# Lecture on LifePo4 cells



## DavidDymaxion (Dec 1, 2008)

It's a great video, highly recommended if you want to learn about how a lithium ion battery works, plus some practical metrics for judging a battery.


JohnM said:


> Anybody posted this video here before? A lot of enlightening things about LiFePo4 batterries.
> 
> http://www.ri.cmu.edu/video_view.html?video_id=60&menu_id=387


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## MalcolmB (Jun 10, 2008)

Watched it this afternoon. First-class information from someone who clearly knows his subject. I wish I'd been able to watch it a couple of years ago. He explains how lithium phosphate batteries are put together and why they behave the way they do. The information on upper and lower voltage and temperature limits is reassuring, and surprising in some ways.

I see Jack Rickard is claiming this video supports his arguments against using a BMS – I guess he pressed the stop button as soon as he'd heard what he wanted to hear. I'd recommend watching the whole thing, but if you're in a hurry the relevant part is from 65 minutes on.


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## pm_dawn (Sep 14, 2009)

MalcolmB said:


> I see Jack Rickard is claiming this video supports his arguments against using a BMS – I guess he pressed the stop button as soon as he'd heard what he wanted to hear. I'd recommend watching the whole thing, but if you're in a hurry the relevant part is from 65 minutes on.


Do you mean that this video does not support Jacks claims ?
I can pretty much hear Jay saing that it is a waste of money to put single cell management in a battery pack, and that the most cost effective way of doing pack management is String management of well matched cells that has been taken to the same state of charge, that being either top, bottom or middle (if you can find a good way of doing that).

Regards
/Per Eklund


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## MalcolmB (Jun 10, 2008)

Hi Per,
He supports Jack's argument to some extent, but that's not the whole story. Put simply, the message I got is that cell-level management doesn't make economic sense to manufacturers of EVs, so they rely instead on closely matching cells, so that they don't drift out of balance. But he also says that cell-level management is the correct engineering approach.

As DIY builders, most of us don't have the luxury of being able to pick and choose cells that are closely matched in capacity and resistance, so we need to manage cells individually, or at least monitor cell voltages and occasionally chivvy the stragglers back into line.

I'm not trying to start another pro/anti BMS argument, just setting things straight. I'd personally love to eliminate the complications of a BMS, and I'm sure that a few years down the line, as production ramps up and quality control improves, they won't be necessary.


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## DIYguy (Sep 18, 2008)

JohnM said:


> Anybody posted this video here before? A lot of enlightening things about LiFePo4 batterries.
> 
> http://www.ri.cmu.edu/video_view.html?video_id=60&menu_id=387


Took me a couple of cracks to get through it... but REALLY GOOD.... especially the last third that was more about application... Thank you!


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## dimitri (May 16, 2008)

pm_dawn said:


> Do you mean that this video does not support Jacks claims ?
> I can pretty much hear Jay saing that it is a waste of money to put single cell management in a battery pack, and that the most cost effective way of doing pack management is String management of well matched cells that has been taken to the same state of charge, that being either top, bottom or middle (if you can find a good way of doing that).
> 
> Regards
> /Per Eklund


It all depends on terminology used and how we understand this terminology. He was way too brief on the subject and the audience was too mesmerized to ask the right questions 

Here is how I understood it. Single cell management is the ability to remove individual cell from the circuit and/or reroute energy around it, such that single cell failure does not stop the whole pack. This requires power electronics ( like IGBT modules ) for each cell, to be able to route energy around bad/empty cell. Some people proposed such BMS schemes, even on this forum, but for anyone familiar with electronics involved its 100% clear that such solution is not cost effective. I think this is what Jay was referring to, he actually mentioned bypassing a cell.

String management, the way I understood him, is monitoring cell level voltages and taking action upon entire pack, i.e. stopping charge/discharge when any cell goes outside of norm. He actually mentioned cell level monitoring using simple cheap circuits. This type of management is precicely what most affordable BMS systems do. Its a string management, based on cell level monitoring.

Sure, if you have the luxury of hand picking cells based on AH and IR match from a large pile of tested cells, and also size the pack such that you never reach top or bottom, then BMS requirement is reduced. However, us DIY folks don't have such luxury.

I'm expecting some rocks being thrown my way because I am the evil BMS pusher  , but I strongly believe there are real benefits in a simple affordable BMS, despite what EVTV crowd is lead to believe by their fearless leader 

BTW, the video was awesome, by far the best info about Lithium batteries in a simple easy to understand form, although it probably helps to have some background in electrochemistry.

I also liked how simple in 1-2 sentences he explained how to balance a new pack before first use. There are pages and pages of forum threads here discussing complicated ways of doing the same thing that can be done in few simple steps. Parallel all cells, charge or discharge to get them outside of flat area, let them sit for a while, then connect in series, done, end of story.


