# New BMS technology



## _GonZo_ (Mar 23, 2009)

I know that this latests weeks has been a lot of discussion on BMS related.
And even a open project has started. (I was hunger to start talking)
I have to say sorry about keeping myshelf away form this posts but we have been working on this very hard for the last months and it was a bit secret, untill the first results start to come out, and they are here.

Let me explain and please let me know your thoughts.

About the actual BMS:
- After a period of time the coherence of the cells will vary, this is a fact. So a BMS or at least a PCM is a must.
- A normal BMS wastes energy, because it burns it throw resistors.
- If the balancing process is carried on as well during discharge as usually then we are loosing even more energy.
- And because we are trying to match all cells from a pack to the one with the lower voltage, higer impedance cell/s and will shut down the pack when this weak cell/s reaches minimum voltage, then what a standard BMS does to the pack is to make it work as a joint of weak cells... 
- Because of this issues as well the battery pack life becomes sorter, up to a 40% shorter in the worst cases.

This issues are well known and have been discussed here before.


Well and is is the new system:
- It transfers energy from higher cells to lower cells when charging and when the pack is discharging. 
- So the pack can draw out all energy of every cell, so then you are not working the pack as its weakest cell/s.
- Actually what we are doing is adjusting the capacity and impedance, tolerances of cells.
- It works at a very low temperature, we still have not defined the full efficiency (quite difficult task...) but it is clear that if it works cold it is very very efficient.
- We are designing it in an scalable way so we can meet any pack size from lets say 1Ah to 1000Ah and now we are starting from 4S to 24S
- Price will be very reasonable. 
- The first working prototype is actually working on the bench continuously.
- The first production unit will be available in a month or so mounted on my kid e-bike.
- In a short time I hope we will be able post prices and more info.

Please let me know your thoughts, opinions, requirements etc. they are all really welcome.

I will try to post a picture of the "prototipe" big bunch of cables and so on... tomorrow (now I am at home )


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## Dolphyn (Nov 17, 2009)

Ideally the BMS would provide some kind of feedback so that weak cells, if any, will be noticed and identified easily!


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## karlos (Jun 30, 2008)

Congratulations on creating a logical and efficient solution! I hope it is a robust and reliable system and stands the test of time.
I could be very interested in trying your system  so please keep us posted with progress.


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## Amberwolf (May 29, 2009)

I like the idea. Need any beta testers? I have a few hundred Li-Ion 18650 cells (2Ah) that I am about to build into packs that I'll need a BMS for.... 

It'll end up running my CrazyBike2, replacing the used SLA UPS batteries that are on there now.
________
Chasey_Show live


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## racunniff (Jan 14, 2009)

_GonZo_ said:


> Well and is is the new system:
> - It transfers energy from higher cells to lower cells when charging and when the pack is discharging.


Can you compare / contrast your design with Lee Hart's Battery Balancer? Tech, price, etc. ?


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## _GonZo_ (Mar 23, 2009)

Dolphyn said:


> Ideally the BMS would provide some kind of feedback so that weak cells, if any, will be noticed and identified easily!


Thank you for the imput, at the moment no, but we are working on extra features now like this one and others like:
SOC, battery life recording, maximun current outputs, etc.

Any way the idea is that the system will be "plug and play" so you can forget about the battery.
I understand that you will like to have more "access" to the information and software but you are a minority, most people and actually I prefer somethig that you install it and works and works OK and do not have to worry any more about it.


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## _GonZo_ (Mar 23, 2009)

karlos said:


> Congratulations on creating a logical and efficient solution! I hope it is a robust and reliable system and stands the test of time.
> I could be very interested in trying your system  so please keep us posted with progress.





> I like the idea. Need any beta testers? I have a few hundred Li-Ion 18650 cells (2Ah) that I am about to build into packs that I'll need a BMS for....
> 
> It'll end up running my CrazyBike2, replacing the used SLA UPS batteries that are on there now.


Thank you for your imputs.

Yes we will need testers.
Let me explain how we want to do it. We will post features in a few days (may be weeks) so every one that is interested as beta testers or end users can let us know if they are happy with that or want more or less features (like you are doing here).
We will study enquires in order to see if they are interesting and can be aplied or not.
Then after a while we will redefine the product, and start first production units.
We will offer them for testing at a very low price (about cost price or less) and we will give a period of around 6 months test to these people, and receive feedback. (money back will be garantee)
Then study feedbacks and redesign if necesary.
In order to be a tester you will have to be able to test the BMS on real world aplications (car, bike, other working system, etc.) not on bench. And will have to give us feedback report mandatory.

We will not give any testing unit for free, the reasons are:
- We are a small company and in this case the design and manufacturing has been done and will be done by outsorthing. (my knoweldge of electronis is limited, I know the basics and know what the sistem has to do but not know how to put all the electronic little pieces... our work shop is prepared for mounting battery packs and not for electronics) so lot of money has already spent on this project.
- We are studing if we let the software and hardware open so the testers can modify it, but it is a big issue with this because a not proper set up can ruin a batery pack... may be we let open some parts of it and not others in orther to make it quite safe use.
- The BMS is "modular" so has to be customized for every use that means quite a big labour cost on every unit.
- We had bad experience on past giving away for free systems to beta testers, some tend to not value the system, even in some cases the systems were not even tested, just lefted on a shelf...
- The testing units can always been send back to us and money will be refound if tester do not want to keep it or want to exchange it for a newer version or system failed due construction problem. (We will set a maximun period of time for this)
- Price for testing units will be much lower that final version (about 50%)
- We will give priority and discount on final versions to testers orders.

