Actually the Nest is powered by 24VAC not 24VDC, so you would need a transformer such as a doorbell one that converts 110AC to 24VAC OR there are converters out there that convert 12VDC to 24VAC for CCTV cameras and I believe you would have to put that power in on the Common C wire of the Nest? I am still working on it but I will get it figured out soon enough
link: https://powerstream.com/inv-12dc-24vac.htm