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## DIYguy (Sep 18, 2008)

I understood pretty much the same thing as Dimitri. I think he was talking about bypassing a cell when referring to the cost and complexity of a BMS. That said, it was very comforting to see most of what this community has already discovered in practice, explained in theory. The application portion of this discussion was really great and supports a lot of what we already have collectively discussed. I think BMS's like Dimitri's are good thing once proven (like his is) and especially for those who want more peace of mind and a less active roll in the technical side of an EV. Personally, I am going without one...at least to start with...who knows, maybe I will change my mind. I recognize the benefits. I've ordered 67 Calb 180 Ah cells (two spares) and am glad that I really pushed for cells that are closely matched in capacity and internal resistance. I hope they give me more than just the words....


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## DavidDymaxion (Dec 1, 2008)

Something I would really like to see is someone wire up all the odd numbered cells in their pack with a BMS, and do Jack Rickard's recommendations on the even numbered cells.

I'm not a chemist, nor do I play one on TV -- so I'm going to flail about for a minute here. Something I gathered from the video is the chemical reaction at the discharged end of the curve is different from the chemcial reaction at the overcharged portion of the curve. Maybe this is something fundamental that would make bottom balancing better than top balancing? Still flailing here, for instance maybe top balancing runs all the cells into an "overcharged" condition, but bottom balancing (and stopping as soon as any cell hits "full") keeps the cells away from a harmful regime?


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## pm_dawn (Sep 14, 2009)

dimitri said:


> It all depends on terminology used and how we understand this terminology. He was way too brief on the subject and the audience was too mesmerized to ask the right questions
> 
> Here is how I understood it. Single cell management is the ability to remove individual cell from the circuit and/or reroute energy around it, such that single cell failure does not stop the whole pack. This requires power electronics ( like IGBT modules ) for each cell, to be able to route energy around bad/empty cell. Some people proposed such BMS schemes, even on this forum, but for anyone familiar with electronics involved its 100% clear that such solution is not cost effective. I think this is what Jay was referring to, he actually mentioned bypassing a cell.
> 
> ...


Hi !
Well I'm no fanatic, and I'm not orthodox follower of MrRickard either. I have had BMS on my cells, but I don't have that now. I have done some stupid thing with my cells but most of them still lives. 
I really think that your BMS is one of the best value for money that you can get in ev land right now. I have the outmost respect for your work both with the BMS and the Display. At the same time much of Jacks talk about bottom balancing makes sense to me. 

BTW this video with Jay was really good and informative, it would have been nice to be there to ask some more question, but he seemed to be in a hurry, or it could be that people at his level simply talks faster because they think faster then most of us. 

Bypassing a cell during chargeing is exactly what your BMS in doing to some extent when it shunts. The shunt lets some of the current past the Cell that is full. For large prismatic cells there is simply no way that it would be economical to be able to take one cell out of the string, for smaller cylindrical cells it would be possible, And as I recall Tesla is pretty much doing that I think with a Fuse on each cell or something like that.

Your BMS is really good in the aspect that you can choose if you want to go with top or bottom balancing, but I don't know if it handles a problems that I have faced with my ThunderSkys. 

One of my cells became a busbar. 0v just plain conductor. The BMS would not spot it, because the voltage was to low. I don't know how long I had the cell that way, but it freaked me a little, that and a shunt that went haywire and started bleeding a cell that was in normal voltage.
From that day I have no BMS. 
I haven't bottom balanced yet but I plan to do.

I used a ver2.6 of the Goodrum/Fechter BMS, Probably mostly my own fault for not using an ESD safe area when soldering and handling the components.

Best Regards
/Per Eklund


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## Jan (Oct 5, 2009)

dimitri said:


> Here is how I understood it. Single cell management is the ability to remove individual cell from the circuit and/or reroute energy around it, such that single cell failure does not stop the whole pack. This requires power electronics ( like IGBT modules ) for each cell, to be able to route energy around bad/empty cell.


Hi Dimitri,

It is not completely clear to me, he meant just that. Shunting during charging could also be interpreted in this way.

And yes, I thought he also says that monitoring is a good thing. But with the nuance he tells earlier in the video about what can go wrong with adding wires and sensors to a pack. 

That is pretty much in accordance with what I think Jack's opinion is.

You must read a lot between the flaming lines to understand Jacks real opinion. He actually is waiting on a wireless BM(onitoring)S. I think.


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## JRP3 (Mar 7, 2008)

He was really approaching the BMS issue from a cost standpoint, which is in line with my thinking. He said that even 10% of cell cost wasn't effective, so if your cells are around 100ah or larger then Dimitri's MiniBMS makes the grade  Obviously the perfect answer is matched cells and that's what we should be pushing for from our suppliers, even if we have to pay a little extra, maybe up to 10% or so, to get them. 
What I found really interesting was his suggestion that pulsing would be beneficial to the cells. I wonder if the charge reversal of regen fits into this category or if he meant higher frequency pulses?


