# Establishing Upper / Lower Acceptance Limits for Impedance



## PhaseShift (Oct 12, 2009)

I am in the beginning stages of establishing some baseline quality metrics for LiFePO4 batteries. The purpose is for pre-shipment auditing of batteries for PEV use and as such, I have to consider as many of the application specific issues as possible. While I may not be a battery expert, I am a very quick study and am unafraid to ask questions- even if they seem to identify me as the village idiot; it is all part of the learning curve. As they say, “you gotta start somewhere”.

So, here is the question I have- 

_Is there a database of manufacturer specific measurable related to battery performance? Short of that, does anyone have solid date on production mean / average when dealing with LiFePO4 batteries, specifically the impedance?_ 

*What I am trying to do is establish a couple of separate, but related metrics for our onsite pre-shipment audits.* 


*A target or centerline value, USL and LSL (Upper Service Limit / Lower Service Limit) for impedance.* The target is easy and of course I do not intend to ask anyone to specify that- we will use typical standard selection procedures for production mean value and go from there. For initial production, we would typically take the advertised target value (“the spec”) and use that until we ascertain whether the production mean is actually centered around that value. The tricky part is establishing the USL / LSL as a percentage and whether that is scaled or fixed. 
In audit data collected so far, we have seen a rather significant parity between USL and LSL and the distribution is quite wide. The variance I am looking at right now is plus/minus 22%. Yep, that number freaks me out a bit- I am looking at an acceptance window a bit wider than 40% and that makes a guy like me ask a lot of questions.
Actually, I am being kind in the above example; seems in every lot, we find a couple of units that are well above or below the trend line- sometimes by as much as 60% which is a significant concern to me and I presume, others. My numbers above are based on kicking out the few units well outside of the trend line and recalculating the group. Obviously, this has to be tempered with reason as kicking out the high / low numbers creates questions as to where to stop. Generally, I am kicking out anything that is 30% or more over / under the mean. 

*Would it be reasonable to assume that we can window the tests and group based on results? *In other words, let’s say I have 600 batteries and within that lot, we can measure and identify a total of six groups where impedance is very close; perhaps as close as 15% total spread. In doing so, I could assign a reasonable centerline value which is absolute, but allow a fairly wide tolerance for acceptability. This keeps the vendor happy, allows the QA team to successfully weed out the real stinkers before they ship and perhaps improves the individual battery pack performance. 
Regardless of what we decide to do, I still need to consider the acceptance criteria; the USL and LSL offset percentages. 

That said and at risk of sounding like a fanboy, does anyone have solid experience or numbers that have proven to produce solid performing battery packs that last through typical cycling in a PEV application? A few weeks ago, I was sure that a 20% window would be the most I was willing to consider, but now, it looks like that may be difficult to achieve without significantly impacting my supply chain. Now, I am leaning more toward a 30% window and trying to get the vendor to tighten up or pre-sort. Even if we have to pre-sort before the pre-shipment audit, I can assign the manpower; that is no big deal and the test gear is cheap. It is more a matter of establishing what we should be concerned with and where we should be looking for other improvements as opposed to getting upset over a non-issue. 

I hope that I have not overlooked this discussion in the forum somewhere. I did a few searches and several internet searches on this and related topics. I could not find anything relevant, so I thought I would post the question here. Be gentle; I am new here and admit that I have a lot to learn.


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## tomofreno (Mar 3, 2009)

You didn't overlook a discussion of this topic, there hasn't been any other than wishing there was some data. How are you measuring impedance of the cells? When you say the variance is +/-22% do you mean variance in the statistical sense, i.e. square of the standard deviation, or do you mean that is the range (max - min) of the data? I don't understand what you mean by "production mean centered around that number" since the mean is just one number. Do you mean the distribution of production data centered around that number, or am I missing something? How many cells have you measured? Is the distribution normal (Gaussian)? If so, what is the standard deviation? I think what concerns people more is the distribution in capacity of cells, since the lowest capacity cell in a series connected pack determines the capacity of the pack. A higher internal resistance results in more voltage across that cell during charging and larger voltage sag during discharge, but isn't typically noticed, whereas capacity is.


