Admittedly you are talking camera batteries but one thing I have had happened to me several times when heading home after dark is for a snap discharge. I have the battery inside, in the warm environment, and then I take it outside and turn on the front or taillight. The taillight isn't as bad as the nickel metal hydride light is. I had one night back in January I left the library with a full charge on the battery for the headlight. I rode 5 minutes and the battery was dead. I knew something was screwy. I come to finally figure out by taking the battery from a warm environment into a cold environment and then turning it on to use it right away causes a fast discharge of the battery. I have much longer AAA battery life if I leave the light on the bike versus in the pack that I bring into the library with me. Anymore I always make sure to let the battery be out in the elements for a nice lengthy period of time before I try to use and I have no fast discharge occur anymore. The more you can leave the battery exposed to the cold the longer the battery life. At least that's what I have seen over the past couple of years.

Sweat, perspiration, whatever you call is also known as 'evaporative cooling'. It's not meant to keep you warm it's designed to cool the body down. Your best bet is to avoid sweat, perspiring, whatever you want to call it, in the first place. I can only imagine what you are going to think when you go to take the plastic bags off your feet at night and the cold air hits your wet feet. Sorry I'll take chilly feet while on the bike and instead I'll try my darnedest to keep the dry.