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## Jan (Oct 5, 2009)

JRP3 said:


> What I found really interesting was his suggestion that pulsing would be beneficial to the cells. I wonder if the charge reversal of regen fits into this category or if he meant higher frequency pulses?


Yep, that part interested me too, but I don't understand it.

I was thinking about Simon’s $200 charger I’m building. Isn’t the Inductor (that’s so hard to obtain) the part of that charger that tries to prevent just that?

And another thing: I doubt that the regen of an AC motor is smoothened like that.


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## dimitri (May 16, 2008)

pm_dawn said:


> but I don't know if it handles a problems that I have faced with my ThunderSkys.
> 
> One of my cells became a busbar. 0v just plain conductor. The BMS would not spot it, because the voltage was to low. I don't know how long I had the cell that way, but it freaked me a little, that and a shunt that went haywire and started bleeding a cell that was in normal voltage.
> From that day I have no BMS.
> ...


First BMS I built was also based on G/F design and I quickly realized that normally open bus is prone to issue you describe and also to wiring failures, etc. This was the main reason I started MiniBMS design from scratch, with normally closed loop being the key feature. NC loop immediately detects "busbar" cell or loose wiring, this is why I laugh when people state loose wiring as the reason they hate/fear BMS.
As for shunting, I always thought it to be the least important feature, basically a small bonus on top of actual BMS functions. Small amount of shunting accompanied by PTC fuse is very safe, so all that baloney about BMS burning cars down is just that, baloney. What burns cars down is people who built them lacking skills, attention to details and respect to physics. In my dealings with BMS customers I personally observed some scary things people do with their EVs, I am amazed we don't see people killed by their EVs on a regular basis. But its no different from amateur mechanics playing with ICE with gasoline and sparks in close vicinity, I wonder how many garages burned down without any batteries or BMSs in them, its all just a matter of perspective. Unfortunately some people with utter lack of perspective sport a pretty big mouth, as if one compensates for the other 

Sorry for sidetracking, its just that you touched on important subject 

Anyway, back to Lithium lecture topic. I also found it interesting to understand the difference between overcharge, which is destruction of electrolite, and overdischarge, which is plating of lithium on the graphite, both of which are permanent damages, but in a different way. Severity of damage of course depends on how much the cell is exposed to these conditions.


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## Jan (Oct 5, 2009)

dimitri said:


> ...I personally observed some scary things people do with their EVs...


I believe you, without a doubt. But the professor in this video was talking about himself, burning down a battery with incorrect wiring. Don't know anymore what he did wrong or what happend exactly. 

Point is, anyone can make a mistake. The stupid and the very smart. And a BMS with all its wiring in such a critical place, is just the part begging for it.


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## JRP3 (Mar 7, 2008)

Jan said:


> And another thing: I doubt that the regen of an AC motor is smoothened like that.


I don't know if smoothing matters, but charge reversal might need to happen faster and more often than occasional regen to have the effects he's discussing.


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## dimitri (May 16, 2008)

Jan said:


> Hi Dimitri,
> 
> It is not completely clear to me, he meant just that. Shunting during charging could also be interpreted in this way.


It could be interpreted this way, but I think professor Jay was focusing on more important points of pack management. I don't think he even thinks of shunting as a BMS function since in his field of large scale economics pack balancing is done at the factory, not in the car. Shunting represents such small part of overall pack management that its not worth all the attention it seems to get in EV forums. BMS spends 99% of the time managing/monitoring bulk charge and bulk discharge, everything else, including shunting/balancing is just icing on the cake. This is why I believe his references to cell management vs string management refers to removal/bypassing a cell during bulk usage, which is clearly not cost effective. String management is what most people do, when done cheaply and effectively it makes sense. When done expensive and complicated it makes no sense.


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## dimitri (May 16, 2008)

Jan said:


> And a BMS with *all its wiring* in such a critical place, is just the part begging for it.


This is where we have a disconnect. I don't know specifically which wiring or BMS's you refer to, but BMS's that I consider effective have just one wire. 10,000 miles after installation this wire is in the same place where I left it, it didn't go anywhere, it didn't touch anything, it didn't beg for anything. It sits there just like 1000 other wires which exist in every one of 100,000,000 cars on the roads today. The wire argument is just silly. Its the same as saying if I piss in the gas tank my gas car will fail. Simple solution to the problem, stop pissing in the gas tank


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## Jan (Oct 5, 2009)

JRP3 said:


> I don't know if smoothing matters, but charge reversal might need to happen faster and more often than occasional regen to have the effects he's discussing.


Maybe. He touches the subject so shortly, we are all left speculating.


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## tomofreno (Mar 3, 2009)

Dimitri, I like your minibms very much as my many comments about it on this site have shown. But in an effort to keep you from getting too big for your pants , I would remind you of one weakness: If the LM 339 chip (quad comparator) on the main board is taken out by a voltage spike, there is no indication the bms is no longer functioning and it will not trigger the relay for HVC or LVC. It happened on mine as you know, and I was lucky I caught it. Because I still don't trust any bms completely, I checked voltages near end of charge with my dvm and found one cell over 3.9V while the charger was still putting out high current. The green LEDs on the cell level boards were on, so everything looked ok. On the smaller pants side , you graciously sent me some optocouplers (4 of them were also taken out) free of charge, and told me the LM 339 was available at Radio Shack for about $2.00! Everything has worked fine since replacing them.


