# [EVDL] Battery theory



## EVDL List (Jul 27, 2007)

although I'm not sure there are many here with serious chemistry 
knowledge I will float the question anyway

is there a fundamental difference, balancing wise, between a large Ah 
cell or several smaller of same combined capacity in parallel?

Dan

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## EVDL List (Jul 27, 2007)

Dan,

I don't have much chemistry theory, but I think there's an electric circuits
explanation.

The small batteries in parallel can have slightly different values of
internal resistance. Because of this, the batteries in parallel will
discharge at different rates, so one will get to empty before the others.

Also, as they discharge at different rates, the internal cell voltages might
no longer be the same, so when you change the load, you could get
circulating currents from one battery charging another. Say one battery is
at 12.5V and the other is at 12.4V and their combined internal resistance is
15mOhm. Then You'll get 6.7A from the one battery into the other battery;
this is just a waste of energy.

Overall, fewer high Ah cells is easier to balance than more low Ah cells in
parallel.

If you use large strings of cells in parallel, it becomes much easier to
deal with these problems, and there are fewer locations where circulating
currents can occur.

This is just using my common sense and what I've read about batteries
online; I'm no expert, but I think that's how it works.

-Morgan
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## EVDL List (Jul 27, 2007)

My big question is how reliable it would be to string together cells into
larger packs without having an individual monitor on every single cell?
Like the person who suggested stringing C-sized Lifepos into a long tube,
you'd only be able to monitor the combined properties of the cells, not the
individual cells themselves. If an individual cell were to go bad in the
middle of the tube, it would be hard to isolate it.

-----Original Message-----
although I'm not sure there are many here with serious chemistry 
knowledge I will float the question anyway

is there a fundamental difference, balancing wise, between a large Ah 
cell or several smaller of same combined capacity in parallel?

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## EVDL List (Jul 27, 2007)

> [email protected] <[email protected]> wrote:
> > My big question is how reliable it would be to string together cells into
> > larger packs without having an individual monitor on every single cell?
> > Like the person who suggested stringing C-sized Lifepos into a long tube,
> ...


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## EVDL List (Jul 27, 2007)

Its very simple to find a bad or low cell in a series string. Takes only 
about 5 minutes to find the bad cell. I use to have ninety 2 volt cobalt 
cells in series for a 180 volt pack.

Take a volt meter reading across the whole battery and lets say it reads 178 
volts. Then read one half of the batteries and lets say one side reads 90 
volts and the other half reads 88 volts.

You then split the 88 volt half into two more halves which you may read 45 
volts in one side and 43 volts on the other. You then keep splitting the 
lower voltage section until you get down to one cell.

This was the methods we use when we want to find a bad street light that 
were wire all in series. Each city block had a return loop at one corner of 
the block to perform this test.

I still perform this test with my 6 volt batteries about every four months 
and found two of them that had a voltage different of 0.02 with the rest of 
the batteries. I than finish charge these two batteries for about 1 minute 
after I do a normal charge with the main battery charger.

Roland


----- Original Message ----- 
From: "Morgan LaMoore" <[email protected]>
To: "Electric Vehicle Discussion List" <[email protected]>
Sent: Tuesday, September 11, 2007 8:25 PM
Subject: Re: [EVDL] Battery theory




> > [email protected] <[email protected]> wrote:
> > > My big question is how reliable it would be to string together cells
> > > into
> > > larger packs without having an individual monitor on every single cell?
> ...


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## EVDL List (Jul 27, 2007)

My guess/hope is that because the LiFePo4 chemistry expects a cc/cv
charge that they will play well together in parallel. During the
Constant Voltage part of the charge they will self equalize. The
critical part is not to go above a certain voltage during charge and or
below a certain voltage during operation. Since voltage is the same by
definition across cells in parallel, this should work well. I cant see
them working in a tube in series easily.

So... take pack voltage and divide by cell voltage to get number of bms
modules
ie 300V/3.2V = 94 boards.
Now to have reasonable charge times, we need a certain percentage of
capacity in balance capability that will determine the amps these boards
must shuttle or dissipate.

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## EVDL List (Jul 27, 2007)

> Jeff Shanab wrote:
> > My guess/hope is that because the LiFePo4 chemistry expects a cc/cv
> > charge that they will play well together in parallel.
> 
> ...


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## EVDL List (Jul 27, 2007)

> [email protected] wrote:
> > My big question is how reliable it would be to string together cells
> > into larger packs without having an individual monitor on every
> > single cell?
> ...


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## EVDL List (Jul 27, 2007)

I won't know until I actually test, but I was hoping that it wasn't
going to be that bad. The lower internal resistance of A123 type cells
will need less of a voltage differential that corresponding lead acids
to self equalize. Barring the catastrophic failure, handled differently,
the under and over voltage protection should still work as in parallel
the voltage is equal by definition. The lowest average capacity will
determine the effective capacity of the paralleled cells. equalizing
will increase during a discharge if the state of charge is imbalanced
And temperature is very important. It becomes important to have well
matched cells that are evenly cooled.(or heated)

Building a fuse into the interconnects will allow a catastrophic single
cell failure to not start a fire, taking that cell out of the pack.

