In preparation for the next time a task puts my head in a dark place, I got a cheap headband LED light:
Unlike most of the others you’ll find, this one has a pair of 18650 lithium cells in the box on the back of the headband:
Contrary to what you might think, the cells are in parallel, with shorting plates connecting the battery compartment terminals. This works well for perfectly matched cells, which is not what arrives in the package.
The 3200 mA·hr capacity claimed (in one line of the product description) doesn’t match the 2200 mA·hr capacity (claimed in another line and) printed on the cells. As expected, both claims far exceed the actual 1500 mA·hr measured capacity.
The 1 A load is somewhat more than the 800 mA I measured at full brightness, but makes for easy comparisons.
I think they put the cells in parallel to reach the claimed 4-6 hours of run time, but in practice the connection discharges the better cell to match the weaker one with no assurance of equal load sharing thereafter.
So I conjured an insulator from the Box o’ Retail Clamshells:
In the unlikely event my head must remain stuck in a dark spot for longer than one cell lasts, I can move the insulator to the dead cell and continue the mission. Charging alternate cells isn’t much of a burden, either.
For unknown reasons, the (anonymous) manufacturer soldered the LED package at a jaunty angle inside the frame:
The lens pulls in-and-out to zoom the focus. The tightest setting (all the way out) projects a bright tilted square out in front, which is somewhat unsettling.
The whole affair cost less than a pair of known-good 18650 cells from a reputable supplier, so ya get what ya get.
The Smell of Molten Projects in the Morning 
Honestly, I’d be terrified to wear that strapped to my forehead. I couldn’t possibly get it off quickly enough if it decided to burn.
The $2 unit at Harbor Freight is surprisingly not horrible once you hammer the battery contacts flat so it’ll accept a standard length AA and not whatever it came with that was 3-5 mm shorter. Also put the battery cover back on with some tape so it doesn’t pop open at an inconvenient moment. I guess together that pretty much means it’s junk. :-)
I have a $40 Black Diamond branded one that I consistently trust for a few days in the woods…
I have had an 18650 in a pocket with my keys. It got shorted and really warm but I could definitely feel it and get it out before there was any flames involved.
Not having the cells in parallel surely cuts the risk of “venting with flame” by at least 90%, right? [grin]
LOLs, I have that exact same light, right down to the tilted LED mounting.
I keep thinking I’ll rig it so the cells are in series with a little constant current LED driver. Maybe a tiny microcontroller that would blink once at 75%, twice at 50%, three times at 25% and maybe four times at 5%.
That way you could confidentiality run them down at full brightness, having notice when the cells need changing, and without worrying about when the switcher would die.
Maybe we should collaborate on the design and I’ll make a tiny PCB :-)