I know a CR123a primary battery is the optimal choice in cold weather, but if I for some reason want to stick to rechargeables, is there a general difference (ceteris paribus) between a 1x18650 and 2xRCR123a configuration?
All thoughts welcome.
An interesting question, but not an easy one to answer, I'm afraid. As Chris says, data are thin on the ground.
However, I can give you an idea of what to look for, if you can track down the data sheets for whichever cells you're interested in.
As examples, here are some data sheets with graphs showing discharge characteristics at various temperatures for three well known Panasonic 18650 cells:
Be warned that the -GA datasheets are hosted by third parties, not Panasonic directly. You can find a list of cells for which information is directly available from Panasonic
here.
Look at the lower left of the page for the -A and -B datasheets, or page 3 of the -GA short version datasheet. Unfortunately, the -GA graph only covers -10°C to 25°C, where the other two cover -20°C to 40°C. Don't ask me what Panasonic were thinking there!
You'll notice that at the lowest temperatures, the cells have trouble getting started, because the cold slows down the electrochemical reactions so much. There's a large voltage drop at first, followed by a substantial recovery as the cells warm up due to power dissipated via internal resistance.
At -20°C, the -A cell drops to ~2.8V at first, so it has significantly more trouble than the -B cell, which only drops to ~3V. Given a choice between the two, you'd want the -B cell.
At -10°C, the -A cell drops to ~3.4V, the -B cell drops to ~3.5V, and the -GA cell drops to ~3.35V. The -GA cell is actually the poorest performer, which could be very significant, especially if those relative standings carry through to -20°C.
This is why it's so important to keep lights and cells as close to your body as possible until you use them. Nice and warm inside your jacket is vastly better than frozen in your car's glove box or at the bottom of your pack. You'll also want spare lights and cells; that's practically mandatory in extreme environments.
Another thing to bear in mind is that the cell voltage at low temperatures will only recover if you're drawing enough current to dissipate significant power via internal resistance. If you're running your light on a low mode, it may never heat up enough to recover.
I have no idea what capacity you'll get out of the cell then, because the data sheets don't cover that case. Those low temperature discharge tests are all performed at substantial currents.
You can also encounter other nasty problems. For example, if you have a light that cuts out at 2.9V to protect the cell from over-discharge, it wouldn't even turn on using a -A cell at -20°C, because the cell would drop straight down to ~2.8V. This also applies to protected cells, although protection circuits often tolerate lower voltages before they cut off.
You'd also be pushing your luck even with the -B cell. If the temperature dropped just a little further, or the cell happened to be getting a bit old, you wouldn't be getting any light that way either.
Although I normally like protected cells and lights with cell protection built in, this scenario is one where I'd seriously consider choosing unprotected cells and lights that drain them to the dregs. Of course, the trade off is that you then become solely responsible for maintaining Li-Ion cell safety.
As ever, it's always a good idea to test the performance of your equipment in safe conditions before you rely on it. Hint: a good household freezer should hit -18°C
On the subject of deciding between 18650 cells and RCR123 / 16340 cells, the first thing I'd say is that Chris is right: 16340s will cost you a lot of capacity, especially in an extremely cold environment. If you still want a two-cell approach, consider 18350s instead. You'll still lose a lot of capacity compared to the 18650, but it won't be quite as bad.
The one place a two-cell approach might pay off is if you want to use the light on a low mode where the cell won't generate enough internal heat to recover a decent working voltage. The extra voltage from two cells might be the difference between a light that works and a light that doesn't.
Again, this is something you would need to test for yourself.
Whichever way you jump, you'd definitely want to carry some primary cells - CR123As - as your last-ditch backup.
