# [EVDL] LiFePO4 Battery Packs Small vs Large Cells



## EVDL List (Jul 27, 2007)

I don't know big or small witch is best but check this out for a tab welder, 
to bad it sounds like you already have bought one. they say for around 100 
bucks http://www.ledhacks.com/power/battery_tab_welder.htm
----- Original Message ----- 
From: "Roger Heuckeroth" <[email protected]>
To: "Electric Vehicle Discussion List" <[email protected]>
Sent: Thursday, July 24, 2008 11:49 PM
Subject: [EVDL] LiFePO4 Battery Packs Small vs Large Cells


> This may be obvious to some of you, but I have been deliberating over
> LiFePO4 battery options for some time now and have come to the
> following conclusions. I figure I'll put this out there and see if
> anyone disagrees.
>
> The conclusion that I have come to is that building packs from
> multiple smaller cells is better that using the larger prismatic
> cells... at least for EV applications. For the following reasons:
>
> 1. Smaller cells are typically capable of higher C discharge and
> charge rates. I don't know why this is, but it seams to be the case
> almost clear across the board. Higher C rates allow for better peak
> performance (acceleration, climbing hills). Also the need for
> expensive Ultra-caps becomes a mute point.
> 2. The smaller cells are manufactured in a more controlled manner
> sometimes by the tens of thousands per day. All by machines. All
> these machines are designed and built in the USA or Europe, even if
> the manufacturing is done in China. I hope I don't offend anybody's
> nationalism, but keeping unskilled labor out of the process would lead
> to less variation from cell to cell within a given batch. Many of the
> larger cells are still built in part with manual labor.
> 3. Cells still vary in performance, however, when you tie a large
> number of them in parallel they have to act as one large cell. The
> characteristics variations from one cell to another are averaged out.
> Statistically speaking you are building an averaging circuit by
> putting them in parallel. All of them share the same voltage from full
> charge to full discharge. This is of course figuring that you are
> putting parallel strings in series, not series strings in parallel.
> 4. By virtue of the averaging of parallel cell performance I believe
> you increase the performance of the whole pack. Theoretically, if all
> parallel strings have the same capacity you don't even need a BMS...
> OK some of you are thinking " oh yes you do." You do need a simple
> charge and discharge limit control, but you do not need an active
> equalization system that shuffles power about while your discharging
> the cells. With a well balanced (averaged out) pack all the cells
> will hit bottom at about the same time. Without active charge
> equalization a series pack of larger cells is limited by its weakest
> cell. It is the first to hit full charge causing wasted charging
> energy as its over-voltage protection circuit shunts current around
> the cell, and its the first to trigger the low voltage shut off
> circuit once it bottoms out. There could still be several kWhrs of
> energy left in the other cells, but to protect the week one from over
> discharge the whole pack shuts down. It takes a very sophisticated
> BMS system to actively shuffle power about while the pack is being
> discharged.
> 5. Small cells give you more design flexibility to custom design the
> pack to fit what space you have available. When I was looking at
> prismatic cells I was going to have split the pack up into several
> areas. With the smaller cell pack it looks like I can fit the whole
> pack 300V, 15kWhr in one spot, and it looks like it will be about 150
> lbs less than the prismatic cells.
> 6. Should the pack need active cooling there is space between the
> cells for air flow by nature of the design.
> 7. Since smaller cells are more of a commodity, their price should
> come down faster than the larger cells in the future.
> 8. Simpler BMS, or did I already say that in #4. Simple over/under
> voltage protection should work fine.
>
> Ok so there are a few disadvantages:
>
> 1. You need a battery tab welder to construct the pack. A bit
> pricey, but actually, for me it will be a business deduction, so that
> helps. Or you have to have someone else construct the pack for you
> that has the right equipment and expertise.
> 2. You have to build your own custom battery case. I'll fabricate my
> own... we do a lot of plastic fabrication in one of my businesses.
> 3. If you don't do a good job at welding the tabs, one could come
> loose throwing the pack off. If the pack is properly built this
> should be easy to diagnose and fix. Not so easy if all parallel
> strings are welded together like the SSI Racing A123 pack (no
> offense). However, get your weld right and there should be no problem.
> 4. More time labor intensive to put the pack together. But then
> again I have probably spent more time researching this stuff than it
> will actually take me to build it.
>
> So, who agrees with the above and who disagrees?
>
>
> _______________________________________________
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## EVDL List (Jul 27, 2007)

I'll be interested to see what the consensus is on BMS for small vs. 
large cell packs. What you say makes sense, but is that how it really 
works? What do they do in the drag bikes that use the A123 cells?

How does pricing compare?

The tab welding is what scares me off the small packs. From what I've 
seen you need a real tab welder to be sure not to damage the cells and 
they are expensive.

