# [EVDL] Li-Ion BMS video: Top vs bottom balancing



## EVDL List (Jul 27, 2007)

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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## EVDL List (Jul 27, 2007)

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.
> ...


----------



## EVDL List (Jul 27, 2007)

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.
> ...


----------



## EVDL List (Jul 27, 2007)

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.
> ...


----------



## EVDL List (Jul 27, 2007)

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 



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

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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## EVDL List (Jul 27, 2007)

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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## EVDL List (Jul 27, 2007)

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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## EVDL List (Jul 27, 2007)

I consistently charge to 3.65 volts. 

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## EVDL List (Jul 27, 2007)

> 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 charge. All cells
are near the same voltage at the end of the drive too. NO BMS. 




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

>
>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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## EVDL List (Jul 27, 2007)

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 <[email protected]>:
>
>>
>>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 poi=
nt
>>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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>

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## EVDL List (Jul 27, 2007)

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 J=E4rvinen <[email protected]>:
> 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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## EVDL List (Jul 27, 2007)

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 <[email protected]>:
> 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 J=E4rvinen <[email protected]>:
>> 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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>

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## EVDL List (Jul 27, 2007)

> 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,
> ...


----------



## EVDL List (Jul 27, 2007)

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----------



## EVDL List (Jul 27, 2007)

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. 




-----
If you don't understand, be patient, you will. Now I understand. 
--
View this message in context: http://electric-vehicle-discussion-list.413529.n4.nabble.com/Li-Ion-BMS-video-Top-vs-bottom-balancing-tp3685346p3687166.html
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## EVDL List (Jul 27, 2007)

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.
> ...


----------



## EVDL List (Jul 27, 2007)

> Gottdi wrote:
> 
> I'm not Bill, but FWIW...
> 
> ...


----------



## EVDL List (Jul 27, 2007)

I know its fine because I do check periodically just to be sure. I also know
because I still get the performance and distance. I drive the same route so
nothing is really different there. I know how far I can safely go and I know
what the performance should be. It is still working just fine. That is how I
know. I am quite frankly getting tired of checking so often. It is the same
each time. I don't fully discharge nor do I fully charge. Now I could get a
BMS and monitor daily and second by second and get a few extra miles from my
pack but I have decided that the system is far to complex and expensive for
that level of monitoring and few extra miles. I prefer to check some times
and to let my charger and controller keep the pack safe. I'd rather not have
all that extra wiring in my pack either. 

If I had a battery that required a BMS then I would use one. 

Pete 

-----
If you don't understand, be patient, you will. Now I understand. 
--
View this message in context: http://electric-vehicle-discussion-list.413529.n4.nabble.com/Li-Ion-BMS-video-Top-vs-bottom-balancing-tp3685346p3687618.html
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## EVDL List (Jul 27, 2007)

Do you monitor the capacity of each cell daily? If yes then why if for the
past few years your pack has shown no issues? Or have you had issues with
the BMS and the LiFePO4 cells? If so why? What was the cause of your issues? 

The cells of mine in their other life were connected to a BMS system from
the manufacturer of the cells. They were connected but out of about 250
cells that were being used and tested and having a BMS hooked up actively
there were about 50 or so that were damages by over charging. Some are still
good enough for a smaller project that requires a small lower voltage pack
and where full capacity is not an issue. Like for my Cushman Truckster which
will be set up as a 48 volt system and used very occasionally for short
drives and shows. 

Some cells were just plain ruined. Good for dissecting and show and tell. I
did not have to pay for those ruined cells or damaged cells. 

Once the cells were ruined by the previous owners they removed the BMS
systems and began bottom balancing the cells and no BMS. Sorry but I do not
trust BMS systems. 


Pete 

-----
If you don't understand, be patient, you will. Now I understand. 
--
View this message in context: http://electric-vehicle-discussion-list.413529.n4.nabble.com/Li-Ion-BMS-video-Top-vs-bottom-balancing-tp3685346p3687651.html
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----------



## EVDL List (Jul 27, 2007)

Well now.. I think you just stated one of the truths. You do not trust BMS.

