21700 Vs 26650 Batteries

I have been considering a new, single cell, flashlight that uses something a bit bigger than the 18650. I have been "on the fence" between 21700 and 26650 batteries. Are most of the currently available 21700 and 26650 pretty similar in their capacity?

For example, the Klarus 21GT-50 lists 5000 mAh. The Klarus 26650 also lists 5000 mAh.

The Nitecore NL2650DW is 5000 mAh. The Nitecore NL2150 is 5000 mAh.

Considering that some typical 21700 batteries are at 5000 mAh and some typical 26650 batteries are, too, is it pretty much "a horse apiece" going with either a 21700 light or a 26650 light?

I am going with 21700 lights as the 26650 lights are too bulky.

26650 batteries are old technology, generally they are not updated to the latest. The 21700 is the hot size that get all the latest chemistries, it do also have smaller volume than 26650.

I would go with 21700 - the flashlight will be less bulky and this battery format will become far more prevalent in the coming years. This is the battery format that Tesla has moved to (they call it 21-70), so I suspect that we will eventually see many of these cells on the market in future and the 21700 should ultimately be cheaper than the 26650 for the same capacity.

FlashKat said:

I am going with 21700 lights as the 26650 lights are too bulky.

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+1

21700 is just right sized. 26650 is the size of the C battery. Just look at a 2C Maglite and you will get an idea. Not a great EDC device.

PLUS Tesla is cranking out a gazillion 21700, which will drastically impact the market.

Most users here, I suspect are using 18650s or 123s at times and we are talking about 16-18mm size.

going from 18mm to 27mm - and that's just the battery diameter. 11mm difference. The outer shell of the actual light is even larger.

I wouldn't EDC anything 27mm for the same reason I would not EDC a C-based Maglite or any other light.

etc said:

going from 18mm to 27mm - and that's just the battery diameter. 11mm difference.

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Choice here is between 21700 and 26650. Since you need some tolerance, that's like 22 vs. 27 mm inner tube diameter. Say you have a smooth battery tube of 1 mm. thickness, then it's 24 vs. 29 mm outer diameter. In terms of feel, probably still a significant difference. But not that much in overall size.

The outer shell of the actual light is even larger.

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Not necessarily. For a light based on bigger cell it's easier to make a powerful head that has same diameter as battery tube. Also bigger light = bigger heatsink.

I suspect improvements in 18650 or 21700 batteries will be 'backported' to sizes like 26650 at some point. But unless that size finds use in similar applications, technology wise it will always be behind the curve. One advantage of 26650 format is flexibility: in a battery tube of ~22 mm diameter you can fit a 3x AAA carrier. But a 26650 based light may be able to fit a 3x AA carrier. And if it takes 70 mm. long cells, takes 21700 as well using a sleeve.

Some (most? ) folks on here don't blink an eye to take a huge 8x 18650 monster out to play. So I guess it comes down to intended use by the topic starter: if light is meant as EDC intended to go into a coat pocket, battery cost is important, or you prefer not to mess with battery size adapters, I'd say go with 21700 format. If light is intended to go into a car trunk / mostly used in garage / backyard, or battery flexibility is more important, then I'd say go with 26650 format and put big honking cell(s) into a big honking light. :laughing:

etc said:

Most users here, I suspect are using 18650s or 123s at times and we are talking about 16-18mm size.

going from 18mm to 27mm - and that's just the battery diameter. 11mm difference. The outer shell of the actual light is even larger.

I wouldn't EDC anything 27mm for the same reason I would not EDC a C-based Maglite or any other light.

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thread necro 2020...

tesla is moving away from 21700 to 46800 (4680).

do you think this is going to mean 2170 won't become the dominant formula!

since when has 27mm-18mm ever equalled 11mm? That's just wrong.

Cheers.

21700 is the new hotness because of the auto industry. The 26650 will always be specialty. The 21700 is about to become one of the most widely-produced batteries on the planet. That translates into lower costs and more choices.

21700s are now showing up in 20v lithium tool battery packs now generally 4Ah though.

