# Cell balancing options - No more Volt Blochers!



## azdeltawye (Dec 30, 2008)

So I was all set to start ordering parts for my DIY LFP BMS and was planning on using the Volt Blochers. But to my surprise when I went to Brian's web page (http://www.voltblocher.com/) to place my order I discovered that this venerable product has been discontinued.  


Any idea on where I could find a similar system for cell balancing and HVC control of the charger for a Lithium battery pack?


Looks like its back to the drawing board to figure out an alternate BMS solution...


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## Overlander23 (Jun 15, 2009)

Depends on how big your pack is, I suppose... but an alternative to the Volt Blocher is the offering from Goodrum/Fechter design from TPPacks.

http://www.tppacks.com/products.asp?cat=26

There's another company that uses these boards and basically does all the construction to sell a turnkey solution. Can't remember who, though. Site search tool will help here...


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## ClintK (Apr 27, 2008)

Ack! Okay, who's going to take over producing Volt Blochers!? EVComponents? Anyone?

I'm maxed out on my spare time right now, but that would be a solid business for anyone willing to do it!


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## JimDanielson (Oct 19, 2008)

http://www.rechargeablelithiumpower...s-c-23.html?osCsid=3jkqaee7l4fo529l1ik6raigf7

this seems to be what you were talking about


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## etischer (Jun 16, 2008)

My coworker and I have been working on a BMS and monitoring solution. Basically a Volt blocher which is networked using rs232. Im hoping to have something to show in about 2 months. The Ebalance will support up to 6 or 12 cells per board, each board will have a communication port, and 2 temperature probes. Up to 12 Ebalance boards can be linked together. A laptop can also be connected, as well as other hmi hardware we are developing.

Supporting multiple batteries with 1 board reduces wiring, and makes it cost feasible to make a nice weather tight enclosure. It's also espcially helpful for batteries mounted under the car since the eBalance can be remotely mounted.


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## dimitri (May 16, 2008)

I find it amusing that my offer in Classifieds section of the same exact system as Brian's ( him and I collaborated on the design, but I ended up ordering my own PCBs ) has 1000 views, but not a single person expressed any interest.

http://www.diyelectriccar.com/forums/showthread.php/custom-made-bms-lifepo4-pack-32464.html

Today there are several choices available and I am not much interested in mass production for same reasons Brian mentioned, its too time consuming with little financial benefit since everyone wants cheapest product, yet demands high quality.


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## etischer (Jun 16, 2008)

I suspect you will be getting more interest now. The nice thing with Brian's Blocher is he offered several different packages depending on budget and needs. His design also fit any size battery. He also had a website that explained how they work and how to connect them. Im sure he gets more hits on his online store than you get readers to the one thread you have started. He is also the original designer and has been making improvements. For example he added a noise filter because I was having problems with PWM noise falsely triggering the shunting circuit. 

Your stated limitations limit you to people with a certain size battery TS160 or SE180, with a Zivan charger. I do like the improvements that you have made though, and if my needs fit in the niche you have filled, Id buy your version, assuming Brian is ok with you selling his design. 1000 views with not a single reply is something you dont see every day.




dimitri said:


> I find it amusing that my offer in Classifieds section of the same exact system as Brian's ( him and I collaborated on the design, but I ended up ordering my own PCBs ) has 1000 views, but not a single person expressed any interest.
> 
> http://www.diyelectriccar.com/forums/showthread.php/custom-made-bms-lifepo4-pack-32464.html
> 
> Today there are several choices available and I am not much interested in mass production for same reasons Brian mentioned, its too time consuming with little financial benefit since everyone wants cheapest product, yet demands high quality.


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## dimitri (May 16, 2008)

etischer said:


> My coworker and I have been working on a BMS and monitoring solution. Basically a Volt blocher which is networked using rs232. Im hoping to have something to show in about 2 months. The Ebalance will support up to 6 or 12 cells per board, each board will have a communication port, and 2 temperature probes. Up to 12 Ebalance boards can be linked together. A laptop can also be connected, as well as other hmi hardware we are developing.
> 
> Supporting multiple batteries with 1 board reduces wiring, and makes it cost feasible to make a nice weather tight enclosure. It's also espcially helpful for batteries mounted under the car since the eBalance can be remotely mounted.


Eric, I wish you best luck with your BMS, sounds like you are combining VB functions with PakTrakr like functions. As a Paktrakr user myself I can tell you how difficult it is to make it work in super high EM noise environment. I also don't see how this is making less wires if you have to attach 12 leads to each board 

You will undoubtly find out that in order to make your product worth your time to produce you will have to price it such that most DIY folks will find it too expensive 

I don't want to speak for Brian, but I am not surprised he quit, you really can't make enough money to justify all the time and effort that goes into even a simple product. 

ClintK, making $2 on repackaging components is not really a "solid business" model, but I might be spoiled by working in IT field for so many years  You have to make $$$ investment in bulk parts only to hope you will get it back soon enough and make some profit and pay off your bills. It only looks simple when someone else is doing it....


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## etischer (Jun 16, 2008)

Each board would have 13 cell connections instead of 24.

The communications for 12 cells would plug in with one input and one output plug. This emliminates 48 communication wires, and potential loose screw connections. One loose connection in a daisy chain could be disaster, with 100 volt blochers you've got 400 points of failure. 

The volt blocher was very time consuming to build, about 15 minutes of labor per board to solder by hand, and this board only serves 1 cell. For 100 cells, you're looking at 25 hours, or $1000 just in labor. 





dimitri said:


> Eric, I wish you best luck with your BMS, sounds like you are combining VB functions with PakTrakr like functions. As a Paktrakr user myself I can tell you how difficult it is to make it work in super high EM noise environment. I also don't see how this is making less wires if you have to attach 12 leads to each board
> 
> You will undoubtly find out that in order to make your product worth your time to produce you will have to price it such that most DIY folks will find it too expensive
> 
> ...


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## dimitri (May 16, 2008)

etischer said:


> Each board would have 13 cell connections instead of 24.
> 
> The communications for 12 cells would plug in with one input and one output plug. This emliminates 48 communication wires, and potential loose screw connections. One loose connection in a daisy chain could be disaster, with 100 volt blochers you've got 400 points of failure.
> 
> The volt blocher was very time consuming to build, about 15 minutes of labor per board to solder by hand, and this board only serves 1 cell. For 100 cells, you're looking at 25 hours, or $1000 just in labor.


There are pros and cons to both approaches, amount of components is similar, so labor will be similar too. I never liked screw terminals, so I used punch-in style on mine. I used single set of 4 wires that runs across all punch-in connectors, to eliminate points of failure.

If your system can replace both VB and PakTrakr, then it might be cost effective, but devil is in details, so we'll have to see what you come up with. Biggest issue with PakTrakr is that its powered from the same cells its monitoring, which is causing major headache with noise.

The idea was to make it cheap and simple to compete with complex and expensive, if you start adding complexity you will inevitably add cost, at which point you will compete with existing products. More products is good for the customer, so good luck, I might just buy yours next time since I am sick and tired of making Paktrakr work right.


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## ClintK (Apr 27, 2008)

dimitri,
The limitation of only TS160 or SE180 cells / NG3 is a no go for me (and I'm sure a lot of people). Otherwise I'm sure you'd get more replies and not just hits.

Here's my take on the business idea of it... it's an existing product, several very satisfied users, fairly simple, each single order was probably $300, still a demand for them, and unemployment is over 10%. Seems like there's at least one unemployed electrician or someone who can solder with spare time to make a few extra bucks on the side.


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## ahambone (Jan 13, 2009)

The Goodrum/Fechter boards that are available at tppacks (as a kit) and at RLP (pre-assembled, but more expensive) can be scaled up to use with a car and they can be used with TSky/SkyE/Headway cells. I'm using a 24s LiFePO4 battery with a Zivan NG-3 charger and the G/F BMS system.

I hate to cross-post to a different forum but I'd really hate to re-type everything in two places. Here's a link to the most recent assembly steps on my pack and a short video watching the Goodrum/Fecther BMS in action.

http://endless-sphere.com/forums/viewtopic.php?f=14&t=12320&start=225#p209942

Cheers,
--Adam


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## dimitri (May 16, 2008)

Main advantage of VB vs TPPack is ability to shunt up to 2 amps, which creates quite a bit of heat. When you try to pack multiple boards in one, you have to compromise on shunting capacity and thermal management. Of course TPPack was designed for smaller packs and chargers, so it does the job, but not many chargers can push out over 20 amps in CC mode and quickly drop to less than 1 amps in CV mode. My 22 amp Zivan can't go less than 1.5 amps in CV mode and other popular PFC chargers are even worse.

If you buy BMS you have to make sure your charger won't overrun it, otherwise you are getting false sense of security, that's why I limited my offer to specific configuration.

I also found that HVC protection is pretty useless since charger's aren't able to react on HVC unless you design additional circuits. LVC also requires additional circuits since every controller is different, so whole thing gets complicated. I got my system to work just the way I wanted it, but I was not interested in selling bags of parts. I'm not complaining, just pointing out that things aren't as simple as they look.

Hopfully one day there will be affordable and functional BMS, meanwhile each does what they can to the best of their ability


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## ahambone (Jan 13, 2009)

I think the complexity of the problem is why there aren't any neat and tidy shrink wrapped solutions. And I agree with the shunting problem - my system is only shunting up to 1 amp. Many users won't find the acceptable.

The tppacks (G/F) system interrupts the charger and throttles it down to ~1 amp with a switching PWM/FET. The tppacks BMS is in-series with the charger. I experienced a problem with the same issue you mentioned - when the BMS started shunting current it choked off the Zivan and the Zivan tripped into an alarm mode the first time. Adding a sizeable buffer capacitor between the Zivan and the BMS fixed that issue.

I guess I'd just saying two things: There is no thought-free simple one-size-fits-all solution. The tppacks solution, like the Volt Blochers, is one where there's a good amount of documentation so you can start thinking about what the BMS needs to do for your application.

Cheers,
--Adam


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## etischer (Jun 16, 2008)

We are using mostly surface mount components and pick & place automation so the labor will be greatly reduced. We have multiple noise filters and voltage regulators to deal with noise. Control power is 12VDC supplied from the cars electrical system. 

The BMS EV-Components sells for their Li Cells only shunts 0.7 watts. We are planning on 7 watts shunting power per cell, roughly the same as the Volt Blocher. I have not heard complaints with the Ethilon system, so perhaps 0.7 watts is adequate for most people. 




dimitri said:


> There are pros and cons to both approaches, amount of components is similar, so labor will be similar too. I never liked screw terminals, so I used punch-in style on mine. I used single set of 4 wires that runs across all punch-in connectors, to eliminate points of failure.
> 
> If your system can replace both VB and PakTrakr, then it might be cost effective, but devil is in details, so we'll have to see what you come up with. Biggest issue with PakTrakr is that its powered from the same cells its monitoring, which is causing major headache with noise.
> 
> The idea was to make it cheap and simple to compete with complex and expensive, if you start adding complexity you will inevitably add cost, at which point you will compete with existing products. More products is good for the customer, so good luck, I might just buy yours next time since I am sick and tired of making Paktrakr work right.


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## dimitri (May 16, 2008)

etischer said:


> We are using mostly surface mount components and pick & place automation so the labor will be greatly reduced. We have multiple noise filters and voltage regulators to deal with noise. Control power is 12VDC supplied from the cars electrical system.