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## tomofreno (Mar 3, 2009)

> I doubt that the regen of an AC motor is smoothened like that.


Here is what it looks like in my car (the "current" pdf), limited by 1 second sample rate:
http://www.diyelectriccar.com/forums/showpost.php?p=208326&postcount=618


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## dtbaker (Jan 5, 2008)

I watched it.... and found it interesting background as well. Also feel that it confirms conclusions I have drawn from other material, which makes me feel better. No big surprises. The high points I heard were:

- balance pack in parallel first... at relatively low C rate, to either high or low side where curves start to get steep so that cell DOD is relatively fixed rather than somewhere in the long flat middle ( some value between 3.6 and 4.0 being good for top-balancing, and probably slightly above where your charger switches from CA to CV is best ). I am comfortable using top-balance since that is where I need cells to be balanced to trigger charger from CA->CV without any one cell going ballistic and I am comfortable with self-monitoring not to over-discharge based on typical range.

- the caveat between the lines is that IF you bottom balance, the capacity of your cells had better be well matched, OR you have to have a cell-level BMS capable of shutting down the charger when the first cell hits HV.

- cell level BMS is perhaps desirable from an engineering standpoint, but tough to justify economically, and active shunting is not required if cells are close to balanced. The corollary being that balance probably should be checked 'occasionally' depending on differences in cell resistivity and manufactures consistency showing up over many cycles... and either manually adjusted at the cell level by drain/charge, or re-wiring in parallel to re-balance the whole pack. 

- the info on monitoring INTERNAL temp to protect electrolyte I found very interesting, and I will position my thermocouples such that they are in contact with a terminal...


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## JRP3 (Mar 7, 2008)

One of the problems that PFC users encounter is the imprecise voltage clamping that results from the way it monitors voltage. With better pack level voltage monitoring and more accurate constant voltage mode there would be better protection, with or without a cell level BMS. I see a market for an external device for PFC chargers that more accurately tracks voltage and shuts down the charger if a preset level is exceeded. Not everyone agrees with me on that


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## dimitri (May 16, 2008)

tomofreno said:


> Dimitri, I like your minibms very much as my many comments about it on this site have shown. But in an effort to keep you from getting too big for your pants ,


Thanks for keeping me in line , Tom. I never claimed my system was perfect, every system has weak points. I'm just trying to help people sort thru terminology and common misconceptions.


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## Jan (Oct 5, 2009)

tomofreno said:


> Here is what it looks like in my car (the "current" pdf), limited by 1 second sample rate:
> http://www.diyelectriccar.com/forums/showpost.php?p=208326&postcount=618


Thanks Tom, but this is not what I was thinking of. I'm not sure what I'm thinking of, actually. I just don't understand the function of an inductor in a charger. I'm prety sure it's not present in an AC motor controller.


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## tomofreno (Mar 3, 2009)

I agree, great video! Very informative. So JRP3 is not alone not doing individual cell management, but in the company of Tesla! ...and the prof recommends an initial "bottom balance". 

I suspect that CALB does the Rs versus capacity plot, groups like cells, though maybe not as close as possible, and does a full charge then partial discharge before shipping. That is because my original (I replaced 4) 32 CALB cells have remained very close in voltage over the past year, despite no balancing on my part and JRP3 received documentation on the capacity of each cell with his.

He does say that monitoring each cell is the best engineering solution and recommends "you guys" do it, just too expensive for low cost volume production.

I also now think I understand why A123 cells are more expensive - thinner coatings/more layers for better Li diffusion, better capacity at higher discharge rates, and maybe some additional special processing of anode material. 

At one point he says keep cells less than 50-60 degree C, at another he says less than 60, to avoid breakdown of the solvent. Less than 50 is worrisome, especially for someplace like Phoeniz, AZ where ambient can be above 45 C, and inside a vehicle much hotter.

The comment 2C discharge is "totally ok" is comforting.

So 1 kWh is about 312 Ah, divided by 0.14 Ah/gm LiFePO4 gives about 2229 gm LiFePO4. Li is about 6.9/149.8 = 4.6% of LiFePO4, or 102 gm/kWh, and a 30kWh pack uses about 3060 gm, or 6.7 lb. I don't think we will run out right away...and we can recycle it when it becomes cost effective with the higher volumes old ev packs will bring.


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## tomofreno (Mar 3, 2009)

> I wonder if the charge reversal of regen fits into this category or if he meant higher frequency pulses?


 At one point he says "100's of Hertz", so I expect the current spikes/reversals with regen we have are too infrequent to prevent the limiting of diffusion/drift of the Li ions to the cathode.


