This week we take a break from the Escalade and the VW Thing and go back to a little bit of battery theory.
I’m learning to use stolen graphics with the green chromakeyer so that I can walk around in front of a green wall waving at things I can’t see while babbling things that could very well be true – or not.
This is a skill I’ve always wanted to develop.
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I have learned the hard way not to read these open online forums like DIYelectricjunk and EndlessFear. They are not good for my head. But I did get drawn into a mailing list I kind of monitor when a gentleman whom should clearly know better started making some preposterous claims agains my dearly beloved LiFePo4 cells. Anyone reading this and acting on it could clearly lose significant ducats on their battery pack and so I spewed forth my usual blunt correction.
It led to some further comments from this “technical” group. The most striking was from the far left field who offered his “opinion” on battery swelling and including a specious appeal to authority via an unknown “university research paper” he was simply unable to locate at the moment.
I was struck by a recent television commercial for an insurance company where a young girl related what she had heard on the internet with the assertion that they would not put that on the Internet if it wasn’t true. And then she introduced her French Model boyfriend she met on the Internet who’s “Bonjour” somehow had faint traces of an Arkansas accent.
Unfortunately, the same world of misinformation that permeates popular EV discussion forums also extends to “university research papers”.
Which leads me to a preposterous personal claim. I’m really good at wading through piles of horseshit in my never ending quest for a pony. I’m not sure why or how. But I have spent years reading and writing technical information, some of it so esoteric it reduces itself both by its volume and obtuse verbiage to an audience of nearly dozens. To quote my hero Sir Winston Churchill, “Sir, your document by its very weight and volume defends itself ably from the dangers of ever being read.”
The form of journaled, peer reviewed scientific papers purports to weed out the noise and leave only gems of true scientific information. It fails somewhat grandly but perhaps DOES arm the reader with more clues. The unfortunate biproduct is it leads to a stilted, “scholarly” writing style that is essentially unreadable. This has become so idiotically ingrained in our University structure that it becomes an end unto itself. My wife’s advisor during her doctorate chided her basically for communicating well and urged her to alter the tone to more scholarly format. In a mirth spasm, I took the offending paragraphs and rewrote them deliberately to produce the most obtuse, schalarly garbage I could concoct, and deliberately altered the nut of information buried within to nonsense and the exact OPPOSITE of what she was saying and handed it to her. I dared her to submit it as a correction. She did and her advisor LOVED it despite the fact that it made no sense at all, and the information within it was totally untrue.
Rigourously structured and referenced documents do not BY LAW have to be irretrievably BAD at communication. They customarily are as an exhibit of poor communication skills on the part of the authors. It’s not technically REQUIRED.
In any event, it can lead to some handy shortcuts. If you include some of their more pedantic terms in your google search, often you will eliminate the total nonsense of the wider Internet and reduce your gleanings to the more narrowly defined nonsense of technical journals. For example, instead of OVERCHARGE use ABUSE TOLERANCE or instead of SWELLING use GASSIFICATION. Etc.
But even once you are down to the elite and magical world of “university research papers” which we all know are the LAST WORD on all things technical, you still have to use a bit of critical thinking to separate the wheat from the chaff. On any particular topic, I wind up with three piles.
NICE. This is quite good. He has a theory. He has observations. THey seem to match. It could work that way. Keeper pile.
SAVE FOR LATER. I don’t know. Save this and see what related papers bubble up over the next few months. This guy has some ideas but they are not very well supported by observation and the observations he does link don’t fit very well. THey COULD mean that. They could mean something else. If we collect more papers on the same topic, this will become clear.
NONSENSE, This paper should be downworded and posted on EndlessFear. He’s striving for a strained theory to explain unrelated observations randomly.
This all derives from the problem that our reach on many things rather extends our grasp. By this, I mean the following:
1. We can BUILD perfectly operational lithium ion batteries. They store energy. Deliver current. Can be recharged. And this can be repeated a lot.
2. We would like to build better batteries.
3. To do that, we need to experiment.
4. To experiment efficiently, we need to understand how the battery works.
5. We don’t REALLY know how the battery works.
Unfortunately, item five extends in all directions to everything. The quantuum mechanics of the atom itself, as described circa 2013 is very very different from that taught in the 1970s. Is it more accurate? Hopefully. It is certainly more complicated.