Are you planning a display unit similar to PakTrakr to go with it? Have you figured out a good SoC algorithm for LiFePo4 to display fuel gauge? How much current will your units shunt?

Sounds interesting, keep us posted on your progress.

Thanks


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## dimitri (May 16, 2008)

etischer said:


> The BMS EV-Components sells for their Li Cells only shunts 0.7 watts. We are planning on 7 watts shunting power per cell, roughly the same as the Volt Blocher. I have not heard complaints with the Ethilon system, so perhaps 0.7 watts is adequate for most people.


You beat my question in your edited post. 

As for Ethilon that EV-Components sells, I have not seen any real reviews at all, so it all looks like vaporware until someone comes along with a review.


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## racunniff (Jan 14, 2009)

azdeltawye said:


> So I was all set to start ordering parts for my DIY LFP BMS and was planning on using the Volt Blochers. But to my surprise when I went to Brian's web page (http://www.voltblocher.com/) to place my order I discovered that this venerable product has been discontinued.
> 
> Looks like its back to the drawing board to figure out an alternate BMS solution...


Bummer. I had just placed an order (yet to be fulfilled, waiting on batteries) for 16 Volt Blochers with EVComponents last month. Now I wonder what will happen...


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## etischer (Jun 16, 2008)

racunniff said:


> Bummer. I had just placed an order (yet to be fulfilled, waiting on batteries) for 16 Volt Blochers with EVComponents last month. Now I wonder what will happen...


I have (25) volt blochers I will be selling in a few weeks, are you looking for 12v 
version? They could be modified for a different voltage with some minor soldering. PM me if interested. I installed larger heat sinks, and capacitors for noise supression.


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## dimitri (May 16, 2008)

Well, since offers are being made here 

I have about a 100 PCBs left from my own design bulk order, I guess I can help someone out if you are interested. 

My modules are exact same schematic as VBs, but a bit different PCB layout since I didn't want any wires, I wanted them mounted directly on cell terminals using metal bars as shown in the picture. My PCB was designed to fit on TS/SE 160/180/200 AH cells ( all 3 have same dimensions ), it won't fit on 100 AH cells as designed, but perhaps you can fit it sideways or group them aside from cells, etc.

Advantage compared to v2 version of VB is that mine are lower profile because they are wider, so it might be good if you have little room at the top of your battery boxes.

Since I am just selling off my remaining stock and not interested in long term business, I will offer them at best price I can manage, but only as assembled units, except you can make your own metal strips for mounting or use wires if you want ( PCB is designed for either bars or wires ).

I can offer basic unit , without HLVC, or full unit with HLVC ( comes with punch-in 4 wire connector ). This offer is for modules only, I don't care what charger you will use, its none of my business 

Please PM me for details, serious requests only, I can accept Paypal and will provide best price based on number of units you buy.


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## etischer (Jun 16, 2008)

Our immediate plan is to use a netbook as the HMI. Our long term goal is to have a 2 line LCD which would could be configured to display highest and lowest cell voltage.

We haven't planned to do a SOC meter but we will have a LVC output . I currently use an idiot light which gets latched on at 280v. This lets me know my AGM pack is about 50% DOD. The LVC and HVC triggers will be software configurable. 









dimitri said:


> Are you planning a display unit similar to PakTrakr to go with it? Have you figured out a good SoC algorithm for LiFePo4 to display fuel gauge? How much current will your units shunt?
> 
> Sounds interesting, keep us posted on your progress.
> 
> Thanks


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## SimonRafferty (Apr 13, 2009)

That's a shame - I must be amongst the last people to get some!

I bought rather than built because Brian was selling a complete kit for less than the components alone cost here!

Good luck to whoever takes up the gauntlet. Etischer's proposal sounds good to me - never know I might even upgrade!

Si


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## CroDriver (Jan 8, 2009)

dimitri said:


> You beat my question in your edited post.
> 
> As for Ethilon that EV-Components sells, I have not seen any real reviews at all, so it all looks like vaporware until someone comes along with a review.


I purchased a 112 cell Ethilon BMS looong ago but still waiting for it


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## EVComponents (Apr 20, 2009)

Otmar is working on a BMS. 

*It is going to be designed with surface mount components so that our pick and place machine can do 50 at a time*. 

But the first priority is Zilla production. Z1K controllers are rolling out this month.


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## jorhyne (Aug 20, 2008)

EVComponents said:


> Otmar is working on a BMS.
> 
> *It is going to be designed with surface mount components so that our pick and place machine can do 50 at a time*.
> 
> But the first priority is Zilla production. Z1K controllers are rolling out this month.


So is this BMS going to be similar in design to what the Volt blochers were?


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## etischer (Jun 16, 2008)

We plan to make them 51 at a time 



EVComponents said:


> Otmar is working on a BMS.
> 
> *It is going to be designed with surface mount components so that our pick and place machine can do 50 at a time*.
> 
> But the first priority is Zilla production. Z1K controllers are rolling out this month.


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## dimitri (May 16, 2008)

While competition between EV Components and Etischer is heating up  I am offering remaining supply of my BMS modules. These are priced even lower than VoltBlochers while supplies last  , although I offer only assembled units since I don't trust people to DIY and then say it doesn't work  , I QC and test each module before shipping.

See details in proper forum section , here is the link http://www.diyelectriccar.com/forums/showthread.php/custom-made-bms-lifepo4-pack-32464.html

(look at latest post in that thread, lots of changes since original offer )

I already have 2 standing orders, so only couple more orders are left to be filled, so think fast 

Send me PM with any questions which aren't answered in my linked post, please don't ask in this thread, we have enough off topic info here as it is


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## Guest (Nov 15, 2009)

I hesitate to post this. But I must. After a conversation with Rich Rudman of Manzanita Micro, I was inspired to a flurry of "balancing" this week using the GEM as a test mule on 24 160Ah Thunderskys. What I discovered was a bit shocking, and worse, it is something that should have been obvious all along.

If you "balance" your sells when charging, you greviously UNBALANCE them at the far end when discharging. This is because the cells have different capacities, and so different "length" discharge curves. If you line them all up at the top, you exacerbate the differences at the BOTTOM curve where they go over the 3.0 v cliff and start a quick descent to zero. The other "longer" cells turn on the weak one and eat it. This all happens at 300 amps of course while we're driving, instead of at 15 when we're charging.

I drove a total of five cells to zero volts this way in a series of three rather gruesomely detailed tests. Two cells "recovered" more or less. The others are dead forever.

Before "designing" a new version of a current shunt active cell balancer, you might want to have a look at the video at EVTV. I really think as soon as you see it you will "get it" and its a little embarassing we've all missed this for so long. http://evtv.me

Jack Rickard


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## JRP3 (Mar 7, 2008)

It sounds as if you ran it without a cell level LV signal, otherwise you would have had warnings of low cells?


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## tomofreno (Mar 3, 2009)

> If you "balance" your sells when charging, you greviously UNBALANCE them at the far end when discharging.


 I think the reason for balancing is to stuff as much charge as possible into each cell without overcharging any of them. If their capacities differ then, as you point out, some will discharge faster than others. That is why usually there is also a LVC signal to warn when the lowest cell reaches a "lower limit" voltage. If you continue to discharge anyway, you will fully discharge some before others as you say, but you are supposed to stop when one hits LVC. You have reported there is no need to balance very often, that your cells in the porsche just don't get that unbalanced. The capacities can't be very different in that case so they should all discharge about the same. 

If you balance at the bottom, then when you charge the cells the ones with lower capacity will reach HVC before the others. If you stop charging then you will not get as much charge in to the higher capacity cells as you could. You will be limited by the capacity of the cell of smallest capacity. Of course the same is true when you balance at the top if you stop discharging when the smallest capacity cell reaches LVC. In either case it seems to me the pack Ah capacity is limited by the cell with smallest capacity.

I think it is good to keep in mind that the population of all cells of a given Ah size produced by a supplier has some distribution in capacity with some mean and variance. Each shipment a customer receives is a sample from this population. If the sample consists of cells which are all close to the mean, then the pack will behave as you describe for the porsche - remaining fairly closely balanced with no active balancing. A sample with a wider variance will of course behave differently. How much variability you will see between samples of course depends on the variance of the population, and the sample sizes. If the population variance is fairly large, results from one sample will not represent all samples very well. From what scant data I've seen, it appears as if the population variance may be fairly small, since some others have also said the cells in their sample seem to remain balanced fairly well without active balancing. But the degree of confidence is small due to the small amount of data. Most of this data has come from you, so thank you very much for that and your stimulation of discussion Jack!


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## JRP3 (Mar 7, 2008)

Since you will always be limited by the cell with the smallest capacity you don't really gain anything by putting more charge into higher capacity cells. Stop charging when your lowest capacity cell is charged and stop driving when it gets near LV. Cell level signaling will give you that without the need for shunting.


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## dimitri (May 16, 2008)

You don't need to destroy any cells to come up with obvious statements, series connected pack will always be as big as its smallest cell, doh....

Balancing at low end is impractical and risky and it gives you zero benefits, since you are supposed to stop driving when first cell hits LVC, or ideally sometime before that 

Due to small differences in internal resistance and capacity there will always be one cell to hit LVC first and one cell to his HVC first, which may or may not be the same cell. BMS is not so much needed for balancing ( although I still argue for long term requirement to balance at HVC to avoid long term drifting apart ) but mostly to avoid overcharge and overdischarge of those "weakest" cells.


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## JRP3 (Mar 7, 2008)

dimitri said:


> Due to small differences in internal resistance and capacity there will always be one cell to hit LVC first and one cell to his HVC first, which may or may not be the same cell. BMS is not so much needed for balancing ( although I still argue for long term requirement to balance at HVC to avoid long term drifting apart ) but mostly to avoid overcharge and overdischarge of those "weakest" cells.


That is quite a bit different than balancing though. All you need is voltage signaling. If cell capacity changes over time you will still be limited to the weakest cell.
After watching Jack's video I must strongly disagree with one of his ideas, that is to disable the controller when a cell gets to a dangerous level. I'd rather kill a cell than be stuck in the middle of traffic, or on a railroad crossing. A loud warning, lights, whatever, but don't cut off the controller.


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## dimitri (May 16, 2008)

JRP3 said:


> Since you will always be limited by the cell with the smallest capacity you don't really gain anything by putting more charge into higher capacity cells. Stop charging when your lowest capacity cell is charged and stop driving when it gets near LV. Cell level signaling will give you that without the need for shunting.


This may not be true if higher capacity cell also has higher internal resistance. Over long time that cell will end up with less and less charge and eventually will fight for a title of "weakest" cell only because you failed to balance it over time.

You have to manage variations in capacity AND internal resistance TOGETHER, not only one or the other.

By giving up BMS and intentionally reducing your working range you are only buying yourself some time before your pack becomes a mess, be it a year or two, but it will happen eventually.


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## JRP3 (Mar 7, 2008)

dimitri said:


> mostly to avoid overcharge


Jack contends that overcharge is not anywhere near the problem that over discharge is. We've seen examples of dead cells from over discharge, are you aware of any from overcharge? I am not.


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## dimitri (May 16, 2008)

JRP3 said:


> That is quite a bit different than balancing though. All you need is voltage signaling. If cell capacity changes over time you will still be limited to the weakest cell.
> After watching Jack's video I must strongly disagree with one of his ideas, that is to disable the controller when a cell gets to a dangerous level. I'd rather kill a cell than be stuck in the middle of traffic, or on a railroad crossing. A loud warning, lights, whatever, but don't cut off the controller.