I can understand that completely. There are more Battery Murdering
Systems out there than real working BMS.

It is fair to admit that we are all learning more every day. Absolute
truth is hard to state.

What we know for sure is that taking LFP cells down or up (temp or
voltage) enough will alter the behavior of the cell. It will not
recover to the state before but it will likely still be working. We
know these cells have some sort of calendar life. So far it seems to
be n fold compared to Cobalt cells. Which is good news to all of us.
Affordable pack compared to previous tech is here.

We do not know for sure how the cells get their individual death
rates. Self discharge and efficiency characteristics. The 'quality' of
a cell might be something at day one and it will be degrading over
years. The correlation of temperature and DOD is recognized but there
has not been any hard curved constant to state it.

So far it is clear the degration of cells is not linear. By using the
cell for years you'll get different performance from it compared if
you just leave it in the shelf.

If you take say.. 2 sets of 5 'identical' and matched cells. Put other
set in series and other in parallel. Set the cycler for 70% DOD with 1
C and let the cycling begin. After every 1000 cycles you take the
cells out and measure individually.

How different these 2 sets are and how they will degrade ?

It's shame it takes 1 to 2 years (!) to run these lab tests. Few times
I've done that but I my self will running out of years before I know
'enough'.

This is why the BMS I wanted to make had the accurate data logging and
all possible sensors in place. Sure... it was expensive to make but
the customers got it for a bargain. I believe we charged about 1 USD /
Ah cells and the BMS. Today.. I could do that with a profit. Nothing
is expensive anymore.

After years and years of reading these logs from few cars in
operation... It is clear to me the BMS at least helps us to :
understand cells better, prevent major failures, 20W individual
balancer DC/DC on each cell is enough, gives better range estimation
to the user and how long the pack might still be valid and working. It
also helps us to know if the user has been tinkering the pack.

There should be BMS on every EV and they should push all possible data
to 'cloud' for us to lear even more.

Going no-BMS route with serious fleets (+10.000 vehicles) would make
the service people sweat and cry.

Would you Pete borrow your car to the next door old lady for 24 months
? No servicing allowed during that period. Would you not feel a bit
uncertain while she charges the car overnight in her garage with
cradles and boxes full of ignitable memories ?

We are constantly working to steal all the marvel of being 'EV dude'
... sorry... soon there will be millions more of us 

-akkuJukka

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



2011/7/22 gottdi <[email protected]>:
> Do you monitor the capacity of each cell daily? If yes then why if for the
> past few years your pack has shown no issues? Or have you had issues with
> the BMS and the LiFePO4 cells? If so why? What was the cause of your issu=
es?
>
> The cells of mine in their other life were connected to a BMS system from
> the manufacturer of the cells. They were connected but out of about 250
> cells that were being used and tested and having a BMS hooked up actively
> there were about 50 or so that were damages by over charging. Some are st=
ill
> good enough for a smaller project that requires a small lower voltage pack
> and where full capacity is not an issue. Like for my Cushman Truckster wh=
ich
> will be set up as a 48 volt system and used very occasionally for short
> drives and shows.
>
> Some cells were just plain ruined. Good for dissecting and show and tell.=
I
> did not have to pay for those ruined cells or damaged cells.
>
> Once the cells were ruined by the previous owners they removed the BMS
> systems and began bottom balancing the cells and no BMS. Sorry but I do n=
ot
> trust BMS systems.
>
>
> Pete 
>
> -----
> If you don't understand, be patient, you will. Now I understand. 
> --
> View this message in context: http://electric-vehicle-discussion-list.413=
529.n4.nabble.com/Li-Ion-BMS-video-Top-vs-bottom-balancing-tp3685346p368765=
1.html
> Sent from the Electric Vehicle Discussion List mailing list archive at Na=
bble.com.
>
> _______________________________________________
> | Moratorium on drag racing discussion is in effect.
> | Please take those discussions elsewhere. Thanks.
> |
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----------



