Li-Ion BMS video: Top vs bottom balancing

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Li-Ion BMS video: Top vs bottom balancing

Elithion
For those of you who use Li-Ion cells, but don't have the Li-Ion BMS book, I am recording a series of videos on Li-Ion BMS issues.

The first set discusses the 8 reasons why only top balancing is appropriate for battery packs for energy use.
http://www.youtube.com/watch?v=rALhwf9ojZQ

I'll announce the next videos as they come out.

Davide
Davide Andrea
Elithion
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Re: Li-Ion BMS video: Top vs bottom balancing

AMPhibian
Hopefully you can do something about the background hissing noise.  I'll be interested to hear the explanations of many of your assertions, as they depart from some real world experiences.  Since I run a bottom balanced pack, with no BMS, and have experienced a very deep DOD, 1.77VPC under load, without losing my entire pack or a single cell, I completely disagree that deeply discharging a top balanced pack will do less damage than a bottom balanced pack.  A top balanced pack certainly would have killed one or more cells in that scenario, I lost none, and have seen no negative effects on any of my CALB cells.

Elithion wrote
For those of you who use Li-Ion cells, but don't have the Li-Ion BMS book, I am recording a series of videos on Li-Ion BMS issues.

The first set discusses the 8 reasons why only top balancing is appropriate for battery packs for energy use.
http://www.youtube.com/watch?v=rALhwf9ojZQ

I'll announce the next videos as they come out.

Davide
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Re: Li-Ion BMS video: Top vs bottom balancing

AMPhibian
In reply to this post by Elithion
OK I just watched the second video and have some problems with that.  You're voltages and watt hour numbers are misleading, you can't use 4.2V or 4.0 volts times amp hours to get watt hours since those voltage numbers only exist with charging current and quickly drop down when off the charger or even under a slight load.  This makes the watt hour differences of the cells in your example quite a bit smaller.  Additionally I don't know of any LiFePO4 cell that is half charged at 3.6 Volts.  CALB's are considered fully charged and TS/Winston are probably 85% charged or more at that voltage, depending on current of course.  
Elithion wrote
For those of you who use Li-Ion cells, but don't have the Li-Ion BMS book, I am recording a series of videos on Li-Ion BMS issues.

The first set discusses the 8 reasons why only top balancing is appropriate for battery packs for energy use.
http://www.youtube.com/watch?v=rALhwf9ojZQ

I'll announce the next videos as they come out.

Davide
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Re: Li-Ion BMS video: Top vs bottom balancing

AMPhibian
In reply to this post by Elithion
Also worth noting that you seem to treat balancing as needing to occur at each charge cycle, but with a well matched pack of cells balancing should be a rare occurrence.  That's how I, and others, get away with no BMS and only an occasional bottom balance.  That bottom balance is done at low current, not the high current you suggest, making it just as accurate as top balancing.  I would like to understand better why TS/Winston cells need to see 4.0V on a regular basis when, as far as I know, all other cells show longer life when consistently undercharged.
Elithion wrote
For those of you who use Li-Ion cells, but don't have the Li-Ion BMS book, I am recording a series of videos on Li-Ion BMS issues.

The first set discusses the 8 reasons why only top balancing is appropriate for battery packs for energy use.
http://www.youtube.com/watch?v=rALhwf9ojZQ

I'll announce the next videos as they come out.

Davide
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Re: Li-Ion BMS video: Top vs bottom balancing

gottdi
In reply to this post by AMPhibian
I too am running BMS free and have a bottom balanced. Pack. No one that I know would mismatch a pack like running a 50 AH cell and a 100 AH cell. But even so the limiting factor is and will always be the small capacity cell. You can't count total capacity because you never use the higher capacity. By bottom balancing vs top you get the safety factor at the bottom of the charge. So his set up is saying that the 50 AH cell will discharge first but it will in fact keep running if allowed on the one cell that still has 50 AH worth of power and will run that through the low cell and you may never know it. In effect ruining the one cell.