I do like the idea that small cells give you a lot more freedom in 
designing the pack form factor.

- SteveS



> Roger Heuckeroth wrote:
> > This may be obvious to some of you, but I have been deliberating over
> > LiFePO4 battery options for some time now and have come to the
> > following conclusions. I figure I'll put this out there and see if
> ...


----------



## EVDL List (Jul 27, 2007)

I came across that on the internet. when doing a search. There's a 
bunch of homebuilt plans out there. Its tempting to build my own, but 
good welds are a critical part of the assembly process. Typically 
there are multiple welds on each battery side.

Years ago I built my own vacuum forming machine for my business. We 
were making parts 1-10 at a time and no professional shop wanted to 
take my business. When they set up a machine they want to do 100s or 
1000s of parts. I also made my own wooden molds. It actually worked 
out terrific. I built the whole machine for under $500. Vs $20,000 
for a used machine. We still use this vacuum former today. Its 
definitely slower than the production machines out there, but this 
particular product is still built in small runs with frequent mold 
changes.

However, a professional dual pulse battery tab welder will probably do 
a much better weld than something than something I throw together. I 
don't really have a surplus of time these days, and my business could 
use some write-offs, so I'll probably go ahead and buy one if I go 
this route. Also, this is my first conversion, but may not be my 
last. I may even get into the custom battery pack building 
business... who knows.




> Jack Riggi wrote:
> 
> > I don't know big or small witch is best but check this out for a tab
> > welder,
> ...


----------



## EVDL List (Jul 27, 2007)

From what I have seen, joining cells in parallel, and then in series 
is how all the pros do it. Parallel strings are forced to run at the 
same voltage, so they hit 3.65 V all at the same time, and 2.5 volts 
all at the same time, so you only have to monitor and control one 
voltage for each parallel string. So BMS is the same architecture as 
with the large cells.

A123s are still super expensive, however, you can get cells that will 
be comparable to TS cells in $/Ahr, and are a hell of a lot more 
potent... 10X current with minimal voltage sag compared to TS blocks 
of the same Ahr.

Yes, welds are critical. Good equipment that is specific to the job 
is key.



> SteveS wrote:
> 
> > I'll be interested to see what the consensus is on BMS for small vs.
> > large cell packs. What you say makes sense, but is that how it really
> ...


----------



## EVDL List (Jul 27, 2007)

> The conclusion that I have come to is that building packs from 
> multiple smaller cells is better that using the larger prismatic 
> cells... at least for EV applications. For the following reasons:
>
> 1. Smaller cells are typically capable of higher C discharge and 
> charge rates. I don't know why this is, but it seams to be the case 
> almost clear across the board. Higher C rates allow for better peak 
> performance (acceleration, climbing hills). Also the need for 
> expensive Ultra-caps becomes a mute point.

I am not sure, but I seem to recall the prismatic cells had
higher internal resistance owing to the way the material was
packed into the corners of the square space. The material is
further away and has a longer path to egress the cell, hence 
higher resistance.

Perhaps someone on the list can confirm that.

Janet





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## EVDL List (Jul 27, 2007)

Here's the company I think I would go with.

http://sunstoneengineering.com/

They can do any type of small spot welds. Fully adjustable.



> Janet Plato wrote:
> 
> >> 1. You need a battery tab welder to construct the pack.
> >
> ...


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## EVDL List (Jul 27, 2007)

> Roger Heuckeroth wrote:
> > From what I have seen, joining cells in parallel, and then in series
> > is how all the pros do it.
> 
> ...


----------



## EVDL List (Jul 27, 2007)

Lee,

Thanks for you feedback.

Its not because we can't get larger cells. Its because the smaller 
cells have a higher C rating with less voltage sag. It would be more 
expedient just to buy TS cells and wire them up, but the performance 
is not there.

I think it would be smart to test every cell first... just a simple 
test to measure discharge / charge capability and to make sure the 
cell isn't a dud would suffice.

I don't understand you theory about paralleled cells not self 
equalizing. Can you expand on that? The way I see it they would all 
act together at the best of their ability, but they are all tied 
together, so they can not work at different voltages. If 3.65 V is 
100% SOC and 2.5V is say 5% SOC then if after you load them for a 
while they will all balance out at a certain voltage and SOC. No 
single cell will hit 2.5V before another. Sure the stronger ones will 
do a higher proportion of the work, but the all are forced to work as 
a team. Help me out here, I don't understand how they can not be 
equalized?

Large cells an develop an internal short also. This normally comes 
from lithium plating if you over discharge a cell. How would a short 
in a parallel pack be any different than a short in a large cell? 
Also if a short did happen because of a failed vow voltage cut off 
circuit for example, it would not accept a charge, and your BMS would 
notice this upon the next charge. We are talking LiFePO4 cells here 
that can be completely shorted out and not cause a fire.

