# 21700 Vs 26650 Batteries



## JAS (Dec 1, 2019)

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?


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## FlashKat (Dec 1, 2019)

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


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## HKJ (Dec 2, 2019)

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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## djkode (Dec 8, 2019)

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.


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## fivemega (Dec 8, 2019)

FlashKat said:


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


*ID of 21700 barrel is about 22mm while ID of 26650 barrel is about 27mm so difference is 5mm.
If 5mm makes it too bulky then 6P is too bulky than E2 because they have about 5mm OD difference too while they both use pair of primary 123.
Today 26650 has little more capacity than 21700 which is a plus.
It is absolutely possible to use wrapped 21700 battery in 26650 barrel while using 26650 battery in 21700 body is impossible.*


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## etc (Dec 9, 2019)

FlashKat said:


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



+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.


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## etc (Dec 9, 2019)

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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## RetroTechie (Dec 9, 2019)

etc said:


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


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.


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:


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## GraXXoR (Nov 18, 2020)

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.



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.


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## gifthunter (Nov 19, 2020)

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.


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## Lynx_Arc (Nov 19, 2020)

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


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## Swedpat (Nov 20, 2020)

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.


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## Katherine Alicia (Nov 20, 2020)

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.


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## Hooked on Fenix (Nov 20, 2020)

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.


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## Lynx_Arc (Nov 20, 2020)

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.


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## idleprocess (Nov 20, 2020)

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.


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## light-modder (Nov 21, 2020)

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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## Katherine Alicia (Nov 21, 2020)

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.




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*


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## busseguy (Nov 21, 2020)

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.





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


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## lumen aeternum (Nov 21, 2020)

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


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## idleprocess (Nov 21, 2020)

lumen aeternum said:


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



Google Images _delivers_.


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## orbital (Nov 21, 2020)

+

I have both & use my 26650s' alot,, but if I had 2 seconds to choose between the two, I'd take the 21700

: all my cell purchases in almost 10 years now, have been unprotected :


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## prof student (Nov 22, 2020)

orbital said:


> : all my cell purchases in almost 10 years now, have been unprotected :





Why is that?


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## orbital (Nov 22, 2020)

prof student said:


> Why is that?



+

My duration of use answers your question.


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## ChrisGarrett (Nov 22, 2020)

prof student said:


> Why is that?



People who have used li-ions for a number of years, who buy quality products and who don't run series lights, don't really have a need for cell protection circuitry, which is ultimately a point of failure.

Chris


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## Katherine Alicia (Nov 23, 2020)

Protection circuits are Great! and really necessary for some applications, such as when you`re modding a light and testing it out on the bench, or you run them in Ican lights that have no circuitry (just battery, bulb and switch) if you accidentaly left one on with out protection you have to throw your new battery away  or as mentioned running them in series (you really don`t want to reverse charge one of these!).
but other than that, you`re Much better off without them, and they`re more likely to fit your light as well! I have a bunch of protected 16340 batts that I can`t use because if I did manage to ram them into the tube, I`d never get them back out again! LOL


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## WalkIntoTheLight (Nov 23, 2020)

Yeah, I almost always use unprotected cells, even in lights that take 4 cells in series. I'm just careful not to run the light after it gives a low voltage warning (in case the cells aren't perfectly matched).

I have some protected cells, and I'm generally unimpressed with them. IMO, the circuitry just adds resistance, and serves very little practical purpose. They're double the price, too.

Get lights with built-in low-voltage-protection, and you never have to worry.


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## prof student (Nov 23, 2020)

orbital said:


> +
> 
> My duration of use answers your question.





Hate to burst your bubble (actually, I don't mind!!!! lol), but no it doesn't!


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## prof student (Nov 23, 2020)

ChrisGarrett said:


> People who have used li-ions for a number of years, who buy quality products and who don't run series lights, don't really have a need for cell protection circuitry, which is ultimately a point of failure.
> 
> Chris



Wellllllllllllll, I've never owned any, sooooooo...................


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## prof student (Nov 23, 2020)

WalkIntoTheLight said:


> I'm just careful not to run the light after it gives a low voltage warning (in case the cells aren't perfectly matched).



Well, I've never had any, so I have no idea about what the threshold is for running lights when battery is low, or at what point it would be considered too low & end up doing damage to the battery




WalkIntoTheLight said:


> I have some protected cells, and I'm generally unimpressed with them. IMO, the circuitry just adds resistance, and serves very little practical purpose. They're double the price, too.
> 
> Get lights with built-in low-voltage-protection, and you never have to worry.



So what is the difference between protected cells & cells that don't have that but do have built in low voltage protection?


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## lumen aeternum (Nov 24, 2020)

WalkIntoTheLight said:


> Get lights with built-in low-voltage-protection, and you never have to worry.



So a series light with low voltage protection in its circuitry will prevent a good battery from being ruined if the other battery fails with high resistance? And it will prevent the good battery from pushing voltage into the bad battery and heating it up and perhaps a catastrophic exothermal reaction?


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## orbital (Nov 24, 2020)

+

Every Tesla car has* a couple thousand 21700s' in it*, do you believe every one of those cells has a pcb?