It is easy to set up a bottom balanced pack and safely run BMS free. If you top balance and always charge to the top most available voltages you only have a tiny bit of extra capacity stuffed in that tiny space which will get burned off in the first couple miles anyway. So I'd rather not over charge my cells like that and would rather know that my cells are going to be fine if I go to the extreme on the bottom of the charge. I will at least know that my cell will still be balanced even when I am creeping along. If I really need more capacity I will just add into the pack an extra cell or two and compensate my charge algorithm. I have built into my charger the ability to add in up to 40 cells and still run a 120 volt pack. Staying both off the top of the curve and the bottom. My pack also being only a few ah out of balance at the top and not 50. Since I never fully charge then I don't' need to worry about mucking up any cells. It truly is unusable power anyway since my low AH cell is the deciding factor.

Thank you much but I will keep running BMS free and keep my cells safe. No extremes on either side.

Pete :)

http://onegreenev.blogspot.com/
No need to wait any longer. You can now buy one off the shelf. You can still build one too.
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Re: Li-Ion BMS video: Top vs bottom balancing

gottdi
Just a reminder, I am stopped before the wooooosh at the bottom of the charge curve. My controller won't allow my to go into the wooooosh zone.
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Re: Li-Ion BMS video: Top vs bottom balancing

gottdi
The BMS issue is really an issue of keeping you from driving pack voltage through the low capacity cells as they reach the low end of the charge. Keep the bottom balanced and they all go into that zone together saving your expensive cells. Bottom balancing is a safe way to keep cells safe. Driving pack amperage and voltage through an empty cell will kill that cell in seconds. You would never even know it while your driving. It is done quietly and quickly. Ask me how I know?
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Re: Li-Ion BMS video: Top vs bottom balancing

gottdi
In reply to this post by gottdi
And where did you dig up the information that ALL these kind of cells MUST be charged to 4.0 volts?
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Re: Li-Ion BMS video: Top vs bottom balancing

gottdi
I consistently charge to 3.65 volts.
http://onegreenev.blogspot.com/
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Re: Li-Ion BMS video: Top vs bottom balancing

Bill Dube
In reply to this post by AMPhibian
 >>>> Self-discharge is the prime cause of imbalance <<<<
The difficulty is that even with perfectly matched cells, the
temperature of each cell is far from matched in a battery pack. The
self-discharge is strongly influenced by the cell temperature.
Self-discharge differences are the prime culprit in cell imbalance.

The pack will indeed go out of balance over time because of
differences in self-discharge.

Unless you take extreme measures, cell temperatures will vary greatly
in an EV pack. One of the main difficulties is the heat transfer,
both in and out, of the power cables. They conduct heat out of the
end cells when the pack is warmer than the surroundings, and conduct
heat into the end cells when the pack is cooler than the
surroundings. Thus, the end cells, and the middle cells, tend to have
a different SOC than the rest of the pack.

 >>>>>>>>> There are three distinct functions of a BMS. "Bottom
balancing" does not provide all three <<<
1) The BMS stops the charger from over-charging any one of the cells.
2) The BMS stops the controller from over-discharging any one of the cells.
3) The BMS balances the cells (typically at the end of charge.)

Bottom balancing only accomplishes #3. It only does this well when
you perform bottom balance procedure.
If the cells get out of balance, say, during an extended storage of
the vehicle between uses, you will over-discharge a cell, since there
is no sensing of the individual cell voltages. Additionally, there is
nothing to stop an overcharge of one of the cells during charging.

 >>>> EVs aren't an application where ottom balancing makes sense <<<

         In a pack used for a radio-controlled airplane, bottom
balancing is a perfectly good approach. Here is why:
1) You fully discharge the pack with nearly every flight. Thus you
can do a bottom balance frequently and easily. (EVs rarely fully
discharge the pack, so the opportunity to bottom balance is rare.)
2) Setting such a small pack on fire is not a life-safety issue. (An
EV pack fire will easily burn down your house. It will burn the cars
parked on either side of it.)
3) The number of cells in series is small, so a single cell voltage
strongly alters the total pack voltage, lessening the severity of the
over-charge and over-discharge.
4) Replacing a pack is inexpensive, so a full cycle life is not worth
the expense of a BMS..
5) EVs typically charge unattended. RCs typically charge while you watch.

         An EV is not the same as an RC. Bottom balancing is not
appropriate for EV applications.

Bill D.