Again, signaling is important to manage difference in capacity, but balancing is important to manage difference in internal resistance. Cell with higher IR even if its largest capacity will be loosing more charge every cycle due to more heat generated, and eventually will become least charged cell. Balancing counteracts this process little by little every cycle.

In my EV, balancing takes few minutes at 1.5 Amps, amount of extra energy absorbed by high IR cell is very small, say 0.1 Ah. If I skip balancing, that cell will loose 10 Ah in just 3 months, or 40 Ah in a year.


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## JRP3 (Mar 7, 2008)

So it sounds as if occasional balancing would be enough to counteract that, once a month or so.


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## dimitri (May 16, 2008)

JRP3 said:


> Jack contends that overcharge is not anywhere near the problem that over discharge is. We've seen examples of dead cells from over discharge, are you aware of any from overcharge? I am not.


Yes I am, when I got my pack together I didn't have VBs yet, but was too impatient and started charging. Even though I was watching it closely I missed first cell getting over 4.25V and that cell is a little swollen now. That cell is now the weakest one in my pack, although it might have had this title even before it got swollen.


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## JRP3 (Mar 7, 2008)

dimitri said:


> Even though I was watching it closely I missed first cell getting over 4.25V and that cell is a little swollen now. That cell is now the weakest one in my pack, although it might have had this title even before it got swollen.


But it's not dead  How high did it get?


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## dimitri (May 16, 2008)

JRP3 said:


> So it sounds as if occasional balancing would be enough to counteract that, once a month or so.


Yes, that would work, but I find it more labor intensive than letting BMS do its job every day.

I acknowledge that most DIY EVs will likely to loose a cell or two over time, no one is protected from failures or defects. Its just a matter of managing risks and some of us find different ways of doing it.

As I have been using my LFP pack for past few months I began to realize more reasons why car makers stay away from EV business. Battery management in a way to be safe for regular non technical consumer is a nightmare. We are putting up with it because its our hobby, but I can see why many consumers would stay away unless the dealer takes full care of the pack, but then it becomes diminishing returns business for them, especially since the pack is so damn expensive to begin with.


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## Tesseract (Sep 27, 2008)

JRP3 said:


> Jack contends that overcharge is not anywhere near the problem that over discharge is. We've seen examples of dead cells from over discharge, are you aware of any from overcharge? I am not.


I bet if you are charging at ~100A _like someone we know _then you might very well damage a cell during charging, but at the 10A or so most chargers can deliver? Nah, not likely. Or, at least, not unless you let them stew for several hours after hitting 100%.

I was just discussing this yesterday with the guys at Rebirth Auto and after giving it some thought - and seeing this thread - I've more or less concluded that there really isn't much merit in either shunt balancing or charging each cell individually. Monitoring each cell's voltage? Sure. But actively balancing? Nah, it's a waste of time. The pack's total Ah is whatever the weakest cell's is, simple as that. Charge them all in series until a cell hits the high voltage cutoff. Discharge them until a cell hits the low voltage cutoff.

I also think that killing the controller upon LVC is a bit drastic. BUT, you could use the LVC signal to reduce the maximum allowed throttle (either by using a relay to switch in additional resistance in series with the throttle pot or adding such a function to a programmable controller.. hmmm..).


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## dimitri (May 16, 2008)

JRP3 said:


> But it's not dead  How high did it get?


I was too upset to take notes of exact voltage, my guess is 4.3V or so. Its not dead, but its no longer capable of high discharge because layers of foil loose some surface contact. It may not be as immediate death sentence as in deep discharge, but its just as dangerous long term damage.


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## JRP3 (Mar 7, 2008)

Tesseract said:


> I've more or less concluded that there really isn't much merit in either shunt balancing or charging each cell individually.


So you agree with Jack


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## JRP3 (Mar 7, 2008)

dimitri said:


> Its not dead, but its no longer capable of high discharge because layers of foil loose some surface contact.


Was it tightly strapped during the charge?


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## Voltswagen (Nov 13, 2008)

Jack
Thank you for that indepth analysis. I have long suspected that a low voltage alarm on the weakest cell is all that is needed and even those who currently run a comprehensive BMS system would agree that their total pack is only as strong as their weakest cell.

I have never heard any argument here or on other forums which document
the weakest cell ever getting stronger. If anything common sense would indicate that over time it could weaken even further.

Is there the possibility that another cell in the pack over time could exchange places with the weakest cell? Possible but unlikely if periodic
measurements are taken and conservative discharge ranges are adhered to.

Now the fun begins Jack. You will no doubt be subject to a barrage of counterclaims and criticisms by vendors who stand to benefit monetarily
by maintaining the BMS scare. 
Keep your cool Jack, because at the end of the day, the fact that you do this testing using your own money and are not a vendor should speak loud and clear to the general public. I applaud your research.

Roy Mann


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## dimitri (May 16, 2008)

Tesseract said:


> ... but at the 10A or so most chargers can deliver? Nah, not likely. Or, at least, not unless you let them stew for several hours after hitting 100%.


Try it with one cell at even 2 Amps for say 20 minutes, see what happens.



> I was just discussing this yesterday with the guys at Rebirth Auto and after giving it some thought - and seeing this thread - I've more or less concluded that there really isn't much merit in either shunt balancing or charging each cell individually. Monitoring each cell's voltage? Sure. But actively balancing? Nah, it's a waste of time. The pack's total Ah is whatever the weakest cell's is, simple as that. Charge them all in series until a cell hits the high voltage cutoff. Discharge them until a cell hits the low voltage cutoff.


Again, you are missing differences in IR. Cell with highest IR will continue to become weaker and weaker over long time since it needs more charge than others to stay on same SOC level.


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## Tesseract (Sep 27, 2008)

JRP3 said:


> So you agree with Jack


Wellllll. Just because the guy was practically frothing at the mouth calling me names in that dc/dc converter thread doesn't mean he can't be right about something else... 

I'm egalitarian like that...


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## Tesseract (Sep 27, 2008)

dimitri said:


> Try it with one cell at even 2 Amps for say 20 minutes, see what happens.


Will do. I should have added that I have no personal experience with this... yet. Frankly, there's not a lot of data on these cells one way or the other, and getting it the hard way is expensive for us, the end-users.




dimitri said:


> Again, you are missing differences in IR. Cell with highest IR will continue to become weaker and weaker over long time since it needs more charge than others to stay on same SOC level.


Maybe. Once again, I have not personally collected data showing a change in internal resistance with final charge voltage. It may very well prove to be inconsequential or, as JRP3 suggests, something that only has to be actively managed once or twice per year.


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## dimitri (May 16, 2008)

> Now the fun begins Jack. You will no doubt be subject to a barrage of counterclaims and criticisms by vendors who stand to benefit monetarily
> by maintaining the BMS scare.


If your idea of fun is making videos of dead cells, sure, no argument here 

My idea of fun is to drive my EV every day and not spend hours with voltmeter every week or month.

Since I was offering product here just like Brian did, I am clearly biased as "vendor who stands to benefit monetarily", so I am no longer going to comment here. Good luck everyone.....


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## Guest (Nov 15, 2009)

Yes, your pack always was limited to the capacity of the cell with least capacity. The point is that there is no point to balancing, but the potential damage sufficient to destroy a cell all happens at the bottom, not the top. The closer you can get the cells on the latter part of the discharge curve, the less likely damage. 

That said, of course a LV warning would be better, and of course limiting discharge to 70% DOD is even better and provides longer life. The trigger here is the premise that if you don't actively balance your cells you will "murder your pack" and "lose cells." 

First, it does not appear to be at all true. Top balancing cells appears to do nothing to avoid cell death whatsoever. In extremis, it actually causes it. But basically, we have a lot of people running around spending money or effort or both to do something that is 1. Unnecessary and 2. Potentially actually destructive in the event you do drive your car to full discharge.

There really isn't any damage possible while charging. It is typically done at 15 or 20 amps max, and if your charger can shut off at an average voltage of 3.6, the variance does not appear to be anywhere in the neighborhood that would lead to a very high (4.5vdc) voltage on any cell. I actually have a Thundersky charger that is fixed for a 72 volt 24 cell system. It charges to 87 volts (3.625 per cell).

But if you "run out of gas" and are drawing a couple HUNDRED amps from a low pack, and a couple of cells are lower by just a few tenths of a volt than the others, they will be immediately driven to zero volts and killed. If you have to balance, bottom balance. 

And yes, a LV alarm would be the most important thing you could do. There is a slight problem with this. The cells routinely drive to 2.0 vdc when under very high current loads momentarily. That's going to set off the alarm. And it is the static or resting voltage we are talking about here. A cell at a charge level of 3.2 volts that sags to 2.0 vdc while generating 600 amps doesn't have a problem and nobody is going to "drive it" to zero volts. But below about 2.8 STATIC, the cell is very weak and will dive for the dirt under any load.

At zero volts, some come back. And some never recover. I don't know what is happening there but I assume it is just a matter of degree.

Jack


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## Guest (Nov 15, 2009)

dimitri said:


> This may not be true if higher capacity cell also has higher internal resistance. Over long time that cell will end up with less and less charge and eventually will fight for a title of "weakest" cell only because you failed to balance it over time.
> 
> You have to manage variations in capacity AND internal resistance TOGETHER, not only one or the other.
> 
> By giving up BMS and intentionally reducing your working range you are only buying yourself some time before your pack becomes a mess, be it a year or two, but it will happen eventually.


I am very curious where you derive such a notion. I have noted no "drifting apart" of cells. Top balancing would do nothing to prevent this in any event. Your pack won't become a mess. I have a year old pack with 5000 miles on it and it doesn't appear to have changed at all that can be detected with test instruments without pulling the entire pack and checking the capacity of each cell.

The cells vary in capacity. Your shunt balancing simply allows the larger capacity cells to be fully charged AFTER the least capacity cells already have been. This moves the ragged edge from the top to the bottom, and it does absolutely nothing else, internal resistance having NOTHING to do with all that.

Your pack capacity was ALWAYS a function of the cell with the least capacity. Top charging accomplilshes NOTHING that I can determine with test instruments whatsoever. If it does, please describe your methodology so I can duplicate this experiment where you derived this information.

The lead acid thinking has permeated our conception of LiFePo4 cells rather naturally. This is all a learning curve. I myself have designed and advocated top charge cell balancing and even posted a schematic for others to do so. I feel badly to have mislead anyone. It kind of made sense, it just turns out to have not been true. These are new devices, and we are all on a learning curve here. I have to fess up. There is NO point in top charging at all, and in fact, if you want to duplicate my test, you will find that it causes you to eat cells, actively destroy them when they would not have been destroyed had you done nothing at all. 

All the above assumes you have NO LV alarm, and are simply going by overall pack voltage as a fuel gage. It can, in the case of top balancing, appear that you have energy left, when in fact one or more cells have ALREADY been destroyed beyond recovery. True, it is not within the 80%DOD level. 

If you charge to 3.4 volts, and only discharge to 50%, you're definitely not getting the max from your pack, but none of this shows up. I'm trying to find the edge here. And I certainly found it. The more I would balance, the more cells I lost. 

LiFePo4 cells are a little bit like redwood sticks as it turns out.

