# Cell comparisons - please contribute



## DIYguy (Sep 18, 2008)

For what it's worth, my 180 Ah Calb cells (67 of them) all have an IR of 0.31 to 0.36 m/ohms.

edit; this was in the Test Report from the factory. I assume it was likely done at "room temperature".


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## GizmoEV (Nov 28, 2009)

drgrieve said:


> · The average voltage sag for x amps which gives the average IR for each cell in a pack


You might want to specify a temperature. The voltage sag when my batteries are at 30°C is much less than when they are below 10°C.


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## Kelmark (Oct 26, 2009)

Bump. I am interested in hearing more about Sinopoly SP-LFP200AHA performance and price. The weight is the same as the 180AH Calb.


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## drgrieve (Apr 14, 2011)

DIYguy,

Do you have any information on pack usage? 

1. I see your min ah was 198. Would you know what it is at 1C? Which brings up a point min is a better number than average as the smallest dictates.
2. In regards to IR, do you have any info on pack voltage sag in normal temps (20 - 30 degrees c)

GizmoEv. Thanks. LMK if you know of any results I can add or if there is anything you'd like to see.


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## bruceme (Dec 10, 2008)

GizmoEV said:


> You might want to specify a temperature. The voltage sag when my batteries are at 30°C is much less than when they are below 10°C.


My 100Ah TS are useless bellow about -5C. They will only pull about 100A before setting off the bms. 

I fixed this by moving my batteries into an insulated battery box I made from aluminum sheet and cut into the trunk. I used 1/2" foundation insulation. Then I hi-temp RTV glue'd a 120V ground heater w/70F thermostat to a seperate floating sheet of aluminum that went under the batteries (between the insulation and batteries.

I plug that in overnight and that keeps them nice and warm all the time. In the summer I take off the top insulation board and open the top so the batteries can cool. Does well and the batteries are happier for it (ballance quicker).


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## DIYguy (Sep 18, 2008)

drgrieve said:


> DIYguy,
> 
> Do you have any information on pack usage?
> 
> ...


Well, I have a bout 3000 km on the pack this summer (even with a late start).
No, sorry, I can't tell you capacity at 1C for sure. Peukerts plays a very small roll in LiFePo chemistry though....as most would say it doesn't exist at all. My suspicion is that they will discharge well over the 180 Ah mark even at 1C. 
Regarding sag, yes, I can tell you that my pack sags to avg 2.6vpc at 5.56C or 1000 amps. This would be at about 25C ambient.


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## Rocketmaker10000 (Dec 7, 2008)

My company is partnered with The Battery Shop listed here. Please let me know if you need any further information. Cells were late in arriving but they will be here this week so they will be in stock. These are High Performance cells! http://www.ecedra.com/thebatteryshop.html 

www.TAEC.co will be listing this on their site shortly.

Please contact me at [email protected] if you need assistance.


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## drgrieve (Apr 14, 2011)

DIYguy said:


> Regarding sag, yes, I can tell you that my pack sags to avg 2.6vpc at 5.56C or 1000 amps. This would be at about 25C ambient.


Thanks for the info.

What voltage is this sag starting from - is it 3.3? Alternatively can you give the sag at another amp draw from the same SOC and I can calculate the IR from the two figures.

Cheers


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## GizmoEV (Nov 28, 2009)

Rocketmaker10000 said:


> My company is partnered with The Battery Shop listed here. Please let me know if you need any further information. Cells were late in arriving but they will be here this week so they will be in stock. These are High Performance cells! http://www.ecedra.com/thebatteryshop.html
> 
> www.TAEC.co will be listing this on their site shortly.
> 
> Please contact me at [email protected] if you need assistance.


The only LiFePO4 cells I see on that site are A123 cells. This thread is about comparing only LiFePO4 cells.


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## drgrieve (Apr 14, 2011)

Updated with some info on ThunderSky 60 cells.


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## GizmoEV (Nov 28, 2009)

drgrieve said:


> Updated with some info on Calb 60 cells.