## EVDL List (Jul 27, 2007)

For the sake of data gathering a sort of BMS monitoring (not managing) system
would be good. Temp, Amps in and out, what ever it needs. Building data
banks would be quite useful. But there is not place to send this
information. It is just there and what would I do with it anyway. For the
OEM's needing real world information it could help to build a better battery
or monitor better and give drivers better information as to distance and
mileage left. Do you have a cloud where this BMS information gathering can
be placed? My controller already does a lot of logging when asked to do so.
It is not on a single cell basis but on a pack basis and I can monitor temp.
I monitor controller functions and volts and amps both in and out. I do not
have it all connected. Now that I have all this information what can I do
with it. Well I know my system is functioning within the parameters. That is
good. What else? Well, its functioning within parameters. Any thing else?
Maybe that I drove it harder today than yesterday. I do that sometimes. Some
times I drive easy and sometimes hard. Same with my Leaf. My Leaf does send
information back to Nissan. That is good.

-----
If you don't understand, be patient, you will. Now I understand. 
--
View this message in context: http://electric-vehicle-discussion-list.413529.n4.nabble.com/Li-Ion-BMS-video-Top-vs-bottom-balancing-tp3685346p3687928.html
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## EVDL List (Jul 27, 2007)

Yes at this point I trust my ride to Grandma. I just don't think she trusts
the ride. 

-----
If you don't understand, be patient, you will. Now I understand. 
--
View this message in context: http://electric-vehicle-discussion-list.413529.n4.nabble.com/Li-Ion-BMS-video-Top-vs-bottom-balancing-tp3685346p3687931.html
Sent from the Electric Vehicle Discussion List mailing list archive at Nabble.com.

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## EVDL List (Jul 27, 2007)

> know. I am quite frankly getting tired of checking so often. It is the same
> each time.


I think this is *precisely* the reason that there should be at a minimum a
battery monitoring system (one capable of shutting things down with
potentially damaging events). The "fun" of carefully watching cells
eventually wears off (or someone gets their hands on the vehicle that
doesn't know what to do or care to do it), and then bad things can happen.
It's like a guard on a watchtower that gets tired of watching as nothing
happens day in and day out. You never know when something is going to happen
or how the situation might change on you unexpectedly.

-Ryan
-- 
- EV Source <http://www.evsource.com> -
Professional grade electric vehicle parts and resources
E-mail: mailto:[email protected]
Toll-free: 1-877-215-6781
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## EVDL List (Jul 27, 2007)

Yes at this point I trust my ride to Grandma. I just don't think she trusts
> the ride. 
>

For a single drive down to the grocery store or as her daily driver without
any intervention by you?

-Ryan
-- 
- EV Source <http://www.evsource.com> -
Professional grade electric vehicle parts and resources
E-mail: mailto:[email protected]
Toll-free: 1-877-215-6781
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## EVDL List (Jul 27, 2007)

As an owner of a Thundersky pack that was seriously unbalanced, and went
through the experience of dragging two cells into the mud, I think I have
some unique perspective.

First, I do see the benefit of bottom-balancing and having all cells reach
the low state of charge at the same time. This lessens the chance of
dragging the lowest cell to zero charge and potentially ruining it.
However, I also realize that a properly functioning BMS should warn about
this event so it isn't done "quietly and quickly."

In my situation, the BMS was doing its job and alerting that I had an
over-discharge situation (audio only). However, the buzzer was so quiet
that I couldn't hear it while driving 60 MPH. I knew I was low, but I
didn't know I had a serious issue until I stopped at the house and heard the
buzzer. I now have a MUCH louder buzzer that I can hear at any speed and a
visual alert (oil pressure light) also. After this event, I found out that
some of my cells were 20-25% off the others. My pack was seriously
unbalanced, obviously. 

However, I don't see how a bottom-balanced pack can always know when to
reliably shut off the charger. While my pack was unbalanced, it was ALWAYS
the BMS that was terminating the charge cycle. There were enough cells that
were low, that the charger (Elcon 2000+) thought it should still keep
charging in order to hit full cell voltage. 

