What i am basically saying is that because Tesla is going to use 21700 does not mean 18650 is dying
Unless it catches on or Tesla sells to everyone and puts serious competitive pressure on other manufacturers to phase out 18650s they are in little danger.
Keep in mind we already have other form factors, 14500, 10180, 26650, 32650 etc. None of them decimated 18650.
Did tesla say why they wanted to go with space inefficient cylindrical batteries rather than prismatic (box style) cells?
You need space in between them no matter what for cooling and I’m guessing they just thought the construction technology had so many advancements to cylindircal cells there wouldn’t be much gain to making prismatic cells with some cooling array setup.
Oh I don’t think that they will go away at all but we may see the “big guys” slow production and development in that size particularly for high discharge cells. We have all sorts of different sizes but because the development has been behind the 18650, the 18650s still crush everything else in power density. Even the best 26650 is nowhere near two good 18650s even though they are the same total volume. This is across the board, we don’t have any 7000mah 26650s and we don’t have any 60A 26650s that sag less than 2 18650s at 30A.
By popular demand
I’m dreading this post already. The 18650 is the biggest Lithium-ion cell manufactured outside of China. I can’t find any in the last 5 years made outside China. This is why top end 26650’s act like the average Chinese 18650 on energy density, same technology. So Tesla engineers take out the slide rules and determine that a particular size gives them the very best edge. They don’t care about the laptops, they make cars. They don’t care about power tools, they make cars. They don’t care about FLASHLIGHTS, they make cars.
I’m guessing their engineers looked at cell density, shell thickness, heat removal, how many connections are needed. Things that engineers with clean slates dream of. I’m pretty sure none of the previous engineers looking at laptop and power tool design thought “What are the flashaholics going to do?”
Now what does a true flashaholic do? He says to himself “Now, what can I build with these?” Flashaholics take whats at hand and exploit it to the fullest.
If we assume 18650 is dead then buy a 26650 light and an adapter
18650 wont die so fast but they will get cheaper soon. demand is much less after tesla and panasonic opens the new plant wich double to total world capacity of Li ion cells.
Bort you missed the point. Have you bought a flashlight in the last 5-10 years. Let me tell you in 5-10 years if Tesla cells start to become common. Lights might be factory built to take advantage of that along with all the electronic and LED improvements. This will probably look like what happened when 18650’s and 26650’s became stand alone products and not just laptop salvage. The AA’s and CR123A’s are not dead the 18650 will not die over night but a new most manufactured cell size looks like it is just around the corner. Whats going to also happen is another choice on the market. I will make one prediction 26650’s are going to be a dead or very cheap product. If new technology goes into 21700 and the 26650 continues to be made with average Chinese technology. Energy density wise a 3500mah 18650= a 5130mah 21700 that’s pretty much a 26650.
Yet we still have alkaline AAs sold by the millions if not billions a year. You can get about a dozen different chemistries of AAs from NiCad, NiMH, Alkaline, lithium primary, lithium rechargeable, carbon zinc, zinc chloride, nickel zinc, alkaline manganese etc.
Just becasue Tesla is releasing a new form factor it does not mean 18650 is dead, otherwise why would we have so many battery sizes
Believe what you will, i doubt Tesla is ending 18650, they are just building a new size for their EVs.
They’re probably just rewrapped WhateverFire cells in drag. 
And even so, get a light that takes a 26650, slap on some of those rubberbandy things that “adapt” 18650s for use in 26650 tubes, and you’ll be able to use 21700s in 26650 tubes as well.
By this logic A 26650 should be 7300mAh.
Can you take a guess why not?
Tell us?
I wouldn’t be surprised if it took 5+ years for these cells to hit the open market. Tesla isn’t going to release them for general sale unless they know for sure that they can meet their own internal needs, and that may very well be never. We’ll probably have to wait until someone like Panasonic, Samsung, LG, or Sony start producing them in their own factories. And for that to happen they will need to see a demand for that cell either from Tesla not being able to produce enough in house or some other car maker or industry adopting that cell. The sales from those of us using them for flashlights probably don’t even show up as a blip on their radar.
Ok people… it’s time to start discarding 18650 flashlights. 
Spec sheet here:
Someone needs to get a pair of those to HJK for testing, STAT!
Clif Notes:
Rated capacity: 4000mAh
Minimum cap: 4050mAh
Typical cap: 4250mAh
Impressively little voltage sag under 15A discharge
~2800mAh capacity after 500 8A discharge cycles.
Edit: max dia = 20.35mm
I wonder how many lights out there have enough body material available such that a enterprising modder with a mini lathe could bore the body slightly to fit a 20700…
There is nothing special about them…
They are in between 18650 and 26650 size, and have between 18650 and 26650 capacity.
It’s just a different size battery.
Why bother making an entirely different flashlight just to fit a different battery if you already have two FAR more widespread options that are the same energy density?
It literally makes no sense….
“let’s make a new shoe size that is between 11 and 11.5, and that fits feet between size 11 and 11.5!”
The difference between 18650 and 26650 is just a few millimeters of diameter, and if you want a larger diameter body you go with 26650, if you want less then you go 18650.
There is no benefit to going in between.
If a new type of battery is to be introduced it needs to bring something better to the table than there already is.
Not “lets make something sized in between, that performs in between”
The 26650 is far from the same energy density as the 18650, I bet they are even worse then the 20700.
4000ma in a high discharge cell is 33% more then you get in a 18650, that is well worth taking note of.
26650 lights are ok but they are large and heavy, only suitable to larger format lights. The 20700 could fit in EDC lights.
The problem with the 26650 is that it has a much lower energy density than the 18650.
Now we have a 21700 with the same high energy density as a 18650 but with the potential to have greater discharge rate and capacity than the 18650 (probably on-par with the 26650) but in a much smaller package then the 26650.
Those are advantages which we flashlight dudes can work with to produce lights which are more powerful than those which run on 18650 but smaller than those which run on 26650.
Also, the 1 inch diameter (25.4mm) battery tubes we now use with 18650 can also fit the 21700 - just bore the hole a bit larger.
The only thing is the tailcap thread diameter area will need to be a few mm larger which is no big deal.
Generally speaking, I bet the lights which run on the 26650 now can also run on the 21700 with the same resulting performance.
If you used the same “better manufacturing” used for the tesla batteries then you would have 26650s with the same energy density as 18650s.
Maybe instead of trying to introduce something in the middle just improve what you already have??
But they are not and will not do that. They have had 26650’s for how many years? Yet not a single major manufacture has produced a 26650 yet. Thus I think it is safe to say they will not be doing so.
The 20700 should reach 26650 levels fairly easy with major brands working on them.