# Prismatic or Cylindrical Cell?



## Karter2 (Nov 17, 2011)

Cylindrical cell manufacture is a very "mature" process.
Well refined and debugged , giving high production rates and consistent high quality.
pouch and prismatic manufacturing is relatively new, slower and as we have seen with various quality disasters, not yet consistent.
In manufacturing, speed is king. It directly influences production costs.
Also, many of the original factors for Tesla choosing multiple small cells for their packs, still apply...Safety, redundancy, thermal management , etc.


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## Hollie Maea (Dec 9, 2009)

The best cylindrical cells have twice the energy density of anything else available--while being cheaper.


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## sholland (Jan 16, 2012)

Cylindrical cells offer very high energy density, but not very high continuous discharge C rates... Just go to a race track with a bunch of Teslas and watch them all get power limited and slow down after a couple of laps!  

For the larger capacity packs that are in Teslas, 18650 cells might work ok (69 parallel cells), but if you are building a smaller pack, say under ~50KWh, you'll find it hard to use 18650 cells while still getting a reasonable continuous discharge C rate for most vehicle applications. That is why every smaller pack from OEMs today are using prismatics.


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## Hollie Maea (Dec 9, 2009)

sholland said:


> Cylindrical cells offer very high energy density, but not very high continuous discharge C rates... Just go to a race track with a bunch of Teslas and watch them all get power limited and slow down after a couple of laps!
> 
> For the larger capacity packs that are in Teslas, 18650 cells might work ok (69 parallel cells), but if you are building a smaller pack, say under ~50KWh, you'll find it hard to use 18650 cells while still getting a reasonable continuous discharge C rate for most vehicle applications. That is why every smaller pack from OEMs today are using prismatics.


This is not true. Tesla uses cells that are optimized for energy. But there are other 18650s that do VERY well on power.

Take the LG HG2, for instance. At 3Ah, it is about 10 percent less energy dense than Tesla's cells, but still higher than any prismatic. But the DC resistance is half of what the Tesla cells are, and with cooling it can do 20A continuous. That's over 6C. When you combine great energy density with pretty good C rate, you get very good power density.


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## sholland (Jan 16, 2012)

I was not aware of these LG HG2... Impressive (eating hat ). 

They would still need to be under ~$3 each to be competitive with what's available in prismatic, without factoring in the added cost and complexity of cooling. What's the lowest cost you can find the LG HG2 for? I am only seeing $5+ each at best...


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## Sunking (Aug 10, 2009)

sholland said:


> Cylindrical cells offer very high energy density, but not very high continuous discharge C rates...


I figured that had to be part of it, but Tesla knew how to work around it and it fit their business model at the time. With the Roadster having such a large capacity (AH, 69P) Means the Tesla or any large capacity battery will only operate in Fractional C-Rates, so they never see those 5 to 10C burst and cruise on C/xx. 

For the larger capacity packs that are in Teslas, 18650 cells might work ok (69 parallel cells), but if you are building a smaller pack, say under ~50KWh, you'll find it hard to use 18650 cells while still getting a reasonable continuous discharge C rate for most vehicle applications. That is why every smaller pack from OEMs today are using prismatics. [/QUOTE]

To sum it up I think you answered my question. 

1. Cylindrical Cells are best practice for large capacity with respect to Energy Density (Wh/L) where space requirements become critical. The larger AH requirements for higher capacity batteries negates High-C discharge rate limitations.

2. Prismatics and Pouches are best practice for High Discharge rates, coupled simple cooling and monitoring techniques. Much less complex. 

My whole issue revolves around 35 plus years of working with battery plants has taught me placing batteries in parallel is asking for trouble. I have also learned the more pieces and connections results in very complex systems prone to failures and malfunction. A battery that requires thermal management does not make one bit of common sense to me. I don't want a parasite in a EV of any kind. I especially do not want to run a Coolant Liquid anywhere near 3000 batteries with all those connections.