Remember that the IR you calculated includes the resistance of all of the connections so your IR value is higher than the true IR of the cell.


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## EVfun (Mar 14, 2010)

I wanted to post a couple of minor corrections. My cells <http://www.diyelectriccar.com/forums...5&postcount=30> are Thunder Sky cells from just before the Winston/Sinopoly split (Feb. 2010 manufacture date.) They are marked as LFP cells but the documentation also lists them as LiFeYPO4 cells (Yttrium cells, like the Winston offerings.) 

I just weighed 2 spare cells that are banded together. I then subtracted the end plates, banding, handles, and screws. I ended up with 10 lb. 0 oz. Each cell weighs 5 pounds on the nose (2.27 kg.) Factory Thunder Sky strapping weighs 1 pound 4 oz plus 1 oz per cell (8 strips of banding growing 2.4 inches for each additional cell.) My Buggy pack weighs just 167 lb.


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## EVfun (Mar 14, 2010)

I thought this might be a good place to link to the video on LiFePO4 batteries presented by Professor Jay Whitacre from the Carnegie Mellon University. He goes into detail about how they work and the limitations of the cells. If you are going to be a student of LiFePO4 cells this is must view material. 

The page with the video is here.

A direct link to download the flash video is here.


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## GizmoEV (Nov 28, 2009)

EVfun said:


> I thought this might be a good place to link to the video on LiFePO4 batteries presented by Professor Jay Whitacre from the Carnegie Mellon University. He goes into detail about how they work and the limitations of the cells. If you are going to be a student of LiFePO4 cells this is must view material.
> 
> The page with the video is here.
> 
> A direct link to download the flash video is here.


The video is definitely worth your time to view more than once if you want to have a better understanding about these cells.


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## spdas (Nov 28, 2009)

Yes a VERY good Video. The last 15 minutes answers a lot of questions posed here on these forums. 

I am wondering about his matching cells on his graph considering Resistance vs. Capacity and putting the cells together that are most similar. He claims that after cycles if you do not have matched cells, (even though you parallel balanced) your pack will eventually go out of balance. 

In my first lot of that I bought cells I have 195ah to 197ah and IR from 0.30 to 0.38 with 2 cells at 0.56.

In the second lot I recently bought cells that have 202ah to 204ah and IR from 0.22 to 0.26. 

Both lots are calb 180ah cells.

With different IR would these cells go out of balance after many charges, and the capacity not be so important?

francis


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## EVfun (Mar 14, 2010)

Francis, are you telling me your new cells only sag down to about 3.15 volts at 500 amps? 

I suspect that the factory IR numbers are actually a measured AC impedance test. If that is the case the cells would sag more under loads in the real world because the capacitance of the cell (LiFePO4 cells are built like electrolytic capacitors) would mask some of the IR.

If they are using an AC impedance test I'm not sure how directly that relates to what you want to match. I would think that you would want to use load tests at a fixed SOC to determine working resistance. Something like fully charging a cell, them removing 10% of the charge at C, then testing for 10 seconds at 100 amps, followed by 10 seconds of rest, followed by 10 seconds at 300 amps. Likely not something we are doing in our garage. 

If those are working IR numbers verified by driving loads I've got to consider switching to 70 amp hour CALB cells (same size as my 60 amp hour TS cells.) 

As far as what to do with your cells, I would recommend not using the 2 cells at 0.56 and then putting the rest together after a top balance. I suspect you will have to much capacity variation (195-204 amp hours) to bottom balance without some going to high at the end of charge. Then monitor them, every day for a week, once a week for a month, and once a month for a year if they are staying together. I don't think they will move apart much.


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## drgrieve (Apr 14, 2011)

GizmoEv said:


> Remember that the IR you calculated includes the resistance of all of the connections so your IR value is higher than the true IR of the cell.


Yes I'm wanting the pack resistance. I'm hoping that the small amounts of data I collect happen to be around the average when applied in a real world situation (ie putting the pedal to the metal).