Roger



> Lee Hart wrote:
> 
> > Roger Heuckeroth wrote:
> >> From what I have seen, joining cells in parallel, and then in series
> ...


----------



## EVDL List (Jul 27, 2007)

Roger,

Actually I think it's more so that no one wants to pay the premium for the 
larger cells. There are larger cells with higher C ratings. TS cells however 
do not make that list. 

Yes, testing every cell would be smart, though it wouldn't answer everything.

>From what I have read, individual Lithium based cells do not equalize well at 
all in strings. This is why Victor (I believe with the Honda CRX?) attached a 
BMS to each cell in his pack. The cells don't all charge at the same rate 
even if they are the exactly the same down to tested specifications. 
Temperature and individual cell wear also effect charging over time.

If stronger cells are strung together with weaker cells the weaker cells with 
take a higher hit on the discharge. These cells would not charge to full in 
the same timeframe and overcharging lithium based batteries (I am told) isn't 
the wisest.

With Lead or Nickel based battery technologies this isn't as much of a problem 
due to overcharge abilities.

To do this properly you really should have a decent battery management system 
worked out. I haven't personally done a lot of testing on these batteries, 
I've just been doing several years of reading. I could be 100% wrong as 
always. 

Christopher



> On Friday 25 July 2008 01:14:41 pm Roger Heuckeroth wrote:
> > Lee,
> >
> > Thanks for you feedback.
> ...


----------



## EVDL List (Jul 27, 2007)

Christopher,

Thanks for your take on it.

What larger cells with higher C-ratings are you referring to? I have 
not come across any large LiFePO4 cells, 50 Ahr or above, that have a 
10 C continuous rating. The best I have seen is 5C, but that was with 
voltage sagging down to 2.6V.

Victor used a series string, no parallel cells in his pack as far as 
I'm aware of.

We use BMS as a catch all term that can mean a lot of things. You 
definitely should have charge regulators to prevent over voltage on 
any one cell. The circuit simply clamps the voltage at some level 
(usually 3.65V) you could leave a cell on this voltage for an extended 
amount of time. It won't over-charge, it will simply not accept any 
more current. If you clamp a parallel string at this voltage the 
current will decrease until there is hardly any current flowing. Each 
parallel string needs one over and one under voltage protection 
circuit. That's all.

Roger



> Christopher Frost wrote:
> 
> > Roger,
> >
> ...


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## EVDL List (Jul 27, 2007)

> Roger Heuckeroth wrote:
> > Lee,
> >
> > Thanks for you feedback.
> ...


----------



## EVDL List (Jul 27, 2007)

Lee,

I'm trying to understand all this.

How are we adding resistance by putting cells in parallel. Doesn't 
resistance follow the rule of

1/Rp = I/R1 + 1/R2 ... + 1/Rn

If each cell had an internal resistance of say 8 mohm, the equivalent 
parallel resistance of say 10 cells in parallel should be 0.8 mohm. 
Am I missing something?

Are you are referring to workmanship issues like sloppy spot welds, or 
not enough weld surface area?

Roger

Lee Hart wrote:


> Lee Hart wrote:
> 
> > It's not really there with the small cells, either. One cell by itself
> > may have a high C rating; but when you bundle large groups of them
> ...


----------



## EVDL List (Jul 27, 2007)

> Roger Heuckeroth wrote:
> > Christopher,
> >
> > Thanks for your take on it.
> ...


----------



## EVDL List (Jul 27, 2007)

Roger Heuckeroth wrote:
> 
> Lee,
> 
> I'm trying to understand all this.
> 
> How are we adding resistance by putting cells in parallel. Doesn't 
> resistance follow the rule of
> 
> 1/Rp = I/R1 + 1/R2 ... + 1/Rn
> 
> If each cell had an internal resistance of say 8 mohm, the equivalent 
> parallel resistance of say 10 cells in parallel should be 0.8 mohm. 
> Am I missing something?
> 
> 

I agree -- however this does explicity support the concept of hammering
the better cells (until they are not . The cell with least resistance
will
supply the most current.





> Are you are referring to workmanship issues like sloppy spot welds, or 
> not enough weld surface area?
> 
> Roger
> 
> Lee Hart wrote:
>


> Lee Hart wrote:
> >
> >> It's not really there with the small cells, either. One cell by itself
> >> may have a high C rating; but when you bundle large groups of them
> ...


----------



## EVDL List (Jul 27, 2007)

So Victor what is your recommendation for those looking for 
higher Ahrs, high C and low ESR on a (reasonable) budget 

Sounds like we are trying to defy the laws of physics (and our wallets  
or should that be :-(

mike






> Victor Tikhonov wrote:
> >
> > Roger Heuckeroth wrote:
> > > Christopher,
> ...


