# Why Parallel Batteries



## Sunking (Aug 10, 2009)

I have discussed this with a few EV folks and no one has given me a good answer other than that is just how we do it. 

Parallel arrangements make the battery system more complicated and expensive than need be. In addition as I have learned in 35 years it cuts down on cycle service due to battery internal resistances, and balancing cable/connection resistances forces one battery to do more work than the other.

Before you say to achieve a greater AH capacity I say NOT because you can get a LFP anywhere from 1 to 300 AH and anywhere in between.

Even EV manufactures exercise this practice and I really want to know why? Take the Leaf for example is a 60 AH battery system constructed with 20 AH modules in a 3pSx configuration. I forget what x equal but does not matter. The battery would be simpler, less complicated, and less expensive with 60 AH modules. A 60 AH cell takes up less space than 3-20 AH cells. 

What am I missing? 

FWIW I am not talking about cylindrical cells, that is another story. Strictly Prismatic Large Format like LFP. Only thing I can possible think of is thermal management. Bu tthat does not compute because the larger the capacity the lower the resistance thus less heat generated.


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## twright (Aug 20, 2013)

I don't have a really good answer, but one small reason is packaging: Smaller batteries batteries are easier to find places for than large ones.


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## Moltenmetal (Mar 20, 2014)

Are the larger prismatic cells not a parallel arrangement of pouch cells in a handsome plastic box? Pardon my ignorance if I'm wrong...


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## Duncan (Dec 8, 2008)

I haven't pulled apart any of the prismatic cells - but are they two large folded sheets of electrodes
Or a set of parallel cells inside the case?


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## Karter2 (Nov 17, 2011)

Damage limitation ?
one small capacity cell fails..you can survive, maybe not notice the difference.
One big cell fails you will certainly notice, and it may even shut down the entire system.

Cost of failure ?
Small cells are cheaper to replace than big ones !

Oh and i dont think this is true..


> ...the larger the capacity the lower the resistance thus less heat generated.


 Because the bigger cell will be carrying much more current, and heat generated is proportional to the square of the current ?


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## onegreenev (May 18, 2012)

Lots of parallel sheets. In the 100ah Hi-Power there are two bunches of parallel sheets. So in other words two 50ah packs paralleled within the plastic container. But they are distinct 50ah pouches. Just not vacuumed like the A123's or the Leaf Pouches.


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## Woodsmith (Jun 5, 2008)

Aside from the internal stricture I have considered this too.

My thoughts for my project is that one 100Ah cell is going to have a smaller volume then two 50Ah cells just due to there being less plastic packaging.

I am working on a balance between pack volume vs required pack capacity for range. Also having fewer cell connections and points for error would make my life easier.


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## Nabla_Operator (Aug 5, 2011)

Duncan said:


> I haven't pulled apart any of the prismatic cells - but are they two large folded sheets of electrodes
> Or a set of parallel cells inside the case?


Pictures here; I see: (b) many sheets in parallel


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## dcb (Dec 5, 2009)

frequently in electronics it is the case that larger single unit doesn't mean cheaper per unit of power. And it isn't hard to rationalize a less expensive choice with a redundancy argument.


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## piotrsko (Dec 9, 2007)

How about early availability? 4 years ago, 300ah cells were not inexpensively around. You got a 100, and by gum, you were happy. Habits get hard to break.


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## Sunking (Aug 10, 2009)

dcb said:


> frequently in electronics it is the case that larger single unit doesn't mean cheaper per unit of power. And it isn't hard to rationalize a less expensive choice with a redundancy argument.


I thought that might be the case so I looked into it before asking. A CALB 60 AH goes for around $80, and a 180 AH around $230 which means larger cells are slightly lower or same in price per watt hour.


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## Sunking (Aug 10, 2009)

piotrsko said:


> How about early availability? 4 years ago, 300ah cells were not inexpensively around. You got a 100, and by gum, you were happy. Habits get hard to break.


Pretty much what I said in the opening post.