I thought I had figured out how many headway cells I'd need to produce 200 kW of power (including the sag) from some posts on battery testing. Then I saw a post on cells built into a pack - the voltage sag was much higher than expected.

So i'm looking to collect some data on pack voltage sag (and ah at 1C but we do have spec sheets for that so hoping they are accurate) so I (and anyone else interested in amp producing performance) can make an educated guess on what cells to choose.

There's a lot to weigh up - energy density, power density, build effort, $$$$.

EVfun's 60ah thundersky pack could (in theory) do 9C keeping voltage @ 2.5. That's almost double as per the spec sheet of 5C. So now I can put their cells back into the possible column. Especially since the energy density of the Sinopoly cells seems to way ahead of Calb.

Also if anyone has any information on any other cell types that are LifePo4 based I'm happy to add them into this thread. (A123, HiPower etc).


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## spdas (Nov 28, 2009)

Yes, the sag is only to 3.15 @ 500amps. (when fully charged)

And yes I am using factory IR numbers.

The video above that I watched, put most importance to capacity balance (196vs204) to keep the cells balanced. I would have thought as u did of replacing the 0.56 cells the IR being more important to keep tighter and therefore more in balance when charging and discharging.

And all my readings are taken through Zillaconfig on my netbook hooked to the Zilla.
Francis


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## EVfun (Mar 14, 2010)

spdas said:


> Yes, the sag is only to 3.15 @ 500amps. (when fully charged)
> [snip]
> The video above that I watched, put most importance to capacity balance (196vs204) to keep the cells balanced.
> [snip]
> And all my readings are taken through Zillaconfig on my netbook hooked to the Zilla.


Wow! I'm gonna have to have a talk with someone over at CALIB about some 70 amp hour cells. If the resistance scales up as capacity scales down they should be about 0.6 milliohms. Usually, the cells do a bit better as they get smaller (a 100 amp hour cell would typically have <2x the internal resistance of a 200 amp hour cell.) 

I wonder how much of my total resistance is connections? I am using the EVworks braided straps, which have 1/0 worth of copper in them. I am using all 1/0 cable, only about 12 feet for about 1.2 milliohms. I am using heavy duty tin plated copper Magna Lugs. No connection gets warm. I have 1 fuse to consider (2.4 milliohms.) I'm adding things up and only getting about 4.5 milliohms plus the crimps and terminal to post resistance. All the contacts where sanded clean and have a small amount of electrical grease and all the crimps where done with a proper hex crimper. If I assume every terminal with its crimp and connection adds 0.1 milliohms, then subtract all of this from my pack total, I find my cells to be around 1.1 to 1.2 milliohms (1.14 milliohms.)

I agree that he emphasizes capacity most, but that would be the expensive one to fix now! Equalizing and watching seems more practical. 

I'm using my Zilla and my Fluke DMM to take the readings. They seem to agree on voltage very well.


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## spdas (Nov 28, 2009)

I am taking the resistance figure for the cell that Calb sent me just to one cell to the other. I am not taking into account connections, or even the 8 foot run (2/0 wire) from the 28 cells in the trunk hooking to the 16 cells in the engine compartment. And I only understand 50% of the previous posts. 
I do have a different voltage reading from the Zillaconfig program and my link10. ZConf seems to show 1.5 volts lower all the time, but amperage draw is the same. 
I also have an issue where the zilla cuts back amperage after 2700rpm, (starts at 650amps @ under 2700rpm, when floored) only dishing out a max of 250 amps at higher rpm's. 

So false readings maybe contribute to the in consistencies that you are noticing.

francis


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## MN Driver (Sep 29, 2009)

bruceme said:


> My 100Ah TS are useless bellow about -5C. They will only pull about 100A before setting off the bms.
> 
> I fixed this by moving my batteries into an insulated battery box I made from aluminum sheet and cut into the trunk. I used 1/2" foundation insulation. Then I hi-temp RTV glue'd a 120V ground heater w/70F thermostat to a seperate floating sheet of aluminum that went under the batteries (between the insulation and batteries.
> 
> I plug that in overnight and that keeps them nice and warm all the time. In the summer I take off the top insulation board and open the top so the batteries can cool. Does well and the batteries are happier for it (ballance quicker).


