# Looking for high Wh/kg LiFePO4 cells



## WarpedOne (Jun 26, 2009)

I'm planing a CanAm Spyder conversion with as high range as possible - hence I'm weight limited. Target max weight of the whole battery pack is 150 kg (330 lbs).

I've looked at prismatics and found they have rather poor specific energy - 
- GWL/CALB 130Ah, 4.4kg: 94Wh/kg (14,1 kWh)
- Sinopoly 60AH, 1.9kg: 101Wh/kg (15,1 kWh)

Cylindrical cells look distinctly better :
- GWL 15Ah, 0.42kg: 114Wh/kg (17,1 kWh)
- Godsend, 22Ah, 0.59kg: 119Wh/kg (17,8 kWh)
- Sffree, 20AH, 0.54kg: 118Wh/kg (17,7 kWh)

Where is the catch? Difference between 14,1 kWh and 17,8 kWh is 3,7 kWh which is considerable amount of energy.
How come some cylindrical cells can store up to 25% more energy per unit of weight than some other prismatics?

Discharge rates aren't a problem, 130Ah pack at ~100V will see average discharge around 0.5C, controller limit is at 440A or ~3C, hence I need 'capacity' cells not 'power' cells. Suggestions?


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## MalcolmB (Jun 10, 2008)

I believe the difference is simply down to the lighter casing. The cylindrical cells usually have a thin, lightweight aluminium can, which is very strong because of its shape. The prismatics have a much thicker and therefore heavier plastic case.

The cylindrical cells may looks as if they have a better energy density at first glance, but to compare the two types of cell fairly you need to include all the additional mounting hardware that's needed with cylindrical cells: spacers, specially cut busbars, nuts, washers and the supporting enclosure. I'm using cylindrical cells myself (LifeBatt) and having built a couple of packs I would definitely use prismatics in future. Cylindrical cells require a lot of additional design work and materials. Their main advantage is in power density, as they generally have higher discharge rates, but prismatic cells seem to be catching up slowly.


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## WarpedOne (Jun 26, 2009)

Thanks!

Is there a list of pack design guidelines? What-to-dos and what-not-to-dos?
Aluminum casing probably means better cell-cooling.


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## MalcolmB (Jun 10, 2008)

I've not come across any design guidelines. The cells vary in construction, so each type has different design requirements. The GWL cells you linked to are sold with their own mounting hardware and I guess the supplier will provide instructions. Headway cells are quite widely used and are available with dedicated plastic spacers that make assembling them much easier.

The main design consideration is to make sure they're well supported, without putting significant strain on the busbar connections.

If minimising weight is really important to you maybe you should look at the pouch cells, such as those from A123 and Kokam. They have better energy densities than cylindrical cells, but also require more complex packaging, as well as being more expensive.


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## skooler (Mar 26, 2011)

FWIW,

Sinopoly 200AH is 110WH / KG

http://www.sinopolybattery.com/ClientResources/201203261556142.pdf

Sinopoly 60AH (B) I get 106WH / KG

http://www.sinopolybattery.com/ClientResources/201203261553412.pdf

As far as prismatic go you don't get much better than the Sinopoly 200AH cells for energy density.

Also bare in mind the that the actual capacity of these cells is almost always 10%+ more than the specified value.

Hope This helps.

Cheers,

Mike


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## dougingraham (Jul 26, 2011)

WarpedOne said:


> Is there a list of pack design guidelines? What-to-dos and what-not-to-dos?
> Aluminum casing probably means better cell-cooling.


I dont know of any published any guidelines but I can give it a shot.

Select a large enough size cell (AH) such that you don't exceed the C rating with your average current. For a light weight EV (under 2500 lbs) this is probably 100AH. A 3C rate would give an average of under 300amps. You can parallel smaller capacity cells to get you above this. Three 40AH cells would give you 120AH or five 20AH cells would give you 100AH. This will keep the batteries from overheating. Pulling 8C or even 10C for 10 or seconds is not going to be too harmful to the battery.

Use enough cells in series to get you to either your controller max voltage or the range you want. An example of this would be 98 cells for the Soliton 1. 98 cells of 100AH would give a pack size of 31.4kwh. At 250wh/mile this would give an estimate range of 125 miles to 100%DOD or 100 miles to 80%DOD. If you need more range you could use larger capacity cells like 130AH and then reduce the number of cells. If you were using a Curtis the limit might be only 38 cells so to get that same 125 miles you would need 258AH cells. So pairing 130AH cells would be just about right.