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## EVDL List (Jul 27, 2007)

> Roger Heuckeroth wrote:
> > How are we adding resistance by putting cells in parallel. Doesn't
> > resistance follow the rule of
> >
> ...


----------



## EVDL List (Jul 27, 2007)

Roger,

Ah, you are right. I generally group chemistry's together in my head. On this 
occasion I was grouping Lithium together. I was thinking of Enax, Kokam, and 
Saft.

Victor made a very good reply, since the others are more experienced I won't 
argue with their answers. 

I agree on the "BMS" term. It would be educational to learn what the best way 
to manage and monitor batteries would be though. I might start looking into 
that. For someone like me, I have a lot of patience and time. For money.. 
well not quite. 2 out of 3 though. ;-)



> On Friday 25 July 2008 03:32:35 pm Roger Heuckeroth wrote:
> > Christopher,
> >
> > Thanks for your take on it.
> ...


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## EVDL List (Jul 27, 2007)

Victor,

Thanks for the explanation. I think the simplest is often the best 
approach. Full equalization charges every night with shunt regulators 
is the way I sill go.


Roger



> Metric Mind wrote:
> 
> > Roger Heuckeroth wrote:
> >> Christopher,
> ...


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## EVDL List (Jul 27, 2007)

Yes, I believe this is very reasonable compromise and small penalty to 
pay (having to wait to charge *completely* preferably every time)
for greatly simplifying the "BMS".

Victor




> Roger Heuckeroth wrote:
> > Victor,
> >
> > Thanks for the explanation. I think the simplest is often the best
> ...


----------



## EVDL List (Jul 27, 2007)

> txhokie4life wrote:
> > So Victor what is your recommendation for those looking for
> > higher Ahrs, high C and low ESR on a (reasonable) budget
> 
> ...


----------



## EVDL List (Jul 27, 2007)

> Lee Hart wrote:
> 
> >...If a cell shorts, the ones in parallel dump *all* their energy
> >into it, creating a fire hazard.
> ...


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## EVDL List (Jul 27, 2007)

I plan on testing that theory. I figure I'll get ten 25650 cells. 
Place them in parallel. Use some kind of current sensor on each cell 
and cycle them for a while. I have a CC/CV power supply capable of 60 
amps. Any suggestions on the test set up?



> txhokie4life wrote:
> 
> >
> >
> ...


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## EVDL List (Jul 27, 2007)

I'm not real comfortable with the other Lithium chemistries. I 
watched a series of test on LiFePO4. These were extreme over-charge 
(50V), crush, puncture, short, high temp then short, etc. I none of 
the tests did these cells come even close to combusting, or even 
smoking. The Kokams look like a nice cell, but I'm not sure about the 
safety, and the cost is 2-3X what my budget is.



> Christopher Frost wrote:
> 
> > Roger,
> >
> ...


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## EVDL List (Jul 27, 2007)

Given the way that I envision EV'ers use their vehicles, this (a complete
charge) would seem to be a small limitation to me.

newbie question: "shunt regulator"="clamper"... true?

Is it inadvisable to parallel the 10ah Headway cells into 10-cell modules,
and then place a clamper on the (3.2v 100ah) module to regulate max charge
voltage? I envision building a 160v 32kwh pack by placing 50 of these
modules (batteries?) in series.

Given that an EV might use 1000 of these cells, this seems to me to enable
the designer to isolate any problems to 1% of the pack - pretty much like
your ACRX.

Given the power density and anticipated lifespan of LiFePO4, it's hard to
simply reject them as a solution. For my long-term goal, I've accepted that
cheap will be secondary to fast and long... but I'm not NASA - I still want
to land on the cheaper end of the expense continuum. 




Yes, I believe this is very reasonable compromise and small penalty to 
pay (having to wait to charge *completely* preferably every time)
for greatly simplifying the "BMS".

Victor






> Victor Tikhonov wrote:
> >
> > Yes, I believe this is very reasonable compromise and small penalty to
> > pay (having to wait to charge *completely* preferably every time)
> ...


----------



## EVDL List (Jul 27, 2007)

> Victor Tikhonov wrote:
> >
> > txhokie4life wrote:
> >> So Victor what is your recommendation for those looking for
> ...


----------



## EVDL List (Jul 27, 2007)

> Roger Heuckeroth wrote:
> > I'm not real comfortable with the other Lithium chemistries. I
> > watched a series of test on LiFePO4. These were extreme over-charge
> > (50V), crush, puncture, short, high temp then short, etc. I none of
> ...


----------



## EVDL List (Jul 27, 2007)

Victor, that's interesting. In a recent thread over on the Thundersky 
board, Jukka advocated charging the Thundersky cells to only 80% SOC, 
and discharging down no further than 20% SOC for maximum cell life. Can 
the cells be equalized each night at 80% SOC instead of 100% SOC (i.e., 
at a lower voltage)?