But I don't see ANY advantage to top cell charging. The cells have fixed capacities, and they don't "drift". All the cells will change in capacity over time, and top balancing won't do ANYTHING about that. You're just sliding the stick around. And if you move it at one end, the other end moves with it.

Jack Rickard


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## Guest (Nov 15, 2009)

Tesseract said:


> Charge them all in series until a cell hits the high voltage cutoff. Discharge them until a cell hits the low voltage cutoff.
> 
> I also think that killing the controller upon LVC is a bit drastic. BUT, you could use the LVC signal to reduce the maximum allowed throttle (either by using a relay to switch in additional resistance in series with the throttle pot or adding such a function to a programmable controller.. hmmm..).


I fear it's about as simple as that. Shut off the charger to prevent overcharge. Shut off the car to prevent overdischarge. The "balancing" gig made sense, just didn't turn out to be true.

Well, I was advocating a BIG RED LIGHT to tell you it was over, and then disabling the controller at a lower voltage to prevent damaging the cells. You're quite correct that that could be dangerous in a driving scenario.

I posed this question several months ago. In a land where 10,000 people per day run out of gas, how do we deal with this with electric cars - particularly for the masses. It got a big yawn online. But if you start to take this problem apart, it gets real icky really quickly. It sounds simple, and yes,, if you've built your own and paid for the batteries, it's not a problem. But what if you want to let your daughter use the car? 

This is what I believe is causing the automotive manufacturers to go to the "range extender HEV." They don't care about range and are fully cognizant that most people who are desperately concerned about range don't drive anywhere anyway. By automatically kicking on a generator, they're saving the batteries that they are on the hook for by warranty. This gives them CONTROL over the discharge cycle. 

In a pure BEV, how do we do the same? 

I'm not at all wed to cutting off the controller. And I'm not philisophically opposed to the concept of "battery management." But what does that mean, and it shouldn't do more harm than good.

1. I don't like fires.

2. I don't want a $25 device I've hired to watch my $200 cell destroy it because IT failed.

3. I want to be able to drive my car the farthest it safely can, even if it means giving up a few cycles.

4. I don't want to get to buy a new cell because I missed on number 3.

5. I don't want to require the driver to get a EE before I loan them my car.

6. I don't want to try to patch the Linux kernel while I'm driving.

ANd lacking any clear leadership or adult supervision from somebody that I can test and verify first, I'm committed to the concept of DOING NOTHING. More so now than I was.

First do no harm.

But some form of LV alarm would be nice. I like the EV power concept of an open/closed circuit through the cells as an alarm because it is inherently EMI resistant. No data bus. It's open. Or it's closed. And we could run 12 v through it (if it's isolated). 

I guess I'm looking for an idiotproof gas gage. In a hundred years, they never developed one for the ICE cars, but it was pretty much no harm no foul if you ran out. Put more in, and restart. For me, its $10K on the refill.

Jack Rickard


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## Guest (Nov 15, 2009)

dimitri said:


> I was too upset to take notes of exact voltage, my guess is 4.3V or so. Its not dead, but its no longer capable of high discharge because layers of foil loose some surface contact. It may not be as immediate death sentence as in deep discharge, but its just as dangerous long term damage.


So measure the discharge capacity in Ah and see what happened.

I don't think it was harmed. 

Jack Rickard


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## JRP3 (Mar 7, 2008)

jrickard said:


> And yes, a LV alarm would be the most important thing you could do. There is a slight problem with this. The cells routinely drive to 2.0 vdc when under very high current loads momentarily. That's going to set off the alarm. And it is the static or resting voltage we are talking about here.


The answer to that might be a time function of some sort, or an average. So 5 seconds or something at 2 volts does nothing but 6 or 10 seconds at 2.9 or whatever gives you the warning. I would think that a weak cell near discharge would sag more under load than a good one.


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## Guest (Nov 15, 2009)

Voltswagen said:


> Jack
> Thank you for that indepth analysis. I have long suspected that a low voltage alarm on the weakest cell is all that is needed and even those who currently run a comprehensive BMS system would agree that their total pack is only as strong as their weakest cell.
> 
> I have never heard any argument here or on other forums which document
> ...


Thanks. Yes, it upsets a few Apple carts. But I don't think they'll kick much. We're all on a learning curve here. The less you know about lead acid batteries, apparently the better off you are. Conceptually it seemed like a pretty good idea. I'm embarassed to point out that if you go to my Picassa slide show photo area for the Speedster, you'll find TWO schematic diagrams for a shunt balancer. I'm as guilty as anybody.

It gets worse. I should certainly have caught this a year ago. But even this week, I'm standing there for HOURS trying to get the last MILLIVOLT of balance like a moron standing out in the rain. This takes ALL DAY. Then I go drive the car, and find I've totally destroyed a cell. 

Then I go do it AGAIN like a moron. ANd lost another cell.

So I do it AGAIN but this time I REALLY balance them. And drove 3 cells to zero volts. One never came back. Don't really know that status of the other two but they took a charge and it drives.

At THAT point I start asking the question. How come the Porsche doesn't kill ANY cells, and I don't do anything, and I've just blown up at least $750 in batteries balancing my ass off here? And with less cells? And with larger cells?

So I start noodling this around, very upset, for ANOTHER day. And it finally hits me. We're just moving the charge level of varying capacity cells around on the table. They have a fixed curve for each cell. What happened to the other end when we lined them all up nice and neat at the top? 

We made the bottom MORE ragged. None of the cells changed at all by putting energy into them. And lining them up MORE neatly at the top isn't going to make it better. You cannot change the capacity or the internal resistance of the cell with a charger guys.

On the Porsche, I have a larger number of cells. And they never were "balanced" at the top. As it turns out by luck they were oddly pretty much balanced at the bottom. I drove 107 miles a couple of weeks ago and EVERYTHING was between 2.8 and 2.9 at the end. Another 300 yards and who knows? But it was looking good. And because they were balanced, my static voltage on the entire pack was very valid indication of "empty."

Once you see it, it's pretty obvious. But I missed it much more badly than anybody else. I had all the indications in front of me and somehow I thought if I REPEATED IT ENOUGH it would get better? Like three times in a week? 

Maybe I should go back and do it again?

Jack Rickard


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## Overlander23 (Jun 15, 2009)

I don't see anything wrong with killing power to the controller if it's the last thing that warns someone over time. It's kind of the way an ICE car works, and what Jack is saying. Why must we have these warnings down at 100% DOD? Have a "low fuel/charge" warning come on at 80% DOD... then shut the system down when you hit the limit. Like running out of petrol. If you still run out... you're dumb, but at least your batteries are protected. 

We can't save people from themselves. I completely understand that if your first warning of impending doom is your car shutting down in the middle of an intersection... but with a more robust system, if you've been warned in advance, there's no saving you anyway.

The difference between the EV and ICE in this case is, providing you haven't caused an accident, you might actually be able to cycle power enough to at least get out of harms way after a bit.

As for LVC measurements on the go, I like the idea of individual cell monitoring... If we're worried about cells normally dipping to 2.0v on an acceptably heavy load, why not have the LVC alarm circuit only come on when the amp load is measured reasonably low?

Yes, this would mean if you were climbing a hill and your pack dipped below acceptable LVC limits you'd never be warned... but there's where that pre-warning comes in. If you still attempt the hill, you ride will shut down.


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## Guest (Nov 15, 2009)

dimitri said:


> Try it with one cell at even 2 Amps for say 20 minutes, see what happens.
> 
> 
> 
> Again, you are missing differences in IR. Cell with highest IR will continue to become weaker and weaker over long time since it needs more charge than others to stay on same SOC level.


Again, the differences don't matter. It's charge level will always be indicated by its voltage. And its capacity and discharge curve, while it will change over time, is NOT going to be affected by your charger. The "weak ones" are the FIRST to hit the voltage.

It's like glasses of water. You can put more water in the larger glasses. But that doesn't change the level of water in the smaller glass. And when you go to take it out, you're not going to get more than the water in the smallest glass. It hits 4 volts first because it is smaller. It hits empty first for the same reason. Holding it there and adding water to the larger glasses "balances" nothing. In fact, it imbalances it - precisely the opposite of what you're trying to achieve.

Jack Rickard

Jack Rickard


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## Overlander23 (Jun 15, 2009)

Here's a question... Let's take the example to an extreme. Say I've just built my EV, and I've got 10 cells... each cell is at an unknown state of charge, but all cells have identical capacities. This is theoretical. 

If I were to charge these all in series and stop charging when the first cell hits HVC then I imagine I might have a bunch of unbalanced cells... unbalanced at the high end and low end.

The question is, do cells drift over time regardless of their capacity if discharged in series? Say, theoretically, that the capacity of all the cells was always the same, over time would the relative charge point of each cell move?


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## Guest (Nov 15, 2009)

dimitri said:


> If your idea of fun is making videos of dead cells, sure, no argument here
> 
> My idea of fun is to drive my EV every day and not spend hours with voltmeter every week or month.
> 
> Since I was offering product here just like Brian did, I am clearly biased as "vendor who stands to benefit monetarily", so I am no longer going to comment here. Good luck everyone.....


Don't go away mad. My idea of fun IS making videos and spending hours, ok not THOUSANDS of hours, but dozens of hours with a voltmeter. And I do froth at the mouth a bit.

But you're not the only savage on the Buffalo hunt. We ALL bought into this line of thought, and certainly I did. I never heard a dissenting voice in two years. Actually ONE guy on Thundersky did warn against "unbalancing" when we were trying to balance. Don't remember who he was, but I should have listened.

You haven't made a mistake. You built a device to do exactly what everyone was demanding. You're a hero. That's what we need. More doers. Less typers.

We were just asking for the wrong thing. It made sense. It just wasn't true.
And in fact, I'll back off of that now that I've gotten spanked. It APPEARS to have not been true. I'm reporting a theory based on some actual measurements. The next shoe to drop is someone will show me how THAT thinking is dorked up as well. 

I spent all day Friday beating up Brian with "what have I missed here. This can't be...they're not going to like me down at the ol' EV shop...what am I missing..."

I would have been smarter, and thinner, and had more hair (and maybe a little less frothy). But I was busy that day.

Jack


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## Guest (Nov 15, 2009)

JRP3 said:


> Since you will always be limited by the cell with the smallest capacity you don't really gain anything by putting more charge into higher capacity cells. Stop charging when your lowest capacity cell is charged and stop driving when it gets near LV. Cell level signaling will give you that without the need for shunting.


In a nut shell. You're all over it as best as I can tell.

Jack


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## Guest (Nov 15, 2009)

Overlander23 said:


> Here's a question... Let's take the example to an extreme. Say I've just built my EV, and I've got 10 cells... each cell is at an unknown state of charge, but all cells have identical capacities. This is theoretical.
> 
> If I were to charge these all in series and stop charging when the first cell hits HVC then I imagine I might have a bunch of unbalanced cells... unbalanced at the high end and low end.
> 
> The question is, do cells drift over time regardless of their capacity if discharged in series? Say, theoretically, that the capacity of all the cells was always the same, over time would the relative charge point of each cell move?


It's an interesting question. But it appears to be two questions.

Certainly you want the cells to be initially "in synch". You don't want a fully discharged cell in a pack of fully charged cells. So some balancing would appear to be in order. I would say balance at the bottom, not the top, because that is where the damage would be done. As I said in the video, there MIGHT be some salutory value to once or twice a year line them all up at 2.9 vdc.