And we are further reduced to the fact that it doesn’t MATTER a whole lot. If our conceptualization through analogy is sufficiently accurate, we can make changes and the outcomes will be consistent with the changes made. And that’s good enough. Indeed some times a simpler analogy serves better to envision changes and their outcomes than a more accurate, but mystifyingly detailed one. And so we deal with the unviverse by analogy – essentially theories. Complex theories or simple theories. But they should be theories consistent with our observations and the observations should be reproducible.
This is the central difference between theories and opinions. Having an opinion about lithium batteries is not a very useful pastime. First, the batteries don’t know your opinions and are curiously disinterested in them. Your opinion will not alter their operation. In fact, unbeknownst to virtually the entire online body politic apparently, the cell won’t be terribly impressed even if you can get LOTS OF OTHER IDIOTS TO AGREE WITH YOUR OPINION. It’s not really a very democratic process. Voting on it doesn’t do any good at all – which at a higher level is the archilles heel of the peer-reviewed journal as well.
But it helps to have a process of observation, recorded data, and then to propose theories to explain those observations. But observations are tricky.
For example, if you find your battery mysteriously swollen, you might suspect something is wrong. If you test the battery by hooking it up, and find that it produces current, from that limited observation you might conclude that battery swelling is harmless and that the battery is operational The problem with that it is its kind of a half a question. You don’t have a theory as to why the battery swelled in the first place. And batteries do more than produce current. They also produce it over a set period of time, storing a set amount of power, and at a potential rate. If the rate of power produced decreases, then the fact that it still produces current is not really a win. If the current production is diminished to milliamps, then the fact that it makes current is almost moot.
If it makes full current, but doesn’t do it very long would indicate a strongly diminished capacity. And if it does it at full current, and at full capacity, but that capacity is now fading quickly with each subsequent charge, the life cycle ability of the cell is reduced.
And so you need to observe ALL the aspects of battery performance by observation to determine that the swelling of the battery was irrelevant. But worse, you really need a theory as to why the battery swelled in the first place.
Prior to measuring ALL the important parameters of the cell, and having a working theory as to how it came to swell to begin with, having an opinion on the matter is TOTALLY inane. It’s idiotic even to claim one.
My favorite papers work like this.
1. We made an experimental change to a cell.
2. It had the following measureable and reproducible result – in most cases a good one.
3. Here’s our theory of how the cell works at all – referenced from these other papers.
4. Here’s our theory of why this change might work.
5. Here’s our theory of how it did work. It improved such and such because of the interaction of a and b. Or it failed to improve such and such because A and B don’t apparently interrelate the way we thought they would in 4.
6. Which kind of calls into question all we know in 3. Or which tends to be consistent with what we know in 3.
The ongoing problem with these studies is scope. If you try to do too much with a test setup, you wind up with too much data that could mean too many different things. A single observation could support three different theories as to why that was observed. If you observe three things we are up to nine theories and we can’t even keep track of them.
If you do too little, you miss interactions with other aspects of the cell operation. Yes, it still makes current. Does it get hot?
I tend to prefer the narrow scope, but it leads to the problem that all answers lead to further questions.
All of this is an attempt to get you in the chair of how hard won the knowledge of lithium cell operation really is. Starting with a basic theory, that polyanions might make more stable cathode materials than simple layered oxides, at the University of Austin they built a cell that WAS more stable and had good battery characteristics as well. They tried several materials and one of them produced a good result. THere were of course 300 other materials they could have tried.
From that point, it is really cool to come up with a more detailed theory of how it all works so you could FURTHER IMPROVE IT. But it is a THEORY. And it is refined and improved continuously.
And you wind up with an entire shopping list of properties of lithium batteries. How fast they can be charged. How fast they can be discharged. How much charge they can hold. How either of those is affected by cold temperatures. Or hot temperatures. This extends endlessly with each question partially answered and leading to further questions. This week we actually look at the difference between cycle life gains from gains in temperature BETWEEN simple cycles of direct discharge from full to empty and the effects of temperature on cycle life on cells that are only PARTLY charged and discharged. Turns out there is a HUGE difference.
And that leads us to serendipity. While opinions don’t matter, luck can. And the advantage of the WIDER scope is that once in a great while even a blind hog gets an acorn now and again.
I meant to do a bit of a chalk talk on the CURRENT THEORY of what happens when you overcharge a cell and why that act can cause a very hot very dangerous fire. We know by observation that using what is PROBABLY the SAFEST of these cells, LiFePo4, we can actually burn a warehouse or a ferry to the ground with a single car. These fires are horrendous and my observation is that it is almost always an overcharging event, usually caused by some well intentioned battery management system, that causes this. But what is the theory of operation of HOW it does this.