I went through a process of bringing all cells up to a common point with a
5A RC charger. (I found out how much my cells were off by noting the
difference in charge that the RC charger put into each cell.) That brought
the cells close enough to the same for the BMS shunting circuit to top
balance the pack. It also brought the cells close enough that the Elcon
charger knows the state of charge correctly and tapers the charge current so
the BMS shunts have more time to do their job. I now have a set of cells
that are matched close enough that all shunt lights come on and the Elcon
charger terminates the charge cycle. However, I still have the BMS system
as a backup in case something goes wrong. The BMS cuts AC power to the
charger, so it is the ultimate backup if the charger misbehaves.

Assuming that several cells could slowly drift out with respect to the
others, how do you notice this before the highest-voltage cells start to be
abused before the charger shuts off? That, as I see it, is the biggest
question in running bottom-balanced, without a BMS. I suppose one could get
around this argument by rejecting the assumption that cells can drift, but I
think I believe in Murphy's Law more than that.

Mike

BTW, the two cells that were discharged down to zero volts seem to have
survived the experience, so far. I charged them slowly and carefully and
took extra care with ventilation to keep them as cool as possible while they
charged. It has been about 6 months of driving since then and they are
still working. I'm obviously keeping a closer eye on them.

> -----Original Message-----
> From: [email protected] [mailto:[email protected]] On
> Behalf Of gottdi
> Sent: Thursday, July 21, 2011 8:35 PM
> To: [email protected]
> Subject: Re: [EVDL] Li-Ion BMS video: Top vs bottom balancing
> 
> 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?
> 
> -----
> If you don't understand, be patient, you will. Now I understand. 
> --
> View this message in context: http://electric-vehicle-discussion-
> list.413529.n4.nabble.com/Li-Ion-BMS-video-Top-vs-bottom-balancing-
> tp3685346p3685796.html
> Sent from the Electric Vehicle Discussion List mailing list archive at
> Nabble.com.
> 
> _______________________________________________
> | Moratorium on drag racing discussion is in effect.
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> |
> | REPLYING: address your message to [email protected] only.
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----------



## EVDL List (Jul 27, 2007)

I've heard numerous discussions about bottom balancing.

Can someone supply a blow by blow description on how this should be done.


Thanks;
Dennis=
=

Elsberry, MO=
=

http://www.evalbum.com/1366
http://www.evalbum.com/3715=
=
=




-----Original Message-----
From: gottdi [mailto:[email protected]] =

Sent: Friday, July 22, 2011 11:34 AM
To: [email protected]
Subject: Re: [EVDL] Li-Ion BMS video: Top vs bottom balancing

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. =





-----
If you don't understand, be patient, you will. Now I understand. 
--
View this message in context: http://electric-vehicle-discussion-list.41352=
9.n4.nabble.com/Li-Ion-BMS-video-Top-vs-bottom-balancing-tp3685346p3687166.=
html
Sent from the Electric Vehicle Discussion List mailing list archive at Nabb=
le.com.



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## EVDL List (Jul 27, 2007)

A caveat: I have heard the theory, but I've never done this myself, so I
would definitely yield to someone more experienced than myself. I've been
going with top-balancing.

Start with the pack already mostly depleted (i.e. drive the vehicle to use
most of the range).

Put a load on each cell (light bulb, resistor load, balancer system, etc) to
take it down to some fixed, known voltage that represents the "bottom" of
the pack. Probably somewhere around 2.5-3.0V.

Repeat for each cell until all cells are at this bottom voltage. 

Charge all cells evenly from there. No top shunts or other equipment to try
and make sure each cell takes as much as possible. Charge each cell to
about 85-90% of fully charged volts per cell. This should ensure all cells
have the same charge capacity so they all reach the bottom and pass out
together. The vehicle simply quits moving if you try to drive too far
instead of taking one or more cells to zero and destroying them.

In general, I'm OK with the theory, but when and how to stop the charger
always seems like the open question to me.