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## idleprocess (Nov 24, 2020)

prof student said:


> Well, I've never had any, so I have no idea about what the threshold is for running lights when battery is low, or at what point it would be considered too low & end up doing damage to the battery


I believe that 2.5V open-circuit is considered the threshold for a li-ion that should absolutely be disposed of with <3.0V considered troublesome. I generally cycle mine into the charger around 3.6V since operating voltage at that point under any significant load will be considerably less.



prof student said:


> So what is the difference between protected cells & cells that don't have that but do have built in low voltage protection?


There is no such thing as a cell with built-in low voltage protection - that is the function of a protection circuit.

In general, a protection circuit performs 2 task at a minimum: under-voltage protection and over-voltage protection. Under-voltage prevents the cell from discharging itself below a danger threshold and over-voltage prevents a charger from over-charging the cell. Both of these were necessary for early applications shoehorning into A) flashlights that lacked any means of regulating their discharge voltage _(i.e. 2x123A designs that happened to be capable of running within single li-ion's 4.2V-3.0V range and still needed for incandescent lights)_ and B) dumb chargers that lacked the means to terminate the charge at 4.2V.

Additionally protection circuits can limit current _(sometimes needed for high-density cells with low max currents in the <3C range)_, and I believe that some will also cut power if cell temperature is too high.

Protection circuits are often eschewed by experienced users since A) they're something to go wrong, B) most LED flashlights of the last several years are built with LVP built in, C) chargers have been available for years with solid li-ion protocols and sensing capabilities baked-in, and D) they're generally not available for the high-current cells many high-performance flashlights demand.


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## idleprocess (Nov 24, 2020)

lumen aeternum said:


> So a series light with low voltage protection in its circuitry will prevent a good battery from being ruined if the other battery fails with high resistance? And it will prevent the good battery from pushing voltage into the bad battery and heating it up and perhaps a catastrophic exothermal reaction?



Short of taking _per-cell_ voltage readings - ala a li-ion BMS system - there's no reliable way to prevent a single cell in a series from going too low and being charged by other cell(s) in the stack. This is what power tool battery packs do - and a large part of why they suddenly cut off when the pack is old or near the bottom of its charge.

Given the ... inconvenience ... of inserting a contact between cells in a typical tubular battery compartment, using protected li-ion cells is recommended for series lights. Lights with LVP generally run on single cells, there there's no ambiguity about state of charge. Some lights might manage LVP on multiple removeable cells - likely using a flat parallel compartment with all cell interconnects made by integrated conductors that can run sense leads to the driver/BMS - but I'm not aware of any.

Insofar as how Tesla packs work, they place 10s to 100+ cells in parallel in a module then line up modules end-to-end to hit their desired voltage. The vehicle's BMS takes readings at the interconnects between modules but takes no notice of individual cells, _many_ of which can fail before a module's performance starts to suffer noticeably.


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## WalkIntoTheLight (Nov 24, 2020)

idleprocess said:


> I believe that 2.5V open-circuit is considered the threshold for a li-ion that should absolutely be disposed of with <3.0V considered troublesome.



It depends on the cell, but 2.5v is a good ballpark if you don't know otherwise.

But, for example, the spec sheet for the Samsung 30Q states that they can be recharged (slowly) even as low as 1.0v. So, read the spec sheet if you want specific safety tolerances.

I've also seen test reports that claim you can even recharge cells that have slight negative voltage (i.e., reverse-charged to below 0v). I certainly wouldn't recommend that, though!


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## idleprocess (Nov 24, 2020)

WalkIntoTheLight said:


> It depends on the cell, but 2.5v is a good ballpark if you don't know otherwise.
> 
> But, for example, the spec sheet for the Samsung 30Q states that they can be recharged (slowly) even as low as 1.0v. So, read the spec sheet if you want specific safety tolerances.
> 
> I've also seen test reports that claim you can even recharge cells that have slight negative voltage (i.e., reverse-charged to below 0v). I certainly wouldn't recommend that, though!



I've seen more than a few _cell harvesting_ pieces that are happy to recharge high-rate cells from 0V - braver souls than myself for sure.


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## Lemurian (Nov 24, 2020)

The upcoming FourSevens 26650 flashlight looks pretty neat.


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## lumen aeternum (Nov 25, 2020)

WalkIntoTheLight said:


> It depends on the cell, but 2.5v is a good ballpark if you don't know otherwise.
> 
> But, for example, the spec sheet for the Samsung 30Q states that they can be recharged (slowly) even as low as 1.0v. So, read the spec sheet if you want specific safety tolerances.



But you would need a "dumb" charger to do it, right? Don't almost all chargers "fail safe" below a minimum voltage?


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## lumen aeternum (Nov 25, 2020)

Lemurian said:


> The upcoming FourSevens 26650 flashlight looks pretty neat.



why? link?


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## GraXXoR (Dec 8, 2020)

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.



I can definitely see the 46800 being epic for camping lights where a long life is the #1 requirement. Their size is also fine for non-pocket uses.

as an aside, I still do think it's a bit of a shame, though, that 26650 never got the love that the 18650 and 21700 got... The size is perfect for 1P handheld arrangements...


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