But understand this is the CURRENT THEORY I can come up with by reading a LOT of “university research papers” and attempting to translate that into something humanoids COULD be expected to get their head around in one brief session. Note that there is no universal agreement on this. There are a LOT of theories. But if you take the more prevalent and cited ones, and kind of artfully put them together into a vague and broad consensus, that’s more or less what it looks like currently. It may be my presentation, but it isn’t really my opinion. It is relating mostly the work of others in as cogent a terms as I can muster.
What you CAN”T have is nonsense with no theory. You can’t have a cell with a voltage of x, apply a higher voltage of y, and then say that you are NOT charging the cell because it is “not higher by very much”. To do that, you have to show a theory of where the excess voltage went. It could go off in heat. It could go off in light. It could go off in furlongs per fortnight. But you can’t simply leave the voltage or the power applied on the table to the whims of the gods. This is the heart of the Boeing problem They have a 3.72 volt battery held at 4.00v and a resulting fire. I think it is from overcharging. That’s a pretty good theory since we demonstrably have by DESIGN a higher voltage than the cell applied for an indeterminate amount of time. I’m quite ok with that NOT being the cause but it can’t be because we didn’t do it enough over voltage. Enough to do it right now?
But I was a little diverted by a fascinating paper I came across actually by one of the students at a university. State of Health Estimation of Li-ion Batteries – Cycle Life Test Methods.
This paper, by Jens Groot is of course entirely directed at something else. He’s developing cycle testing methodologies to aid in developing some sort of long term state of health estimation algorithm. But a couple of things jumped out. The first was the dramatic difference between the cycle life of cells that are charged to 100% SOC and subsequently discharged to 11% SOC, and the alternate case of cells charged to 50% SOC and discharged to 24% SOC. The former was depleted to 80% of its initial capacity after 2000 cycles. The later was at 9000 cycles and still enjoying 85% of original capacity.
We had long talked of this, as a theory, and here was a fairly rigorously crafted test that showed it in glaring form. It’s not a few percent. It’s four and a half times greater cycle life.
And it implies that our 2000 cycle cells could easily go 10,000 or 15,000 cycles driven conservatively. It might also imply a DIFFERENT operating procedure to extend battery cycle life – don’t ever fully charge it.
Secondly in that comparison was the fact that the 2000 cycle version was done at 3.76C, while the 9000 cycle version went as high as 23C current rates. I had heard many times that high power output of a cell reduced cell life. I have never been able to muster any observation to support that, but was vaguely ok with it as it “sounded like it made sense.” The implication in this test, not totally proved but certainly there in the data, no problem with high current levels.
The other thing that jumped out was that on the SAME driving cycle A, there was a hugish difference between 23 degrees centigrade and 35 degrees centigrade. And the direction CONTRAVENED everything I’ve seen written about the effects of temperature on cycle life. All my OWN observations in testing show a battery working bettter in all respects at higher temperatures of 35 or 40 degrees centigrade, so my question was, “why then would cycle life be shorted from higher temperatures.” Something short term ought to be “worse” at 35 or 40 degrees if those temperatures are bad long term.
Well apparently not. Same for same, he got LONGER cycle life from 35 centigrade than from 23.
Why does all this matter?
Operating procedures. How should you charge your cells. How far should you discharge them. Should you cool them if they get hot? Should you heat them if they get cold? And the answers matter. If you are using cell capacity, to cool cells so they will last longer, when they will actually last longer hot, you not only lose cycle life from the lower temperature, but also from the use of energy from the cell to cool it. You get a double debit in EXACTLY the opposite result your are attempting to achieve, with additional weight and complexity in your car to do a dumb thing.
At the same time, it might make sense that the reverse situation is also true, when in fact it can be totally false. We DO know that cells too cold when charged are damaged. So the yin and yang and logical ones and zeros that are attempted in online discussions because they “make sense” just are not the basis for cell care. You can’t really thought experiment your way into “logical” results. Every operation must be related at least to a working THEORY and the results should be both observable and reproducible. The cells are what they are.
Why am I excited? Because every time we touch one of these cells we learn something new. In fact, in every case where someone ELSE touches one of these cells, we learn something new – if they follow some basic methodologies and accurately report their observations.
I was kind of eggregiously reminded this past week that some of the online poseurs unfortunately will get so wed to a position that they will actually and quite deliberately present “observations” never made nor ever possible to make to support their theory. It is true I suffer fools with exceptionally poor grace. Even quite publicly.