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## PhaseShift (Oct 12, 2009)

My explanation could have been better; I was a bit tired when I hacked this post out. Same as now actually. 

To shed more light on the data population-

34 batteries, all LiFePO4, 160 Ah. Each measured using a Hioki 3554 (good calibration stamp) and a Fluke 187 (also good stamp); both known good / accurate pieces of gear, but not confusing either with lab grade bench equipment. We are talking EOL test data here, but taken by a couple of very experienced and qualified QA professionals using known good equipment. Ambient temp about 70F and approximately 36 hours since the batteries were fully charged. The Hioki is giving us voltage and impedance readings and we are double checking the voltage with the 187 at the same time, then again an hour later. Hioki is not exactly a common name for portable gear in the states, but I have used their equipment in a number of different applications related to T&M and always found them to be well built, reliable and accurate within their application range. 

Voltage ranges from 3.306 to 3.309 with the Hioki and 3.3057 to 3.3092 with the Fluke. Overall average voltage of the data pool is 3.3084. 
Impedance ranges from 0.210 to 0.320 if we kick out the one unit that was well off the upper trend line. Average impedance is calculated to be 0.2642mOhm, again with that one odd-ball kicked out of the population. The one unit of concern has a measured impedance of 0.384mOhm. 

The vendor gives us nothing other than a verbal specification for acceptable deviation range on impedance; originally indicated to be +- 0.1mOhm which I am fine with and consider that to be workable if not a bit wide considering the numeric quantity we are going to figure pass / fail dispositions on. Still, we are starting from nothing here, so for lack of a better method, we take the average impedance and consider it as the nominal value and we offset that by the indicated tolerance. While this may not be the way we will look at this for a production unit, we have to start someplace and using the average will ultimately result in a higher yield as opposed to selecting an arbitrary number. 

We are dealing with a limited sample size here and frankly, not having a nominal value puts my QA inspectors at a disadvantage. We have to take a less than optimal situation and work through it to establish reasonable values for nominal impedance, USL and LSL. 

That said, even if I had no nominal value and had to make a decision to accept / reject a battery from this lot based on the impedance, I would have to consider rejecting the 0.384mOhm unit because it is significantly different than the average and even well beyond the distribution of the bell curve (which is already pretty darn wide). Problem is, I also have to ask _“am I rejecting the one good battery and accepting nonconforming units?”_. The reason is that without the nominal value, we had to select a value that potentially represents a nonconformance. 

Like I said, this is a less than ideal situation and maybe I should have left the data out and simply asked- _“What would you guys consider to be a reasonable tolerance for impedance when dealing with LiFePO4 batteries?”_

PCB


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## tomofreno (Mar 3, 2009)

> originally indicated to be +- 0.1mOhm


 Do you know how they are measuring this? I am not familiar with the Hioki and how it measures impedance. I'm wondering how representative the measurement is of impedance at the high current draws in evs.



> am I rejecting the one good battery and accepting nonconforming units?


 Possible of course, but highly unlikely if most of the distribution is centered around a mean significantly different (more than 1 std dev) from this cell. A sample size of 34 should give a decent estimate of the population mean as long as it is a representative one. Are you really measuring impedance Z = R +iX, or just resistance? I think the only thing most of us could do is estimate cell resistance by measuring voltage across the terminals while charging at a known DC current. As far as I know, no one bothers, they are mainly interested in capacity (Ah).


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## dimitri (May 16, 2008)

tomofreno said:


> As far as I know, no one bothers, they are mainly interested in capacity (Ah).


I disagree. Maybe no one bothers because they don't have a clue of how significant it is to have matched IR cells in a pack or they have no way of measuring it, or they have no choice in the matter since they get cells delivered from reseller and reseller does not match them or does not care to match them. Some customers get cells direct from China and have even less say in the matter.