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## tomofreno (Mar 3, 2009)

> I never claimed my system was perfect, every system has weak points. I'm just trying to help people sort thru terminology and common misconceptions.


 Your still one of my hero's Dimitri.


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## tomofreno (Mar 3, 2009)

> With better pack level voltage monitoring and more accurate constant voltage mode there would be better protection, with or without a cell level BMS.


 I agree, though I don't think I would trust it (maybe anything ) enough to not monitor it. But he did make it sound like if you had cells matched to within say less than 1% in Rs and capacity, you would need no monitoring other than pack voltage, so just charge it to say 3.45n, n being number of cells, and quit, and they shouldn't change much over time. Bet they would though if you are regularly stressing them at 3C or more discharge. Have you noticed any drift of yours - since you have done some 5.5C discharges?

The breakdown of the solvent resulting in plating of the anode sounds like it could be a real problem if it starts as low as 50C, 122F. The over-disharging and resultant plating of the cathode with lithium doesn't occur until less than 1 volt, so should be much more easily avoided.


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## EscapeVelocity (Nov 12, 2010)

pm_dawn said:


> I used a ver2.6 of the Goodrum/Fechter BMS, Probably mostly my own fault for not using an ESD safe area when soldering and handling the components.


I had a Goodrum/Fechter V2.4 on my motorcycle. It took two of my 40Ah Thundersky cells (out of 24) to 0V. It's been off my bike since. I am now looking into bottom balancing the pack at 3.0V and perhaps using a celllog's HVC to disconnect my charger.


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## EscapeVelocity (Nov 12, 2010)

Jan said:


> You must read a lot between the flaming lines to understand Jacks real opinion. He actually is waiting on a wireless BM(onitoring)S. I think.


Having dealt with solar photovoltaic systems, why isn't there a monitoring system that communicates through the powerline? In the PV world there are these hot new micro inverters that attach to the backside of each panel and communicate to a master unit through the same wire that is carrying the current.

I do think the miniBMS modules are pretty close though, using a daisy-chain style communication method.


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## JRP3 (Mar 7, 2008)

tomofreno said:


> Have you noticed any drift of yours - since you have done some 5.5C discharges?


I haven't done a bottom check lately but during charging they all seem to have the same relationship that they've had all along, i.e. the same cells that filled up first in the beginning are still the same ones now, and by the same percentage. Of interest is that one group of 12 cells sits in the motor compartment which heats up from the motor and controller, while the others sit in the trunk which stays cooler, so for part of the time they probably have greater differences in internal resistance as they are living in different environments. So between that, pulling my cells to 1.77 volts once, charging some to 4.0 volts, and pulling 5.5C from them occasionally, I'm doing everything wrong  Fortunately for me, but unfortunately for information gathering, since I won't drive it in the winter this abuse will probably be masked by the limited cycling these cells will see.


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## tomofreno (Mar 3, 2009)

> I had a Goodrum/Fechter V2.4 on my motorcycle. It took two of my 40Ah Thundersky cells (out of 24) to 0V.


 This will not happen with a minibms, unless you let the cells sit for a very very long time, due to current limiting resistors in all paths between cell electrodes. These limit current in event of high leakage through a semiconductor device in the path. There is also a fuse to prevent too high current flow. Its a pretty bullet proof and clever design. Thought I would jump in here since Dimitri probably won't say anything since he is the designer/seller.


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## aeroscott (Jan 5, 2008)

great to finally get a good understanding of lithium batteries . i would love some more information like that .


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## Jan (Oct 5, 2009)

JRP3 said:


> I see a market for an external device for PFC chargers that more accurately tracks voltage and shuts down the charger if a preset level is exceeded. Not everyone agrees with me on that


If you want to measure the voltage during charging, and whant to filter out the resistance of all the wiring, you have to measure all the battery boxes individually. The wiring between the boxesm in the front and back of the car also consume pieces of the voltage with increasing amps. Or not?

A simpler solutian is stop charging so now and than, wait a while, and measure the 'resting' voltage.


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## JRP3 (Mar 7, 2008)

The PFC's allow the set voltage to creep up a few volts over time which was not a problems with lead acid but can be with lithium. A simple pack level voltage monitor that more accurately clamps the voltage is all that should be needed. Your way would be even better but more complicated. Mind you, this issue is really only a problem if you are trying to get closer to full SOC.


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## Jan (Oct 5, 2009)

JRP3 said:


> The PFC's allow the set voltage to creep up a few volts over time which was not a problems with lead acid but can be with lithium. A simple pack level voltage monitor that more accurately clamps the voltage is all that should be needed. Your way would be even better but more complicated. Mind you, this issue is really only a problem if you are trying to get closer to full SOC.


