[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?
> 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.

Without battery monitoring/equalization, once the cells start to get
unbalanced, you'll overcharge some while charging and over-discharge
others while driving; this will really shorten the life of your
battery pack.

You can just cut slits in the side of the tube and put in tabs for the
battery management system there. You still have to get the BMS,
though; I don't know of an existing solution that you or I could buy.
For anything mass-produced, it would be stupid (economically/range
wise) to not use BMS, but for the first hobbyist solutions there may
not be many available/affordable BMS options.

-Morgan

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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.

Laptop battery packs generally parallel cells directly (because it's
cheap and easy). The drawback is that if a cell shorts, the others in
parallel also dump into it, leading to fires or explosions.

> During the Constant Voltage part of the charge they will self equalize.

I'm not so sure. The voltage-vs-SOC charge curve is so flat that the
self-balancing effect may be negligible. Even for lead-acids it takes
days for cells in parallel to self-balance to the same state of charge.
With lithiums it could take weeks or months.

--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in -- Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net

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[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?

With lithiums, you need to have individual monitors on every single
cell. If you don't, you have a high risk of catastrophic failure (fires,
explosions, etc.)

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

Yes. When it's all one big cell, it tends to all be at the same
temperature, and all parts of the cell were obviously made at exactly
the same time and of the same material composition.

With many small cells in parallel, they can be at different
temperatures, and from different production runs or batches of
materials. Some battery chemistries (like lithium) do a poor job of
staying at the same state of charge if simply connected in parallel. If
you connect (say) an 80% charged and a 20% charged cells in parallel, it
takes an impractically long time for them to move to the same state of
charge (50%-50%).

The big problem in balancing lithium cells is that you must prevent each
and every cell from every being overcharged or undercharged. This is
hard to do if you directly connect them in parallel.

There are also problems if one cell shorts or opens. If you can't detect
such failures and keep using the pack anyway, they can lead to much
bigger failures.
--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in -- Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net

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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
> to self equalize.

Try an experiment. Get two cells. Charge one, and discharge the other.
Connect them both in parallel for (say) 24 hours. Now separate them, and
load test each one to see how many amphours it contains.

If you're right, each cell would contain 50% capacity. My hypothesis is
that they will be more like 10%-90%.

--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in -- Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net

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>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
>> to self equalize.
>
>Try an experiment. Get two cells. Charge one, and discharge the =

other. =

>Connect them both in parallel for (say) 24 hours. Now separate them, =

and =

>load test each one to see how many amphours it contains.
>
>If you're right, each cell would contain 50% capacity. My hypothesis =

is =

>that they will be more like 10%-90%.

I think that this really would be the outcome. Li-Ions have so flat =

voltage curve that they would self-equalize very slowly.
- But does it really matter? Try experiment no. 2 ( let's say that the =

cell capacity is 100 Ah each):

First:
>Get two cells. Charge one, and discharge the other. =

>Connect them both in parallel for (say) 24 hours. =


Now, one cell has 10 Ah and the other 90 Ah.

Next, _don't_ separate them. Just connect a load resistor, start to =

discharge and see what happens. =

The outcome should probably be somewhere between the two extremes:
a) you get out 20 Ah and one dead cell
b) you get out 100 Ah and two healthy cells

I don't have even a guess where the result will be, but this would be =

a very interesting experiment!

Seppo

>
>-- =

>Ring the bells that still can ring
>Forget the perfect offering
>There is a crack in everything
>That's how the light gets in -- Leonard Cohen
>--
>Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.
net
>
>_______________________________________________
>For subscription options, see
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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
> charged cell:
>
> http://www.zeva.com.au/Equalisation.jpg

Thanks! This is the way to proceed, folks. We can speculate all we want,
but actual data is the only way to move forward with confidence.

--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in -- Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net

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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_
> connected to each other.

It still matters, because the cells are never identical.

Let's suppose their amphour capacities are identical, but their internal
resistances are different. Start with both fully charged. Discharge them
in 1 hour. If one has twice the internal resistance, it only supplies
half the amphours; so it ends up with one cell at 33% and the other at
66% SOC.

> If you started charging your buddy paired cells, one starting off fully
> charged, the other starting off fully discharged, as the more-charged
> cell reaches full, it's internal impedance goes up, and it will draw
> less current than the discharged cell.

But what if the internal impedance does *not* go up as it reaches full?
For nicad and nimh cells, their impedance goes *down* when they reach
full! For two cells in parallel, the fully charged one has the *lower*
voltage -- it hogs more current, so it overcharges more, gets hotter,
gases more, etc.

> Yes, the paralleled cells will always be at different
> temperatures (nothing is ever exactly the same), will
> have different capacities, and hence different states
> of charge. But their impedances will always be
> different, too, so they will draw and put out
> different amounts of current.

For lead-acids, the hotter the cell, the lower its internal resistance
and the higher its amphour capacity. This means the hotter cell hogs the
load and/or charging, causing it to get still hotter, etc.

All we're saying is that things like type of cell, internal resistance,
temperature, and all the other variations between them complicate
things. You can't blithely assume they will "play nice" together. You
have to *confirm* it through testing, and then have systems in place to
catch problems and deal with them before they become disasters.

--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in -- Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net

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Lee Hart wrote:
> 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
>>charged cell:
>>
>>http://www.zeva.com.au/Equalisation.jpg
>
>
> Thanks! This is the way to proceed, folks. We can speculate all we want,
> but actual data is the only way to move forward with confidence.
>


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

>
> 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?
>

That was the point I was trying to make. While I have
not made direct measurements (can't really put an
emeter on each of the two cells, I think the
resistance of each shunt would be significant), I
believe the above to be true. I guess I could
parallel the full and discharged cells, wait, and then
charge the buddy pair for various amounts of time,
then disconnect and discharge, measuring how full they
were, but I barely have enough time to read the EVDL
list, much less do all the experiments I'd like to.

I think what it boils down to, what can you do instead
of paralleling several smaller cells? Yes, you can
parallel each string of cells, but then you would need
a BMS module for _every single_ cell. For what
currently interests me (2.3 amp hour A123 cells), that
would be too many, keeping in mind that the odds of a
BMS failure rise exponentially with the number of
components added. For example, the Killacycle has 110
cells in series, and the latest pack will have 11 in
parallel. Right now, we need 110 BMS modules. If he
managed each string individually, we would need 1,210
modules. We can't afford the cost, size, or weight.
So that's not an option.

I am not claiming to be an expert, so if someone
disagrees with me, please let me know, so we all can
learn. I hope I don't sound like I'm saying "any
other way is too hard, so my way is best", 'cause
that's not a logical argument.

- Steven Ciciora

<snip>


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