I think PhaseShift is starting an excellent practice of quality control of LFP cells before they go to the customer. If I had a choice of LFP resellers I would definitely buy from a guy who has similar QC procedures, rather than just repacking from big boxes into smaller ones.

PhaseShift, IMHO there is nothing wrong with cells having 0.1 or even 0.2 mOhm higher IR than others, as long as you collect them into a separate pack where all cells are in that IR range. Perhaps you can sell them to customers who don't need 3C rates, such as solar storage or marine trolling, etc, perhaps at a slightly less cost.

IMHO, the actual IR value is not as important ( as long as its less than 1 mOhm for example ) as having cells in one pack matched for both IR and capacity.

My gut feel is that 15%-20% deviation of IR is acceptable and 5% of capacity deviation is acceptable within one pack. At least that would be my personal requirement next time I buy LFP pack.

I got screwd on my current pack of 40 cells, where 5 out of 40 came from totally different production batch and exhibit weaker performance than the rest, although they reportedly had similar IR values. For this reason I think its also important to match production batch by serial numbers, which will most likely match capacity at the same time.

I can't claim warranty replacement on my 5 cells since reseller never promised to match them and they still work, just not as well as others under 3C load. If reseller matched my cells by production batch I would not have these issues.

Hope this helps.


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## tomofreno (Mar 3, 2009)

> I disagree. Maybe no one bothers because they don't have a clue of how significant it is to have matched IR cells in a pack or they have no way of measuring it


 You are the only person I have seen talk much about IR Dimitri, because you have some cells where it is an issue. Some have a bms such as the Elithion, or data logger like the cell log8 that display cell voltages, but I've not seen anyone report significant differences in voltage sag other than you. I believe you have said you don't see significant sag below around 2C, correct? In that case, people may not see it if they accelerate moderately. It may also not be detected in QC if the measurement is made at low current. If made at high current, I wonder how much cell to cell variability the terminal connections add. That is why I asked how the measurement is made, its not clear to me what it tells you. What is the difference in resistance of your good and bad cells, and what difference in voltage sag does it cause at 3C? Would be good data for Phaseshift. You could measure voltage drop across terminal connections during charging, and if significant, subtract out the voltage drop that resistance would cause at 3C to get a more accurate estimate of cell voltage sag.


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## dimitri (May 16, 2008)

Tom, I guess you are right, if it wasn't for 3C accelerations I wouldn't worry about IR as much. I guess it was a mistake to plan my pack for 3C, but I believed TS datasheets, which clearly was stupid of me 

A lot was learned in past year about LFP cells, that is why I keep pointing out to people not to plan on frequent 3C discharge with TS cells if they want them to last, despite what datasheets say.

I guess in retrospect I wish I just had all my cells from same batch, no matter what their IR was. At least then I would get consistent behavior across the pack.

Another reason not a lot of people report these things is that you need cell level data analysis while driving, which I get from Paktrakr. Now that those CellLoggers become best thing since sliced bread more data will be published I hope.


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## PhaseShift (Oct 12, 2009)

I am already learning something- indications are we may not have been accurately measuring an important metric. That is one of the reasons I posted; to learn where we are not doing an adequate job, where our methods or capabilities need improvement as well as where we are doing the right thing. A lot of folks come into forums like this one trying to prove how smart they are; that may not be the best approach. I prefer to come at this from the angle that I have a lot to learn and the best way to learn is to ask appropriately and articulate the questions so that they pique the interests of the most experienced minds on the forum. 

Are their established and documented methods that I can apply to ascertain the true state of the batteries, particularly the metrics that will allow us to develop solid QA acceptance criteria moving forward? Clearly, there will be batch to batch differences and there are going to be differences between models, vendors etc… However, what is learned and established as a test methodology should at least apply to other similar chemistries and assembly formats. 