I DO know from experience that you want to marry in NEW cells into a pack pretty carefully.

Cells do drift over time IN their capacity. The voltage and internal resistance is largely a reflection of that. What happens when they diminish in capacity differently, say for example, because they are in different places in the car and get different cooling from the motion of the car?

I don't know.


Jack Rickard


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## Guest (Nov 15, 2009)

dimitri said:


> You don't need to destroy any cells to come up with obvious statements, series connected pack will always be as big as its smallest cell, doh....
> 
> Balancing at low end is impractical and risky and it gives you zero benefits, since you are supposed to stop driving when first cell hits LVC, or ideally sometime before that
> 
> Due to small differences in internal resistance and capacity there will always be one cell to hit LVC first and one cell to his HVC first, which may or may not be the same cell. BMS is not so much needed for balancing ( although I still argue for long term requirement to balance at HVC to avoid long term drifting apart ) but mostly to avoid overcharge and overdischarge of those "weakest" cells.


Balancing at the low end is no more or less impractical than the top end. I can't do a little circuit that does it automatically if that's what you're referring to. But you can do it manually.

Jack Rickard


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## cycleguy (Oct 7, 2009)

Jack, I've read your post and also watched your latest Friday video, I totally agree with your assumptions and analysis, however, it would stand to reason that if top balancing is inaccurate due to differences in cell capacities, then bottom balancing would seem just as inaccurate. For example, bottom balancing all cells to the same 2.8V would be no different than top balancing them to 4.0V. 

I think the key is to balance each cell to the same point in it's capacity curve, whether it's at the top or bottom of the curve. 
To do this, it would be necessary to measure the rate of voltage drop or voltage rise during either the charge or discharge cycle to balance each cell to the same point in the knee of the curve regardless of what voltage that ends up being. This would assure all cells are at the same point in their capacity curve. You can make note of the bottom and top voltages of each cell and then set your max. charge and min. discharge voltages according to weakest cell in the pack.

Developing a single cell charger or dis-charger that measures the rate of voltage change and then displays that voltage once that set rate is reached would be a great tool to balance each cell before assembling your pack.
It would most likely be best to determine this at the bottom discharge knee since I agree that over discharge is more harmful to the cell.

I think that once each cell is balanced this way and accurate low and high voltage cut-offs are determined, an active battery management system would not be necessary. A simple low voltage battery cell monitor would be the only thing needed. This can then be communicated to the controller to either shut down or limit throttle for a limp mode strategy. 

The most elegant solution would be to integrate all this into an external battery charger with shunts for each cell mounted to a large fan cooled heat sink assembly. The charger would monitor the rate of voltage rise of each cell during the charge cycle and shunt voltage once each cell reaches the same point in the knee of the curve. This would balance each cell to the same point in it's capacity curve rather than to the same voltage.
The on-board charger would not be able to do this but would still work fine if set to a more conservative 3.7-3.8 Volts per cell. This setup would eliminate the potential hazards of carrying on-board shunts at each cell and increase the reliability of the shunts by making them more robust with better heat dissipation abilities since they would now be external. This would allow balancing without having to remove each cell individually.

I'm not an electronics expert and have no idea how hard it would be to build something like this, but would think it would be something worth creating.


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## Jeff (Sep 25, 2008)

I read the thread with a bit of amazement at the amount of time spent on the subject 

(1) If you require charge balancing to the degree that you risk damage to cells that drop out early in the discharge cycle, you've got bigger issues to worry about. Seriously, your pack is either: (A) total junk, and you're screwed (B) you are discharging far too deeply, or (C) forgot to integrate some means of low voltage cutoff, and it'll soon be back to (A).

(2) If your pack requires more than a few watts dissipated to balance (assuming it is mature and undamaged), you need to find a new battery supplier. Someone has sold you a pack of mismatched junk.

A high voltage clamp (shunt) on each cell during charge is critical, and a low voltage cutoff on each cell during discharge is critical. Nothing has changed here. 

If you are experiencing trends that indicate an early failure mechanism, it might be wise to publish that data for others to read.

Regards, Jeff


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## JRP3 (Mar 7, 2008)

I think at the low end if a cell is at 2.8 volts or so it's getting near empty, no matter what it's capacity, just as a cell at 4 volts or so is getting near full, regardless of capacity. So if balanced at the bottom to 2.8 all cells would be at the same point on the discharge curve, I think.


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## JRP3 (Mar 7, 2008)

Jeff said:


> A high voltage clamp (shunt) on each cell during charge is critical, and a low voltage cutoff on each cell during discharge is critical. Nothing has changed here.


Except that Jack has about 5000 miles on a pack with no high voltage shunt, and has had no issues, because, as you point out, the cells are all very close to each other to begin with and he doesn't take the pack to the ragged edge.


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## Guest (Nov 15, 2009)

cycleguy said:


> Jack, I've read your post and also watched your latest Friday video, I totally agree with your assumptions and analysis, however, it would stand to reason that if top balancing is inaccurate due to differences in cell capacities, then bottom balancing would seem just as inaccurate. For example, bottom balancing all cells to the same 2.8V would be no different than top balancing them to 4.0V.


Actually there is a large difference between the two ends. At the top, we are using a charger at typically a few amperes of current. It is true, you could conceivably damage a cell by overcharging, but it hasn't been much of an issue in practice.

At the bottom, I'm somewhat dependent on the pack voltage to show me I'm running out of gas. Actually, I have a balance indication, and in fact an Ah meter, with the EVision.

Worse, at the bottom we are still driving the car. Picture 300-500 amps through the pack, instead of 3 to 5 amps through the pack. 

Keep in mind that this is still somewhat theoretical in practice. The reason it hasn't come up earlier, is anyone with $10K in a battery pack (except me, duh) doesn't drive it to 100%DOD. Or even 90%. The convention is 80% DOD and often 70% for those trying to maximize pack life. Note that this is all still better than the lead acid days where 50% DOD was the story.

But the result was we had a lot of people spending effort and money top charging to NO good effect I can determine. Since they didn't discharge 100%, it didn't show up as a problem for them. Classic Blue Elephant Gun. DOes, it work? Do you see any Blue Elephants? Then it works. Buy two. Two would work even better.

And none of us WANT it to show up as a problem for them. I can fall on my sword. You guys can watch the video. And there we are....

The shunt regulators appear to be useless. In extremis, they can be damaging. Pick one.

LV. I havent' addressed this, because we all use different controllers. But MOST controllers have some form of LV detect built in. They'll either shut down or effect a "limp" mode to get home. This is actually the proper place for this, since the controller knows whether it is applying power or taking a static reading. UNFORTUNATELY, the controller is detecting total pack voltage, NOT individual cell voltages. 

IF you have it balanced at the bottom, your controller is more likely to be effective in this role. If you top balance, it will be LESS likely to be effective in this role. And as I say, it varies from controller to controller.

I would like to see a series of individual cell measurement boards, with some sort of 12v bus or better a simple open/close loop, hooked up to the controller, where the controller knows when a cell hits a certain voltage, which could be calibrated on the cells. Then software programmable in the controller as to what it does with the information (lights LED, goes into LIMP, shuts down, throws up, or changes case color).

If a sturdy bus could be devised that reports voltages, even better build all the intelligence into the controller where you can program several stages of alarm/shutdown based on several programmable software levels. If ANY cell reached x, do y. If any cell reaches v, do w. If any cell reaches t, do u. Etc.

The charger is the wrong place for all this. You only charge while sitting. Your big current draws are while driving. That's where I think the damage is done. Some chargers are pretty stupid, and so some thing has to be rigged to shut them off to prevent overcharge. But that's really overcoming a shortcoming from a badly designed charger, or when using the wrong charger for your pack.

I think it's the controller.

http://evtv.me


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## tomofreno (Mar 3, 2009)

> Again, you are missing differences in IR. Cell with highest IR will continue to become weaker and weaker over long time since it needs more charge than others to stay on same SOC level.


 Dimitri, I don’t think internal resistance matters as far as how much charge you put into a cell or take out of it. It does of course affect how much energy you get out of the cell. During charging or discharging you have the same number of ions per unit area and time flowing to the cathode, and same number of electrons per unit area and time flowing to the anode in each series connected cell (current continuity). The charge transfer into or out of each cell is the same. The voltage, V, required, and the work done, qV,to move that charge differs in cells with different internal resistance. That just results in different energy output from the charger during charging or different energy output from the cells during discharge, but all the series connected cells gain the same amount of charge during charging, and loose the same amount in discharging.

I don’t think it matters at what voltage you first balance the pack, but I think it should be initially balanced. If not, then the pack capacity will be a function of both cell capacity and cell state of charge. If by chance your lowest capacity cell has the highest initial soc, your pack capacity will be even more limited by this cell.


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## tomofreno (Mar 3, 2009)

I should have said ions and electrons per unit time, not per unit time and area. The latter is flux density, and this may well be different between cells due to differences in effective collecting area of the electrodes, but the total flux, ions and electrons per unit time, should the same due to current continuity (current is the product of charge and flux). 



> The cells routinely drive to 2.0 vdc when under very high current loads momentarily. That's going to set off the alarm.


 Even at soc greater than say 40%? I was thinking they would set it off at about 30% soc or less when pulling high current so it would give you a warning your low capacity cell was getting low, but you would still have enough charge to get to a charging station or home.


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## Russco (Dec 23, 2008)

jrickard said:


> I hesitate to post this. But I must. After a conversation with Rich Rudman of Manzanita Micro, I was inspired to a flurry of "balancing" this week using the GEM as a test mule on 24 160Ah Thunderskys. What I discovered was a bit shocking, and worse, it is something that should have been obvious all along.
> 
> If you "balance" your sells when charging, you greviously UNBALANCE them at the far end when discharging. This is because the cells have different capacities, and so different "length" discharge curves. If you line them all up at the top, you exacerbate the differences at the BOTTOM curve where they go over the 3.0 v cliff and start a quick descent to zero. The other "longer" cells turn on the weak one and eat it. This all happens at 300 amps of course while we're driving, instead of at 15 when we're charging.
> 
> ...



Jack, back to the beginning. Since your conversation with Rudman, whom manufacturers battery chargers with his famous Rudman shunt regulators for both lead acid and lithium batteries, _what does Rudman have to say about using shunt regulators for end charging of lithium batteries?_


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## azdeltawye (Dec 30, 2008)

Wow! I had no idea this thread would cause such a ruckus.

While I agree with Jack on the premise that "bottom balancing" has more merit than "top balancing" I see no easy way to automate this process. And I think at this point nobody really knows how far individual cells will drift over the life of the battery pack. I feel it would be prudent to error on the conservative side and ensure that the pack is balanced periodically, even if it is only by means of "top balancing". Regardless of if "top balancing" is the preferred method or even necessary at all I have much better things to spend my time on than manually "bottom balancing" my traction pack several times a year… 

Perhaps we should table this discussion for five years or so and see what mortality rates the various methods of battery management yield…


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## dexion (Aug 22, 2009)

I seem to be ok working to the weakest cell in my pack (#3 on my car and #19 on my scooter.) I use a simple volt meter I got off ebay.

What we need is a way to charge the weakest cell(s) off the pack when they start to go below x voltage. A dc-dc converter to 3.3V say from pack voltage and something to float the charge around to the weakest cells. Like a spare tire for a flat that sort of thing. But, I suppose it really isn't that necessary if you are that close all the time you need a bigger pack. Or, a way to disable cells from the pack when they are below x voltage as well. That way the strong cells can still run the car and balancing at the top makes some sense. But this all sounds more expensive/complicated than just replacing weak cells or building a bigger pack.