At 08:15 PM 7/21/2011, you wrote:

>Also worth noting that you seem to treat balancing as needing to occur at
>each charge cycle, but with a well matched pack of cells balancing should be
>a rare occurrence.  That's how I, and others, get away with no BMS and only
>an occasional bottom balance.  That bottom balance is done at low current,
>not the high current you suggest, making it just as accurate as top
>balancing.  I would like to understand better why TS/Winston cells need to
>see 4.0V on a regular basis when, as far as I know, all other cells show
>longer life when consistently undercharged.
>
>Elithion wrote:
> >
> > For those of you who use Li-Ion cells, but don't have the Li-Ion BMS book,
> > I am recording a series of videos on Li-Ion BMS issues.
> >
> > The first set discusses the 8 reasons why only top balancing is
> > appropriate for battery packs for energy use.
> > http://www.youtube.com/watch?v=rALhwf9ojZQ
> >
> > I'll announce the next videos as they come out.
> >
> > Davide
> >
>
>
>--
>View this message in context:
>http://electric-vehicle-discussion-list.413529.n4.nabble.com/Li-Ion-BMS-video-Top-vs-bottom-balancing-tp3685346p3685769.html
>Sent from the Electric Vehicle Discussion List mailing list archive
>at Nabble.com.
>
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Re: Li-Ion BMS video: Top vs bottom balancing

gottdi
This post was updated on .
1) The BMS stops the charger from over-charging any one of the cells.
2) The BMS stops the controller from over-discharging any one of the cells.
3) The BMS balances the cells (typically at the end of charge.)
1. My Elcon Charger stops the charge quite fine at the set voltage of 3.65 volts. No cell ever goes over 3.8 volts.
2. My Synkromotive controller prevents the use of the car below a set point of 2.5 volts also limits amperage  draw below a specific set point too.
3. When I bottom balanced my cells I no longer had any problems. My pack runs cool and my cells are all in balance at the end of discharge. No cell is below 2.4 volts at termination. NO BMS.


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Re: Li-Ion BMS video: Top vs bottom balancing

Bill Dube

>
>1. My Elcon Charger stops the charge quite fine at the set voltage of 3.65
>volts. No cell ever goes over 3.8 volts.

Does the charger or system measure individual cell voltages? If not,
how do you actually know what the maximum cell voltage is?

If the "set voltage" is 3.65 volts, how is it possible for some of
the cells reach 3.8 volts? Do you measure the individual cell
voltages continuously, or do you just assume what they are based on
pack voltage or occasional spot checks with a voltmeter?

>2. My Synkromotive controller prevents the use of the car below a set point
>of 2.5 volts also limits amperage  draw below a specific set point too.

The Synkromotive controller has no provision for sensing the
individual cell voltage. How would you actually know if a cell went
below 2.5 volts?

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Re: Li-Ion BMS video: Top vs bottom balancing

Jukka Järvinen-2
I love the tread here ! :)

Bottom balancing and No BMS surely works. For a while.

I did that with LiNiCoO2-cells too until the temperature and
efficiency differences drove the pack off balance. With LiFePO4 you
get away with it for a longer time and possibly without dramatic exits
to afterlife.

Until eventually the weakest cell determines the capacity of the pack.
Then you can replace that cell and go on until the next one needs to
be changed.

You'll eventually swap all cells and will be doing that for a hobby.
Or you can just change all cells every 5 years. Calculate how much
it'll cost to you go that route.

For a commercial company that is not an option. Car has to work 15
years without issues. That's a standard what existing vehicle industry
requires from new start-ups.

If I could I'd go for actively balanced pack (politics here). Matching
the DOD and SOC % according the efficiency differences. That route has
provided excellently working packs for years (+6) at customers hands
on daily drivers.

I'd say that there's a LOT to be learned here. Keep open mind and
never say never. As we learn something about the cells they've just
changed the production methods or materials. Constantly changing maze.

BMS has to learn and it has to be very versatile computing machine. If
you ask my opinion.