Thanks.

Bill Dennis



> Metric Mind wrote:
> > Yes, I believe this is very reasonable compromise and small penalty to
> > pay (having to wait to charge *completely* preferably every time)
> > for greatly simplifying the "BMS".
> ...


----------



## EVDL List (Jul 27, 2007)

Thank you for that.

I had intended to add "in two strings" - but that brings up another noob
question: 

Given the need to assemble (1000) 3.2v 10ah cells into a 32kwh pack
comprised of reasonable subassemblies, it seemed to me that linking 10 cells
in parallel into (100) 3.2v 100ah units was a good first step.

Those 100 units could then be linked in series to obtain a 320v pack, but it
seemed to me that max power draw might then become a limitation.

Is there any benefit to two packs of 160v wired in series? Is it better to
create units comprised of 20 cells?

Maybe (50) regulated 3.2v 200ah subassemblies (arranged in series into a
160v 200ah pack) is better?

Jeff




> Stephen West-2 wrote:
> >
> > 50 x 100Ah x 3.2V = 16kWh
> >
> ...


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## EVDL List (Jul 27, 2007)

Everybody's trying to sell something in this world. It doesn't make 
us all liars.

Lead acid batteries can explode if you miss use them. My father was 
in a submarine in WW2. They had lead acids, and if the battery bay 
got flooded they would short out and explode, at least that's what 
they were told. I read that the US Navy has adopted LiFePO4 because 
they are the safest technology out there. They can be shorted out, 
shot, lit on fire, and they still won't turn into fireworks.

Have you actually witnessed A123 array catch fire, or for that matter 
any LiFePO4 cell pack? Or are you talking theoretically?



> Lee Hart wrote:
> 
> > Roger Heuckeroth wrote:
> >> I'm not real comfortable with the other Lithium chemistries. I
> ...


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## EVDL List (Jul 27, 2007)

Hi Bill,

It depends on when (which SOC) the voltage of the particular cells start to
rise during charging.
For the thundersky LiFePO4 batteries, this happens to be around 90%.
Once the voltage is above 3.4v (assuming 0.2C charge rate) they are about
90% full. 
Above this point, the voltage rises rapidly, so loading each cell
independantly (say with a zenner and a resistor) so they settle around
3.6vpc means they are all more than 95% full.
The voltage curve at such a high SOC is steep, so a variation of as much as
0.1vpc either side doesn't matter so much as its just surface charge.

That's what ive found with my 48v lfp40aha pack.

Obviously at faster charge rates you need to raise the voltage, just stay
below 4vpc for long periods of time for decent service life is what ive
heard. It doesn't matter if the voltage hits 4.2vpc occasionally for a few
seconds before the BMS cuts ive also heard. Ive taken my own cells to 4.2vpc
many a time, but only for a few seconds as my BMS trips at this voltage and
the zenner and resistor take the cells back down to below 3.6vpc.

When the pack is just sitting there with no charge current at all, they
hover around 3.35vpc.

Matt

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf
Of Bill Dennis
Sent: Saturday, 26 July 2008 10:23 AM
To: Electric Vehicle Discussion List
Subject: Re: [EVDL] LiFePO4 Battery Packs Small vs Large Cells

Victor, that's interesting. In a recent thread over on the Thundersky
board, Jukka advocated charging the Thundersky cells to only 80% SOC, and
discharging down no further than 20% SOC for maximum cell life. Can the
cells be equalized each night at 80% SOC instead of 100% SOC (i.e., at a
lower voltage)?

Thanks.

Bill Dennis



> Metric Mind wrote:
> > Yes, I believe this is very reasonable compromise and small penalty to
> > pay (having to wait to charge *completely* preferably every time) for
> > greatly simplifying the "BMS".
> ...


----------



## EVDL List (Jul 27, 2007)

I think it makes a lot of sense to split the packs up into manageable 
sub-packs that are wired in series. As long as each sub-pack has its 
own BMS circuit, and they all report back to one central controller. 
That is pretty much what Valence does.



> lumberjack_jeff wrote:
> 
> >
> > Thank you for that.
> ...


----------



## EVDL List (Jul 27, 2007)

> txhokie4life wrote:
> 
> > Victor Tikhonov wrote:
> >>
> ...


----------



## EVDL List (Jul 27, 2007)

> Bill Dennis wrote:
> > Victor, that's interesting. In a recent thread over on the Thundersky
> > board, Jukka advocated charging the Thundersky cells to only 80% SOC,
> > and discharging down no further than 20% SOC for maximum cell life.
> ...


----------



## EVDL List (Jul 27, 2007)

> On 25 Jul 2008 at 21:23, Metric Mind wrote:
> 
> > Some are driving small cars, others think it isn't worth to jeopardize
> > life doing that.
> ...


