly, they're using Nikon D3 and Quantum battery packs which are kept inside their "wooly bears" (mid layer clothing)then connected by electrical harness.
Looking for something similar, but it doesn't seem to exist anywhere

The best way to prevent fogging is to acclimate the camera to the outside conditions before you use it. Placing the camera and lenses in the outside conditions in a zip lock bag, minus batteries, will help to prevent fogging. Keep the lens caps on. Here in FL, we go from a cool dry (air conditioned) environment to a hot, moist one and it results in instant fogging. Enroute to where I am going, I open the windows of the car and allow the camera and lenses to adjust to the outside environment before use goes a long way. I am not aware of any external battery packs for digital cameras

Acclimatize the camera as STM said. Have some good moisture absorbent towels with you. When you bring the camera inside, let it dry before turning it back on. I would just get spare batts if i were you. Not much more you can do i think. Just keep it dry. In winter, it can get down to -50c with wind and my cams have survived.

You can also keep dessicant bags inside the plastic ziplock bag when acclimating, especially from going from cold to warm again, which is more important to guard against condensation than from warm to cold. I'll be looking forward to some amazing pics.

Winter photography tips: Protect your gear in extreme cold from Adorama Learning Center