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## tomofreno (Mar 3, 2009)

> What we need is a way to charge the weakest cell(s) off the pack when they start to go below x voltage. A dc-dc converter to 3.3V say from pack voltage and something to float the charge around to the weakest cells.


 See Lee Hart's design at:
http://www3.telus.net/nook/balancerland/


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## CroDriver (Jan 8, 2009)

Jack, this makes sense, thank you for sharing your experience and knowledge!

So a really really good BMS should monitor the voltage and current that's coming in and out of each cell, calculate it's overall capacity and capacity per, let's say 0,1V and then charge each cell independent of each other based on the collected data. 

I think that we will need to change the whole idea of a battery pack that is charged by a standard charger and monitored/balanced by a separate device (BMS). A intelligent charger with a integrated "BMS" is probably the only long-term solution for people that don't want to spend their time with voltmeters and individual cell chargers.

In Jack Rickard's case the pack is fine without a BMS for a few hundreds (thousands?) cycles but I think this won't be possible in my car since I'm pulling very high currents. Since it looks like I won't get my Ethilon BMS that I purchased I'll find out the harder way.


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## JRP3 (Mar 7, 2008)

dexion said:


> What we need is a way to charge the weakest cell(s) off the pack when they start to go below x voltage. A dc-dc converter to 3.3V say from pack voltage and something to float the charge around to the weakest cells. Like a spare tire for a flat that sort of thing.


Not a horrible idea if you have one or two cells that you know are low, you could have a 3.3 volt charger boost them, but it would have to be able to push some amps if it only comes on if LV is triggered when driving.


> But, I suppose it really isn't that necessary if you are that close all the time you need a bigger pack.


Yup.


> Or, a way to disable cells from the pack when they are below x voltage as well.


 That will end up stressing the rest of your pack and probably killing the next lowest cell.


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## JRP3 (Mar 7, 2008)

Ultimately this is about saving our money. Even a low cost setup such as the VBs, at about $20 per cell, would cost me $720 for my 36 cell pack. That's 6 extra 100ah cells. With the more expensive units it would be even more cells. All for "protection" that may not be necessary, and might, as in Tom's case, actually cost us some cells. Either way it seems to be something of a gamble, so I'm going to hang onto my cash and see what happens. I can kill 5 cells and still be better off financially.


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## cycleguy (Oct 7, 2009)

I think what we are all looking for here is a cost effective, safe and practical way of charging and balancing our packs. I want to treat the entire pack as if it were one cell. Why not simply use a separate single cell charger for each cell. You can get a $15 smart charger that charges to 3.8V with led indicator and .5Amp output that shuts off by itself. Why not simply use one of these for each cell and pack them all together into one enclosure. I don't think it would be much larger than my current charger. A 30 cell pack would run $450, 15amp output, about the same as my current 96V onboard charger, but now you'll also have built in cell balancing and no need for shunting. This combined with an individual cell low voltage warning device is all you'll need to keep your pack safe and healthy.
This seems like too simple of a solution, am I missing something?


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## JRP3 (Mar 7, 2008)

A few things. 3.8 is really too high for SE cells, but fine for TS cells. But you aren't charging at 15 amps, you're charging at .5 amps, which is too slow. Faster charging increases the usability of my vehicle. It might be possible to use individual cell chargers along with a high output charger such as a PFC30. The PFC does the fast charge to a certain level then the individual chargers finish and balance. This is essentially what Jack is doing with his PFC75 and Brusa, though not on individual cells. What charger are you referencing?
Of course Jack is basically saying balancing at the top end on a regular basis doesn't really get you anything, since once your smallest cell is full there is no more range to be had by charging the others higher.


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## JRP3 (Mar 7, 2008)

If you do want to balance one of these per cell would be even cheaper, and 2 amps at least for finish charging http://www.all-battery.com/TenergyLiFePO4BatteryCharger01300.aspx


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## Buckster (Nov 4, 2009)

From what I understand a traction battery is basically a multiple cell pack that has been prebuilt into a single module and individual cell balancing is not employed anyway, I am not sure what advantage a BMS or blotchers offer in real terms and it seems the jury is still out judging by the comments here, I can see the advantage however of individual cell monitoring with regards to effective charging and fault reporting so perhaps that is where an effective BMS wins out, can someone clarify how individual cell balancing would work in a series system? Under load I cannot see how it can function effectively as current still has to flow through any affected cell while the current is being drawn from the pack, so any cell that displays a lower energy density than the others is going to discharge quicker than the others regardless of what you do, the only effective way to bolster that cell is to place another battery or energy source in parallel to it. This actually draws me to another consideration about running two 360v packs in parallel, it would be better if the make up was based on running two parallel cells at a time in series with another two parallel cells and so on rather than just two series blocks of cells in parallel. (I know that sounds confusing but I know what I mean).


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## Guest (Nov 16, 2009)

CroDriver said:


> Jack, this makes sense, thank you for sharing your experience and knowledge!
> 
> So a really really good BMS should monitor the voltage and current that's coming in and out of each cell, calculate it's overall capacity and capacity per, let's say 0,1V and then charge each cell independent of each other based on the collected data.
> 
> ...


No. You're trying to resolve this to do exactly the same thing again. You do NOT need to do it at all. Quit trying to "balance" your pack with a charger. 

I do a LOT of current out of my pack. At highway speeds, 540 amps. 

This is pretty natural lead acid thinking. The difference with LiFePo4 cells is this "hook" in the discharge curve at the end. That's the problem. It would be nice to detect individual cell voltages at the bottom end, and do something useful with that information.

It is probably salutory to initially balance at the bottom to basically validate your fuel gage and ensure all the cells get there "more or less" in concert.

Lacking the ability to do either of those, DO NOTHING. It's a LOT better than top balancing.

Jack Rickard
http://evtv.me


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## Guest (Nov 16, 2009)

cycleguy said:


> I think what we are all looking for here is a cost effective, safe and practical way of charging and balancing our packs. I want to treat the entire pack as if it were one cell. Why not simply use a separate single cell charger for each cell. You can get a $15 smart charger that charges to 3.8V with led indicator and .5Amp output that shuts off by itself. Why not simply use one of these for each cell and pack them all together into one enclosure. I don't think it would be much larger than my current charger. A 30 cell pack would run $450, 15amp output, about the same as my current 96V onboard charger, but now you'll also have built in cell balancing and no need for shunting. This combined with an individual cell low voltage warning device is all you'll need to keep your pack safe and healthy.
> This seems like too simple of a solution, am I missing something?


Yes. You are essentially using separate chargers to do the same "top balance" we're discussing. I recommend you avoid that. But it is of course your batteries.

Jack Rickard


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## cycleguy (Oct 7, 2009)

jrickard said:


> Yes. You are essentially using separate chargers to do the same "top balance" we're discussing. I recommend you avoid that. But it is of course your batteries.
> 
> Jack Rickard


Jack, I understand the concept of bottom balancing and agree with you, however if you bottom balance all the cells to 2.8 volts, how do you really know that is the bottom of each cell, given the variance in cell capacities. It would be no different than assuming 3.8V is the top. My earlier point was to try to balance all the cells to the same point in their discharge hook regardless of what that voltage ends up being, 2.8 or 2.5V. Then note the weakest cell and make sure you don't go below that. Since the voltage in the cell starts dropping at a faster rate once you approach this hook, you could determine this rate and then stop discharging the cell once this rate is reached. This would assure all cells are at the same point in their discharge curve and balanced regardless of the voltage reading.

On your stick example in the video, you illustrated the difference in capacity at the bottom by lining up the tops of each stick. In that example you could actually see were the top of each cell was and lined them up, but we can't see that in a real cell unless we adjust each cell to the same point in it's discharge curve and then start from there. That point may be 2.8V for one cell and 2.5 for another. The bottom line is that during discharge, all cells would reach the same point in their discharge hook at the same time regardless of what that voltage would be, and I think that was what you were trying to accomplish.


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## CroDriver (Jan 8, 2009)

jrickard said:


> No. You're trying to resolve this to do exactly the same thing again. You do NOT need to do it at all. Quit trying to "balance" your pack with a charger.
> 
> Jack Rickard
> http://evtv.me


I was actually thinking to charge them to different voltages with regards to each cell capacity since the "BMS" would know the capacity of each cell if it would follow how much current is comming in and out of each cell and compare that with the voltage drop/rise.

If cell #4 has 100 Ah and cell # 8 has 110 Ah cell #4 should be charged to 3,8V and cell #8 to 3,7V so they would balance at the other end, at 20-30% SOC.

It's quite complicated but it might could work(?)


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## Guest (Nov 16, 2009)

cycleguy said:


> Jack, I understand the concept of bottom balancing and agree with you, however if you bottom balance all the cells to 2.8 volts, how do you really know that is the bottom of each cell, given the variance in cell capacities. It would be no different than assuming 3.8V is the top. My earlier point was to try to balance all the cells to the same point in their discharge hook regardless of what that voltage ends up being, 2.8 or 2.5V. Then note the weakest cell and make sure you don't go below that. Since the voltage in the cell starts dropping at a faster rate once you approach this hook, you could determine this rate and then stop discharging the cell once this rate is reached. This would assure all cells are at the same point in their discharge curve and balanced regardless of the voltage reading.
> 
> On your stick example in the video, you illustrated the difference in capacity at the bottom by lining up the tops of each stick. In that example you could actually see were the top of each cell was and lined them up, but we can't see that in a real cell unless we adjust each cell to the same point in it's discharge curve and then start from there. That point may be 2.8V for one cell and 2.5 for another. The bottom line is that during discharge, all cells would reach the same point in their discharge hook at the same time regardless of what that voltage would be, and I think that was what you were trying to accomplish.



You're in WAY to deep. What you describe may be possible, but it is way past what I can measure with available equipment I know of.

The ASSUMPTION is that the charge and discharge curves of the cells are essentially identical in shape. What will be different from cell to cell in a series where cell capacities vary is the LENGTH of the curve. The assumption, and I think we are QUITE safe in this for all practical purposes, is that 2.8 vdc represents the precise same point in a precisely identical curve from cell to cell. Similarly 3.65 vdc represents precisely the same point in a precisely identical curve on the other end. 

The reason you REACH 3.65 vdc on one cell first while charging is the overall LENGTH of the curve is different - not the voltages we detect to define it. Cells with less Ah capacity would naturally reach 3.65vdc BEFORE cells that have larger Ah capacity.

Similarly, at the other end, cells with smaller capacity would reach the 2.8 vdc point first. That's the problem.

By "balancing" the cells at the 2.8 vdc level, we are attempting to assure that they all march into hell in lockstep. As the entire voltage of the pack decreases in unison, we can detect this and determine that we are running out of fuel. And we don't have the situation of one of the cells running down to zero volts while others are still above 3 volts and still making good power.

But given that they have different "lengths" or Ah capacities, when we charge them, the differences will be exacerbated at the top of the charge cycle. That's where I want them for two reasons. 

1. We are dealing with the cells at a 10-20 amp charge current level instead of at the 200-500 amp current level. (NEDRA guys read 1200-2000 amp level). Destruction is an order of magnitude less imminent.