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## EVDL List (Jul 27, 2007)

> Metric Mind <[email protected]> wrote:
> > Bill Dennis wrote:
> >> Victor, that's interesting. In a recent thread over on the Thundersky
> >> board, Jukka advocated charging the Thundersky cells to only 80% SOC,
> ...


----------



## EVDL List (Jul 27, 2007)

Enax and Kokam both have testing information available, I know Enax has test 
information in the following PDF (these two manufacturers have similar 
chemistries)

http://www.extraenergy.org/files/JanSteffenLang-ENAX.pdf

Whether it's a123, Kokam, Enax, Saft or even Valence they will cost a bit more 
than the bargain alternatives. Those who opt for them do end up with a better 
battery though. Has anyone looked into bulk pricing for any of these 
manufacturers? Perhaps one of them would be able to get the price down a bit?

I might toss some emails around and see what I can come up with for best price 
per watt.



> On Friday 25 July 2008 09:27:12 pm Roger Heuckeroth wrote:
> > I'm not real comfortable with the other Lithium chemistries. I
> > watched a series of test on LiFePO4. These were extreme over-charge
> > (50V), crush, puncture, short, high temp then short, etc. I none of
> ...


----------



## EVDL List (Jul 27, 2007)

A smart BMS would be able to narrow it down to a particular bank. Thermal
monitoring would be a good way to detect overworked cells.

As far as replacing a bad cell, that would not be easy with the way some of
these packs are made, however, that is something you could work into the
design.

These are all good points and I appreciate the input. If I'm going to do
this I want to do it right.


Brad Baylor wrote:
> 
>


> Lee Hart wrote:
> >
> >>...If a cell shorts, the ones in parallel dump *all* their energy
> >>into it, creating a fire hazard.
> ...


----------



## EVDL List (Jul 27, 2007)

> Roger Heuckeroth wrote:
> > Everybody's trying to sell something in this world. It doesn't make
> > us all liars.
> 
> ...


----------



## EVDL List (Jul 27, 2007)

Very interesting --- testing is sure more fun when it's someone else $$$ 
Thanks for sharing.

The way I see it is there are two problems trying to be solved.
Range (total energy storage), and acceleration (instantaneous delivery).

I was trying to think of a solution of two paks in parallel maybe
physically, 
or switched in somehow:

However:
With LiFePO4 it seems that if you solve the needs for acceleration, by 
the shear amount you need for acceleration, you have effectively addressed
range, however the reverse is not true. Acceleration needs the Amperage
at a rate that the large Ah but low C cells cannot deliver. Caveat -- this
is not entirely true,
but the Watt-Hrs need for acceleration demand such a battery solution, that
it tends to cover the Range requirements (i.e. a lot of Watt-Hrs, whether
NOW 
or in time).

I think one thing I haven't quite got my head around yet is exactly how much
is needed
NOW and what the relative difference is between instantaneous amps, and
total aHrs needed.

I think that gets back to seeing a low of range calculators -- but still
struggling with
an acceleration calculator.

thanks for the discussion folks!

mike





> Lee Hart wrote:
> >
> > Roger Heuckeroth wrote:
> >> Everybody's trying to sell something in this world. It doesn't make
> ...


----------



## EVDL List (Jul 27, 2007)

Very interesting --- testing is sure more fun when it's someone else $$$ 
Thanks for sharing.

The way I see it is there are two problems trying to be solved.
Range (total energy storage), and acceleration (instantaneous delivery).
I'll add that Range == $$, Acceleration == $$$ (or more $).
So there seems to be a desire to engineer/comprehend a solution
which makes the economic trade-offs.

I was trying to think of a solution of two paks in parallel maybe
physically, 
or switched in somehow:

However:
With LiFePO4 it seems that if you solve the needs for acceleration, by 
the shear amount you need for acceleration, you have effectively addressed
range, however the reverse is not true. Acceleration needs the Amperage
at a rate that the large Ah but low C cells cannot deliver. Caveat -- this
is not entirely true,
but the Watt-Hrs need for acceleration demand such a battery solution, that
it tends to cover the Range requirements (i.e. a lot of Watt-Hrs, whether
NOW 
or in time).

I think one thing I haven't quite got my head around yet is exactly how much
is needed
NOW and what the relative difference is between instantaneous amps, and
total aHrs needed. I.e. what is the ratio of the two

I think that gets back to seeing a lot of range calculators -- but still
struggling with
an acceleration calculator.

thanks for the discussion folks!

mike





> Lee Hart wrote:
> >
> > Roger Heuckeroth wrote:
> >> Everybody's trying to sell something in this world. It doesn't make
> ...