Box voltage measuring... I think I must remember that. If you have a lot of cells, say 85 or so, it's hard to notice a bad cell in the total voltage. Box voltage measuring is a interesting compromize between cell level monitoring and string level monitoring... Relativly easy to add to the $200 Si charger. And just a few wires. Only (number of boxes - 1) extra. Must remember this.


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## Jan (Oct 5, 2009)

And another thing... The warming up of the batteries by discharging them shortly with high amps... If you insulate them good in the winter. And use this method. But not just by discharging them, but by storing it in a few ultra caps and quickly charging it back into to batts. I've no idea how that should work. Probably with very powerfull dc/dc converters. What makes it a useless idea. But if it could be done simple, you can heat them up with very little energy loss.


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## JRP3 (Mar 7, 2008)

Jan said:


> And another thing... The warming up of the batteries by discharging them shortly with high amps...


Hard acceleration should take care of that, along with some regen braking if you have it.


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## Jan (Oct 5, 2009)

JRP3 said:


> Hard acceleration should take care of that, along with some regen braking if you have it.


Uhm yeah. That wasn't very intelligent, yep. It's the age.


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## jeremyjs (Sep 22, 2010)

Neat. So basically if you have closely matched cells, less than 1%, in both resistance and capacity there's no major need for a BMS if you bottom balance and don't discharge too deeply. You can basically get away with a pack level HVC and LVC, but if they're not that closely matched a BMS is almost a necessity. I wonder how calb, thundersky, etc. meet up with that demand.

What I really thought was interesting was the talk of pulse charging to achieve a much higher charge rate without excessive heating or undue stress on the cells; the frequency of which greatly depended on the exact cell chemistry.


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## rwaudio (May 22, 2008)

I'd like to start out by saying I like Jack Rickard, but why is he so closed minded!?! How many new guys are going to be miss guided into thinking that they can go BMS free and do whatever they want, and think their pack is indestructable. (I know Jack checks his pack at least once in awhile and has the know how to correct something if it comes up, he doesn't have an electrical BMS he has a human BMS) I asked him a few more questions, and he showed me the link to the video, saying it backed him up 100%. I watched it a few days later and replied to him that it doesn't entirely support him, and the comments that do agree with his stance don't entirely agree with him because you can't take the statements out of context. The guy in the video says that if the cells are close enough there is no need for a BMS and I think that most people in this thread have agreed with that. BUT and this is the part Jack doesn't seem to think was even said in the video, is that smaller batches of cells where you might have 1-2 extra they could be all over the map from a "budget" company like Thunder Sky. I made the comment that the guy actually recommended a BMS or in the very least a voltage monitoring system for the guys using TS in an application no different than any of us DIYer's. Jack replied saying you don't understand english and that it couldn't be made any clearer. 

I politely replied stating that I disagree because of the context in which he pulled the statements doesn't fully apply to the DIY converter because we don't have the pool of cells to match as close as may be required for a build the pack and never look at it again type of approach.
His final reply:
_"We do NOT "agree to disagree" . ; IT's not a democracy, it's not a vote, no you are NOT entitled to your own opinion, it doesn't mattery WHAT you think, it doesn't change anything AT ALL._

_They're YOUR batteries and you can do whatever you like with them. Pour gasoline on them, say a prayer, and drop a bic lighter on the whole mess if it makes you feel better."_

I guess none of us are "allowed" to disagree with the almighty Jack or so much as have our own opinion, even based on things said in the very video that he is claiming backs him up. 

Again, I like Jack and I think his show is helpful to the EV community, but I'm sure the next episode will be all about how right he is because some guys comments when taken out of context agree with him. 

I think (there I go thinking again, not allowed to do that) the ideal solution is to go BMS free, it's not always the right solution however, and it's our duty as the designers of our conversions to make the call as to whether or not some form of BMS is or is not required based on the cells we choose, the level we charge and discharge to, the quality and assembly of our packs.

My rant is over, I would highly recommend everyone watch the entire video for themselves and not just take someone elses word for it.


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## EVfun (Mar 14, 2010)

rwaudio said:


> _"IT's not a democracy, _[...]_ you are NOT entitled to your own opinion_"


It *is* still a democracy and you *are* entitled to your own opinion (just not your own facts.) 

What I took from the video is that even 10% upward cost to add a BMS is to much for major manufacturers to undertake in large packs. If you where buying 10,000 cells and making packs of 100 you would be better off creating matched sets, but that cell level monitoring was good for non-matched sets. He seemed against taking cell level action (take pack level action.)


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## MN Driver (Sep 29, 2009)

I watch Jack's show. He is an stubborn guy who acts just like any other old stubborn guy. Not all old guys are stubborn however, but he is about as stubborn as they get, to the point where he gets angry at the slightest disagreements. I watch his stuff at 1.5x speed for his speech pace to seem normal to me, but if nobody else is talking I can understand it fine at 2x. Saves me plenty of time watching his stuff and I'll slow it down when there is something like a machine or car driving so it is in the proper speed to fully get it. That's beside the point though. I think there are going to be many people who are going to destroy their packs by 'taking them out of the box' and just using them. I'm not sure why he assumes they will be at the same point. He said on his forum that .5v difference at the bottom is to be ignored. There is no way they are magically at the halfway point likes they think they are. Someone is going to either charge up and overcharge a number of cells or on the first discharge murder a dozen.