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## Hollie Maea (Dec 9, 2009)

sholland said:


> I was not aware of these LG HG2... Impressive (eating hat ).
> 
> They would still need to be under ~$3 each to be competitive with what's available in prismatic, without factoring in the added cost and complexity of cooling. What's the lowest cost you can find the LG HG2 for? I am only seeing $5+ each at best...


I can't give out exact prices, but the prices are competitive when buying in bulk. Of course buying onesies from Vape shops is VERY expensive...

They're new cells so not surprising you hadn't heard of them. Before that, the only 18650s at that power level were 2.5Ah, which is still very dense.


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## Karter2 (Nov 17, 2011)

Sunking said:


> the Tesla or any large capacity battery will only operate in Fractional C-Rates, so they never see those 5 to 10C burst and cruise on C/xx.


Tesla's S85P delivers 590 kW from its 85kWhr pack of 18650s.
Which will mean 6-7C discharge.!



Sunking said:


> For the larger capacity packs that are in Teslas, 18650 cells might work ok (69 parallel cells), but if you are building a smaller pack, say under ~50KWh, you'll find it hard to use 18650 cells while still getting a reasonable continuous discharge C rate for most vehicle applications. That is why every smaller pack from OEMs today are using prismatics.


I am not aware of any OEM EV that has a pack rated as high ( 6+C) as the Tesla's .
Most of the commonly used aftermarket LiFePO4 prismatics are also only 3-5 c rated.




Sunking said:


> To sum it up I think you answered my question.
> 
> 1. Cylindrical Cells are best practice for large capacity with respect to Energy Density (Wh/L) where space requirements become critical. The larger AH requirements for higher capacity batteries negates High-C discharge rate limitations.
> 
> ...


There is nothing "simple" about the Volt cooling system for its Prismatics.
Energy density and discharge rates are dependant on cell chemistry, not the form factor.
There are 18650s rated for 30C , and prismatics designed for 0.1C discharge.
There is no simple choice for an EV designer.


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## sholland (Jan 16, 2012)

Karter2 said:


> Most of the commonly used aftermarket LiFePO4 prismatics are also only 3-5 c rated.


Most LiFePO4 prismatics are rated 3-5C continuous, and much higher for peak discharge current (10-12C for CALB, way higher for A123). The consensus among those who have characterized the Leaf or Volt cells seems to say 4C continuous and 7-9C peak. The testing folks have done on Tesla P85D Panasonic cells are speculating a rating of ~1-2C continuous and 6C peak, and that's with cooling. Even with that cooling, the Model S's I was behind on Laguna Seca at the recent Refuel event were all slowing way down after just a couple of laps due to battery thermal limit being reached.

Just looking at cell impedance tells the story... The latest Samsung or LG 18650 is still ~20mOhm. The Panasonics are higher. All the prismatics are under 1-2mOhm.




Karter2 said:


> There is nothing "simple" about the Volt cooling system for its Prismatics.
> Energy density and discharge rates are dependant on cell chemistry, not the form factor.
> There are 18650s rated for 30C , and prismatics designed for 0.1C discharge.
> There is no simple choice for an EV designer.


I wholeheartedly agree. For the typical conversion builder though you cannot disagree that building a large pack from 1000's of cylindrical cells vs prismatics, with proper design for cell contact, bus-bars and cooling is much, much easier with prismatic cells. 

I'm not even getting into the potential boondoggle it can be to try and find the cylindrical cell that is what it is spec'd to be, and at a reasonable price.


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## sholland (Jan 16, 2012)

Here's a spreadsheet I have put together to try to compare all the different commonly available cells and come up with a solution. Just adjust the green values on the first sheet, and the numbers will update on the second sheet to reflect the necessary cells needed for your requirements. I add more rows with parameters as I need them and more columns as I find different cells. The pricing may not be the latest for some of them, but it's close, and your own purchase power might differ from mine... 

The conditional formatting was for my own needs, but you can change those for yours.

I hope this helps.