But beyond that, we live in an exciting time. These energy storage devices are modern day miracles and indeed appear miraculous to me BECAUSE we do not fully understand them. The process of gleaning little bits of information on them a tiny bit at a time is actually very intensely pleasurable to me personally. ANd it implies a future of vastly improved energy storage – totally silent, without emissions, with no moving parts. Not just cars, but flight, levitation, all things become plausible as we learn to store and retrieve usable energy in useful amounts at the atomic level. It is the true promise of “atomic energy.” And I get kind of jazzed up at that. When I consider what an unfathomable amount of data can now be stored on a tiny thunb drive so small I lose it IN my pocket, at a price of less than $20 the concept of energy storage on the same scale is just explosive. Literally. Cranial detonations. I can’t get my head around it.
And so just a brief peek into my ongoing and very passionate love affair with batteries. For me, they are a peephole glimpse into the very land,,, the shadow of the figertips…. of God.
Below are the rest of the slides used in today’s show which you may examine at your leisure….
45 thoughts on “SHADOW OF THE FINGERTIPS”
LOVE IT JACK!
Not everyone is technical on this site. However you have the ability to convey thoughts in the most non-confrontational manner on very in depth subject matters. I love these white paper type videos!
All the best.
Brusa NLG513 being discontinued: http://www.brusa.biz/index.php?id=55&L=1&tx_ttnews%5Btt_news%5D=181
Thanks for the change of pace with this weeks show. I enjoyed it.
In the chart for Cycle D it says for SOC: Average of 54.9%, Min of 28.8%, and Max of 39.1% Since this one seemed to get the best cycle life I was particularly interested in the particulars of that test. I am confused by how the average state of charge could be higher than the Max SOC. An average of 54.9% would imply a max somewhere near 81% but certainly above 55% I am guessing it is a typo and the average SOC should be around 34%. This would be a utilization of 10% of the capacity of the cell so you might expect it to last 10 times longer than a cell that was cycled to the limits and extrapolating the charted data out would seem to indicate this might be the case. Of course for miles driven they would be about the same. 2000 cycles to 100% DOD gives the same miles driven as 20000 cycles using only 10% of the charge.
He was testing so many things that the only real conclusion I can come to is the same as yours. The cells like to operate at 35C better than at 23C.
A bit cavalier on the concept here that if it takes one woman nine months to make a baby, then 9 women should be able to team up on the project and make one in 30 days.
You imply that it is accepted fact that 2000 100% cycles means 20,000 cycles at 10%. I don’t really care WHAT APPLE puts on their AppleCare documents. I’m unaware of any such documented relationship. It is pretty apparent that lesser cycles last longer and the specs provided by some of our suppliers imply 2000 cycles at 80% and 3000 cycles at 70%. I’ve never seen any hard data on THAT figure of merit.
So this IS a bit of news to me at least.
Beyond that, yes he had a very busy test going on but his purpose was to compare cycle testing methodologies to help develop some sort of long term state of health algorithm. As in many tests, often other things fall out. He did note that the inverse relationship to temperature was not as expected. The scope was a little wide for me, but often that is where the surprises jump out.
Like most things, the answers bring on more questions. For example, would alternating discharge and charge like a driving cycle produce longer cycle life than continous discharge followed by continuous charge at the SAME 24-50% SOC range. Every time we finish a test, it points to further tests. And the results, as I pointed out, very much CAN matter in how we deal with the cells and what we expect from them.
In any event, it is interesting.
I agree with you and I didn’t think I was implying anything. I am just reading the data and now crunching some numbers. From his tests he was seeing 1200 deep discharges from 100% to 11.4% (Cycle type C). It looks like he would be seeing as many as 12000 shallow discharges from 39.1% to 28.8% if he continued the testing out that far (Cycle type D). Cycle type E gave about 2800 discharges from 100% down to 13%. Cycle type A appears to give about 8500 discharges from 50% down to 22.6%. If the distance you could travel with type C is = n miles before the battery wears out then the distance for cycle type D would be 1.16*n and with cycle type A it would be 2.19*n and finally with type E it would be 2.29*n. This assumes I am not completely confused by the meaning of the table entries. This bears out your assertion that not going to the ends is quite a lot better in that you can more than double the distance you can travel with a given pack before it wears out. This doesn’t address the economics however because to get twice the utilization from a pack the pack needs to be more than twice the necessary size. It may turn out to be better to size the pack for an 80% discharge, get the 2000 cycles and then replace the pack with a better version after 6 years than to spend twice the money necessary up front so you can do the shallow discharges and keep it for 14 years. There is also the fact that a larger pack weighs more which will cost some efficiency. By chance I sized my pack between these two so I expect perhaps 10 years life based on what I am seeing here. If I could charge at work I should be out to 15 years of useful life.