2. I'm in the garage and can manage this. Not driving down the road wondering what's going on. If I routinely charge to 3.625 vdc average and at the end of the charge cycle I'm seeing individual cell voltages above 4.25 vdc, I can modify the charge level I'm bringing the pack to in order to accommodate this. 

I also have great visibility as to what the "balance" or variation is in my cells. If I really did have an unusually "short" stick that goes above 4.25 vdc way out ahead of the others, I can replace it with a single cell of higher ampacity and thereby improve my pack and ultimately my range. 

But the assumption is that the 3.65 vdc points, and the 2.8 vdc points, are accurate references representative of the curve. Since 90% of the capacity is in the long flat range of 3.3 to 3.0 vdc, I think this is a very safe assumption. 

In truth, you are technically correct in that there will be some minute variation there. But it is a very very micro percentage of the total ampacity and curve we are trying to deal with here - effectively below the noise threshold of what we are measuring.

In other words, the length of the flat discharge part of the curve is so much larger than the length of the sharp hooks on the ends, that slight differences in the curvature at the ends are swamped by the differences in overall length. 100:1. 1000:1. Somewhere between those two.

Jack Rickard


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## cycleguy (Oct 7, 2009)

Thanks Jack, I was getting a little too deep into this, it is safe to assume that two cells at 2.8V would be at the same point on the curve. With this in mind, we don't need to make this more complicated than it is.

I'm leaning towards no BMS, initial cell balance at 2.8V, a Paktrakr or similar device to monitor each cell, which would simply shut off the charger once the first cell reaches 4V or 4.2 and also warn when a cell reaches 2.8V and a cut-off at 2.5V. There would have to be a delay built into the cut-off, perhaps 30sec. to prevent cutoff during voltage sag. You can check and compare all your cell voltages periodically to weed out a weak cell.
I think this is all we really need.


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## DIYguy (Sep 18, 2008)

Please excuse me for slightly changing the train of thought here.. but since the thread is called cell balancing options, I must assume that it was not meant to discuss only lithium related issues...?? 

Am I still correct in assuming that overcharging as related to VRLA chemistry is likely an application where shunting at the top end is perhaps more warranted?? I know that overcharging these devils causes irreparable damage due to boiling/venting. Yes-no?


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## JRP3 (Mar 7, 2008)

cycleguy said:


> I'm leaning towards no BMS, initial cell balance at 2.8V, a Paktrakr or similar device to monitor each cell,


You need to be careful with what you use to monitor the pack. I think the Paktrakr draws power from some of the cells, effectively unbalancing your pack.


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## JRP3 (Mar 7, 2008)

DIYguy said:


> Am I still correct in assuming that overcharging as related to VRLA chemistry is likely an application where shunting at the top end is perhaps more warranted?? I know that overcharging these devils causes irreparable damage due to boiling/venting. Yes-no?


Yes. But LA is so 20th century


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## Buckster (Nov 4, 2009)

Batteries are the most crucial and annoying part of an EV, I am amazed how much of a minefield it is and how diverse the approach of even the large manufacturers is, I have learned so much from reading this board.


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## DIYguy (Sep 18, 2008)

JRP3 said:


> Yes. But LA is so 20th century


I didn't recall asking for that....


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## Propellator (Oct 13, 2009)

allow me to throw in some experience from a different field of use.
For the last 5 years or so I use LiPo cells in my radio controlled models.
In the beginning balancing was virtually unknown and my first packs were charged and discharged without balancing. But since they were hilariously expensive I never really charged them to the top and never discharged them completely. Those cells still work today.
After the top notch sponsored pilots started to balance their packs (at the top) this technology quickly spread to the point where battery packs came with balancing cables already attached (to pick up single cell voltage for the charger). 
LiPos became more and more affordable and so, when a cell in a pack became pregnant, we considered the cells to be of bad quality and that his would be the statistical failure one encounters once in a while. So we continued to balance at the top and accepted the occasional cell failure.
Never even considering that the problem might be the preprogrammed cutoff voltage (measured across the whole pack) of the controllers we use.

Last year I started to use a controller which employs the single cell voltage pickup (because it was available and fit the model) and from then on I never had a cell failure again.
The problem was solved, I thought, because cell quality became better and better. Sadly my grey matter didn't allow me to make the right conclusion.

So I have to say that, when watching Jacks video on Sunday, jaw dropping and forehead slapping, it was an extreme pleasure to witness one of the rare eureka moments in engineering. 

I'm now off to check whether the radio control guys knew about the balancing issue all along.


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## 86Honda (Apr 15, 2009)

dimitri said:


> I was too upset to take notes of exact voltage, my guess is 4.3V or so. Its not dead, but its no longer capable of high discharge because layers of foil loose some surface contact. It may not be as immediate death sentence as in deep discharge, but its just as dangerous long term damage.


Mine went to 4.7 V and is permanently dead. Though I can still use it as a prop for education programs, or a $100 doorstop.


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## Voltswagen (Nov 13, 2008)

Bob
Your garage page said you installed Volt Blochers. How did you get to 4.7?


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## Dolphyn (Nov 17, 2009)

Going back to the original topic of this thread, I think the EV Power cell modules from Australia would be a good replacement for Volt Blochers. No need to buy the "master unit", since the modules can be used on their own (or with a wire to detect if any cell has high or low voltage). They can be purchased here. I inquired about shipping to the United States, and the cost is reasonable. Enjoy!


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## 86Honda (Apr 15, 2009)

Voltswagen said:


> Bob
> Your garage page said you installed Volt Blochers. How did you get to 4.7?


Once upon a time I moved the clutch cable out of the way of the hood latch, and in doing so I unknowingly pushed over the heat sink on one of my cells. That snapped two of the three leads to the sink, and it stopped shunting. I did not realize this until after two charge cycles. By then it was too late. The red LED showed shunt condition, but the heat sink was cold. Result - 4.7 volts and a swollen and dead cell.


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## jondoh (Sep 8, 2008)

actually, i don't think the volt blochers can stop a cell from going beyond its upper voltage limit. It doesn't necessarily stop the flow of power into the cell. It diverts some of it through the resistor. 

you sure the cell wasn't damaged on over discharge?


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## 86Honda (Apr 15, 2009)

jondoh said:


> actually, i don't think the volt blochers can stop a cell from going beyond its upper voltage limit. It doesn't necessarily stop the flow of power into the cell. It diverts some of it through the resistor.
> 
> you sure the cell wasn't damaged on over discharge?


It diverts at least 2 Amps, and that's what my charger reduces to. And yes, charge energy can still get through, but at a very slow rate. This cell never went below 2.4 V at any time it was measured, even after range tests.


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## Dave Koller (Nov 15, 2008)

Sorry if this is off thread - or maybe not - but have you guys been here?

BMS DIY

http://www.aeva.asn.au/forums/forum_posts.asp?TID=900


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## tomofreno (Mar 3, 2009)

> Sorry if this is off thread - or maybe not - but have you guys been here?


 Thanks for the link, some good info. Looks like the simple circuit is setup to start shunting current at a voltage below the HVC signal so the charger keeps running to balance cells. That of course works well if you are charging less than or equal to the shunt current, but if not, you need some signal to throttle the charger current back prior to the shunt turning on. And your high current charger has to be capable of reliably cutting back to a current less than the shunt current. Also, as someone in the thread pointed out it is good to have current limiting resistors in all current paths between the cell terminals, since if a semiconductor junction fails shorted, it may short the cell. Dimitri describes this and other issues in his cheap bms thread.


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## bblocher (Jul 30, 2008)

There is a lot of good talk going on here. It's hard to be on the cutting edge and trying to figure all this out so I wanted to throw in my experience as well as it seems to be quite different than others are describing here. I have 10k miles on my pack now and the characteristics of the cells have changed over time.

When my cells were new the VBs had quite a bit of work to do to get them all topped off. I bet it took a day at least. After that each charge there wasn't much time needed to balance and you could argue that a little balance every month, or maybe never again would be fine.

Seven months and 10k miles later it's completely flipped. It takes 80% (technically a bit less) of my pack to get to work (35 miles up hill). I noticed I started getting cells that were lower and lower in voltage when I arrived to work as the days went on (it's also cold now). Eventually one cell hit 0v on the way up and was lost, it wouldn't charge. I had a couple of other cells that I could see would be next. I manually charged these cells and they were fine for a few days (even being the first to charge up instead of being the last like before). Within a few days they were the lowest cells during charge again and low when I arrived to work. I ended up shunting even longer during a charge to give these cells more time to charge and now all is good. I can make it to work without them dropping too low in voltage.

So it seems the internal resistance of these cells are all changing at their own pace as they age and creating a lot of problems for series charging. It's even requiring more shunt balancing than I expected. I guess you could just replace these cells, but I'm not sure if others would soon follow or not. There is more to learn here on how cells change over time, but I couldn't disagree more that nothing needs to be done. Maybe if you don't need but half of your capacity this is the case.

I know this goes against some theories that a few of you are tossing around and I'm not trying to discredit anyone. This is just my experience thus far.

It would seem to me that individual cell charging (of course what a pain for an EV) is the best solution for new and aged cells.


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## Voltswagen (Nov 13, 2008)

Brian
When you first assembled this pack did you determine your weakest cells?


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## bblocher (Jul 30, 2008)

Voltswagen said:


> Brian
> When you first assembled this pack did you determine your weakest cells?


That's a good point, I did not. I got the capacities from Elite Power Solutions for all my cells. I didn't have a good way at the time to test them myself. All of the cells were 162-167Ah assuming those are correct. I'll have to find that list and see if the lower Ah cells are the ones lower now as well.

I'd still like to do some tests now and determine capacity (@75F) for each cell.


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## Voltswagen (Nov 13, 2008)

Brian
Thanks for being honest about that. Yes, definetly try to measure the capacity now.
You're scaring the crap outta me as you & Dimitri are the guys we all look to for real world experience before we mortgage our kids and follow your lead.
You're a bright guy and I'm certain you will discover the cause.
Roy


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## dimitri (May 16, 2008)

Thanks Brian for jumping in here 

My guess is that your pack is undersized for your hills and commute distance, which forced you to get too low SoC on regular basis, which in turn killed a cell and reduced capacity on others, which in turn snowballed in further reduction and imbalance, etc. Lower temp just added insult to the injury 

From my experience I think we gave TS too much credit with 80% DoD, especially when temps are dropping, they seem to be dying from deep discharge and even more at lower temps.

Even here in flat and warm Tampa, I am observing reduced pack performance when temp is at high 50's in the morning, and I am almost never below 50% SoC, but on those rare occasions when I do get to lower SoC I can see nasty voltage drops more and more.

I guess bottom line is - size your pack as large as you can and keep temps as high as you can and never go below 20% SoC.

Oh and also, please think really really hard before jumping into low end balancing, its not as simple as it looks on TV... don't cut the branch you are sitting on....


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## CroDriver (Jan 8, 2009)

Man, I would kill someone to know how the Tesla guys solved this problem! 

Any info on that?


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## etischer (Jun 16, 2008)

I think many problems disappear when you put cells in parallel. 

If you have 10 cells in parallel, one or two weak cells don't cause a failure. Cells in series cause the weak link effect. 



CroDriver said:


> Man, I would kill someone to know how the Tesla guys solved this problem!
> 
> Any info on that?


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## CroDriver (Jan 8, 2009)

etischer said:


> I think many problems disappear when you put cells in parallel.
> 
> If you have 10 cells in parallel, one or two weak cells don't cause a failure. Cells in series cause the weak link effect.