Please let me know your thoughts


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## _GonZo_ (Mar 23, 2009)

racunniff said:


> Can you compare / contrast your design with Lee Hart's Battery Balancer? Tech, price, etc. ?


The idea is about the same:


> The *Battery Balancer* is a system that tries to keep the set of batteries in an electric vehicle "balanced" by shuttling charge to the weaker/lower capacity batteries in the pack so that the batteries will all reach empty at about the same time when they are discharged and full at about the same time when they are recharged.


I did not knew about it, I have to check closer in order to see if we have achieved the same results by the same way or by another way.
I am sure that if we founded it before will have save us a lot of time and headaches...

Wil let you know if it is similar or not when we look at it closer.


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## racunniff (Jan 14, 2009)

_GonZo_ said:


> The idea is about the same:
> 
> 
> I did not knew about it, I have to check closer in order to see if we have achieved the same results by the same way or by another way.
> ...


OK, thanks. Note that Lee's Balancer is very heavily DIY - the user has to source the components and build it up - so if you offer more complete kits, that is still an advantage in your favor


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## Tesseract (Sep 27, 2008)

_GonZo_ said:


> ....
> - After a period of time the coherence of the cells will vary, this is a fact. So a BMS or at least a PCM is a must.


What parameter(s) of the cells will drift? Internal impedance, overall capacity? Both? Doesn't seem like there is very good (or much) data on this.




_GonZo_ said:


> - A normal BMS wastes energy, because it burns it throw resistors.


Sure, but it might not even be desirable or necessary to balance at the end of the charge (aka "top balancing"). Jrickard here makes a pretty persuasive argument against top balancing.




_GonZo_ said:


> - If the balancing process is carried on as well during discharge as usually then we are loosing even more energy.


True, but the point to "bottom balancing" is that you would only have to do this occasionally. Perhaps only once per year, because you would then discharge them to equivalent end points every use and only long term drift in impedance/capacity would throw them out of balance again.




_GonZo_ said:


> - And because we are trying to match all cells from a pack to the one with the lower voltage, higer impedance cell/s and will shut down the pack when this weak cell/s reaches minimum voltage, then what a standard BMS does to the pack is to make it work as a joint of weak cells...


True, but there's not a lot of difference in capacity between cells, so practically speaking it might not be worth doing anything about this.




_GonZo_ said:


> - Because of this issues as well the battery pack life becomes sorter, up to a 40% shorter in the worst cases.


Huh!? So far the differences in cell capacity reported are in the range of 1-3Ah out of 100-200Ah cells. That's more like 1% difference.




_GonZo_ said:


> - It transfers energy from higher cells to lower cells when charging and when the pack is discharging.


Charge shuttling is a valid method of balancing, but is gaining an extra 200-400Wh of capacity in a 30kWh pack worth the tremendous increase in complexity required in the BMS to do this?

Don't get me wrong - I think charge balancing is very elegant, and certainly the "best" method if judged by performance alone, but my intuition says it is overall more cost effective to either buy one more cell or live with driving 2km less... Strictly an informal opinion and I certainly don't mean to discourage your effort, I just wonder if you've considered the cost/benefit ratio of the charge shuttling system over less sophisticated methods like simply shutting off the charger when the first cell hits a high voltage cutoff (HVC) and cutting back or inhibiting the controller output when the first cell hits low voltage cutoff (LVC).

Definitely give this some thought before committing too much to the development effort.


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## _GonZo_ (Mar 23, 2009)

Thank you very much for your input Jeffrey, you were on my list to contact as tester so it is very welcome that you give your ideas.

Let me see if I can give answer to all of what you say on your post. with not too manny words...



> What parameter(s) of the cells will drift? Internal impedance, overall capacity? Both? Doesn't seem like there is very good (or much) data on this.


All cells are different just when manufactured, there is diferences on internal resistance, capacity, weight and shape.
When packs are constructed on a profesional manner every cell is checked individually in order to match them on IR and capacity, althought there is still differnces between them, but we try to keep them in a very narrow limits. (usually <1-2%)
During cicling the cells anode and cathode experiment losse of properties so internal resistance increases and capacity is lost as well. (over-chargers and over-discharges, high currents and high temperatures, speed up this looses)
With the bad thing that the weaker cell suffers from this more than the stronger cells.
So after a while even if the cells in a pack were very well matched upon construction they will start to be more and more different between them with every use.
A normal BMS avoids that the weakest cell suffer from over charging and over discharging but does not help the cell in high currents delivery.



> Sure, but it might not even be desirable or necessary to balance at the end of the charge (aka "top balancing"). Jrickard here makes a pretty persuasive argument against top balancing.


On this case I have to be on discrepancy with Jrickard, why: Because of experience. The first balancer we designed about 7 years ago (I think the first balancer for Lipoly cells on the world) we made it that way, bottom balancing (it was straight thinking and very easy to do...) and did not work, the results actually were quite bad because it was not protecting the pack at all so no at the end some cells were over-charged and that is not only bad for the pack it is actually very dangerous if working with Li-ion Lipoly cells... So we changed it for a top balancing one "Equalipo V2" and in this case it worked as protecting and balancing and doing a good job.
As well bottom balancing requires to add maintenance labour to the final system.
So sorry but no good option in our opinion.



> True, but there's not a lot of difference in capacity between cells, so practically speaking it might not be worth doing anything about this.
> Huh!? So far the differences in cell capacity reported are in the range of 1-3Ah out of 100-200Ah cells. That's more like 1% difference.