Mike

> -----Original Message-----
> From: [email protected] [mailto:[email protected]] On
> Behalf Of Pestka, Dennis J
> Sent: Monday, July 25, 2011 8:38 AM
> To: Electric Vehicle Discussion List
> Subject: Re: [EVDL] Li-Ion BMS video: Top vs bottom balancing
> 
> I've heard numerous discussions about bottom balancing.
> 
> Can someone supply a blow by blow description on how this should be done.
> 
> 
> Thanks;
> Dennis Elsberry, MO
> http://www.evalbum.com/1366
> http://www.evalbum.com/3715
> 
> 
> 
> -----Original Message-----
> From: gottdi [mailto:[email protected]]
> Sent: Friday, July 22, 2011 11:34 AM
> To: [email protected]
> Subject: Re: [EVDL] Li-Ion BMS video: Top vs bottom balancing
> 
> 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.
> 
> 
> 
> 
> -----
> If you don't understand, be patient, you will. Now I understand. 
> --
> View this message in context: http://electric-vehicle-discussion-
> list.413529.n4.nabble.com/Li-Ion-BMS-video-Top-vs-bottom-balancing-
> tp3685346p3687166.html
> Sent from the Electric Vehicle Discussion List mailing list archive at
> Nabble.com.
> 
> 
> 
> _______________________________________________
> | Moratorium on drag racing discussion is in effect.
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> |
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## EVDL List (Jul 27, 2007)

The other method is to put all the cells in parallel and then
discharge to about 2.8V and leave them there for a few hours to make
sure they are all at the same voltage then charge them like Mike said.
The advantage of paralleling all the cells is that it eliminates most
if not all the potential variation between cells when balancing them
individually.

On Mon, Jul 25, 2011 at 11:17 PM, Mike Nickerson


> <[email protected]> wrote:
> > A caveat: I have heard the theory, but I've never done this myself, so=
> I
> > would definitely yield to someone more experienced than myself. I've b=
> ...


----------



## EVDL List (Jul 27, 2007)

Perhaps I'm missing something - I've done zilch with lithium - but could you 
accomplish much the same thing this way?

1. Measure the capacity of all cells; the lowest capacity cell is your pack 
capacity

2. While driving, measure amp-hours withdrawn; stop when you've used 80% of 
the measured capacity

3. Charge until you've replaced all the amp-hours used, plus a calculated 
percentage for efficiency

4. Periodically recheck capacity to ensure that you're still using the 
correct parameters

This scheme uses actual state of charge, rather than voltage, which is 
usually a rather inaccurate way to measure SOC.

David Roden
EVDL Administrator
http://www.evdl.org/


_______________________________________________
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| Please take those discussions elsewhere. Thanks.
|
| REPLYING: address your message to [email protected] only.
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## EVDL List (Jul 27, 2007)

Don't put them in parallel. If you do all you have done is made on large
cell. I drove my car until I was at a low SOC and stopped before any cells
were too low. The I bring up the low cells to a specified voltage then do it
again. Next time bringing the pack down and again finding the low cells and
brining them up meet the others. Do this until all your cells are matched
closely at the bottom. Do not rematch then at the top because if you do you
will have just undone all your work. Then if you want you can find your
lowest capacity cells and do AH counting. Charge your pack to only 3.65 and
stop at that voltage. Any more than that will result in no real gain in
miles. Maybe 1 or so. Really. Have your controller stop or cut way back at
like 2.6 volts. No more. But since your bottom balanced you won't need to
worry. No need to drive further than that low knee either because you really
only have a mile or so anyway like at the top. Stay within the boundaries of
the curves and you will be fine. Balance at the low end because you drive
your car below full anyway and you WILL drive your car someday to the danger
zone. Best be prepared for that or you WILL loose cells. 

Or you can drive your car until you normally stop then take each cell down
to a specific static voltage like 2.5 to 3.0 volts and make sure they all
match at that voltage. I chose 2.4 for my cells and cut off the charger at
2.5. Cut back amperage at 2.6. I have had no troubles since. 

-----
If you don't understand, be patient, you will. Now I understand. 
--
View this message in context: http://electric-vehicle-discussion-list.413529.n4.nabble.com/Li-Ion-BMS-video-Top-vs-bottom-balancing-tp3685346p3695946.html
Sent from the Electric Vehicle Discussion List mailing list archive at Nabble.com.