I've seen photos of Lead Acid conversions where they make these massive structures out of angle iron to restrain the batteries. Lithium cells are a lot lighter and smaller so you dont need the huge structures. I made my boxes out of fiberglass and foam because it strong and isn't conductive. It also acts as an insulator for use in my cold climate in the winter. In a crash event it will absorb a lot of energy when it comes apart. I think most people are using steel or aluminum and there is probably nothing really wrong with this except it is going to be heavier than you would like and it is a conductor. It was the idea of an accident where the box shorts out the pack or part of a pack that worries me.

Use a fuse on at least one cable where it leaves the battery box. You need at least one fuse per box. The most likely problem where the fuse would be needed is in a crash where the cables leaving that battery box get pinched together or to the chassis. You want the fuse opening up in that situation.

It is probably a good idea to use somewhat flexible cell interconnects. A solid copper bar 1/8" thick would be a good conductor but could cause issues with vibration and the battery posts long term. There would certainly be an issue in a crash where the battery posts could be ripped out of the cell.

There are probably others but when using prismatics it is pretty easy to be successful.


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## jeremyjs (Sep 22, 2010)

WarpedOne said:


> Thanks!
> 
> Is there a list of pack design guidelines? What-to-dos and what-not-to-dos?
> Aluminum casing probably means better cell-cooling.



Don't know about guidelines, but almost all of the designs that end up using cylindrical cells or pouch cells, of the same chemistry, end up weighing nearly/just as much as using prismatics for a similar battery pack capacity; because of the need for all of the extra packaging/mounting hardware plus you almost always get more than the rated capacity with the prismatics; which you usually don't get with the cylindrical and pouch type cells. Other than the need to make irregularly shaped packs, such as in a motorcycle conversion, or needing the really high rates some of the pouch and cylindrical cells can produce, it really doesn't seem to make a whole lot of sense to use anything other than the prismatic cells unless you have a very specific set of circumstances. Especially with higher rate prismatic cells that have been coming out, like the Calb CA's.

Again. I'm not saying the pouch (A123) and cylindrical cells don't make a lot of sense in certain builds. A123 pouch cells seem to be great for short range builds that don't sacrifice on power and cylindrical's seem great for having to fill irregularly shaped spaces; such as in motorcycle builds.


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## DIYguy (Sep 18, 2008)

Here's one way to do it. Pretty good density. About 2mm between them in a staggered format. Lexan box. Fan cooled. Heavy terminals.


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## Ziggythewiz (May 16, 2010)

Nice pics. Do you have them in higher res?


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## mharvey (Oct 30, 2009)

Very sexy. That white material used on the inside is also some sort of plastic?

Also with prismatics, you need to consider the sandwich material. which is typically aluminum with strapping to prevent the sidewalls from blowing out which also adds to the weight. I'm pretty sure they don't include that in the cell weight specs.


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## JRP3 (Mar 7, 2008)

Actually with prismatics you don't really need strapping and end plates if you just make a tight tolerance battery box. They don't really seem to want to swell unless you are abusing them beyond design.


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## JRP3 (Mar 7, 2008)

WarpedOne said:


> I've looked at prismatics and found they have rather poor specific energy -
> - GWL/CALB 130Ah, 4.4kg: 94Wh/kg (14,1 kWh)
> - Sinopoly 60AH, 1.9kg: 101Wh/kg (15,1 kWh)
> 
> ...


You aren't looking at the best energy density prismatics, and you aren't taking into account the extra packaging you'll need with cylindrical cells.
As others have mentioned the Sinopoly 200ah's are better, as are the CALB 100's and 180's, which usually come in around 110ah and 195ah actual.


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## DIYguy (Sep 18, 2008)

Ya, I'll take some better pics soon as I finish them up. Those pics were taken with BB handheld I think. I don't have much to do really. I've just had them paralleled for bottom balance and have to do the series connects and install the output connection and fan wires. 
The inner support board is expanded foam PVC. Very light and thick enough for good support. A dab of hot glue on each side of a cell stiffens it right up. PVC is not so nice on fire, but hopefully that won't happen . . And its enclosed in 5 mm Lexan. I've tested these cells to 75C.


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## WarpedOne (Jun 26, 2009)

> Here's one way to do it. Pretty good density. About 2mm between them in a staggered format. Lexan box. Fan cooled. Heavy terminals.


Yes, I was planning something similar without fan s - an L shaped bussbars for parallel packs and a rubber bands between the cells to space them appart about a mm. I would rotate them for 90 degrees for air to move axialy through the pack. And create a "holding enclosure" out of 1mm aluminum.



> As others have mentioned the Sinopoly 200ah's are better, as are the CALB 100's and 180's, which usually come in around 110ah and 195ah actual.