You are right if the pack is well done there should not differences over 2% between the cells, but as commented before that small difference at the beginning starts to grow with the time and cycles.
Why, because on a battery pack with no BMS, PCM or at least a balancer the weakest cells will suffer form overcharging, over-discharging and will have to deliver more current than the rest in relation with their real size (smaller capacity so smaller power) on every cycle unless you are very careful and never fully charge / fully discharge the pack, although even been so careful the differences of the cells will grow. As there is other factors that influence on this like the cells relative position, vibrations, temperature differences, etc.
So in order to achieve a long pack life and little maintenance work a balancer, PCM or a BMS is a must.
Lets make some numbers:
Lets say that we have a pack (I am always talking about battery packs, bare cells is a different issue) that is suppose to work 2000 cycles to 80%
To give it a size lets say it is a 100Ah rated capacity when new, the weak cell then will be 99Ah.
The weak cell will have a lost being conservative of 0,1% of capacity on every full cycle due over charging and over discharging (we have reordered up to a 4% looses per cycle on Lipoly cells on extreme uses, luckily LiFe cells suffer less form over charging or over discharging)
If user only does full cycles one of every 10 cycles that gives a result of a loose of capacity on that cell of a 1% every 100 cycles. 
So after 1000 cycles that cell will have lost 10% of capacity due damages caused for over discharging and over charging plus the normal cycle lost that is another 10% so 20% in total.
So after only 1000 cycles on the pack a cell in the pack is already almost dead. As you may understand the damages on the cell with the time will be greater and grater, because the difference is bigger.
The solution to the problem is obviously change the damage cell for a new one as you say, but we go back again to the problem of maintenance...
With a BMS, PCM or at least a balancer this does not happen or at least is minimized.



> Charge shuttling is a valid method of balancing, but is gaining an extra 200-400Wh of capacity in a 30kWh pack worth the tremendous increase in complexity required in the BMS to do this?
> Don't get me wrong - I think charge balancing is very elegant, and certainly the "best" method if judged by performance alone, but my intuition says it is overall more cost effective to either buy one more cell or live with driving 2km less... Strictly an informal opinion and I certainly don't mean to discourage your effort, I just wonder if you've considered the cost/benefit ratio of the charge shuttling system over less sophisticated methods like simply shutting off the charger when the first cell hits a high voltage cutoff (HVC) and cutting back or inhibiting the controller output when the first cell hits low voltage cutoff (LVC).


What you describe there is a PCM sistem.
Well you may be right and may be cost is too high for some cases but I think that if now every one is trying to get any extra mile form every droop of petrol with very high cost control and optimization systems, why we on the EV market should not try to do the same with every mAh.
Another thing is that the way I like things is simple as simple as possible "a good design is when you are not able to take more parts away" so this system is done with the same politics, as well I am sure that you will be surprised with the final prices.


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## Overlander23 (Jun 15, 2009)

I think the key here is "over-charging" and "over-discharging". Seems to me that those having problems are pushing their packs and the individual cells too far.

I like the idea of charge-shuttling, too, but if a BMS really did just what Jeff said... limit charge to the first cell that hits HVC and limit discharge to a reasonable level above the weakest cell's LVC, then the cells wouldn't be pushed. Maybe the likelihood of cell drift would go down with these practices.

The Toyota Prius' pack has proven to be long lived... but it only uses about 10% of the available capacity in the middle of the DOD. Not only are the cells not pushed at all, but any drift has a long way to go before it becomes a problem. Impractical for our packs, but the concept is sound.


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## JRP3 (Mar 7, 2008)

Overlander23 said:


> I think the key here is "over-charging" and "over-discharging". Seems to me that those having problems are pushing their packs and the individual cells too far.
> 
> I like the idea of charge-shuttling, too, but if a BMS really did just what Jeff said... limit charge to the first cell that hits HVC and limit discharge to a reasonable level above the weakest cell's LVC, then the cells wouldn't be pushed. Maybe the likelihood of cell drift would go down with these practices.


I agree. I argue that instead of a complex and expensive BMS you are better off just sizing your pack properly. Spend some of the money you save on slightly larger cells, unless you are extremely close on size and weight and need to use all available capacity. Of course this will shorten your number of cycles anyway, even with a BMS. Cells will last longest by discharging them as little as possible.


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## _GonZo_ (Mar 23, 2009)

At the present moment our BMS just does this:
1- Balance the cells transfering energy form one to others.
2- Protects on charge shuts down chare if any cell reaches the limit and/or full pack reaches limit.
3- Protects the pack from over discharging shut down power when any of the cells reaches minimun voltage and or full pack reaches the limit.
4- Protects form over current discharge or charge.
5- Protects from shorcuts.

We were thinking in more things to be added like temperature monitoring, life record, charge gauge, etc...

But from your latest opinions may be is a good point to stop adding features and let it like it is. That will make it cheaper  than if we add fancy staff to it.

Do you think that with those 5 points are then enough?


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## _GonZo_ (Mar 23, 2009)

JRP3 said:


> I agree. I argue that instead of a complex and expensive BMS you are better off just sizing your pack properly. Spend some of the money you save on slightly larger cells, unless you are extremely close on size and weight and need to use all available capacity. Of course this will shorten your number of cycles anyway, even with a BMS. Cells will last longest by discharging them as little as possible.


Completly right, to go cheap on batteryes is bad politics. and the end is expensive.
My advise is always that you have to save weight on every thing, in order to be able to install a bigger battery pack.


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## JRP3 (Mar 7, 2008)

_GonZo_ said:


> 4- Protects form over current discharge or charge.