It is very interesting and I am glad you found this.
You are quite correct. There is a little problem with D. Refer to the text description of D in the Jens Groot paper and the discussion subsequent.
Jack I’m right with you on “… peephole glimpse into the very land…” Some things fill you with awe and rejoicing at their simple elegance. For me it is the squirrel cage motor (rotation from 3 phase electricity with a few random chunks of iron and copper) and the stone arch (crossing a big space with a material that doesn’t do tension or bending)
Yes. It is a glimpse into perfection.
The promise of the atomic age was unlimited power. In some ways, it lead to an unlimited mess. Crudely splitting heavy atoms to release energy has a lot of problems.
Lest I get too poetic in peering into Infinity and Beyond……
But really. Storing energy in the interstices between individual atoms is ENORMOUS to me – far far beyond what is immediately in front of us – putting a few kiloWatthours into 1700 lbs of aluminum copper and carbon.
Imagine if you can store energy by intercalating substances INTO this atomic level, and retrieve them BY COMMAND FROM this atomic level location, it is a step not to far to glean the energy FROM those atoms directly. Imagine if you would for a moment, instead
of crudely splitting the nucleus of heavy isotopes, if we could tease the flow of electrons gently and quietly from ordinary substances directly. And that the adroit manipulation of energy from matter and matter from energy in a very safe and controlled
fashion could be learned? Lithium cells and photovoltaic cells and photocatalytic cells are really all manifestations of the same process that inevitably lead to that. Atomic level energy control. Were that mastered, almost everything becomes immediately possible. I cannot even conceive of it nor comprehend it. It would make the sum total of the industrial revolution, the technical revolution, and the information revolution look like almost irrelevant prelude.
The slavery question was a very real problem of the 1840’s and 1850s woven intrinsically throughout our economy and way of life in those days and seemingly intractable. It actually almost looks nonsensical in terms of 2013. Similarly the problems of today
will appear nearly quaint in such a future. Why did hundreds of millions of people give away their stuff….for oil?
Good day gents from South Africa.
I have been a silent disciple of Jack’s work from the start of EVTV. I can comfortably state that I am in full congruence with practically all his observations and opinions concerning LifePo cells, and the CALB CA cells in particular. I have worked with a few of these cells albeit in relatively small packs (typically 13 or 17 60Ah series packs), as I currently focus mostly on golf cart conversions. I strictly don’t qualify to comment under Jack’s definitions of “science experiments”, but thought I would share a few tips/observations resulting from my work with these cells.
We typically select and group cells based on reported capacity in the first instance and internal resistance in the second. These cells are however much like children. Besides coming from the same gene pool, they are all different with unique characteristics.
We only charge through constant current protocol, and never bother with top balancing, but bottom balancing is in my view the absolute key.
Initial charge of a pack is done unbalanced, but carefully observed by cell to get a feel for where the pack is as a team of cells, and identify the weakest player.
Careful bottom balancing is then done down to 2.5V through individual discharge and then parrallel balancing. I find it requires at least 72 hours in parrallel mode to ensure the cells are all stable at the correct (2.5V) bottom voltage.
The initial balanced charge is then done by individual cell monitoring until the first cell reaches 3.7V. This usually absorbs about 60 Ah on cells all rated at 64Ah. Pack voltage is then set albeit that the cells are quite unbalanced at the end of charge. Some would be at 3.4 others at 3.5 etc.
This is however key, as it must be realised that by bottom balancing, you are amplifying any differences in individual cells at the top end.
For example on my current 13s pack the cut-off voltage is 45.1V (average per cell of 3.47V) , and not exceeded on subsequent charges, as the weakest child then hits our pre-set limit of 3.7V.
Trust this makes some sense. Will hopefully move into accredited Jack territory soon with a conversion on a 1957 Lotus Eleven.
We are increasingly leaning toward your 3.47volts Anthony. Jens Groot excellent efforts would tend to imply that even lower charge terminations might be economically advantageous if cycle life is the main consideration.
Not a bad strategy. Start with small packs and golf carts to learn what is and what isnt’. Then move up to a 1957 Lotus Eleven. I would predict that process will ultimately lead to a very satisfying car for you with a very long battery pack life.