What voltage does your BMS cut off at? I'd be willing to allow voltage to sag lower in the cold considering they will heat themselves up more that way and won't overheat anyway if they are cold to begin with and being subjected to sag as long as things don't get too carried away. So its important to know where they are sagging to and what amperage. So far to me, knowing what temperature to heat to and what temperature won't require heating and how much insulation to use is very important to pack design yet this type of information exists nowhere yet. I'm trying to get my hands on two 100Ah cells, one being Winston and another CALB to test but the person who has them didn't seem to understand that sag is a problem in the winter and now is nervous about trying to test them, not entirely sure why as its important info.


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## Yabert (Feb 7, 2010)

Hi

I have new information about Headway cells. They come from the manufacturer.

40160S: _T__ypical impedance is about 4~5mohm_.

38120HP: _T__ypical impedance is less than 3mohm_.... That sound to me like a good power capability! More than 1800w / Kg


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## patrickza (Dec 28, 2009)

Those are both fairly good looking numbers, any changes to the 38120s cells? If the 38120s outperforms the 40160s then to me it seems like a better option, as the cost per AH is virtually identical. The only downside is working with more cells.


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## spdas (Nov 28, 2009)

Aloha, I figure my 180 calbs are at least 180 ah at 1c. Also, I am sure the factory uses the best conditions and lowest C rating to boast the 198ah. I tested mine down to 2.5v and got [email protected] 
I consider my 180ah calbs to be available 150ah batteries and treat them that way.
francis


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## Yabert (Feb 7, 2010)

patrickza said:


> If the 38120s outperforms the 40160s then to me it seems like a better option.


In fact, for the same battery pack (example 10 Kwh), the 38120S have a better power capability, but the 40160S have a better power density and a better volumetric density. 
So, a 10 Kwh battery pack build with 40160S will weight less and take less space, but will have a lower peak power.


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## jackbauer (Jan 12, 2008)

Yabert , do you have a pdf or other high res version of that datasheet? Very interested to see the curves and data on the 40160s. I run a 10kwh pack of 192 40160s cells.


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## Yabert (Feb 7, 2010)

Absolutely Jack

Do you try some impedance test with your cells?


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## drgrieve (Apr 14, 2011)

Added voltage sag figures from valerun for Calb 100ah. 

If anyone can report pack voltage sag for any cell types I'll add them in.

Also what would be a way to calculate a real world volumetric density of headway cells so you can compare them to Calb cells for example.

Thanks.


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## GizmoEV (Nov 28, 2009)

drgrieve said:


> Also what would be a way to calculate a real world volumetric density of headway cells so you can compare them to Calb cells for example.


I assume you are referring to the cylindrical cells. What I would do is measure the distance between the centers of the terminal screws between two adjacent cells mounted in the spacers and then measure the length of the cells including the spacers. Volume = (terminal distance)^2 x (length) This is the volume of the "square" rectangle taken up by the cylinder. If you want the actual volume of the cylinder the formula is Volume = pi x radius^2 x length.

The square rectangle volume would make the most sense to me since it represents the volume needed for the pack in a vehicle.


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## sokon (Sep 15, 2011)

> The square rectangle volume would make the most sense to me since it represents the volume needed for the pack in a vehicle.


I dont know whether it makes any much sense to compare (relatively) small cylindrical cells to large prismatic cells in volumetric energy density at all. Especially, since pack building from small cells is much more flexible in shape than pack building with larger prismatic cells. Depending on the shape of the volume that u want to fill with your pack, small cells might be better than large ones, inspite the fact that the large ones offer better vol. energy density. For example consider triangle shaped packs to fit in a bycicle frame as offered here: http://www.emissions-free.com/catalog/i29.html


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