Shouldn't that already be covered by your charger, controller, fuses and circuit breakers?


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## _GonZo_ (Mar 23, 2009)

JRP3 said:


> Shouldn't that already be covered by your charger, controller, fuses and circuit breakers?


You are right it should be... 
But I have seen so manny things already on battery world


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## _GonZo_ (Mar 23, 2009)

If every thing goes well, I think that I will be able to posts today ( a bit latter on) the especifications of this new balancer. 
I am gonig nevious.


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## _GonZo_ (Mar 23, 2009)

_GonZo_ said:


> If every thing goes well, I think that I will be able to posts today ( a bit latter on) the especifications of this new balancer.
> I am gonig nevious.


Sorry I had to pospone the upload of the info due 2 causes, one is that I relised that with minor changes (not big changes now because PCB are already done...) we can even simplify the sistem and make it even more modular.
Other is that we just received form A123 news about theyr new prismatics cells.


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## _GonZo_ (Mar 23, 2009)

Sorry for the delay, but finally I can start to release information aout this new BMS.

We have give it the name of "SHUTTLE BMS" it was inspired to mi by Tesseract from this form, thank you 

You can see overall specificaions from attached PDF.


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## kcameron (Oct 16, 2009)

Gonzo,

I just want to voice my support for the idea of your BMS keeping the cells in balance by shifting energy between them. The concept is certainly sound; though I can't endorse your specific implementation until you publish details on the approach.

The alternative of adding more cells or using larger cells to avoid over-discharging is insufficient. For one thing, a larger capacity pack will cost more which is all the more incentive to protect it. Also, the larger pack will still be subject to unnecessarily early degradation unless it is protected by the best possible BMS.

Here's how I look at it:

I'm sure all of us here would agree that some cells in a pack will inherently have a smaller capacity than the rest (let's call them "runts"). I believe we'd also all agree that the degradation a cell experiences due to cycling will depend on the depth of discharge (DOD: usually expressed as a percentage of full capacity). We usually think of DOD as a single value for the entire pack. However, without a BMS, the runts will see a deeper DOD than the pack average. Since the runts are being cycled more deeply, they wear out more quickly than the rest of the pack. In other words, the weak get weaker.

Another factor in predicting a cell's rate of degradation is to consider the charge/discharge currents. Similar to the above, these currents are expressed as a multiple of a cell's amp-hour capacity. All cells in a series-connected pack will inherently see the same absolute current but will see differing currents relative to their respective capacities. For instance, the strong cells in a pack may see .99C discharge at a time when the runts are seeing 1.01C. Again, the increased strain causes the weak cells to get weaker as the pack is cycled.

The usual BMS method of diverting a cell's charging current to a resistor as it approaches maximum voltage is one way to reduce the total cycling current through the runts and therefore marginally increase their lifespans (besides the primary purpose of preventing over-charging). It doesn't do anything to address the runts' increased relative DOD and charge/discharge rate issues, though.

My Ford Ranger EV pickup is on its third AGM lead-acid battery pack. This last time, I used better quality batteries and it's been holding up well for the last year. Still, I'm sure that I would find one or two batteries that are starting to show noticeably reduced capacity relative to the others if I were to bother checking. For the last year, I've been informally researching how to build a BMS for the Ranger which would keep the 26 14V batteries balanced whenever the truck is running or charging. I've gone through a few iterations and have a basic circuit in mind (though not on paper yet). The simple version of the idea is that it would divert varying amounts of power from the higher-voltage batteries to the others. The amount of power diverted from each battery would depend on how much higher its voltage is from the average at any time. This sounds a lot like what you're doing, Gonzo. So far, I've been too lazy to actually implement such a system for my truck. It seems your BMS might work for me though I can see why it might not since you're balancing 3.6V lithium cells as opposed to 14V AGM batteries. Still, I'll be monitoring this thread for progress.

Keep up the good work!

Kev


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## _GonZo_ (Mar 23, 2009)

Thank you very much for your input, I was feeling alone in this project 

My mother tong is not English so it is dificult for me to express my ideas correctly and acurate in this language, but you did it perfectly about the overall idea of the project, thank you. I will use part of your text f you do not mind on the project description in English if you do not mind. 

There is only some differences from what you say:
_
- The amount of power diverted from each battery would depends on voltage differences between each cell in the pack. It automatically varies from 100mA to 2000mA that is enough up to 200Ah good quality packs (this can be encresad for larger packs or especial uses)
- On first prototipe we are managing 3.3V lithium cells (LiFePo4)
3.6V or 3.7V Lithium cells is posible as well with very litle modification.
- In order o manage LeadAcid batteries 12V, 14V, 24V, 48V we may need big modifications unless we do it directly from each cell in the battery (2.0V) I am sorry but it is not in our plans to work in that line as we know that LeadAcid cells is a almost dead battery chemistry because it is using lead and I know that in at least EU they are preparing laws that will forbide them for common uses (ie. cars) in few years..._

I just finished yesterday all the rough tests I wanted to carry on the first prototipe. It is a BMS for a 6S LiFe pack.
I will post pictures today a bit latter on.
One thing that I want to say before you look at the pictures is:
- First: It is a protitipe so no carrying case is placed and a big mess of cables is around it in order to have easy access to working parameters so do not expect a very nice finishin or so. So sorry for the mess. Finished units will have a proper case and very little cables to be conected to the pack, charger, speed controler and gauges.
- Second: Some of you may be surprised of the small size of it, it is so small because we do not need bulky heat sinks (It does NOT heat up). We are not burning current, we are just need to move current form some cells to anothers.