Many thanks for the reply Jack. Appreciated. Apologies for my manners, but by introduction to my first post I should have stated my sincere appreciation for the excellent work that you and the team have EVTV have put in over the last years. The videos and blogs are a source of inspiration and have at least educated me to the point that I know what I don’t know. That is at least a start!
That’s useful corroboration. One question: you say “…Pack voltage is then set albeit that the cells are quite unbalanced at the end of charge. Some would be at 3.4 others at 3.5 etc….” I observed exactly that if you measure voltages at the end of charge. However on the three packs I’ve worked with so far they fell back into line either after a rest period or after a miniscule amount of power was taken out. There is an example at http://tovey-books.co.uk/attachments/Image/Cycle_500_1.jpg which is a voltage and current trace of the charge phase. Is this consistent with what you observe?
The voltage measurements I refer to are taken just before charge is terminated. Soon after charge current is removed, even before any load is applied, the voltages all fall back into line, normally within 10-20 millivolts. They usually end up between 3.30V and 3.32V, even the weakest cell in the pack. You only see the amplified differences while charge current is still applied.
We normally charge at C/4 rates.
Yep – exactly what I am seeing. As you will see from the chart, the voltages seem to writhe around like a bundle of snakes at the top of charge – some combination of capacity, impedance and diffusion delay
I have looked at your data, and you did some excellent work.
As you mention, there are several factors which are more pronounced during this final part of the charge.Warburg,internal resistance, localised thermal variance, capacitance and probably a few others, which is why it is safer not to mess around in this area too much if you want to safeguard the flock!
Hi Jack, et al,
Thanks for posting this very interesting paper.
I have a couple of comments on Jack’s points, some agreeing, and some disagreeing with his conclusions or inferences:
1) The study shows that 35 degC is better than 25 degC for running these cells: I think that is fantastic, and Jack’s comment that people should take it into consideration when designing their systems is right on,
2) There is clearly a difference in cycle life between the 90% (approx) DOD cycled cells (cycles C & E), and the much lower DOD range of cycles A, B, & D. I agree with Jack’s 4.5 times approximation. If you look at lifetime Ahr output, then (to the 80% EOL level taken in the paper) cycle C high DOD gives 1350cycles*(100%-11.4%) = 1196 full capacity equivalent cycles (fcec), whilst cycle A gives 8500 * (50% – 22.6%) = 2329 fcec. That’s still almost 2 times the total life-time capacity or energy output. So clearly smaller DOD cycles are considerably better than bigger. (I believe this is what Doug was saying in his comments).
3) Given point 2), the tradeoff between using a bigger battery capacity to get a lower DOD for the same distance, or accepting a lower distance to get that lower DOD for the same size battery becomes critical, and will be very dependent on the use factor for the vehicle.
4) Although the paper definitely shows that lower DOD gives greater cycles (and greater life-time output), I don’t think that Jack’s statement (“It might also imply a DIFFERENT operating procedure to extend battery cycle life – don’t ever fully charge it.”) is necessarily a correct inference from the data. I don’t see anything in the paper that suggests that where on the SOC range the DOD is initiated has any effect on the cycles e.g. it might be that starting at 100% SOC and discharging to 80% is the same as starting at 45% and discharging to 25% from a cycle life perspective. I don’t necessarily think that is correct, and I agree generally with Jack’s statement, I just don’t think the paper presents data on that. If anyone can point me to a paper that does address that topic, I’d be greatful, (BTW, I did notice Jack’s use of the term “might” in that sentence),
5) I don’t agree with Jack that (“The implication in this test, not totally proved but certainly there in the data, no problem with high current levels.”). Here you’re comparing Cycle C (at 2000 cycles) with Cycle A (at 9000 cycles) to make that statement. But those 2 Cycles have 2 significantly differing variables: the power draw (as you state), and the DOD. If you’re looking at the impact of charge/discharge rates on cycle life, within this test it would probably be more appropriate to compare Cycle A with Cycle D because the DOD is quite similar and so would have less effect than with Cycle C, and so it’s coming closer to isolating just 1 variable. Comparing these 2 Cycles: Cycle D clearly has better cycle life than Cycle A, which to me suggests that there is a correlation between peak current and cycle life. Groot also draws that conclusion at several points in the paper.
Anyway, my comments are made in the spirit of non-egotistical discussions of facts, which Jack strongly espouses, and in the spirit of a vigorous pressure-testing discussion of our inferences & conclusions based on those facts.