He still to this day doesn't understand why top balancing failed for him. While most of us understood clearly that you need HVC/LVC protection of some kind, he completely ignored it and then got angry at people when they explained it to him as he struggled to explain why he was right with his way and the rest of the world is dead wrong.

He also doesn't realize that GM is in fact using a BMS, there are pictures showing it online with 4 banks of centralized components mounted on top of the pack with leads coming off of them. Jack didn't understand what the person giving the speech was saying, that the auto manufacturers don't separate cells out of their series configuration. ...Tesla has a BMS too, not sure of any automaker who wouldn't have one. Might work for an EV enthusiast but John and Jane Doe wouldn't have a clue and couldn't manage a 8 year 100k or longer warranty in adverse conditions without something in place. I could go on but I really shouldn't pull this into a long rant so I'll just stop.


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## tomofreno (Mar 3, 2009)

> I wonder how calb, thundersky, etc. meet up with that demand.


 Well in the case of CALB in my experience. That is exactly how I have been running for about a year and over 7k miles (Edit: Whoops, I missed that "pack level" qualifier. I am operating with cell level HVC/LVC - the minibms. Sorry for the confusion). The cells seem very closely matched, but I don't know how well. The four cells I replaced due to a mishap however, are a bit different, and have required individual balance charging on occasion to keep them within < 0.02V of the others. The original 32 are usually within < 0.002V of each other.


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## JRP3 (Mar 7, 2008)

jeremyjs said:


> Neat. So basically if you have closely matched cells, less than 1%, in both resistance and capacity there's no major need for a BMS if you bottom balance and don't discharge too deeply. You can basically get away with a pack level HVC and LVC, but if they're not that closely matched a BMS is almost a necessity. I wonder how calb, thundersky, etc. meet up with that demand.


My SE/CALB cells varied less than 4% and after some charger tweaking and occasional monitoring I'm running without a BMS. Most of the cells were within 2% of each other and it should not be difficult to specify a pack within that range.


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## dimitri (May 16, 2008)

I think a lot depends if you are lucky to get all cells from same manufacturing batch or not. From my 45 TS160 cells, 40 were from one batch and 5 from totally different manufacturing date, several months older. Those 40 are practically twins, but significantly better ( both capacity and IR ) than last 5. 

If there is a way to request all cells from same batch/day from your cell dealer chances are you will get very similar cells. You can easily tell batch/date by serial numbers, at least on TS cells that I know of.

Funny how we went from not being able to buy Lithium cells to picking and choosing our options in just 2 years , great progress for EV world indeed.


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## tomofreno (Mar 3, 2009)

So maybe the next step in choosing our options is to push for data for Rs and capacity for each cell, and less than 2% spread in each of these. Or at least get some cell suppliers to investigate the cost of this option.

Btw I think the video should be made a sticky with some title like lithium battery fundamentals.


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## JRP3 (Mar 7, 2008)

tomofreno said:


> So maybe the next step in choosing our options is to push for data for Rs and capacity for each cell...


Didn't your cells come with serial numbers and a data sheet listing both? I agree we should push for closer grouping, and it shouldn't be that hard to get, at least for capacity, which is probably more important than resistance. I was wondering if resistance is somewhat self regulating, higher resistance will generate more heat, which will then lower the resistance somewhat.


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## tomofreno (Mar 3, 2009)

> Didn't your cells come with serial numbers and a data sheet listing both?


 No, just Rs, but I think that is a function of the supplier, not CALB.


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## DIYguy (Sep 18, 2008)

JRP3 said:


> I agree we should push for closer grouping, and it shouldn't be that hard to get, at least for capacity, which is probably more important than resistance.


During my discussions, and also when I ordered my Calb cells, I stressed that I wanted a pack with all cells as close in capacity and resistance as possible. I didn't offer any additional money...but I did say that it was a significant issue and that it would get attention by members of this forum. (this was before the video and this discussion) The replies were positive. There were also comments of how busy the factory was right now. (Im hoping this is a plus as there would be more cells to pick from  ) I mentioned that I would be happy with a pack that was close in capacity and R over one that had a total Ah count that was higher. As long as it met or exceeded spec. They should be ready for shipment from the plant to the port on Dec 17th so at this time I should get the spec sheets... I think. I will advise outcome.


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## dtbaker (Jan 5, 2008)

DIYguy said:


> They should be ready for shipment from the plant to the port on Dec 17th so at this time I should get the spec sheets...


you ordered direct from CALB?


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## DIYguy (Sep 18, 2008)

dtbaker said:


> you ordered direct from CALB?


No, I was going to...but I got a better price elsewhere, believe it or not.