Unfortunately 200Ah are too big and heavy, 180Ah also.
I want at least 32S string, 160Ah cells is max - Winston 160Ah offer 'abysmal' 88Wh/kg.


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## DIYguy (Sep 18, 2008)

WarpedOne said:


> Yes, I was planning something similar without fan s - an L shaped bussbars for parallel packs and a rubber bands between the cells to space them appart about a mm. I would rotate them for 90 degrees for air to move axialy through the pack. And create a "holding enclosure" out of 1mm aluminum.
> 
> 
> Unfortunately 200Ah are too big and heavy, 180Ah also.
> I want at least 32S string, 160Ah cells is max - Winston 160Ah offer 'abysmal' 88Wh/kg.


If you want the higher energy density, you may want to see if you can get your hands on the new Panasonic 18650's that Tesla is using. I think they are one of, if not the highest in energy density at this time. They also have built in protection for charge/discharge IIRC.

I would put more thought into conductive battery enclosures for any cells that have protruding terminals . . and perhaps even those that do not. Just something to think about. 

Of course, if you are not drawing higher Crates and heat is no issue, no need for fans.


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## skooler (Mar 26, 2011)

WarpedOne said:


> I want at least 32S string, 160Ah cells is max


What defines these specs?

Is a 32s string (102.4v nom) all that the controller can handle? the minimum voltage for your performance goals?

Why is 160AH the 'max'? size, weight?

I don't think anything here is very clear. If I told you i could give you a 1000AH cell that weighed 1KG would you discard it because it is above the 160AH 'max'? (see what I'm getting at)

If you let us know what motor and controller you are using and perhaps the maximum weight of your pack we can give much better advice 

Cheers,

Mike


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## frodus (Apr 12, 2008)

I think he said he was looking at the AC35 in another thread, which is an HPEVS system and they're good for up to 130VDC max (about 36 cells after a full charge).

Battery size is an issue because he's wanting to put it in a Can Am Spyder, so there's not tons of room for 200Ah cells.


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## JRP3 (Mar 7, 2008)

Seems as if the new CALB CA 100ah cells might be a good fit. They look to be able to push out good C rates, even better than the SE series which were already pretty good, and you avoid the assembly issues of cylindricals. 36 cells would be about 250 lbs.


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## WarpedOne (Jun 26, 2009)

> What defines these specs? Is a 32s string (102.4v nom) all that the controller can handle? the minimum voltage for your performance goals?
> Why is 160AH the 'max'? size, weight?
> I don't think anything here is very clear. If I told you i could give you a 1000AH cell that weighed 1KG would you discard it because it is above the 160AH 'max'? (see what I'm getting at)
> If you let us know what motor and controller you are using and perhaps the maximum weight of your pack we can give much better advice


Sorry, you are right. I've openned the thread with partial question not giving the full frame why it is important to me. I'm speccing a conversion of CanAm Spyder. My first choice were AC35 and Curtis 1238, meanwhile I switched to similar combo that is awailable from Italy, avoiding the shipping costs from USA to middle Europe - is there a distributor for HPEVS in Europe?. 

Controller is 96V nominal. So 30S string covers nominal system voltage, add a bit to cover voltage sag under load and 32S is here. I haven't found what is the max voltage for this controller yet.

Weight is limited by GVWR of CanAm Sypder - 540kg. I don't have exact engine weight so I can't say exactly how heavy the batts can be, my estimation is that they must not exceed cca 150kg without exceeding the GVWR me included 

My goal is a pack with as much kWh as possible (with 'normal' price) using LiFePO4 chemistry. I'm not confident with LiPoly yet.


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## frodus (Apr 12, 2008)

what controller are you using with that motor? The link was just to the motor, not the combo.


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## WarpedOne (Jun 26, 2009)

Controller is Sigmadrive ACT950L.
2min 460A, 1h 240A, 96V nominal, 6.1kg


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## mharvey (Oct 30, 2009)

JRP3 said:


> Actually with prismatics you don't really need strapping and end plates if you just make a tight tolerance battery box. They don't really seem to want to swell unless you are abusing them beyond design.


True you don't need it technically. But tight tolerance boxes don't allow much air for cooling. Practically speaking strapping into smaller bundles (like the GBS cells are shipped) provides flexibility in the install allowing for air flow that cram packing blocks into tight boxes can't. 

And besides, who doesn't abuse their cells?


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## JRP3 (Mar 7, 2008)

I'm not aware of anyone reporting cell cooling to be necessary with the LiFePO4 prismatics, but that may depend on your climate.


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