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## bwjunkie (Jul 31, 2013)

Cells are fragile, and the bigger ones are more fragile for the reason already mentioned "less plastic packaging per AH". I'm on my 4th version of battery box always wondering how things will hold up in a medium speed crash, I have to say I'm kind of happy I have smaller batteries at 60ah. I did have to double them up into 2P parallel because I moved to a heavier chassis, and it would have been nice to own 90ah batteries.

in short-
A more fragile battery requires a better box design.

josh


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## Duncan (Dec 8, 2008)

Back to Sun's initial comments
From the pictures of the disassembled cells a large cell is simply a number of small cells already paralleled up

So the option of using large single cells like Lead Acid does not actually exist


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## Karter2 (Nov 17, 2011)

I still believe there would be thermal issues with extra large cell assemblies.


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## Woodsmith (Jun 5, 2008)

So are the ribs on the cell case partly thermal management, to allow air flow around the cell?
What happens when all the cells are in a well insulated battery box to keep them warm?


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## Karter2 (Nov 17, 2011)

I suspect the ribs are multipurpose..
Separation to facilitate cooling and prevent abrasion damage
Strength reinforcement ribbing to aid resisting internal pressure.

A little simplified math...
Using data taken from Wynston cells ,
40 Ahr @ 0.7 mOhm IR,
.. and 160 Ahr @ 0.4 mOhm IR
Assume internal heat generated during discharge is the IR multiplied by the current squared...( not exactly i know...but good enough for this comparison)

At 1C, (160 amps) load the, 160 Ahr cell will generate..
160x160 x 0.0004 Watts ..= 10.24 W
For the same load 160 Amps, the 4 paralel 40Ahr cells will generate ..
4X 40x40x 0.0007 Watts ...= 4.48 W

So the parallel cells (same total capacity) are generating less than half as much total heat, and have much more surface area and a shorter thermal path, to dissipate that heat.
No big deal at 1C with only a few watts to lose, but if you ate punching at 10C then your losses are an extra 550W from the big cell. !
And if you happen to have a pack of say 50 in series, then at max power (10C) you are putting an EXTRA 27kW of heat into those cell..
...and losing those 27kW of output. ? 
....( which is roughly 10% of total battery power)


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## Sunking (Aug 10, 2009)

Did the same for Calb CA series using 60 AH vs 180 AH and got similar results and is the answer I was looking for.

CALB 60 AH Ri - 1 milli-ohm
Calb 180 AH = .6 milli-ohm

# of the Calb 60 AH in parallel Ri = .33 milliohm

Not only is there a heat issue, but also voltage sag and the power losses associated with it. 

Is it huge, only while accelerating does it become significant in my application running 48 volts and at cruise only looking at running 50 to 60 amps or .3C.

Cost waise running 3 in parallel vs 1 larger is not much as it works out to about $2 or $3. Hardware another $7 per S and that can add up even on a 16S pack.

I opted to use GBS 200 AH using 100AH cells for the golf cart. 



Karter2 said:


> I suspect the ribs are multipurpose..
> Separation to facilitate cooling and prevent abrasion damage
> Strength reinforcement ribbing to aid resisting internal pressure.
> 
> ...


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## Karter2 (Nov 17, 2011)

Yes, i was going to include the extra voltage sag ..(~10% more with the larger cells at 10C ) ..but i thought it would make an already lengthy post even more complex !


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## Caps18 (Jun 8, 2008)

I'm kind of stuck with the parallel arrangement of batteries as they were packaged. It also dictated the voltage I had to work with. Since I wanted to use 120-130Vdc instead of 360-390 Vdc.

It does make the wiring more complicated, and I am just hoping that it won't cause any problems.


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## major (Apr 4, 2008)

Sunking & Karter2,

I would not put much value on calculations using Ri numbers from spec sheets. See the article from which I took this quote: 



> Unfortunately, very few manufacturers specify true DC resistance. Many specify impedance at 1 kHz instead, which easy to measure, but is useless to the user, and is quite unrelated to DC resistance.


http://elithion.com/wp_short_discharge_time.php 

Furthermore, notice how SDT is a single value for Calb CA, Calb SE, or Winston, etc. This implies to me that the real internal resistance is the same when normalized for capacity for product, meaning four 40Ah cells will waste the same energy as a single 160Ah cell for a particular C-rate.


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## Sunking (Aug 10, 2009)

Thanks Major that is what I needed to know. Based on that single string should yield the best performance.


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## Moltenmetal (Mar 20, 2014)

Given that they're just parallel pouch cells in a handsome plastic box, there is no reason that external versus internal paralleling will make any difference to efficiency.