_______________________________________________
| Moratorium on drag racing discussion is in effect.
| Please take those discussions elsewhere. Thanks.
|
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| Multiple-address or CCed messages may be rejected.
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----------



## EVDL List (Jul 27, 2007)

That's essentially what I do with my bottom balanced BMS-less pack. I watch
the AH counter while driving and stay away from the bottom, under charge the
pack to stay away from the top, (though I do use voltage to stop the
charger), and occasionally check cell voltages at the top and bottom, where
voltage does correlate well to SOC. So far they have not changed in
relation to each other.




> EVDL Administrator wrote:
> >
> > Perhaps I'm missing something - I've done zilch with lithium - but could
> > you
> ...


----------



## EVDL List (Jul 27, 2007)

> Mike Nickerson wrote:
> >
> >
> > However, I don't see how a bottom-balanced pack can always know when to
> ...


----------



## EVDL List (Jul 27, 2007)

How old are your cells and how many full cycle equivalents have they done?



> AMPhibian <[email protected]> wrote:
> >
> > Mike Nickerson wrote:
> >>
> ...


----------



## EVDL List (Jul 27, 2007)

Purchased in 2009, installed in 2010, about 4K miles on them, so relatively
new. I don't drive much and it's been off the road since Dec.



> David Nelson-5 wrote:
> >
> > How old are your cells and how many full cycle equivalents have they done?
> >
> ...


----------



## EVDL List (Jul 27, 2007)

My pack has a manufacturer date of November 2009. They have 100 full
cycle equivalents on them. A year ago I top balanced them and then
only charged them to 3.485vpc. After 11 months, 12,000Ah (200Ah pack),
and a little over 5000 mi the high and low cells were 0.103V apart. I
did discover that some dust can still get into the box and that water
had been getting in too. I think that most of the time the water was
getting on the low cell. I know it did one time for sure because the
top was full and overflowing  so I'm thinking that had more to do
with the imbalance than anything. I've resealed the lid and did
another top balance. Will be interesting to see what happens now. At
the moment I don't have my BMS boards attached which bothers me since
I don't have a bottom balanced pack. I do have a split pack voltage
monitor in place, however. The monitor only draws 0.35mA. I discovered
that my BMS boards only draw about 1mA. Black Sheep Technology really
did a great job on the low current draw! I usually hear 6mA or more
with other individual board setups.



> AMPhibian <[email protected]> wrote:
> > Purchased in 2009, installed in 2010, about 4K miles on them, so relative=
> ly
> > new. I don't drive much and it's been off the road since Dec.
> ...


----------



## EVDL List (Jul 27, 2007)

Since our mileage is similar, I'll throw another data point at this:

40 160Ah TS Cells manufactured 11/2009.
Total Ah removed: 10,000
Av discharge: 52Ah
Deepest: 124Ah
Cycles: about 200
Miles: 5200
More detail: http://www.evalbum.com/3145

I am using a BMS to top balance the cells. My pack is split, so when I
charge, the cells at the rear the car are a different temperature that the
cells in the front of the car. This is especially true is the car has been
sitting in the sun. At the end of charge, you can see the inside (warmer)
cell balancers shunting while the inside cells (cooler) are not shunting. I
tried to document this with a video
http://www.youtube.com/watch?v=3Dlhn4IDePAGc.

Just the facts, draw your own conclusions, not trying to argue.

Travis



-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf
Of David Nelson
Sent: Sunday, July 31, 2011 9:21 PM
To: Electric Vehicle Discussion List
Subject: Re: [EVDL] Li-Ion BMS video: Top vs bottom balancing

My pack has a manufacturer date of November 2009. They have 100 full
cycle equivalents on them. A year ago I top balanced them and then
only charged them to 3.485vpc. After 11 months, 12,000Ah (200Ah pack),
and a little over 5000 mi the high and low cells were 0.103V apart. I
did discover that some dust can still get into the box and that water
had been getting in too. I think that most of the time the water was
getting on the low cell. I know it did one time for sure because the
top was full and overflowing  so I'm thinking that had more to do
with the imbalance than anything. I've resealed the lid and did
another top balance. Will be interesting to see what happens now. At
the moment I don't have my BMS boards attached which bothers me since
I don't have a bottom balanced pack. I do have a split pack voltage
monitor in place, however. The monitor only draws 0.35mA. I discovered
that my BMS boards only draw about 1mA. Black Sheep Technology really
did a great job on the low current draw! I usually hear 6mA or more
with other individual board setups.