----------



## EVDL List (Jul 27, 2007)

> txhokie4life wrote:
> 
> > I think that gets back to seeing a low of range calculators -- but still
> > struggling with
> ...


----------



## EVDL List (Jul 27, 2007)

Here's an interesting analysis of fire involving a Prius conversion
and A123 cells. Apparently a bad connection caused it. Also
interesting is that they don't recommend paralleling cells without
fusing due to the reason Lee stated. Here's a quote from the
analysis:

"The full-sheet bus bar system does not provide for cell-by-cell
fusing as is recommended for parallel cell assemblies. In parallel
assemblies, a short from one cell to another cell in the next series
string causes the energy of all the paralleled cells to flow through
the one cell and into the short."

http://file.sunshinepress.org:54445/toyota-prius-a123-car-fire-investigation-report-2008.pdf

Brad Baylor


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## EVDL List (Jul 27, 2007)

David,

It was only an illustration of different perception
of term "reasonable" by different people. I could
(should?) have picked another example.

I wasn't planning to discuss cars safety at all.

Victor



> EVDL Administrator wrote:
> > On 25 Jul 2008 at 21:23, Metric Mind wrote:
> >
> >> Some are driving small cars, others think it isn't worth to jeopardize
> ...


----------



## EVDL List (Jul 27, 2007)

Wow -- that is a good read -- and quite intimidating.

Quite a concern with all the home brew battery pack building
that is going on.

Does anyone know whether the high Ahr cells from TS that look 
like claymore mines, multi-cell -- or one "contiguous" cell.

The more cells there are the greater the possibility for failure.

M




> Brad Baylor wrote:
> >
> > Here's an interesting analysis of fire involving a Prius conversion
> > and A123 cells. Apparently a bad connection caused it. Also
> ...


----------



## EVDL List (Jul 27, 2007)

They are multi-plate cells, plates bolted together along
one edge (of course neg. are separate from pos., they alternate.

If you take a TS cell apart it looks like a book with pages you can flip.

Victor



> txhokie4life wrote:
> > Wow -- that is a good read -- and quite intimidating.
> >
> > Quite a concern with all the home brew battery pack building
> ...


----------



## EVDL List (Jul 27, 2007)

> Lee Hart wrote:
> 
> >>
> >
> ...


----------



## EVDL List (Jul 27, 2007)

I read this report, and I found the A123 System Design Guideline in 
the appendix to be great information.

In case you did not read the report, it was concluded that the fire 
was due to loose nut and bolt type connections. It was not due to 
paralleling the cells.

The A123 guidelines give us good insight to the levels of redundancy 
that they expect their system integrators to provide. Basically, they 
are calling for both a hardware and a software control over 
voltages. In other words, they would like to see over-voltage and 
under-voltage protection on each "series element". That is each cell 
or parallel group of cells. Plus they want to to monitor voltage on 
each series element with an A/D converter and then use software to 
monitor the same to assure that the hardware protection is working. 
That sound like good advice.

Read the report on the other things they expect in the BMS. Sounds 
like solid BMS guidelines regardless of what Lithium batteries you are 
using.



> Brad Baylor wrote:
> 
> > Here's an interesting analysis of fire involving a Prius conversion
> > and A123 cells. Apparently a bad connection caused it. Also
> ...


----------



## EVDL List (Jul 27, 2007)

I also thought it interesting that they want to track charge and discharge
values 
over time. That way if there was a consistant high volt and low volt they
could hone in on problems.

Man that would be a lot of work building that into things.

m



Roger Heuckeroth wrote:
> 
> I read this report, and I found the A123 System Design Guideline in 
> the appendix to be great information.
> 
> In case you did not read the report, it was concluded that the fire 
> was due to loose nut and bolt type connections. It was not due to 
> paralleling the cells.
> 
> The A123 guidelines give us good insight to the levels of redundancy 
> that they expect their system integrators to provide. Basically, they 
> are calling for both a hardware and a software control over 
> voltages. In other words, they would like to see over-voltage and 
> under-voltage protection on each "series element". That is each cell 
> or parallel group of cells. Plus they want to to monitor voltage on 
> each series element with an A/D converter and then use software to 
> monitor the same to assure that the hardware protection is working. 
> That sound like good advice.
> 
> Read the report on the other things they expect in the BMS. Sounds 
> like solid BMS guidelines regardless of what Lithium batteries you are 
> using.
> 
>


> Brad Baylor wrote:
> >
> >> Here's an interesting analysis of fire involving a Prius conversion
> >> and A123 cells. Apparently a bad connection caused it. Also
> ...


----------



## EVDL List (Jul 27, 2007)

The largest military lithium battery that I'm aware of is about the size of a
30ah Genesis battery. It's a primary battery with built-in condition monitor
and self-discharge feature. The latter is both for safety in disposal and
denial of resources to the enemy. There is a NiMH version in the same
physical package for use in training.