If one of the bolted external parallel connections were to come loose, the risk of cell damage would increase versus the presumably more durable method by which the pouch cells are paralleled inside the plastic box of a larger cell. If you have a BMS, if it is wired correctly it should warn you in time to avoid damage.


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## lithiumlogic (Aug 24, 2011)

I know there are LFP cells in many different AH capacities, but in practice

1) you might want to stick with a brand or vendor you trust, restricting selection

2) packaging requirements dictate the height and width of the pack. Especially height, having batteries fit the right way up under the boot floor/cargo deck/rear bench seat. You'll measure the space you've got and pick a cell that comes as close as possible to this... not tall enough your conversion ends up with a smaller pack, too tall obviously even worse. So you pick the cells you want and worry about how to arrange them in series and parallel afterward.

3) controllers with a lower max voltage, like for an AC50 system, pretty much guarantee having to go parallel for a big enough pack.


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## Karter2 (Nov 17, 2011)

major said:


> Furthermore, notice how SDT is a single value for Calb CA, Calb SE, or Winston, etc. This implies to me that the real internal resistance is the same when normalized for capacity for product, meaning four 40Ah cells will waste the same energy as a single 160Ah cell for a particular C-rate.


 Fair point, however, the fact that there is only one value for SDT ( and hence RI) may imply they are all the same....or it may suggest that measurements were not taken from different cell sizes !
Whilst i respect greatly the Elithion resource, i fear they may have also fallen foul of some dubious data to base their calculations on.


> For example, we can pick two points in the following discharge curve for a LiFePO4 cell.
> 
> 
> 
> ...


 If you have seen the curves for Wynston cells you may have noticed that they are strangely * IDENTICAL* for all cells, with discharge quoted as "C" value rather than amps ??? ..suspicious and very misleading, not to say highy unlikely !
Its also interesting that Wynston are very careful to quote specific RI figures for each different cell size in their official data sheets.
Why would they bother with AC data ?
In your experience, is there ANY correlation between AC and DC IR measurements ? ...i cannot believe AC figures are totally random ! ...different and inaccurate maybe, but random seems unlikely


Any other sources of actual DR IR measurements on cells of same chemistry but different size ?


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## major (Apr 4, 2008)

Karter2 said:


> ......Why would they bother with AC data ?
> In your experience, is there ANY correlation between AC and DC IR measurements ? ...i cannot believe AC figures are totally random ! ...different and inaccurate maybe, but random seems unlikely
> 
> 
> Any other sources of actual DR IR measurements on cells of same chemistry but different size ?


Why AC? Its easy to measure with an instrument. 

I've tested the DC internal resistance it is cumbersome to do and seemingly not 100% repeatable so I rarely publish the numbers. I've not seen correlation to published AC figures although I never really looked at that. For that matter, I often found no correlation to published DC figures for Pb-Acid batteries I did years ago. Spot checks of DC cell resistance of Lithium Ion I've done in the past few years have come close the manufacturers' spec, IIRC.

I don't know of any DC Ri tests on various capacity cells of a product. Maybe Jack R has done some. And I'd be surprised if Davide (eLithion) hasn't done it. It'd be nice nice to have him chime in on a few of these threads.


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## Karter2 (Nov 17, 2011)

Odd so little data is available, RI seems easy to deduce .. since most folk have chargers that can generate a discharge curve, and most manufacturers publish curves at various loads, from which it is straight forward ( as pointed out in the Elithion page ) to calculate the DC RI.
I know care is needed to be consistent with SOC and measurement accuracy, but its not difficult,
... but it seems folk dont realise the importance of RI as an indicator of cell performance and health.
Shame that those Wynston charts are so suspicious !


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## dmwahl (Aug 25, 2014)

Another reason is that it can lead to better matching of each cell block in terms of capacity and internal resistance. For the sake of illustration, assume you can manufacture cells where the capacity and Ri are +/-5% regardless of capacity. Assume also that the distribution of cells in a batch follow a normal distribution (bell curve). If you use 100Ah cells, they will range from 95Ah-105Ah. If you parallel 10x10Ah cells together, they will individually range from 9.5-10.5Ah, but statistically as a whole they will be better matched than the 95-105Ah of your single cell. For DIYers, statistically reducing variations isn't a big deal, but for a fleet of cars like Tesla has, I bet it's a very big deal.