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## dtbaker (Jan 5, 2008)

DIYguy said:


> No, I was going to...but I got a better price elsewhere, believe it or not.


I had heard that there were some direct-from-china outfits that had special incentives to sell.... but also that there were some questionable practices regarding import/customs fees and shipping methods.

would you care to let us all know what kind of a deal you got, from where, and then most importantly, let us know how happy you are when you have your order sitting on your doorstep!

as you are aware, the source, wholesaler, retailer chain for batteries has been questionable recently with only a few sources remaining credible since the whole EVC non-delivery fiasco with Morrison and the continuing fallout from that.


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## DIYguy (Sep 18, 2008)

Hi Dan, Yep, I'm aware. Been scheming for months... Rather than take this thread off track...u can read about it here.. http://www.diyelectriccar.com/forums/showthread.php/anyone-purchase-powerscaner-yeti-46890.html

Actually, u did read about it... but maybe u didn't read on after...  (that was me...


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## JRP3 (Mar 7, 2008)

tomofreno said:


> No, just Rs, but I think that is a function of the supplier, not CALB.


My cells came from EVC, the first shipment I think. Weren't yours from EVC as well? I actually have a PDF from SE for all the cells in that shipment I think.


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## DIYguy (Sep 18, 2008)

dtbaker said:


> I had heard that there were some direct-from-china outfits that had special incentives to sell.... but also that there were some questionable practices regarding import/customs fees and shipping methods.
> 
> would you care to let us all know what kind of a deal you got, from where, and then most importantly, let us know how happy you are when you have your order sitting on your doorstep!
> 
> as you are aware, the source, wholesaler, retailer chain for batteries has been questionable recently with only a few sources remaining credible since the whole EVC non-delivery fiasco with Morrison and the continuing fallout from that.


You know what is really ironic in this whole battery thing.... it kind of hit me right between the eyes when I was searching for reasons to trust..... the only people getting screwed out of batteries (and other stuff) and exhibiting unscrupulous behavior are Americans....not Chinese....lmao.... Primarily LionEV and this Morrison fellow. Remember also...that all these re-sellers....are doing what everyone is afraid of...buying from Chinese... I know it's no secret.


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## JRP3 (Mar 7, 2008)

Part of the suspicion stems from the early buyers of LiCo cells from TS who got screwed, by TS.


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## DIYguy (Sep 18, 2008)

JRP3 said:


> Part of the suspicion stems from the early buyers of LiCo cells from TS who got screwed, by TS.


I understand the suspicion...but they're was never a intent to not deliver anything. There is also some thoughts now that perhaps there never was anything wrong with those LiCo cells....but more a problem with the users methods....kinda like top balancing before BMS perhaps??......


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## tomofreno (Mar 3, 2009)

This journal paper adds some to the discussion:

http://www.che.sc.edu/faculty/Balbuena/PDF%20files/Tao-Li.pdf

The CM video left out the fact (due to time constraints no doubt) that the lithium ions are solvated, so diffuse through the solvent as Li(PC)4 (lithium bonded to 4 propylene carbonate molecules) for example. One of the results of the above paper is that Li(EC4) has a smaller radius than Li(PC)4 and so a higher diffusivity, resulting in lower cell resistance. 

So looks like there is a lot involved in picking the "right" solvent mix (EC is a solid at stp, so has to be mixed with other solvents) for high diffusivity, high b.p., and low m.p., and probably other things like breakdown at electrode surfaces. I would guess vapor pressure of the solvents would affect cell temperatures at which cell swelling occurs. I would also guess this is the main driver of swelling - though possibly lower v.p. components are produced when solvent breakdown occurs. 

You would think that once this occurs cell performance would be very poor because the anode/separator/cathode sandwiches are no longer in intimate contact. I recall Jack R. saying he used a clamp to squeeze a swelled cell back together and recovered some performance. Makes sense if there was only minor breakdown of the solvent, and the main affect was separation of the layers due to increased solvent vapor pressure at elevated temperature. The cell will never be the same though for sure.

The above paper also makes you wonder about the heterogeneous reactions at the anode and cathode surfaces, where the Li ion must be de-solvated and intercalate into the electrode material. Got to be some contribution to cell resistance from this also. As with everything, the deeper you dig the more complex it gets.


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## rochesterricer (Jan 5, 2011)

Bumping this up because I just found it. One thing about the video I didn't understand. He said that they don't make the electrodes thinner because it drastically increases the cost of production, but he doesn't explicitly say why. Does it use more of the expensive electrolyte? Is it because it requires more separator membranes, and those are expensive?


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## JRP3 (Mar 7, 2008)

I think the increased number of components, including more separators, adds to cost. Electrolyte should remain about the same I would think.


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## jeremyjs (Sep 22, 2010)

It also reduced the a/h; because of the volume of separator becomes a larger percentage of the battery rather than the anode and the cathode. So basically material costs go up and energy density goes down. He thouches on this briefly.


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