> AMPhibian <[email protected]> wrote:
> > Purchased in 2009, installed in 2010, about 4K miles on them, so
> relatively
> > new. I don't drive much and it's been off the road since Dec.
> ...


----------



## EVDL List (Jul 27, 2007)

Good data, Travis. I'm blessed with a single battery box and so far I
haven't seen any pattern to the highest or lowest cells. I have
noticed that the energy to recharge them increases when they are
colder than when warmer. I have a graph of energy from the wall vs.
distance distance driven and see that when the batteries are cold
(<10=B0C) I get about 5.3mi/kWh but when they are warm (>17=B0C) I get
over 6mi/kWh. I have also noticed that when the batteries are cold the
end of charge cycle takes much longer than when they are warm. This
would seem to agree with your observations. The question is, since we
charged to a fixed voltage are the cells getting over charged when
warm and undercharged when cold? Also, what would happen in your case
if you used 3.4-3.5vpc as your cutoff voltage? Would the warm set get
fully charged and the cold set get under charged? Someone on DIY said
they saw the same sort of behavior you are seeing. Another question,
if you were to charge to say 3.5vpc and hold that until the current
dropped to under 1A, then equalize the temperature between the sets of
batteries, would their ocv's be the same? What would happen if they
were then put on charge until all your BMS boards were shunting?

All interesting questions which need data to answer.

On Mon, Aug 1, 2011 at 7:55 AM, Travis Tyler <[email protected]> wrot=
e:
> Since our mileage is similar, I'll throw another data point at this:
>
> 40 160Ah TS Cells manufactured 11/2009.
> Total Ah removed: 10,000
> Av discharge: 52Ah
> Deepest: 124Ah
> Cycles: about 200
> Miles: 5200
> More detail: http://www.evalbum.com/3145
>
> I am using a BMS to top balance the cells. My pack is split, so when I
> charge, the cells at the rear the car are a different temperature that the
> cells in the front of the car. This is especially true is the car has been
> sitting in the sun. At the end of charge, you can see the inside (warmer)
> cell balancers shunting while the inside cells (cooler) are not shunting.=
I
> tried to document this with a video
> http://www.youtube.com/watch?v=3Dlhn4IDePAGc.
>
> Just the facts, draw your own conclusions, not trying to argue.
>
> Travis
>
>
>
> -----Original Message-----
> From: [email protected] [mailto:[email protected]] On Beh=
alf
> Of David Nelson
> Sent: Sunday, July 31, 2011 9:21 PM
> To: Electric Vehicle Discussion List
> Subject: Re: [EVDL] Li-Ion BMS video: Top vs bottom balancing
>
> My pack has a manufacturer date of November 2009. They have 100 full
> cycle equivalents on them. A year ago I top balanced them and then
> only charged them to 3.485vpc. After 11 months, 12,000Ah (200Ah pack),
> and a little over 5000 mi the high and low cells were 0.103V apart. I
> did discover that some dust can still get into the box and that water
> had been getting in too. I think that most of the time the water was
> getting on the low cell. I know it did one time for sure because the
> top was full and overflowing  so I'm thinking that had more to do
> with the imbalance than anything. I've resealed the lid and did
> another top balance. Will be interesting to see what happens now. At
> the moment I don't have my BMS boards attached which bothers me since
> I don't have a bottom balanced pack. I do have a split pack voltage
> monitor in place, however. The monitor only draws 0.35mA. I discovered
> that my BMS boards only draw about 1mA. Black Sheep Technology really
> did a great job on the low current draw! I usually hear 6mA or more
> with other individual board setups.
>
>


> AMPhibian <[email protected]> wrote:
> >> Purchased in 2009, installed in 2010, about 4K miles on them, so
> > relatively
> >> new. I don't drive much and it's been off the road since Dec.
> ...


----------