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## Duncan (Dec 8, 2008)

_For DIYers, statistically reducing variations isn't a big deal, but for a fleet of cars like Tesla has, I bet it's a very big deal._

That is a very good point!
And may relate to why the large prismatic cells are actually a lot of smaller cells in parallel


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## dmwahl (Aug 25, 2014)

Duncan said:


> _For DIYers, statistically reducing variations isn't a big deal, but for a fleet of cars like Tesla has, I bet it's a very big deal._
> 
> That is a very good point!
> And may relate to why the large prismatic cells are actually a lot of smaller cells in parallel


It's also why (I suspect) Tesla can get away with such a small balance current on their modules, only ~100mA. If they're so well matched they don't need to worry much about large balance currents.


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## Sunking (Aug 10, 2009)

Karter2 said:


> In your experience, is there ANY correlation between AC and DC IR measurements ? ...i cannot believe AC figures are totally random ! ...different and inaccurate maybe, but random seems unlikely


For one AC is impedance taken at some frequency, and is not the DC resistance which will be lower, significantly lower. Telecom use some pretty expensive Battery MHO meters to use strictly on AGM batteries. The readings are fairly benign and do not mean much other than an indicator of a cell or string on its way out. You have to establish a Base Line reading when the batteries are new at a given temperature after the batteries have been fully charged up and acclimated to its environment. Unlike FLA where battery health is accurately determined by specific gravity, AGM batteries are sealed and there is no access to the electrolyte. Mho's and cell voltages are the only indicator of an AGM. Very useful if you have a new set, but the readings do not really mean much. If you have a baseline it is a very good indicator to let you know it is time to break out the check book and replace them. Having said that most telephone companies just replace all VRLA batteries every 7 years as policy because no VRLA has ever passed a capacity test after 7 years. Only cells that can last that long are are FLA Pure Lead cells.

But FWIW Mho's can be applied to any battery chemistry and makes a very good SOC meter. The trick is to make a data point table of SOC and temperatures. There are a few companies that offer such Battery SOC meters that come with tables for many batteries but still require some fine tuning and calibration by the user. It will let you know when your batteries are going to the Great Boat Anchor in the Sky. When you start seeing the Ri going up from base line, its over.


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## rmay635703 (Oct 23, 2008)

Sorry I had to post it, the main reasons have been discussed but as far as OEMs are concerned smaller cells makes it easier to reach the numbers required for proper quality controls.

In this case it is easier by volume to get better quality by the numbers.

Ah well.


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## Karter2 (Nov 17, 2011)

Sunking said:


> For one AC is impedance taken at some frequency, and is not the DC resistance which will be lower, significantly lower.....


 Interesting background.. but.
if you use Elithion,s SDT Data and formula for SDT =3600*Ahr*DCIR/Cell voltage.... to deduce the DCIR, for a known cell (EG:, Wynston 40Ahr)..
...you arrive at a DCIR figure much BIGGER ( 3.3mOhm) than the Wynston data of 0.7mOhm !
That does not agree with the assumption that the Wynston data is AC impedance ( which suggests the DC IR should be " significantly lower" )...!
Further, why would any battery manufacturer go to the bother of accurately measuring and recording a performance parameter, ..call it "_*Internal RESISTANCE" *_in mOhms, ....when infact it is AC impedance, which is in practical terms useless as a indicator of condition, quality or performance.
I, for one , am still not convinced those Wynston figures are not actual DC resistance data. !

EDIT:... 
An Email question to CALB recieved a reply confirming their Internal Resistance figures are derived from DC current and voltage measurements.

Edit 2 !..
well , i take it all back.
When queried on the exact method of measurement, it turns out they use a Hioki IR meter, which just happens to use a 1kHz AC, "4 point ", measurement protocol !
So AC measurements they are after all !

But now i have to come back to that comment of Sunkings ...


> ....DC resistance which will be lower, significantly lower...


 Are you sure about that relationship, Sunking ?
since these AC IR readings i have seen from a test of 36CALB cells all ranged between 0.4 and 0.55 mOhm , it would suggest the DCIR figure would be lower still !....which puts them even more at odds with the Eithilon SDT data. ????


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