That said, where can I learn how to precisely measure the impedance of the battery in a meaningful way- as in a bench test as opposed to a EOL test? I am not an EE, so I need a procedure that I can then locate or develop a test system for and calibrate appropriately before starting tests. Also note that we understand the need to have the right test equipment and will budget appropriately. Long term, we will have to outfit a bench specifically to the challenge of measuring the batteries and establishing a meaningful, reliable correlation between the bench measurements and those made at an EOL station or an OQC / pre-shipment inspection environment. 

What is clear is that I have to develop a better system for accurately measuring the performance metrics of cells and batteries and apply that information to a database whereby we can begin to establish standards, tolerances and in some cases, floating limits. 

So, any advice on where I can find a documented procedure for correctly measuring impedance and other metrics on the bench?

Phase


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## Guest (Feb 4, 2010)

dimitri said:


> Tom, I guess you are right, if it wasn't for 3C accelerations I wouldn't worry about IR as much. I guess it was a mistake to plan my pack for 3C, but I believed TS datasheets, which clearly was stupid of me
> 
> A lot was learned in past year about LFP cells, that is why I keep pointing out to people not to plan on frequent 3C discharge with TS cells if they want them to last, despite what datasheets say.
> 
> ...



We've bumped heads before. I ROUTINELY do 3C in the Speedster and have on EVERY DRIVE for over a year. it is not a problem, and it is NOT why you have been losing cells Dmitri. 

I'm not losing them, and I'm driving today in February. And yes, right at 3C. They do it gladly and without complaint.

It simply is not true that these cells will not do 3C. These cells are 16 months old and have 7500 km on them. No losses. Not even any questionable cells.

Jack Rickard


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## Guest (Feb 4, 2010)

PhaseShift said:


> I am already learning something- indications are we may not have been accurately measuring an important metric. That is one of the reasons I posted; to learn where we are not doing an adequate job, where our methods or capabilities need improvement as well as where we are doing the right thing. A lot of folks come into forums like this one trying to prove how smart they are; that may not be the best approach. I prefer to come at this from the angle that I have a lot to learn and the best way to learn is to ask appropriately and articulate the questions so that they pique the interests of the most experienced minds on the forum.
> 
> Are their established and documented methods that I can apply to ascertain the true state of the batteries, particularly the metrics that will allow us to develop solid QA acceptance criteria moving forward? Clearly, there will be batch to batch differences and there are going to be differences between models, vendors etc… However, what is learned and established as a test methodology should at least apply to other similar chemistries and assembly formats.
> 
> ...


It is not terribly difficult, but as pointed out, I would do it at meaningful current levels. That's a little pricey on a constant current load but not wildly so.

If you want a really indicative number it is not too difficult. I think it's better to measure between two current levels than to static voltage.

So set a constant current load at 1/2C and measure the voltage, and then set it at 2C and measure the voltage. Take the difference in voltage and divide by 1.5C. 

Jack Rickard
http://evtv.me


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## dimitri (May 16, 2008)

jrickard said:


> We've bumped heads before. I ROUTINELY do 3C in the Speedster and have on EVERY DRIVE for over a year. it is not a problem, and it is NOT why you have been losing cells Dmitri.
> 
> I'm not losing them, and I'm driving today in February. And yes, right at 3C. They do it gladly and without complaint.
> 
> ...


First off, I have not lost any cells, I simply have a few cells weaker than others. Those weaker cells are from different batch and different manufacturing date and even have slightly different shade of casing. I only know they are weaker because I monitor cell level voltages via Paktrakr and I observe significanly deeper voltage sag on those cells under 3C, even more so at colder temps. THIS IS ALL I AM SAYING.....

- If I wasn't monitoring them I wouldn't know they are any different, because they deliver 3C just fine, only at much lower voltage than others
- If I had all cells from same batch I wouldn't know they are different because I would have nothing to compare with

So, all I am reporting is what I see with my own eyes. My report is no more true or false than any of your reports, so let's not pretend that either of us have any more authority than the other on the subject.

The only points I have been trying to bring across is that TS cells aren't all perfect and its better to have all cells matched than having mixed cells in one pack. Its not a MUST HAVE, but its NICE TO HAVE.

