Tbone are you taking into account the thickness of the surface? Calculations are hard to nail, but understanding the equations can be useful. Anyway, for conduction/convection, which matters most at non-crazy temps, thickness matters. It's not just about the "different surfaces".
Yes, I will. The thickness of the surface is one part of the overall internal thermal resistance. The result is a reduced surface temperature = less cooling.
I still have to get a feeling of how important each effect is. I need to crunch some numbers to see if everything fits.
Well the result of a thick surface layer is less cooling. But the result of a thin enough one less cooling, but not necessarily measurably less.
You don't a need full calculation to figure the coating effect though.
It looks like the thermal conductivity of some bad epoxy is around 0.2 W/(m*K), or resistivity is 5 m*K /W or 500cm*K/W. The surface area of an S2 is what 100cm^2 maybe (3*pi*10, rough guess). So dividing, the linear resistivity is abobut 5K/(cm W). Ok, for an 0.1cm thick coating that's 0.5K/W or 0.5C/W if you prefer. 1mm seems pretty thick, and 0.5C/W seems pretty un-catastrophic.
That thermal conductivity was pulled from some random article but polyester is only 4 times worse and air is only 10 times worse. That means having 1mm of closed cell foam would only produce a 5C/W increase. Ok, that's quite a bit for a 10W light, but that's 1mm of (non-convective) air! Real coatings are probably 0.1mm of something 10 times more conductive.
I think someone just blew up about nothing.
EDIT.. forgot the per watt in the last setp. that's important of course: fixed.
I know in myth busters there's always the one guy with the chalkboard and someone else is like, hey.. lets' just go blow stuff up and see what happens. djozz blew this one up real well already.
Djozz I actually believe in freezing temps it would stay cool enough with atmosphere and hand cooling. Its not apples to apples. I have a x6 triple xpl v6 3d DD. And around 60-70F+ within a few mins it gets scortching on turbo with 30q/hg2/GA etc with good cells. But when its 20-30F outside. I can hold it on turbo and never have to set it down. Especially if I’m walking around or just a windy. I know it has more aluminum but talking a lot more heat then one emitter could put out on dd
I just hope you understand that under cold ambient temperatures your DD is far from your DD when it is warmer or hotter.
Li-ion simply cannot perform in freezing temps, it performs very bad.
You wont get any high current at all that is why everything is so much cooler.
I just hope this graph makes sense to people, even after the cell warms up still heavily under-performs.
I get what your saying. I usually use 30q or hg2. But if I’m using for hours at the park I will put what I have with me in it. but use a combo of all three. My light isn’t at freezing when its turned on. It stays inside my house which in winter averages 74-75degrees. Its not kept in the car or anything. And if I carry its in my pocket against my warm leg. So its not the same as pulling a light out the glove box at 20 degrees. I’m not saying the light is running cool. I’m saying the light is tolerable to not have to put down. The head is still hot. But the handle doesn’t get to scortching levels at around 20-30 degrees. I have thick calloused hands from shoveling, weedeating, manual labor daily. And then from barbells in the gym. it takes a lot to burn my hands. When I put a light down it’s hot.
Another good reason to like buck drivers.
@Flintrock, that was my inital feeling. The powder coat is very thin and the area (compared to the thickness) is big. If the surface of the other light would not be black but reflective a powder coat may have the better cooling by emission.
I will know more soon. Stay tuned.
Another old CPF post where I was suggesting how to assess the likely performance of different cells (mostly Li-Ion 18650s) at very low temperatures.
Thank you 
I’m basically too lazy to build all those nice test rigs you make!
This was a very informative read. Any idea how the BLF A6 would compare, with its FET +1 driver?
The XP-L led with FET+1 driver draws about 5A, so 1.66 times the heat in a comparable package. Still assuming that everything is lineair, the calculation is 20+70*1.66=136.2 degC. That is the pill temperature, right under the led will be hotter. A bit too close to unsoldering temperature of the led for comfort.
Did I not do this calculation already somewhere above in this thread? Edit: found it, post #44: How hot does an S2+ get at 2.75A? Comparing an anodised Convoy S2+ with a fluorescent yellow powdercoated Convoy S2+ - #44 by djozz
Yes, you did. But someone else mentioned that the release of heat from the body (convection, thermal radiation) wasn’t fully taken into account. I guess I am really wondering if you or anybody else actually tested it with a FET driver, or in a similar light like the BLF A6.
I know the result will be “freaking hot!”. I’m just curious if it really does get hot enough to fry any electronics or melt solder.
I see, may do that at some point. At this moment I’m behind with hobby-related things (I hate that) so it gets at the end of the list.
the amount of heat transferred to air and radiated as IR light is not linear to the temperature of the light
so with more heat generated from the LED you cant calculate this linear
At the moment I am waiting for my S2+ with XM-L2 U6-4C and 7135x8.
Are there any measurings for the heat transfer rate from the star/pill to the head on a stock flashlight?
What I am afraid of is that the head is below 60degC and I think everything is allright, while the LED gets too hot and solders out itself or something else.
Best regards
a DTP star with proper heatpaste has a pretty low thermal resistance to the pill,
or even if you solder it to the pill
the most critical part in heat transfer is the tiny thermal pad of the LED
You are right, but what kind of star uses convoy normally for the LEDs?
Are they good enough for this purpose?
The newest ‘tan’ S2+ from Simon came with a DTP star, but older S2+ Convoys have a non-DTP star which under normal use are fine up to 3A or even higher. But a test like in the OP might not be very healthy for the led on the non-DTP star.
Thanks djozz, that was the information I was searching for!