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## kcameron (Oct 16, 2009)

_GonZo_ said:


> I will use part of your text f you do not mind on the project description in English if you do not mind.


No problem. Feel free to use it. Since I'm too lazy to build one myself, I'll consider it my contribution to the state of the art.


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## stormconnors (Aug 19, 2008)

"If you can't make it right, make it adjustable," was the advice of a friend of mine. It would seem that this device could be made to handle various types of cells or even batteries. Each of the setpoints could be set with individual components or adjustable pots.


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## JRP3 (Mar 7, 2008)

Kcameron, it seems to me that your description merely changes from slightly deeper discharging of the smallest capacity cells to increased cycling of the smallest capacity cells by essentially "recharging" them more often using energy from the larger cells. Either way they are likely to die first, and you've gained nothing in the long run, but you've probably spent a significant amount of money on a complex BMS.


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## Coulomb (Apr 22, 2009)

JRP3 said:


> Kcameron, it seems to me that your description merely changes from slightly deeper discharging of the smallest capacity cells to increased cycling of the smallest capacity cells by essentially "recharging" them more often using energy from the larger cells. Either way they are likely to die first,


The lowest capacity cells are going to work the hardest, I guess. So yes, this won't prevent the weakest from dying first.



> and you've gained nothing in the long run, but you've probably spent a significant amount of money on a complex BMS.


I disagree. Now if you have one cell that is say 10% less capacity than the average, you can now get most of that 10% back, i.e. you can now use the average (actual) capacity of your pack instead of the minimum. That could be a significant extra chunk of range.

Also, if one (or a few) cell(s) is(are) low in capacity, propping them up with stronger cells instead of "eating them" (causing them to go close to reversing) would presumably be very good for the longevity of the pack.

If this can be done for not much more than the cost of a standard BMS, it sounds quite worthwhile to me.


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## JRP3 (Mar 7, 2008)

Coulomb said:


> I disagree. Now if you have one cell that is say 10% less capacity than the average, you can now get most of that 10% back, i.e. you can now use the average (actual) capacity of your pack instead of the minimum. That could be a significant extra chunk of range.


I'd say if you have a cell that is 10% off from the rest you should probably replace it, but even if you gain back that 10% you aren't talking about much. That's 5 miles on a 50 mile pack.



> If this can be done for not much more than the cost of a standard BMS, it sounds quite worthwhile to me.


I think most BMS's are rather pricey, other than the simple ones, and something this complex that can shuttle charge between different cells will not be simple or cheap, in my opinion.


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## kcameron (Oct 16, 2009)

JRP3 said:


> Kcameron, it seems to me that your description merely changes from slightly deeper discharging of the smallest capacity cells to increased cycling of the smallest capacity cells by essentially "recharging" them more often using energy from the larger cells. Either way they are likely to die first, and you've gained nothing in the long run, but you've probably spent a significant amount of money on a complex BMS.


I was trying to simplify the algorithm description but perhaps I went too far. Here's some more detail:

The weaker cells would always see less current than the others and would therefor be protected from accelerated degradation. Basically, the BMS would attempt to keep the voltage equal across all the cells during charging and discharging. During charging, the voltage on the weak cells will tend to increase faster than the stronger cells. In that case, my BMS would shunt just enough current from the weak cells to the stronger ones to keep the voltages equal. Note that this would apply not only when connected to the "charger" but also during regenerative braking. During discharge, the voltage on the weak cells would tend to drop quicker than than the others so current would be shunted from the stronger cells to the weaker ones. In order to prevent efficiency loss and increased cell wear due to circulating energy, one constraint would be that the amount of shunted power would never be so much that some cells are discharging while others are charging.



> I'd say if you have a cell that is 10% off from the rest you should probably replace it, but even if you gain back that 10% you aren't talking about much. That's 5 miles on a 50 mile pack.


Adding new cells to a used pack would make it even more out of balance though a good BMS might eventually even things out. Another factor is that it implies more maintenance. One of my goals is to create a pack that needs little attention from the owner.

JRP3, you're right that one must weigh the costs of a BMS against the benefits. Since a LiFePO4 pack can cost $10K or more, I say that it warrants spending something to protect it.

I think I could build such a LiFePO4 BMS for about $30 per cell plus perhaps $80 for the master controller. That's within reason for a $100 cell. I'm also thinking about controlling 2-4 cells per node which should make it more cost effective.


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## Coulomb (Apr 22, 2009)

myself said:


> The lowest capacity cells are going to work the hardest, I guess. So yes, this won't prevent the weakest from dying first.


I've just realised, after reading the above post, that this isn't true either. If a cell is being charged while it is being discharged, the charge current doesn't flow into the cell and immediately out again, just less current comes from the cell.

So the weaker cells can have an easier time, causing the stronger ones to weaken relatively more, until they the stronger ones are no longer significantly stronger. So they all end up with much the same capacity. This is a very desirable property, it seems to me; no matter what the cells are like now, they will tend to wear out evenly.

Now: can this be made economical. $30 per 40 Ah ($44) cell is too much, but $30 per four $44 cells is reasonable. (I'm thinking high voltage, lower capacity, large number of cells here; obviously any BMS is more economical for a 120 V pack).


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## JRP3 (Mar 7, 2008)

The problem is the voltage differences you are talking about will only show up at the very ends of the charge/discharge curve. Your smallest cell will not show a significant voltage difference through most of the cycle, so your added expense and complexity will only come into play for a small fraction of the time. Spending $30 on a $100 cell makes no sense as you could have gotten a significantly larger cell which would give you more range and would be less likely to reach the ends of the curve. It makes even less sense on a smaller cell.