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## Karter2 (Nov 17, 2011)

Excellent spreadsheet ! that must have taken a lot of time to compile.
Thanks for posting.


sholland said:


> Most LiFePO4 prismatics are rated 3-5C continuous, and much higher for peak discharge current (10-12C for CALB, way higher for A123). The consensus among those who have characterized the Leaf or Volt cells seems to say 4C continuous and 7-9C peak. The testing folks have done on Tesla P85D Panasonic cells are speculating a rating of ~1-2C continuous and 6C peak, and that's with cooling. Even with that cooling, the Model S's I was behind on Laguna Seca at the recent Refuel event were all slowing way down after just a couple of laps due to battery thermal limit being reached..


 Tesla's cells are not selected for max discharge rate , but rather for capacity and life cycles..and Tesla seem to play cautious with their temp limits !
( there are other 18650's with higher C rating ,..10C continuous is not uncommon.)
Just for info, incase you missed this ..
https://endless-sphere.com/forums/viewtopic.php?f=14&t=67721




sholland said:


> . For the typical conversion builder though you cannot disagree that building a large pack from 1000's of cylindrical cells vs prismatics, with proper design for cell contact, bus-bars and cooling is much, much easier with prismatic cells.
> .


 Agreed, but there are pre built "sub assemblies" of 18650S available from most manufacturers, ..if required.
But yes, its a shame that there are not more high capacity ( 10-20+ Ahr ) quality cylindricals like the A123s , available


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## Hollie Maea (Dec 9, 2009)

sholland said:


> Just looking at cell impedance tells the story... The latest Samsung or LG 18650 is still ~20mOhm. The Panasonics are higher. All the prismatics are under 1-2mOhm.


That's a little misleading. Impedance by itself isn't the full story--you need impedance per capacity.

If for instance you have a 3Ah 18650 with a 20 mOhm impedance and a 60Ah prismatic with a 2mOhm impedance, the 18650 is actually better since when you parallel 20 of them together to get 60Ah the overall impedance drops to 1 mOhm.


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## sholland (Jan 16, 2012)

Hollie Maea said:


> That's a little misleading. Impedance by itself isn't the full story--you need impedance per capacity.
> 
> If for instance you have a 3Ah 18650 with a 20 mOhm impedance and a 60Ah prismatic with a 2mOhm impedance, the 18650 is actually better since when you parallel 20 of them together to get 60Ah the overall impedance drops to 1 mOhm.


Yes, you are right, the impedance is inversely proportional with the capacity, mainly because internally a prismatic is constructed of many parallel 'cells' already, just like an 18650, only larger with greater surface area.

I suppose my point is that the complexity of building and evenly cooling 20 parallel 18650 cells will be much more difficult than cooling a single prismatic. A couple of parallel prismatics will be lower impedance still, and still be relatively easy to thermally manage.


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## Sunking (Aug 10, 2009)

Hollie Maea said:


> That's a little misleading. Impedance by itself isn't the full story--you need impedance per capacity.
> 
> If for instance you have a 3Ah 18650 with a 20 mOhm impedance and a 60Ah prismatic with a 2mOhm impedance, the 18650 is actually better since when you parallel 20 of them together to get 60Ah the overall impedance drops to 1 mOhm.


Perhaps on paper and theory but with 60 connections and their resistance has not been accounted for. That is my beef with parallel cells you just increase your chances of a problem 60 fold. Instead of 45 cells to worry about, you now get 2700 cells to worry about.


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## Hollie Maea (Dec 9, 2009)

Sunking said:


> Perhaps on paper and theory but with 60 connections and their resistance has not been accounted for. That is my beef with parallel cells you just increase your chances of a problem 60 fold. Instead of 45 cells to worry about, you now get 2700 cells to worry about.


If the cells are welded together--which is how they should be--then this becomes utterly irrelevant.


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## Karter2 (Nov 17, 2011)

Sunking said:


> Perhaps on paper and theory but with 60 connections and their resistance has not been accounted for. That is my beef with parallel cells you just increase your chances of a problem 60 fold. Instead of 45 cells to worry about, you now get 2700 cells to worry about.


...BUT.. If you have one cell fail in that 45 cell series pack, you are dead in the water.
One cell (or connection). failing in a 60P group is insignificant.


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