Again, thanks for the interesting link and fascinating discussion.
I agree Cycle D is interesting. Unfortunately, I kind of have a problem with Cycle D all around. His description of Cycle D is quite clear with a SOC range of 29 to 46 or 47%. None of the numbers in the Table for Cycle D at all resemble the description in the text. So I’ve just kind of factored Cycle D out of the results in my mind. I don’t really know WHAT he did on Cycle D.
If it is narrow SOC range but also more limited current, that difference in current draw between D and A MIGHT represent the difference in cycle life between A and D. However, the 29% to 46% is also a much narrower range of SOC from A’s 22% to 50%. D is a 17% range and A being 28%. That difference would appear commensurate to the difference in cycle life between A and D. Under that assumption, the difference in current draw between A and D would again appear moot.
There is a further problem with your analysis in that respect. IF we attribute the difference between A and D to be IN the current draw, then how do you account for the difference between A and C?
You see SOC range either IS the significant factor or it is not. Assuming it is accounts for the differences in both directions. A is less than C but D is less than A in SOC range and that correlates with the stack in both directions.
But A is dramatically greater than both C AND D with regards to peak charge and discharge currents, yet D has longer cycle life, and C has shorter. Not indicated.
Bottom line on your point 5 is that the DOD is NOT “quite similar”. One is 17% SOC range and one is 28%, or so it would appear in the cycle text descriptions. Again, this is kind of muddied by what he has in the table. But would appear to match the direction of impact on cycle life and the degree. Leaving us again with the level of peak charge and discharge currents apparently DON’T MATTER in cycle life.
But your comments dramatically illustrate the degree of the problem. This kind of analysis requires some VERY critical thinking processes and the more you peer at the data, the more things move around on the page. I have you at a disadvantage because I do this a LOT and have other inferences gleaned over time – the more you do it the better at it you get. But it goes quite beyond that.
We develop a hypothesis then that peak current level has no impact on cycle life. All we have to do is set up an experiment at say 1C charge and discharge and 10C charge and discharge. We don’t need to go all the way to 23C.
Then we have to run a couple thousand cycles. And even with a 40 Ah cell, one of them is cranking 400 Amps. This wears the shit out of the equipment. You blow up constant current loads. Contactors simply weld together after a certain number of cycles.
Yes, we coudl do it with small 18650 cells. If you notice, MOST of these papers are done with 18650’s or button cells. The problem I have with that is that larger cells display anonmalies much better and are more forgiving. TIny errors in test set up can get entirely magnified all out of proportion when you are using tiny cells and tiny voltage and current levels.
John Hardy presented a superb example of this just this week. He had a series of cells on cycle. Understand this takes WEEKS in many cases. But he had two errant cells. At a glance, I could tell he had ONE errant cell, but the other apparently errant cell had almost EXACTLY the same curve as the other cells, but was displaced from the others by a set amount. I asked him to try an experiment and swap that cell with another. As I suspected, the displacement stayed in the same TEST POSITION now on a DIFFERENT cell. The problem was in the test connections.
Not only is cell testing much like watching paint dry, you can have a LOT of time and effort invested in a test, only to learn that ONE SCREWUP VOIDS THE ENTIRE THING and of course you get to start all over. Often, the level of effort just swamps the value of what you are trying to learn. A component failure can send you back to step one.
I’m not trying to paint this all heroic. I’m trying to provide a glimpse into how hard won these little bits of knowledge really are as this explains how 23 years after the first graphite anode Li-ion cell we still don’t really KNOW how they really work – even with regards to a simple set of characteristics that matter to YOU.
Discharge cutoff voltage.
How to size your pack to your maximum acceleration current load.
These are very real very pragmatic very NON academic exercises in getting competent performance in a car from a battery pack that may well have set you back $15,000. Do you know how old I was before I ever got to drive a CAR that cost $15,000?
For the record, my first licensable motor vehicle was a 1954 Dodge Coronet that I purchased for $60 cash money hard come by. It was in such a state that my fellow high school students cruelly pried the D and the E from the front emblem leaving my car to spell DO G across the front. Kids can be cruel. Worse, while it was a three speed manual it had fluid drive and no working parking brake so I had to chain it to the basketball post in the parking lot to keep it from rolling away. The parking lot of course featured brand new Mustang, Camaro, and Z28 hot rods all aglitter in new paint. I was mostly red lead primer. But I digress.