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## Coulomb (Apr 22, 2009)

JRP3 said:


> Spending $30 on a $100 cell makes no sense as you could have gotten a significantly larger cell which would give you more range and would be less likely to reach the ends of the curve.


Well, remember you won't get a 30% bigger cell, since you (surely) need some BMS. Low and high voltage cutoff would be the bare minimum.

But I take your point: you could get a 20% bigger cell and a simple $10/cell BMS, and likely get better range. Perhaps even 25% and $5.


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## JRP3 (Mar 7, 2008)

Actually I don't think you need anything on the cell level if you size your pack properly. Since cell level only comes into play during the very last bit of charging and discharging, size your pack so you never get there, and you should be safe. This is exactly what Jack has done with his Speedster, what Tom is doing with his SwiftE, and what I'll be doing with my Fiero. If you use your BMS money to purchase a 20-30% larger pack you should easily avoid the ends of the curve, as well as drawing a lower C rate at the same amp draw, which will also extend the life of the cells. It's a win all around as far as I can tell.


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## Coulomb (Apr 22, 2009)

JRP3 said:


> Actually I don't think you need anything on the cell level if you size your pack properly.


Huh? So how do you know when to stop charging? When the _average_ cell voltage gets to say 3.65 V? You could easily overcharge 1/10 of your cells. When the average gets to 3.40 VPC? Then you are losing range... possibly a fair bit of range. but I suppose you overcome that with the extra pack size.

But the danger of overdischarge is too high, surely. You're pretty much guaranteeing unbalance, and some cells will get eaten.

It's a big gamble. Too big in my opinion, but if I'm proved wrong, it will be great for EVs: BMSs add a lot of complexity. I'm glad you guys are taking the gamble, and I can look on for free.


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## JRP3 (Mar 7, 2008)

It looks as if an average charge to 3.45 gives a good balance of safety and range. There seems to be very little capacity above that, and in the real world you may be only giving up a couple of miles. Same thing on the low end, there isn't much usable energy below 3 volts, so don't go there. You'll get a lot more range by up sizing your pack 20-30% than you'll get with a BMS.


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## racunniff (Jan 14, 2009)

JRP3 said:


> It looks as if an average charge to 3.45 gives a good balance of safety and range. There seems to be very little capacity above that, and in the real world you may be only giving up a couple of miles. Same thing on the low end, there isn't much usable energy below 3 volts, so don't go there. You'll get a lot more range by up sizing your pack 20-30% than you'll get with a BMS.


That may be, but I *already* cram as many Wh into a car as I can manage. A BMS takes up less volume. And I think you absolutely need a per-cell HVC / LVC monitor at least. Individual cells can go flaky, and you don't want to take your whole pack down when one goes.


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## JRP3 (Mar 7, 2008)

Yes if you need every ah you can fit then you need to take your cells to full charge and discharge, and you need cell level management, no doubt. However by taking your pack to the edge every time you will be shortening pack life. As for monitoring for flakey cells, I plan to monitor two halves of the pack. Any significant difference in voltage between the two will tell me I have a problem.


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## tomofreno (Mar 3, 2009)

> You're pretty much guaranteeing unbalance


 I'd be very interested in any data you know of that demonstrates this Coulomb. Cells with balancing circuits on them remaining in balance is not evidence that balancing circuits are required for them to remain balanced. It could well be that cells drift out of balance over time, particularly as they age, but I haven't seen any data demonstrating that. Jack's cells are fairly well balanced after a year of aggressive use. If he had a bms on them, many would claim the bms kept them balanced. 

I plan to eventually install HVC and LVC as insurance. I expect they will rarely if ever trigger, but might if cells drift over longer term and I start checking them less frequently. It depends on how much attention you are willing to give to your pack. If you monitor cell voltages with a dvm during charging you can determine which cell gets to the exponential rise in voltage first. If you have a charge counter you know how much charge it took to get there from it's starting voltage, so you know how many hours you can charge at a given current from that starting voltage on that cell before it hits HVC. From then on you just go out and start checking cell voltages starting about 20 or 30 minutes prior to that time (or different time if it started at a different voltage) to ensure that cell is behaving the same and no others are at higher voltage. You won't overcharge this way. If that cell was within several mV of the other cells prior to charging, then that is the lowest capacity cell. Avoiding overdischarge is then simple if you only discharge to 30% soc of that lowest capacity cell - and you know how many Ah that is and can track it with your "Coulomb counting" gauge. After a few charge/discharge cycles the charger can be set up so that it enters cc mode and stops at a given soc of that highest voltage cell, say 95%. To be safe you would still need to check with your dvm near end of charge to ensure nothing has changed significantly.

If you don't want to give this much attention, and/or you want to squeeze every Wh out of your pack you can, then yes you definitely require some sort of automated protection. It seems that most commercial suppliers of evs, phevs, or hybrids do not charge to 100% nor discharge to 20% normally, even though they use a bms, in order to prolong cell life.

How often cells require balancing is an open question to me. Someone like JRP3 who is willing to monitor his pack as above, and also willing to occasionally (say every several months) individually charge cells to balance if necessary, can very likely operate with no bms at all without a problem. Eventually cells will weaken, and likely not all the same, so some will have to be replaced sooner than others. HLVC will warn you of this if you aren't paying close enough attention to see it yourself.

I think the disagreement on this just stems from how much time and attention people are willing to spend monitoring a pack and what socs they should operate between. There are a number of possible approaches.