-akkuJukka

http://www.google.com/profiles/jarviju#about



2011/7/22 Bill Dube <[hidden email]>:

>
>>
>>1. My Elcon Charger stops the charge quite fine at the set voltage of 3.65
>>volts. No cell ever goes over 3.8 volts.
>
> Does the charger or system measure individual cell voltages? If not,
> how do you actually know what the maximum cell voltage is?
>
> If the "set voltage" is 3.65 volts, how is it possible for some of
> the cells reach 3.8 volts? Do you measure the individual cell
> voltages continuously, or do you just assume what they are based on
> pack voltage or occasional spot checks with a voltmeter?
>
>>2. My Synkromotive controller prevents the use of the car below a set point
>>of 2.5 volts also limits amperage  draw below a specific set point too.
>
> The Synkromotive controller has no provision for sensing the
> individual cell voltage. How would you actually know if a cell went
> below 2.5 volts?
>
> _______________________________________________
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Re: Li-Ion BMS video: Top vs bottom balancing

David Nelson-5
Jukka,

You and Davide are the only ones I've heard anything about TS cells
needing to be charged to 4.00V either periodically or on every charge.
Do you have any more information about that or is it just something
you discovered by testing?


2011/7/21 Jukka Järvinen <[hidden email]>:

> I love the tread here ! :)
>
> Bottom balancing and No BMS surely works. For a while.
>
> I did that with LiNiCoO2-cells too until the temperature and
> efficiency differences drove the pack off balance. With LiFePO4 you
> get away with it for a longer time and possibly without dramatic exits
> to afterlife.
>
> Until eventually the weakest cell determines the capacity of the pack.
> Then you can replace that cell and go on until the next one needs to
> be changed.
>
> You'll eventually swap all cells and will be doing that for a hobby.
> Or you can just change all cells every 5 years. Calculate how much
> it'll cost to you go that route.
>
> For a commercial company that is not an option. Car has to work 15
> years without issues. That's a standard what existing vehicle industry
> requires from new start-ups.
>
> If I could I'd go for actively balanced pack (politics here). Matching
> the DOD and SOC % according the efficiency differences. That route has
> provided excellently working packs for years (+6) at customers hands
> on daily drivers.
>
> I'd say that there's a LOT to be learned here. Keep open mind and
> never say never. As we learn something about the cells they've just
> changed the production methods or materials. Constantly changing maze.
>
> BMS has to learn and it has to be very versatile computing machine. If
> you ask my opinion.
>
> -akkuJukka
>
> http://www.google.com/profiles/jarviju#about
>

--
David D. Nelson
http://evalbum.com/1328

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Re: Li-Ion BMS video: Top vs bottom balancing

Jukka Järvinen-2
Both. Seen, learned and developed theories. Tested and found them working.

After some life on the cells (3-4 years) you see the capacity drop and
internal resistance grow. Dependes on cell of course.. symptoms can
vary. Charging to formation voltages you'll regenerate some life back
to the cell. You do not need to stay up there for hours. Just visit
the voltage and very high SOC. Just enough to eat out some of the
extra electrolyte. Then again.. depends on cell design if there is
extra. I believe A123 do not for example.

Battery chemists say otherwise because there's nothing in chemical
equation that would support this claim. Then again in mechanical
structures and the construction of the cell.. it does make sense.
Battery engineers are not chemists. And other way around.

Also another thing that chemists do not support is the Lithium
transit. It's the method how the electrolyte can actually transport
Lithium Ions between electrode pairs. And this makes ALL the
difference between small paralleled cells and one electrolyte
compartment where are more pairs paralleled.

I'm talking here battery chemists as they would be one united front..
I talk about the people I've met and had pleasure to argue.

-akkuJukka

http://www.google.com/profiles/jarviju#about



2011/7/22 David Nelson <[hidden email]>:

> Jukka,
>
> You and Davide are the only ones I've heard anything about TS cells
> needing to be charged to 4.00V either periodically or on every charge.
> Do you have any more information about that or is it just something
> you discovered by testing?
>
>
> 2011/7/21 Jukka Järvinen <[hidden email]>:
>> I love the tread here ! :)
>>
>> Bottom balancing and No BMS surely works. For a while.
>>
>> I did that with LiNiCoO2-cells too until the temperature and
>> efficiency differences drove the pack off balance. With LiFePO4 you
>> get away with it for a longer time and possibly without dramatic exits
>> to afterlife.
>>
>> Until eventually the weakest cell determines the capacity of the pack.
>> Then you can replace that cell and go on until the next one needs to
>> be changed.
>>
>> You'll eventually swap all cells and will be doing that for a hobby.
>> Or you can just change all cells every 5 years. Calculate how much
>> it'll cost to you go that route.
>>
>> For a commercial company that is not an option. Car has to work 15
>> years without issues. That's a standard what existing vehicle industry
>> requires from new start-ups.
>>
>> If I could I'd go for actively balanced pack (politics here). Matching
>> the DOD and SOC % according the efficiency differences. That route has
>> provided excellently working packs for years (+6) at customers hands
>> on daily drivers.
>>
>> I'd say that there's a LOT to be learned here. Keep open mind and
>> never say never. As we learn something about the cells they've just
>> changed the production methods or materials. Constantly changing maze.
>>
>> BMS has to learn and it has to be very versatile computing machine. If
>> you ask my opinion.
>>
>> -akkuJukka
>>
>> http://www.google.com/profiles/jarviju#about
>>
>
> --
> David D. Nelson
> http://evalbum.com/1328
>
> _______________________________________________
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Re: Li-Ion BMS video: Top vs bottom balancing

AMPhibian
In reply to this post by Bill Dube
Bill Dube wrote
Unless you take extreme measures, cell temperatures will vary greatly
in an EV pack. One of the main difficulties is the heat transfer,
both in and out, of the power cables. They conduct heat out of the
end cells when the pack is warmer than the surroundings, and conduct
heat into the end cells when the pack is cooler than the
surroundings. Thus, the end cells, and the middle cells, tend to have
a different SOC than the rest of the pack.
I have not seen this yet with my pack.  Some cells are in the motor compartment and some are in the trunk, have not seen any relative drift and they remained balanced.
 
Bill Dube wrote
Bottom balancing only accomplishes #3. It only does this well when
you perform bottom balance procedure.
If the cells get out of balance, say, during an extended storage of
the vehicle between uses, you will over-discharge a cell, since there
is no sensing of the individual cell voltages.
I have stored my vehicle for months, have not yet seen self discharge causing out of balance conditions.
Self discharge in the large format prismatics seems very low to non existent.
 
Bill Dube wrote
>>>> EVs aren't an application where bottom balancing makes sense <<<
  Possibly, though in my admittedly limited experience I've seen no evidence of that statement being true.
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Re: Li-Ion BMS video: Top vs bottom balancing

AMPhibian
In reply to this post by Jukka Järvinen-2
Jukka Järvinen-2 wrote
With LiFePO4 you get away with it for a longer time and possibly without dramatic exits
to afterlife.

Until eventually the weakest cell determines the capacity of the pack.
 The weakest/smallest cell always determines the capacity of the pack.
Jukka Järvinen-2 wrote
Then you can replace that cell and go on until the next one needs to
be changed.

You'll eventually swap all cells and will be doing that for a hobby.
Or you can just change all cells every 5 years. Calculate how much
it'll cost to you go that route.
 
It might be less than the cost of most BMS's.  Think about that, you pay for a BMS to protect your cells because they are expensive, but if the cost of the BMS is high enough and the failure rate without one is low enough a BMS makes no financial sense.  I understand there may be safety issues that a BMS might prevent, but there have been safety issues that BMS's have not prevented and may have even caused.  We do know that some BMS's have killed some perfectly good cells.  So not only did people pay for a BMS they had to pay to replace cells that the BMS actually killed.
Your first line of defense should be to get closely matched cells so that even if you use a BMS it has very little to do on a daily basis.  I think batteries have gotten better and companies are doing a better job of matching packs from the recent orders I've seen.  If you have a cell that's gotten out of line you might be better off replacing it than trying to nurse it along with a BMS.
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Re: Li-Ion BMS video: Top vs bottom balancing

Bill Dube
In reply to this post by AMPhibian
The key is that you don't actually make the measurements on every
cell on a continuous basis.

When you actually make the measurements on every cell on a routine
basis, you will see the problem. You don't make the measurements,
(and the cells survive in the short term,) so you don't see that
there is any problem.

