2170 Format (aka 'Tesla style') cells coming in a 21 Amphour battery pack

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scrambler

Active Member
It would be nice to know what 21700 cells are being used, because last time I compared the top 18650 with the top 21700, I could not find a big energy density gain per volume or weight (they do have other advantages).

Below were my calculations, feel free to point out any error on my part.

The highest capacity 21700 I could find was the Samsung 50E and the highest capacity 18650 was the Samsung 35E.
Ans so I calculated the following.

18650 Samsung 35E:
Dia 18.55mm x Length 65.25mm = Volume of 17,634 mm3
Weight 50g
Capacity 3.5Ah => 0.000198 Ah/mm3 and 0.07 Ah/g

Samsung 50E:
Dia 21.1mm x Length 70.7 mm = volume of 24,722 mm3
Weight 69g
Capacity 5Ah => 0.000202 Ah/mm3 and 0.0725 Ah/g

That is only 2% more Ah per volume and 3.5% more Ah per weight for the 21700 vs the 18650

So right now, I am not seeing a huge gain in either battery size or battery weight.

Am I missing something obvious here?
 
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CityExplorer

Well-Known Member
Energy is measured in Wh, not Ah. Wh should be obtained from the MF or a proper calculation and not the multiplication of two approximate numbers. Not saying it will make a difference in your conclusion, but it will at least make your explanation more valid. There is a lot that goes on in building a proper battery, especially one that is discharged at a high wattage. some of these factors may not be significant for some batteries or use cases. The Energy of a battery needs to be considered in its particular application if the operating parameters fall outside of the conditions used to establish the nominal capacity of the battery as reported my the manufacturer. The battery energy is not simple N*Ecell, unless conditions were identical in establishing Ecell, which is not typically done since they do not operate in isolated environments.
 

scrambler

Active Member
The two cells have the same Voltage, so the Wh will be directly proportional and wont affect the energy density calculation in any significant manner.

As I said, I agree that there are other aspects of a battery that may benefit from the 21700 vs 18650, but so far it wont make the battery any significantly smaller or lighter.
 

CityExplorer

Well-Known Member
The two cells have the same Voltage, so the Wh will be directly proportional and wont affect the energy density calculation in any significant manner.
This is incorrect because as I said voltage is not a single number and not a constant. if you lookup discharge curves you will see this. we simplify things with single numbers on what is reported as nominal capacity. the output wattage at a specific discharge rate will vary for different cells even of the same type 18650 and rated for the same Ah capacity. That is why there are OK, good, and great cells. However that classification depends on matching the cell to the correct application. it's not just about Ah.
 

CityExplorer

Well-Known Member
.... but so far it wont make the battery any significantly smaller or lighter.
It certainly can. In a given space you design your battery for a give Wh, voltage (min/max/average), and maximum discharge current. To align all those parameters it may be that you can meet those parameters with less cells of one size vs another because the parameters related to discharge maybe different given the cells, even when the cells use similar construction such as typical 21700 and 18650, because some goals could be met with 2 21700 vs 3 18650, and one has to consider the total area which includes empty space as well as other structural elements that may be required.

That is not to say it always does for a given design, but it is certainly possible.

A simpler way to look at it is some batteries could be made with a single 21700 that could not be made with a single 18650, but would require 2. So the single 21700 battery pack could be smaller over all. Sure if you could probably make a bigger pack that might meet or exceed the specification with 18650 cells, maybe even all specifications, but it would not fit in the same space.
 

scrambler

Active Member
I will be happy and interested to be presented with a concrete case of a 21700 based Ebike battery that would have a significantly (15% or more) smaller or lighter battery with the same Wh than an 18650 based one, or significantly more Wh for the same weight or size :)
 

Johnny

Well-Known Member
I will be happy and interested to be presented with a concrete case of a 21700 based Ebike battery that would have a significantly (15% or more) smaller or lighter battery with the same Wh than an 18650 based one, or significantly more Wh for the same weight or size :)
You are right. That %20 number seems to be based on Musk's claim that these cells will be 6000mah but so far they have been 5000mah.