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## Coulomb (Apr 22, 2009)

tomofreno said:


> I'd be very interested in any data you know of that demonstrates this Coulomb. ...
> I think the disagreement on this just stems from how much time and attention people are willing to spend monitoring a pack and what socs they should operate between. There are a number of possible approaches.


Oh, just that the without transferring charge from one cell to another, the weak will always get weaker, and the strong stay strong. So they have to diverge. Jack's pack is a sample of one, and even at one year old, that's only 10-20% of the life of the pack, and it's the uneventful end. I guess another year might be more telling.

You seem to be way more relaxed than me about not getting the maximum performance out of your cells. Also, you seem prepared to put in a lot of multimeter time as the batteries are charging. We'll see how much you like that when the novelty has worn off...

Edit: so I pretty much agree with your last sentence about the source of this disagreement.


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## JRP3 (Mar 7, 2008)

Coulomb said:


> Oh, just that the without transferring charge from one cell to another, the weak will always get weaker, and the strong stay strong. So they have to diverge.


By what mechanism? As I've mentioned the only time any charge shuttling will be happening is at the end of the curve, something you should be avoiding no matter what. Besides, the strong will get weaker as well, especially since with shuttling you will be drawing more power from them to support the weaker cells.


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## Coulomb (Apr 22, 2009)

JRP3 said:


> By what mechanism?


By the mechanism that the weak see larger discharges every cycle in proportion to their lower capacity. Greater depth of discharge, I think we all agree, leads to shorter life. The way lithium cells die, if they die of old age (usually) is to just have their capacity lowered to the point where you won't put up with the reduced range.

So my uinderstanding is (and please correct me if I'm wrong), something about extreme %SOC (very high or very low) leads to a very small but irreversible and therefore cumulative lowering of capacity. Since lithium cells can have a 2000 cycle life at 80% DOD and still have 80% of their original capacity left, the capacity reduction for the weakest cell would be of the order of 20/2000 % = 0.01% per cycle. But the stronger cells will be reducing their already larger capacities by say 0.0095% (edit: of their relatively higher capacity compared to the weakest cell's capacity) over the same cycle, so the gap between the lowest and highest capacity cell gets very slightly higher over time.



> As I've mentioned the only time any charge shuttling will be happening is at the end of the curve, something you should be avoiding no matter what. Besides, the strong will get weaker as well, especially since with shuttling you will be drawing more power from them to support the weaker cells.


Yes, this thread is about a way of shuffling energy from one cell to another. In this unusual (so far) case, the strong will work harder to do some of the work of the weak, making them a little weaker, so in the case of a charge suffling "balancer", you'll get the capacities of the cells converging. It took me a while to realise that.

Without the charge shuffling, the usual case, with or without top balancing, the capacities will diverge.

Sorry if my post was confusing; I think I forgot which topic I was in there.


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## tomofreno (Mar 3, 2009)

> Oh, just that the without transferring charge from one cell to another, the weak will always get weaker, and the strong stay strong.


 So in other words, you have no data. (Edit: I don't mean data you generated. I mean any data you know of from anyone.)


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## JRP3 (Mar 7, 2008)

Ultimately the point of a BMS should not be to save cells, but to save money. If it costs less to replace a few cells over time than it does to install a BMS, the BMS doesn't make sense. A $20 BMS on a 34 cell pack is the same cost as 6 100ah cells at todays prices, probably more cells in the future.


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## Coulomb (Apr 22, 2009)

tomofreno said:


> So in other words, you have no data.


No, I don't have data. This is all based on theory; experience from years of use of lithium is hard to come by. Does that make it any less valid?


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## MN Driver (Sep 29, 2009)

Years of LiFePO4 is hard to come by, however if you look for information on Lithium Ion, which discusses the Lithium Cobalt chemistry, you can find quite a bit of information. The R/C community has quite a bit of information but much of it involves a great deal of high rate abuse causing early failures versus more standard mechanisms of failure such as that which can be found outside of forums.

Taking a look through my own experiences, which goes along with different sources of information that can be found online, indicates that Lithium secondary cell chemistries age with cycles as reduced capacity as well as degrading internal resistance. High rate electronics get affected most by the degrading internal resistance which causes the device to shut off when it sags under voltage. Most of the time this can be seen with common household electronics. For example, I have an older laptop from 2004 where, when new ran for 4 hours surfing the web, which is pretty light use. Running a video benchmark benchmark it got about 1 hour 45 minutes. If I do the same with it now, it will surf the web 45 minutes, but if I run the benchmark and pull the plug with a charged battery it drops out before a minute will pass, it doesn't even have enough power to get into standby mode, but it tries to. So we have a good idea on how Lithium Cobalt fails through heavy cycling as I ran mine through two cycles a day and it was pretty much down to 1/2 of its original capacity in a year and I bought a new battery after 1.5 years for it, after 3 years I didn't get a new one and just lived with trying to find an outlet versus having the convenience of using the battery more often. So you have a real life example of a Lithium Cobalt being cycled to death at 90% DOD. It didn't even take 300 cycles to bring its capacity below 80% but I'd imagine that laptop manufacturers run with lowest bidder cells. I've had similar experiences with my smartphone batteries failing. I use them online with WiFi and cellular internet often, I'll get an extended battery after the first one doesn't last 2 hours with a Youtube video, which oddly enough only took a year to go from 3 hours to 2 hours of battery performance. With the extended battery I can usually charge the battery when its at about 30% whenever I use it for the internet and as a phone heavily. I haven't been disappointed with adding extra capacity for any device I've owned. I notice that the batteries last much longer, but that is mostly because they live an easier life with deep cycles less often and less amperage draw.