John


On Sat, 26 Jul 2008 21:46:10 -0400, Roger Heuckeroth <[email protected]>
wrote:

>
>


> Lee Hart wrote:
> >
> >>>
> >>
> ...


----------



## EVDL List (Jul 27, 2007)

Wow. I read through the entire report and if they're findings are
accurate, what a bone-head mistake.

With the plastic in there, it very well could have been conducting
through the bolt only, no surface area of terminal to surface are of
fuse. This was a fire waiting to happen.

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## EVDL List (Jul 27, 2007)

But....
It was usefull in triggering and displaying other failure modes. Good
info for design changes.

If they don't want to change they're layout I would start by sending the
nickel sheets to the laser cutter to have relieves cut in them. They
would allow the vents to blow more gracefully and create the intercell
fuseing.

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## EVDL List (Jul 27, 2007)

> Roger Heuckeroth wrote:
> >> The specific incident with A123 cells occurred with a 6p4s array...
> 
> > Very interesting. Thank you for sharing that. Is it your opinion
> ...


----------



## EVDL List (Jul 27, 2007)

hi interesting ,, got me wandering worring ,, think im adding more lock
nuts on my batteries and cable ends .. lonnie


Brad Baylor
> Here's an interesting analysis of fire involving a Prius conversion
> and A123 cells. Apparently a bad connection caused it. Also
> interesting is that they don't recommend paralleling cells without
> fusing due to the reason Lee stated. Here's a quote from the
> analysis:
>
> "The full-sheet bus bar system does not provide for cell-by-cell
> fusing as is recommended for parallel cell assemblies. In parallel
> assemblies, a short from one cell to another cell in the next series
> string causes the energy of all the paralleled cells to flow through
> the one cell and into the short."
>
> http://file.sunshinepress.org:54445/toyota-prius-a123-car-fire-investigation-report-2008.pdf
>
> Brad Baylor
>
>
> _______________________________________________
> For general EVDL support, see http://evdl.org/help/
> For subscription options, see http://lists.sjsu.edu/mailman/listinfo/ev
>
>

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## EVDL List (Jul 27, 2007)

On Sat, 26 Jul 2008 14:01:16 -0400, Brad Baylor <[email protected]>


> wrote:
> 
> >Here's an interesting analysis of fire involving a Prius conversion
> >and A123 cells. Apparently a bad connection caused it. Also
> ...


----------



## EVDL List (Jul 27, 2007)

Going through old messages here...

Here are some pictures of my abuse test with A123 cells: http:// 
www.havina.fi/a123.htm. One cell did catch fire in my (very 
sofisticated construction) nail test. This is the explanation I got 
from A123:

> ....the liquid electrolyte within the cell is an organic
> solvent and is flammable if introduced to spark or flame. However
> this is very different than the chemical reaction and combustion
> that takes place with traditional oxide based Lithium ion.

Osmo



Roger Heuckeroth kirjoitti 26.7.2008 kello 6.08:

> Have you actually witnessed A123 array catch fire, or for that matter
> any LiFePO4 cell pack? Or are you talking theoretically?
>
>


> Lee Hart wrote:
> >>
> >> I can tell you for sure that a densely packed array of A123 cells
> >> *can*
> ...


----------



## EVDL List (Jul 27, 2007)

The biggest problem for larger quantity smaller cells assembling together is
that it would defenitely cause imbalancing problem, even you have the best
BMS system. And this BMS would also be difficult to design. So no cycle life
can be ensured for these small cells. 

-----
China Hipower, best lifepo4 producer in China.
www.chinabatteries.net
[email protected]
-- 
View this message in context: http://www.nabble.com/LiFePO4-Battery-Packs-Small-vs-Large-Cells-tp18645172p19103698.html
Sent from the Electric Vehicle Discussion List mailing list archive at Nabble.com.

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## EVDL List (Jul 27, 2007)

The problem with these cells is that they are limited to 2C maximum 
discharge. This forces you to go with a large battery pack to get the 
power you need for acceleration and to climb hills. Now if they were 
5C rated then they would be just what the doctor ordered.



> James Massey wrote:
> 
> > G'day Milo (All)
> >
> ...


----------



## EVDL List (Jul 27, 2007)

> James Massey wrote:
> >
> >
> >
> ...


----------



## EVDL List (Jul 27, 2007)

Actually it is 3C now for 1 minutes.

-----
China Hipower, best lifepo4 producer in China.
www.chinabatteries.net
[email protected]
-- 
View this message in context: http://www.nabble.com/LiFePO4-Battery-Packs-Small-vs-Large-Cells-tp18645172p19118223.html
Sent from the Electric Vehicle Discussion List mailing list archive at Nabble.com.

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