 
A friend of mine who's building e-bicycles takes hunderts of 10Ah cells, test them ALL and makes packs (2-5 cells in parallel) with cells that have similar capacity.

I doubt that he would do it if there is no need for it.

RC racers are doing the same as far as I know.


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## dimitri (May 16, 2008)

CroDriver said:


> Man, I would kill someone to know how the Tesla guys solved this problem!
> 
> Any info on that?


By throwing money at it 

Seriously, IMHO, cylindrical cells are much more stable against foil layer separation than prismatics and as Eric pointed out, when they are paralleled it improves even more against single cell failure.

Also, they likely have hard limits on LVC and its set to much safer levels, my guess is 30% SoC hard cutoff to save the pack. I am talking about actual cell SoC, not what's displayed to the driver. Its likely to be Display 0% SoC = 30% actual SoC in the pack.

Pretty much throwing money at the problem all the way around


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## Voltswagen (Nov 13, 2008)

Hmmmmmmmm...parallel?
Jack's Porsche uses 2 - 36 batt strings of 90ah


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## etischer (Jun 16, 2008)

I'm talking cells in parallel, not strings of series cells in parallel. I don't know how Jack is wired. 



Voltswagen said:


> Hmmmmmmmm...parallel?
> Jack's Porsche uses 2 - 36 batt strings of 90ah


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## Dave Koller (Nov 15, 2008)

etischer said:


> I don't know how Jack is wired.


LOL  won't touch that... but I bet he could enlighten the crowd - be interesting to know...


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## tomofreno (Mar 3, 2009)

> My guess is that your pack is undersized for your hills and commute distance, which forced you to get too low SoC on regular basis


 He said 80% dod. According to the TS spec they should get something like 2000 Cycles at this dod right? He is charging twice a day, 10k miles at 70 miles per day is about 140 days, so maybe 280 cycles,1/7 the spec.


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## etischer (Jun 16, 2008)

if capacity is reduced in cold weather he might be gonig beyond 80% dod. He may not reach 100% charge in cold weather too. 



tomofreno said:


> He said 80% dod. According to the TS spec they should get something like 2000 Cycles at this dod right? He is charging twice a day, 10k miles at 70 miles per day is about 140 days, so maybe 280 cycles,1/7 the spec.


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## Voltswagen (Nov 13, 2008)

Etischer
If you parallel all cells in a pack....the Ah is additive but the voltage is not. How will you power say a 120v motor on 3.2v nominal?


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## etischer (Jun 16, 2008)

Group 18 cells in parallel, then connect 100 of those groups in series. If you get a weak cell, there are 17 others to balance the load. 




Voltswagen said:


> Etischer
> If you parallel all cells in a pack....the Ah is additive but the voltage is not. How will you power say a 120v motor on 3.2v nominal?


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## CroDriver (Jan 8, 2009)

etischer said:


> Group 18 cells in parallel, then connect 100 of those groups in series. If you get a weak cell, there are 17 others to balance the load.


Yep, that's what I'm doing now. 13 Headway 10Ah cells in parallel and 90 of them in series

Should be better than the ThunderSkys I have now


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## Voltswagen (Nov 13, 2008)

So you are suggesting very small cells for example...the A123 cells?


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## Voltswagen (Nov 13, 2008)

CroDriver
So your system is 130AH at 288v?


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## CroDriver (Jan 8, 2009)

Voltswagen said:


> So you are suggesting very small cells for example...the A123 cells?


A123 are very expensive and it's not easy to make packs out of them



















I prefer Headway cells, they are bigger (2,3 Ah vs. 10Ah) and are easier to connect since they have screw terminals. 10C continuous and 20C peak is enough for a car.

You won't need A123 cells unless you're racing or building a very small pack for a bike, go-cart or something like this...



Voltswagen said:


> CroDriver
> So your system is 130AH at 288v?


Yes, maybe I add more 130Ah paralleled cell strings later for higher voltage


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## etischer (Jun 16, 2008)

Voltswagen said:


> So you are suggesting very small cells for example...the A123 cells?


The original question was asking about the Tesla pack. I read though Teslas pdf too quickly... 
"The Tesla uses 186540 form-factor cells", 6800 of them. 

18650 = 18mm diameter, 650mm length.


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## JRP3 (Mar 7, 2008)

6831 actually


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## tomofreno (Mar 3, 2009)

> "The Tesla uses 186540 form-factor cells", 6800 of them.


 Wow, a lot of development and reliability lab effort must have gone into making low contact resistance, reliable connections.


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## lnstech (Nov 20, 2009)

I have a mail order company that sells electronic kits (www.techkits.com). I am also an EV builder for hobby. I would be willing to sell the Lead Acid version of the Volt Blocher kits if I could get Brian's permission & schematic. I have tried to contact him by email, but with no response yet.


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## Guest (Nov 21, 2009)

Propellator said:


> allow me to throw in some experience from a different field of use.
> For the last 5 years or so I use LiPo cells in my radio controlled models.
> In the beginning balancing was virtually unknown and my first packs were charged and discharged without balancing. But since they were hilariously expensive I never really charged them to the top and never discharged them completely. Those cells still work today.
> After the top notch sponsored pilots started to balance their packs (at the top) this technology quickly spread to the point where battery packs came with balancing cables already attached (to pick up single cell voltage for the charger).
> ...


I'll add an interesting aside to that - and I get a LOT of info from RC guys. I probably have nearly a dozen of their little balancers here. Yes, some do charge at the top. By far the majority of what I have, do a very small 100 ma or 200 ma BLEED to the LOWEST cell voltage. Cute little aluminum cases. Typically do 6 cells. I have one that does 8. If you hook it up without a charger, it bleeds each cell in turn in a kind of cycle until they are all at the level of the lowest cell.

Jack Rickard


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## Guest (Nov 21, 2009)

Dolphyn said:


> Going back to the original topic of this thread, I think the EV Power cell modules from Australia would be a good replacement for Volt Blochers. No need to buy the "master unit", since the modules can be used on their own (or with a wire to detect if any cell has high or low voltage). They can be purchased here. I inquired about shipping to the United States, and the cost is reasonable. Enjoy!


Actually, I suspect the head unit is the most important part. What I THINK they're doing (I don't have mine yet) is summing optoisolators. The forward voltage on these is something, I don't know what. But say 1/2 volt. Over a hundred of them in series that's 50 volts. And you're not really isolated from teh pack.

I think the head unit converts this to a relay switched. Actually, it can do several things. But the most important is it isolates you from the optoisolator loop. I know what isolators are SUPPOSED to do, but you have to power them somewhere, and to do it with minimum wires you have to series them.

Jack Rickard


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## Guest (Nov 21, 2009)

bblocher said:


> There is a lot of good talk going on here. It's hard to be on the cutting edge and trying to figure all this out so I wanted to throw in my experience as well as it seems to be quite different than others are describing here. I have 10k miles on my pack now and the characteristics of the cells have changed over time.
> 
> When my cells were new the VBs had quite a bit of work to do to get them all topped off. I bet it took a day at least. After that each charge there wasn't much time needed to balance and you could argue that a little balance every month, or maybe never again would be fine.
> 
> ...



Ahemmm.... I hate to be insensitive. And I've kind of made you the poster boy. Understand, my own personal belief is your a HERO that designed a circuit to do something quite well, it was just the wrong thing, although EVERYONE, including me, thought it was the RIGHT thing at the time.....

But you top balanced. And you have a 0 volt cell that can't recover. With 10,000 miles. And a couple more going that way....

That's EXACTLY what got me started thinking. I had a smaller set of cells, right in front of me, in a vehicle where it was NO problem to reach any terminal, with a voltmeter in my hand, and I had to go through this THREE TIMES in as many days. So I'm real sympathetic here. The old thinking doesn't just go away by itself....it takes actually QUITE a bit of whiskey....

Here's a couple of graphs that might help. 





















Jack Rickard
http://evtv.me


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## Dave Koller (Nov 15, 2008)

jrickard said:


> Actually, I suspect the head unit is the most important part. What I THINK they're doing (I don't have mine yet) is summing optoisolators. The forward voltage on these is something, I don't know what. But say 1/2 volt. Over a hundred of them in series that's 50 volts. And you're not really isolated from teh pack.
> 
> I think the head unit converts this to a relay switched. Actually, it can do several things. But the most important is it isolates you from the optoisolator loop. I know what isolators are SUPPOSED to do, but you have to power them somewhere, and to do it with minimum wires you have to series them.
> 
> Jack Rickard


Jack:
Isn't this the schematic for those at (first page) http://www.aeva.asn.au/forums/forum_posts.asp?TID=900


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## JRP3 (Mar 7, 2008)

Jack, any chance you could resize that for better readability? I'm scrolling up and down and back and forth just to see it.


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## Guest (Nov 22, 2009)

Dave Koller said:


> Jack:
> Isn't this the schematic for those at (first page) http://www.aeva.asn.au/forums/forum_posts.asp?TID=900



I don't know for sure. But a device built to that schematic would do precisely what they're describing on their web site.

Jack Rickard


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## Guest (Nov 22, 2009)

JRP3 said:


> Jack, any chance you could resize that for better readability? I'm scrolling up and down and back and forth just to see it.


There. How's that?

Jack


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## JRP3 (Mar 7, 2008)

Better, thanks.


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## JRP3 (Mar 7, 2008)

Jack, I sent you a PM a few days ago but never heard back, you may not have PM notification turned on.


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## Dave Koller (Nov 15, 2008)

jrickard said:


> I don't know for sure. But a device built to that schematic would do precisely what they're describing on their web site.
> 
> Jack Rickard


my last post here:
http://www.diyelectriccar.com/forums/showpost.php?p=151680&postcount=72

I think one could add a cmos pulser circuit (15 cent chip say 4 gate 4093 ) and read all those needed parameters into that opto-isolator (in the afore mentioned schematic) and string them in Dimitri's ideaq of a NC loop to a simple led on the dash! as mentioned in the above link to the other thread.. real simple on off pulses - no emf - similar to what you showed but easy homebrew (err DIY).. BMS ; but I know this thread is about balancing but I think they are all of the same family..


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## Guest (Nov 22, 2009)

JRP3 said:


> Jack, I sent you a PM a few days ago but never heard back, you may not have PM notification turned on.


I may not even know what PM is? Was it after noon?

Jack Rickard


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## Dave Koller (Nov 15, 2008)

jrickard said:


> I may not even know what PM is? Was it after noon?
> 
> Jack Rickard


I believe it means private message..


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## Roy Von Rogers (Mar 21, 2009)

jrickard said:


> Balancing at the low end is no more or less impractical than the top end. I can't do a little circuit that does it automatically if that's what you're referring to. But you can do it manually.
> 
> Jack Rickard


Sure one can build a circuit to do that. It just has to be the opposite what a Goodrum/Fechter balancer would do. Use the resistors to drain the cells and use the TC54VX29 to cut off the the drain resistor at that cell, untill all reach that TC54 level. Whether its even practical to bother with such a circuit, considering the few time one would need to do that, is another matter.

Jack your absolutly correct in your testing and observations about these batteries. Not to mention trying to push these batteries past their knee curve and using energy (money), and bleeding off energy (money), in the process to balance them past that curve, for what return ?? The little energy value we get pushing them up there, doesnt even justify the energy we waste to get them there, it just doesnt make any sense.



Roy


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