has anyone used them when its cold, say -20? what is the coldest people have used them?
Power. The electronics should work... might work differently than posted in the various specs (ie sensitivity will be really HIGH for warm bodies).
But the Batteries = No Joy.
Even lithiums will lose the ability to produce power in relatively short order in sub-zero temps.
I'd think if you have an AC power source nearby... and plug a DC converter (6v, or whatever your cam requires).
Or, find an environmental way to generate power (windmill?)