I understand the popularity of 21700 size.
It has significantly more capacity than 18650 while almost as much as 26650(yes the highest capacity same as average 26650). So far I have only one 21700 light: Thrunite T2 and the size is good in the hand. I still think it's a bit too large and heavy for a true EDC and here I consider 18650 the upper limit.
And for a light with large reflector and the head much wider than the body I find no reason not to use 26650, especially if you can use Keeppower 6000mAh cells.
26650 body is perfect in the hand, in my opinion.

I`m with Swedpat too, 18650 or less is great for EDC, 21700 is more of a Purse or Coat pocket light (a lot of Pocket Rockets come in 21700 size), and I find that 26650 tend to be for more specialised use, for instance I have 4 26650 lights, one`s good down to 100m underwater, the other is a Thrower (ft03), one`s a LEP and the other can light up a whole room really bright (1400lm) for ages and not get warm! non of these are EDC material though.

21700 is the cell with most money invested right now for research and development. Follow the money for the best batteries. You'll get the cheapest and highest capacity cells. 26650 cells however will fit in some 18650 chargers where 21700 cells won't. I'd go with the 21700 personally for a flashlight. For a headlight, I prefer nothing larger than 18650.

I think that 18650s and 21700s will both improve about the same as both are useful sizes one is good when you need smaller size with the most power and the other when you just need a little more power in a single cell. I see a large market in tool batteries and power banks and some rechargeable lights and lanterns for both cell sizes in the future. Tool batteries need both sizes with 18650 being entry size 1 and 2 layer packs and 21700 for larger tools that need more power.

As far as I know, Tesla still makes the Model S/X with 18650 cells, thus it's likely they will continue making the Model 3/Y with 21700 cells for some time. Throw in the power tool OEMs transitioning to 21700 and I'd expect the formfactor to see some additional development over the next few years much like the 18650.

I do not believe that the 46800 cell has manifest itself in a Tesla product yet. Given that 46mm D x 80mm L is larger than a D cell (33.2mm x 61.5mm), that's going to make for a rather chonky flashlight body relative to the 18650 lights that are rather common these days although two end-to-end would bear some resemblance to the 2D maglite formfactor that was common ~20 years ago. With ~5.4x the volume, one might expect those cells to clock in at 26Ah!

EDIT:

I ran new metrics using average values for the best 18650/21700/26650 density/power cells I'm aware of / could find:


I used values from separate cells for peak Ah and peak I. The Synthetic C value, thus, is mostly for the sake of having something to work against on the back of this napkin.

My sense is that capacity (Ah) should scale in a mostly linear fashion assuming similarly efficient construction relative to existing cells.

Peak current for any possible high-performance version of this formfactor is another thing altogether - I doubt that a single cell will tolerate pushing >200A - especially when its surface area:volume ratio is but ~44% that of the 18650. Thus while the naïve handwaved metric suggests 8.5C is possible, I imagine that <4C is more realistic while still providing a heck of a lot of angry pixies.

Of course, Tesla has their own Secret Sauce™ for their specific cells and is apt to be interested almost exclusively in energy density since their designs throw massive parallelism at the problem which provides more than enough current to accelerate an EV at a satisfactory pace. Whether the rest of the industry picks up on the formfactor is very much an open question.

Sounds like those 46800 cells would be the bees knees for larger lights. I'd love a modern take on a 2D mag. Maybe a bigger head though.

light-modder said:

Sounds like those 46800 cells would be the bees knees for larger lights. I'd love a modern take on a 2D mag. Maybe a bigger head though.

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yeah! a pair of those batts and an XHP90.2 in the head would rock!

someone would have to design a new charger for these though *looks over at XStar and flutters eyelashes*

HKJ said:

26650 batteries are old technology, generally they are not updated to the latest. The 21700 is the hot size that get all the latest chemistries, it do also have smaller volume than 26650.

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Thank you for the info. I'm not real familiar with these style batteries yet and just assumed the 26650's had higher capacity than the 21700 due to the larger number. I guess I got a lot to learn. LoL

I'd like to see a side by side & top down photo of each battery, to understand their relative sizes.