Right now Tesla build their packs with thousands of 18650 batteries. But they want to switch to 2170 to increase density. That’s an odd size cell. Why not go further to the more common 26650?
Currently only the Chinese make a 26650 cells. Since Tesla is making their own cell they get to set the specs. Chances of these cells finding themselves in power tools, lap tops or flashlights any time soon to be slim to none. If wide spread use does happen then smaller 21mm cells will make for thinner products. Tesla wants to make several billions cells that would put any size they pick as a new standard.
Optimal ratio of battery pack density to capacity.
Probably too much empty space in the pack with the larger 26650 size.
26650 may have more energy per cell, but there will be more unused space between as the curves grow larger.
EDIT: found some good info on the topic here
https://endless-sphere.com/forums/viewtopic.php?f=14&t=72208
So basically they expect the extra efficiency will be perfect enough to forget the notion of supplying consumer demand for the old standards. They won’t be able to sell batteries for existing products, but that is ok with them. Cool.
Tesla builds the cells for it’s cars - it couldn’t care less about preexisting standards. Not that other major 18650 manufacturers particularly care about flashlight market - some of them (like Sony) even actively fight against sales of their 18650 cells to consumers.
It is about sheer numbers. Just looked it up under “Forum Categories”: BLF has 18,419 users.
I have 40+ lights of different sizes, and 21 batteries size 18650. If all BLF members are the same, together we own 386,799 batteries. A 85 kWh Tesla battery-pack has 7,104 cells. So combined we own as many cells as 54.4 Tesla cars. Tesla sold 14,820 cars in 2016Q1. We are quite a large community, but flashaholics are no big players on the battery market. And, as stated above, when you make them yourself, you can do your own specific optimalisation on capacity, heat production and heat dissipation, et cetera.
Thank you, all are very good answers and links! 
Over 500 million 18650 cells were sold in 2010. Let’s not consider that trivial. And that’s a long time ago. Dewalt didn’t even introduce the lithium pack until 2013. I sure had never seen a 18650 in the year 2010. God knows what the sales are today.
Sold to whom? Companies like Tesla?
Tesla plans to sell 500,000 cars per year by the end of this decade. That is 3,500 million cells. Plus their domestic housing storage plans. That’s just one company. There are more (car)manufacturers. Think big 
One added bonus for Tesla is you will only be able to buy standard replacement packs from them :money_mouth_face:
Cheers David
From my own experience with 26650 and 18650 batteries, I found the 26650 battery is more prone to shock than a smaller 18650.
I have dropped 26650 batteries on a soft wood floor and wrappers cracked and need to be re-wrapped. 18650 batteries when I dropped had no problem.
I am very careful now not to drop batteries.
IIRC, the large cell diameter you go, the harder it’s to do internal electrical connections or something like this. This is the reason why 26650 of the same generation have only 1,5 times better characteristics while being twice the size of 18650.
For some reason I got a feeling this article reeks of fanboyism.
You get twice the capacity with a 26650 over a 18650. Highest capacity Chinese 18650 cell is about 2600 mah, 5200-5300 for 26650. Repeat only the Chinese make 26650 cells. Panasonic, Samsung, Sanyo and LG don’t and their are the ones with cells above 2600 mah capacity in 18650’s Now if they can develop or steal enough expertise than maybe we’ll get 7000 mah 26650’s. I personally prefer 26650’s over 18650’s in single cell lights.
The best 26650 isn’t twice the capacity of the best 18650 and typically only one and a half times the capacity but slightly more than twice the volume and can’t be packed in the same space even if it was equal in power/volume ratio. Capacity/volume seems to rise approaching 18650 and declines above that but how far above or below that I don’t know. Maybe they’ve tested various sizes for capacity/volume and found a scosh more helps. If a 5% gain in capacity can eliminate 5% of the cells with each cell costing only 1% more then it would make sense. I’m sure they looked at it from every angle imaginable.
Quoted from another forum:
“It is all about optimization and finding the best ratio between cost, energy, dimension.”
Tesla needs money.
Talking about a new standard (with easy to understand pics like the one below) and his they want to sell 500.000 cars sounds aot better then not having a pitch like this.
Sure a bit bigger means more powahh and that is also good for the range is an issue for EVs.
And well who wouldn’t trust them now with their dream of selling 500.000 cars?
They serve the more high end market just getting to the lower end of the market. And it’s not like known and huge companies like Volkswagen, BMW, Renault/Nissan, Chevrolet/Opel are making things in this spectrum of the market right?
And seeing the lonely reign in the higher segment sure they have no problem selling all the cars they want because Mercedes, Daimler, Rolls Royce, BMW, Porsche or Lexus will not bother to give them competition right?
And besides the traditional car powerhouses it is not like Chinese cars have gone from dangerous Ly failing safety test to doing pretty decent, right?
Oh wait…hmm…well good they can show their 10% bigger, 33% more powerful and 25% cheaper cells then so journalists and investors have something else to focus on then financial troubles and competing cars
I think market will quickly gravitate toward this new 21700 size. Tesla isn’t even the first to settle on this size, Samsung started production of 21700 cells for E-bike market some time ago and Panasonic is already producing NCR20700A and NCR20700B. The primary market for these cells, just like for 18650 cells, is electric cars, powerwalls, E-bikes, power tools, etc. which all use battery packs, not individual cells. Flashlight market, where individual cells are used, is a very small part of it, but I think it will also benefit from this new format. After all, there are quite a few existing flashlights which can use this cell right now: many lights accept 70mm long cells because protected 18650 and 26650 are often 70mm, and any light designed to accept 26650 or 3xAA holder in place of 18650 is wide enough to accept 21mm cell.