Put a temperature logger on each of the end cells and on a few middle
cells. Record for a few weeks, and then tell me there is no
difference in temperature across the pack.


At 07:09 AM 7/22/2011, you wrote:

>Bill Dube wrote:
> >
> > Unless you take extreme measures, cell temperatures will vary greatly
> > in an EV pack. One of the main difficulties is the heat transfer,
> > both in and out, of the power cables. They conduct heat out of the
> > end cells when the pack is warmer than the surroundings, and conduct
> > heat into the end cells when the pack is cooler than the
> > surroundings. Thus, the end cells, and the middle cells, tend to have
> > a different SOC than the rest of the pack.
> > I have not seen this yet with my pack.  Some cells are in the motor
> > compartment and some are in the trunk, have not seen any relative drift
> > and they remained balanced.
>
>
>Bill Dube wrote:
> >
> > Bottom balancing only accomplishes #3. It only does this well when
> > you perform bottom balance procedure.
> > If the cells get out of balance, say, during an extended storage of
> > the vehicle between uses, you will over-discharge a cell, since there
> > is no sensing of the individual cell voltages.
>I have stored my vehicle for months, have not yet seen self discharge
>causing out of balance conditions.
>Self discharge in the large format prismatics seems very low to non
>existent.
>
>Bill Dube wrote:
> > >>>> EVs aren't an application where bottom balancing makes sense <<<
> >   Possibly, though in my admittedly limited experience I've seen no
> > evidence of that statement being true.
>
>--
>View this message in context:
>http://electric-vehicle-discussion-list.413529.n4.nabble.com/Li-Ion-BMS-video-Top-vs-bottom-balancing-tp3685346p3686642.html
>Sent from the Electric Vehicle Discussion List mailing list archive
>at Nabble.com.
>
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Re: Li-Ion BMS video: Top vs bottom balancing

gottdi
Bill,

Of course there will be changes. The temps change but do you really need to monitor your cells every second of every day to determine if there is a problem? So far there has been no problem since my bottom balance and the settings on my charger and controller are doing just fine and the cells are not getting HOT. They do warm when used as expected. I actually expect some differences in use. So what. No big deal. Once you know that things are working you only need to check once and awhile. I am continuing to get full charge and expected performance and distance. All without needing to check on a second by second basis. Cells fluctuate. It is a normal thing. It is you don't want them to fluctuate too much.

What do you consider the short term. Week, month, year, decade? So far there are some who are and have been running daily for nearly a few years now and have seen no issues all with out checking on a second by second basis. So when will they crap out? Who the heck knows. No one has been using the long enough yet to know. Not you guys who insist that a BMS be present and not those who insist you don't need one. So the whole argument is moot. You base your need for a BMS on what? Old battery technology and just transfer to the LiFePO4 cells and insist it is true. Sure your cells may work just fine with one but they also work find without. My cells have had a BMS on them in their previous life and like I said earlier many cells did get ruined by top balancing them and having the BMS fail. The cells got over charged. Way over charged after the BMS failed to shut off the charger.

So what difference in SOC have you measured in your pack from the cells being placed in different parts of the car and on a section being a tad warmer than the other.

Or are you just assuming there is a different SOC.


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Re: Li-Ion BMS video: Top vs bottom balancing

AMPhibian
In reply to this post by Bill Dube
I know there are different temperatures, I'm saying that my pack has remained balanced so far in spite of the temperature differences.  So far when I've done a deep discharge to check the bottom balance they are still in line.  Since I'm bottom balanced they are of course not in line if I charge near 100% DOD but the cells remain in the same relationship to each other and have not shifted amongst themselves.  I rarely charge near 100% SOC since it would seem that prolongs the life of the cells, unless the 4.0V charge recommendation that has been quoted for TS cells is accurate and pertains to other cells, which is new information that I've not heard before, and still have yet to see any technical explanation for.

Bill Dube wrote
The key is that you don't actually make the measurements on every
cell on a continuous basis.

When you actually make the measurements on every cell on a routine
basis, you will see the problem. You don't make the measurements,
(and the cells survive in the short term,) so you don't see that
there is any problem.

Put a temperature logger on each of the end cells and on a few middle
cells. Record for a few weeks, and then tell me there is no
difference in temperature across the pack.

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