The spec sheets for LiFePO4 seem much better, they still display the internal resistance with age in the charts that come with the specsheets though, which is a primary concern of mine because I've read an experience of someone using Lithium Cobalt and the eventual demise of those packs was a function of not being able to pull the same current to the point where the pack was done with at that point. They said they would use Lithium again as it didn't have the pitfalls that they experienced with Lead Acid, and are currently using two different brands of LiFePO4 at the moment, but time will tell for LiFePO4. I'm a bit concerned over calendar life too as a charged Lithium Cobalt cell degrades with time and I'm not sure if this carries over to LiFePO4 or not and since nobody has had this around long enough to tell their tales, we just don't have the information yet. So, it seems that data and experiences exist for Lithium Cobalt, but its the new technology that is difficult to find, and I'd except to come across lab data for life testing but it seems that the only independent stuff I could find was the FMA Direct's PDF with A123 information that pointed out that under high rate draw that if the internal cell temp is above 140 degrees F or 60 degrees C, your cycle life swirls around the toilet bowl and goes down the drain. It's hard to tell what temperatures the core of these batteries are running at since we can only really tell by the outside of the case and from the terminal bolts. I'm assuming the highest temp that we can find is via the terminal bolts as its the only thing with a metal connection to the actual internal cell components. Small cylindrical cells make it easier because of the thin metal case and little distance between the center and the outside. I'd imagine the inside of the wad in a prismatic gets a bit warmer under high rates but we don't know how much.

We do have some data on LiFePO4, at least certain brands. From this forum we have people who have ran 6C on Thunder Sky cells and found they aren't fond of that after awhile as they fail to zero volts or vent, I'm assuming that they develop an internal short and have a feeling it could be internal heat related but I don't know specifically for that situation. Over 10C at 50% DOD(if I remember right) and they sag to around 2 volts. Cold temperatures and they sag more than warmer temperatures. Discharging them too far kills them, an exact point isn't known but I don't know of anyone daring enough to bring them below 2 volts under any circumstance.

Long term data with cells that were treated well and didn't have sudden premature failures. That information we do not have. I'm hoping that we find those answers later, rather than sooner, as this means that they are able to last longer.


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## s2156945 (Apr 15, 2009)

MN Driver said:


> It's hard to tell what temperatures the core of these batteries are running at since we can only really tell by the outside of the case and from the terminal bolts.


Can we not do some sums?

E.g. voltage drop * current = cell heating power ?
combine with specific heat of batteries (Theoretical :Anode material / cathode material / LiFePO4 ; Experimental: fridge one for a few days, and then place it in a known quantity of known temperature water in an esky for a few days)

So a 100Ah TS cells putting out 1000A at 2 volts is putting 1.2V * 1000A = 1200W into heating the cell.

Assume worst case, only half the cell weight is LiFePO4, and all the heat goes into it. So 3kg cell = 1.5kg LiFePO4, which is mostly Iron, 0.45 J/g.K. (Call it 0.4 for easy maths)

1200W = 1200 J/s
1200 / 0.4 = 3000 g.K / s

So each second at 10C could result in a 2 degree temp rise.

It could be worse than this, the heat could all be happening at the anode + cathode, only some (10%?) of the LiFePO4 could be getting heated, each second at 10C could be a 20 degree temp rise for that 10%.

Anyway, enough maths after midnight 

Happy New Year


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## _GonZo_ (Mar 23, 2009)

My God, I just been some days out and all this happened. (Sorry but had to attend something important)



> By what mechanism? As I've mentioned the only time any charge shuttling will be happening is at the end of the curve, something you should be avoiding no matter what. Besides, the strong will get weaker as well, especially since with shuttling you will be drawing more power from them to support the weaker cells.


The BMS is always working during full charge and discharge not only at the ends of the curves.
Because there is weaker and stronger cells and because there is different Internal Resistance cells in a pack.



> Originally Posted by *tomofreno*
> _So in other words, you have no data.
> 
> _
> No, I don't have data. This is all based on theory; experience from years of use of lithium is hard to come by. Does that make it any less valid?


Yes we have data:
A pack of Lixx cells becomes unbalance after a period of usage.
If not controled some cells of the pack will be dead earlie.
A Lixx pack should not be ever be charged or discharged with out LVC and HVC control and balancer is a must if it is need to get the maximun spam life and power of the pack.


About the complexity and price:
The "Shuttle BMS" is way complex inside but it is not complex to install and will not be expensive, we are calcualting that will be way under 10% of the full pack cost.


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## JRP3 (Mar 7, 2008)

_GonZo_ said:


> The BMS is always working during full charge and discharge not only at the ends of the curves.
> Because there is weaker and stronger cells and because there is different Internal Resistance cells in a pack.


As long as you are within the good part of the curve I'm not sure it matters if some cells are a few hundredths of a volt apart from each other. What does it matter if one cell is at 3.22 during discharge and another cell is at 3.24? It does matter if one cell is at 2.6 and the rest are at 2.8 but you shouldn't be taking any of your cells that low if you want maximum life anyway.


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## _GonZo_ (Mar 23, 2009)

Here are some pictures of the device.
Sorry for the delaly but things do not go as smooth as we will like...
I tried to make a video but did not come out well, tomorrow I will try again.

Picture 1 is a closer look of the BMS right side board is the balancing unit and left side board is the sunts board.
Picture 2 is the BMs performing a test.


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## CroDriver (Jan 8, 2009)

Any news?


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