Perhaps you got lucky with your good cells, perhaps I got unlucky with my weak ones, we simply don't know because we don't have enough statistical data to decide either way. Just because our cells survived 300-400 cycles, does not mean they will survive advertised 2000-3000 cycles. So far I found TS datasheets to be full of crap, especially their discharge graphs. Your own bar graphs you posted all over Internet confirm it as well, since the curve at 1C load you found is on the lower level than TS is showing. I bet if you had 3C test it would be even more different from TS 3C graph. So, if we can't believe one piece of data on TS datasheet, what makes you think we can believe any of the data, especially life cycles?

Don't get me wrong, I really like LFP cells. Its just that datasheets have to be verified before anyone can depend on them. And you have to accept that quality control is not perfect, so resellers should have their own quality control procedures, which is precisely what this thread is about.


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## PhaseShift (Oct 12, 2009)

jrickard said:


> It is not terribly difficult, but as pointed out, I would do it at meaningful current levels. That's a little pricey on a constant current load but not wildly so.


Any advice on a source for the constant current load that is accepted as an industrial or lab grade device?

Phase


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## Guest (Feb 5, 2010)

dimitri said:


> First off, I have not lost any cells, I simply have a few cells weaker than others. Those weaker cells are from different batch and different manufacturing date and even have slightly different shade of casing. I only know they are weaker because I monitor cell level voltages via Paktrakr and I observe significanly deeper voltage sag on those cells under 3C, even more so at colder temps. THIS IS ALL I AM SAYING.....


Well then I misread your message. I thought you said that no way would these cells do 3C and if you try to do 3C you WILL lose cells.





dimitri said:


> So far I found TS datasheets to be full of crap, especially their discharge graphs. Your own bar graphs you posted all over Internet confirm it as well, since the curve at 1C load you found is on the lower level than TS is showing. I bet if you had 3C test it would be even more different from TS 3C graph. So, if we can't believe one piece of data on TS datasheet, what makes you think we can believe any of the data, especially life cycles?


I've found the TS spec sheets to be quite good and quite precise. But you do have to interpret them a little bit intelligently. Like the voltages shown are at the current rates shown and so forth. 

I found the curves matched rather nicely. But TS has 3 or 4 rates on a little bitty chart and I blow it out to a much more detailed one focusing JUST on 1C for example, they might look a little different but I think if you do a point to point analysis they're pretty representative.



dimitri said:


> Don't get me wrong, I really like LFP cells. Its just that datasheets have to be verified before anyone can depend on them. And you have to accept that quality control is not perfect, so resellers should have their own quality control procedures, which is precisely what this thread is about.


I guess I pretty much agree with you, although it would be expensive. If they simply did a select/sortation by capacity, on a large enough shipment of cells. But it is time consuming to do such tests. Which would you prefer, a set of $210 220Ah cells from the grab bag, or a matched set at $350 per cell?

You can just buy more cells than you need and do your own select. There's no free lunch in all this.

But I do agree that a matched set would be much easier to live with.

Jack Rickard


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## etischer (Jun 16, 2008)

jrickard said:


> I've found the TS spec sheets to be quite good and quite precise. But you do have to interpret them a little bit intelligently. Like the voltages shown are at the current rates shown and so forth.
> 
> I found the curves matched rather nicely. But TS has 3 or 4 rates on a little bitty chart and I blow it out to a much more detailed one focusing JUST on 1C for example, they might look a little different but I think if you do a point to point analysis they're pretty representative.


Jack, 
looks like you would be pulling upwards of 6C if your eSpeedster is 144kw. 

144kW / 120V = 1200A 
2 parallel packs of 90Ah = 6.6C

What kind of voltage sag do you see at 1C, 2C, 3C...? 

The voltage sag I experience with my TS cells make the TS graph look quite optimistic. 

At 2.5C, I sag to 2.95V, TS spec says it should be 3.20V. 

That's a huge difference, do you see the same amount of sag?


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