Mountaineering headlamp?

[Agro]

@Pavlo

In fact, to keep things even lighter, I would consider something like a Zebralight H600w or Armytek Wizard Pro Nichia and use it with a cold weather 18650 battery such as this Fenix which is rated down to –40 C/F.

The weight savings would be significant, and it would be a significantly smaller, simpler light with less that can go wrong.

I would just avoid the cheap 18650 headlamps that aren’t regulated or temperature controlled.

That Fenix uses Panasonic NCR18650F cell under the wrap. I suggest a look at the datasheet discharge curves at different temperatures:

NCR18650F PDF

What you can see is:

• that Panasonic rates it to –20C. Fenix lies about –40.
• at –20C the cell has very high internal resistance. Turning on a fully charged cell with dicharge current of 2.6A (which is quite a lot) drops the cell voltage below 3V. This would already trigger over-discharge protection of many flashlights. Does Fenix add a protection circuit? It may trigger as well. Then internal resistance warms up the cell and voltage recovers and the cell delivers 1800 mAh before dropping below 3V again. But:
  • 2.6A is a lot, a lower mode would work better
  • that’s with a fully charged cell, half-discharged will be unusable at –20C unless warmed up
  • that’s with a new cell, a worn one would probably work worse

I would still recommend a light with remote cell placed under clothes.