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## EVDL List (Jul 27, 2007)

> Jeff Shanab wrote:
> > I won't know until I actually test, but I was hoping that it wasn't
> > going to be that bad. The lower internal resistance of A123 type cells
> > will need less of a voltage differential that corresponding lead acids
> ...


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## EVDL List (Jul 27, 2007)

>----Alkuper=E4inen viesti----
>L=E4hett=E4j=E4: [email protected]
>P=E4iv=E4m=E4=E4r=E4: 14.09.2007 17:29
>Vastaanottaja: "Electric Vehicle Discussion List"<[email protected]>
>Aihe: Re: [EVDL] Battery theory
>


> >Jeff Shanab wrote:
> >> I won't know until I actually test, but I was hoping that it wasn't
> >> going to be that bad. The lower internal resistance of A123 type =
> 
> ...


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## EVDL List (Jul 27, 2007)

[No message]


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## EVDL List (Jul 27, 2007)

[No message]


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## EVDL List (Jul 27, 2007)

[No message]


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## EVDL List (Jul 27, 2007)

> Ian Hooper wrote:
> > Well, speculate no longer! I set up the experiment (sort of) on my
> > battery tester, just using a fully discharged cell as the load from a
> > fully charged cell, and got the following discharge curves out of the
> ...


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## EVDL List (Jul 27, 2007)

> Steven Ciciora wrote:
> > I don't believe this is as big of a problem as some
> > people might think it is. It shouldn't matter how
> > long it takes, if the multiple cells are _always_
> ...


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## EVDL List (Jul 27, 2007)

yes, how refreshing, someone actually doing something. thanks!



> Lee Hart wrote:
> > Ian Hooper wrote:
> >
> >>Well, speculate no longer! I set up the experiment (sort of) on my
> ...


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## EVDL List (Jul 27, 2007)

Self equalize was a bad term. I am talking about equalization when in
parallel and exposed to a charging voltage.
There must be a differential or current won't flow.

li-ion like CCCV charge.
hypothetically we start with these two cells at 20% and 90% and apply a
regulated charge voltage to them while connected in parallel.

wouldn't the 90%full tend to climb rapidly and take less current while
the 20% cell would tend to draw more charging current?

And I disagree with the 90/10% estimate. I have a pdf here of a study
that illustrates this is not the case.

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## EVDL List (Jul 27, 2007)

> --- Jeff Shanab <[email protected]> wrote:
> <snip>
> 
> >
> ...


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## EVDL List (Jul 27, 2007)

Thanks Ian!
I have two questions.
Hey, how did you test that, what hardware/software?
Do you have the information for the cell next to it?

Here is the test I would like to perform, but don't have the stuff.

Although in reality I believe it would be hard to get here, connecting
them together and letting them stabilize over night should put us at the
worst possible SOC difference between 2 cells in parallel.

Cell A discharged
Cell B charged
measure amps and volts of each after connection and then a side by
side comparison of individual charge currents and estimated state of
charge as a charge voltage is applied of 3.65 until current drops below
blah,blah,blah. Whatever ever the recommended charge is.

I also expect an asymptotic graph on the first cycle.

Now discharge and charge daily, I think you will see them converge not
diverge. That is the real question. How do they behave over numerous cycles.

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## EVDL List (Jul 27, 2007)

[No message]


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## EVDL List (Jul 27, 2007)

> Jeff Shanab wrote:
> > Hypothetically we start with these two cells at 20% and 90% and apply
> > a regulated charge voltage to them while connected in parallel.
> > Wouldn't the 90% full tend to climb rapidly and take less current
> ...


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## EVDL List (Jul 27, 2007)

After thinking about it two cells may not tell us what we need to know.
2 items in parallel looks an awfully lot like 2 in series LOL
But seriously, the current from one battery has no choice as to where it
will go. WE may need to have 3 or more in the test to really get a good
picture of how they behave.

Nice job on the software! What 'api' is it useing? I wanna know if I can
compile it for linux  wxgtk? cocoa, mono?





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## EVDL List (Jul 27, 2007)

I put the pdf about the study on the server

http://cvevs.jfs-tech.com/li-ion%20in%20series%20and%20parallel%20strings.pdf

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## EVDL List (Jul 27, 2007)

You left out /PDFs/ like this:

http://cvevs.jfs-tech.com/PDFs/li-ion%20in%20series%20and%20parallel%20strings.pdf

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## EVDL List (Jul 27, 2007)

*Re: [EVDL] Battery Theory*

I ran out of disk space and had to move the the link slightly.

http://cvevs.jfs-tech.com/PDFs/li-ion%20in%20series%20and%20parallel%20strings.pdf

can always just go to http://cvevs.jfs-tech.com  Sorry folks

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