@Ravi Kempaiah is the Battery expert, I think he may have good insight on this.
 

Stefan Mikes

Well-Known Member
Region
Europe
City
Brwinów (PL)
A question to @scrambler and @CityExplorer:

Take the series of Specialized UI (SBC) batteries. Available as 460, 500, and 604 Wh version, same form factor.
Take the Giant EnergyPak Smart batteries: 500 or 625 Wh, same form factor.
All these batteries are 36 V nominal.

(Or, Bosch. For long long time they used to offer 500 Wh batteries then out of sudden 625 Wh Bosch batteries appeared in the market.)

How comes? How could Specialized or Giant achieve completely different battery capacities while maintaining the same battery dimensions? Isn't it possible these manufacturers used the same cell structure/architecture but different type of cells? Just asking.

Are you sure it is 4p not 5p?
Not sure of anything @Johnny. Just thinking the easiest way to keep the same battery dimensions and similar internal design would be just replacing 18650s with 21700s as the latter have bigger energy density. Note: These large batteries are insanely expensive.
 
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scrambler

Active Member
The fact they share the same box and mount does not say anything about the actual size of the battery pack.
The number of cells is what tells you the actual size of the battery pack.
Also depending if you want more power or more capacity, they may use cells with different Ah capacity, so two packs with the same number of cells could have a different Wh capacity.
What we were discussing is that for a given battery need (power & capacity), 18650 and 21700 cells will not have a very different energy density (today).
So if you want to build the same battery (same specificity) , one with 18650 cell and one with 21700 cells, you are not going to gain significant energy density with the later. You may however be able to better optimize other aspects of the battery using the 21700 cells.

But if you have a fixed volume, you wont get much more Wh by using the best 21700 for the job instead of the best 18650 for the same job.

I think the main reason they don't yet live to the initial hype of increased energy density, is the fact the 18650 technology has been refined for years now so we can get cells that are pushing the envelope of what the 18650 form factor can deliver. The 21700 are still fairly recent and not widely used yet, so less development and optimization has taken place and we are not seeing the full energy density they may eventually achieve.
 

Stefan Mikes

Well-Known Member
Region
Europe
City
Brwinów (PL)
So you think Scrambler these manufacturers just use 18650 cells but of different capacity for different batteries?
 

Johnny

Well-Known Member
625 Wh Bosch batteries appeared
Still 18650 cells, just a larger pack, that's why it doesn't fit the bikes designed for 500wh.

easiest way to keep the same battery dimensions and similar internal design would be just replacing 18650s with 21700s as the latter have bigger

21700 cells are larger you can not replace 18650 with 21700 unless you also change the case dimensions.

Bosch 400 and 500 are pretty much the same pack with different capacity 18650 batteries (400 cells were around 2.75ah, 500 use 3.4ah cells)
 

kmccune

Active Member
The less cells the better, the larger volume on the bigger cells of course would translate into more AH, tell me this will the 46800 be availble on EBikes in the future and please what are the AH of each cell( I never had a problem on buying the EBayrectangles because I always derated the claimed capacity by at least 25-30%.
 

kmccune

Active Member
Thank you Johnny and Scrambler. Very enlightening!
Uh, Stefan, I have battery shopped for quite some time now and occasionally you can find so real honest battery manufacturers that will give you a fair idea using different cost cells and the capacity. Some of the cheaper Chinese cells are rated fairly eg., 2.5 vs 3.0 etc.
That Scientist in disguise that does the comparisons on virtually everything we like on You tube, knocked the "socks off" of cell capacity claims, with of course most of the "Panasonic and better quality cells" doing much better than the generic Chinese cells with outragous Mah claims on the wrappers. When I buy a cheaper brick battery I try to go with the ones with the higher cell count( and still derate my expectations) I had one 48 volt battery that did extremely well.
Have thought about building my own battery packs until I add the cost up in components,I know Micah loves doing it( He seems to have time) it doesn't seem cost-effective to me and Heaven forbid I would try to ship a lithium battery on my own.
I wish there was a list of "geeks" and builders in various locales whom one could engage to design and build a custom system( bike and power).