The point is things like bottom balancing and tinned braided battery straps, nordlock washers, non-shedding sanding blocks are hard won. But they DO matter. Heroic engineering to monitor temperature and voltage of each individual cell do NOT seem to matter in teh car and are very upsetting to this actually quite delicate process of making durable connections that wear well in an environment of constant vibration and thermal cycling. Top balancing using shunt balancers looks to me like Voodoo – you might as well cut a pigs throat on an alter during a full moon and work on a specific chant language while randomly shorting cell terminals with a screwdriver.
In the backdrop of all that, I see new people coming to gain real enthusiasm and a passion to do their very first conversion. And the first thing they run into is a lot of people sharing their very valuable opinions and declaring themselves in “agreement” in very sage tones of learned wise men WITHOUT EVER DOING ANY OF THE WORK TO LEARN ANYTHING AT ALL. Worse, a select set of them have devised a way of gleaning another $2000 from this wide eyed group of new enthusiasts under the theory that if they are going to spend $10,000 on a battery pack, isn’t it worth another $2000 (parts cost $125) to “protect” that investment? Shylocks and superstition. Deadly combination.
And what are they to believe? Here’s the majority of the online constituents, with a very elaborate obviously complicated and highly designed technical miracle they all agree is necessary to operate these cells, and on the other hand there’s this really fat old guy with a bad cough waving a $9 battery strap he obviously stole from an antenna ground. Oh better. He has a $1 3M sanding block in his other hand.
Fortunately, I often get a second audition AFTER they’ve destroyed six or eight cells of their original pack with the rest swollen which they’ve been told is normal – UNfortunately enthusiasm now a little dampened.
First do no harm. A friend was put OUT OF BUSINESS after investing hundreds of thousands of dollars because his homeland, Norway OUTLAWED conversions subsequent to a Qashquai fire on a ferry – absolutely caused by the “Battery Management System.” We face severe shipping restrictions and increasing expense for our cells because of very basic misunderstanding (slander actually) of these cells and I know of one enthusiast who was NOT ALLOWED to participate in a fun event with his car because he had NO BMS and the “professor from the local university” who ran it could not risk a FIRE. We could very well face a federal REQUIREMENT for a BMS to avoid fires, that would of course lead to MORE fires. Alice in Wonderland, all from very lazy “experts” pontificating about things they know squat about shit.
It is true I often fail in my polite graces when dealing with this online. Regularly accused of outright “rudeness.” Guilty.
The process is hypothesis, test, publish, independently verify, theory. Not chant, vote, agree, and form an opinion.
Bottom line. You can’t type yourself smart using a keyboard and a screen. I totally distrust “wisdom of crowds”, ALL “university research papers”, experts, professional battery engineers, and yes the United States Government.
It takes a mighty big dog to weigh a ton. I’m from Missouri. Show me.
Quite right on the errant cell. I had unwittingly got a bit of ground wire shared between the power and measurement circuits which was messing up one of the voltage traces by about 50 millivolts. As the real voltage variance was only about 15 millivolts across most of each cycle the signal to noise ratio was rather poor.
Awww, look at what you two have done. Made me too late to cut my hedges.
So basically, what you are saying is if you leave the cell on the shelf, it has more life.
What an excellent presentation! Thanks
I have a system with 30 100ah cells, due to voltage constrains on the system I’m only using 28 at this time, I installed 30 cause the fit exactly in to the area were 5 lead batteries were located, so I have two spares now. The charge is setup for 98 volts, 3.5v per cell, the pack is bottom balanced. I want to marry in the other two cells, for exactly the reason that Jack so aptly stated. But it occurred to me since the charger (Elcon type, non can bus) is a voltage operated device, that this may no longer work to charge the cells correctly at 30 cells.
Let me explain my thinking. So if I charge 30 cells at 98v that means 3.266 per cells, which is good for I’m not fully charging, which if the claims are correct, and I have no reason to believe it is not, should give me longer cycle life on the pack.
We know that the cells have a flat voltage between the two ends, which means that it may (will) reach the shut off voltage long before the cells are charged to proper capacity. So at this point my thinking is in order to do this correctly, one would have to have a higher voltage charger, and now use an ah counter (JLD404) to turn off the charger at the desired capacity , for using an ending algorithm will no longer work.
Correct me if I’m missing something.
Roy, others may well me more qualified to comment, but I will give you my 2 cents worth.
Setting any cut-off voltage on the flat part of the charge curve is close to useless, as you will never know what state of charge you are at. At 3.266V, you may well be at only 15% SOC under charge current of 1C or even less and trigger your cut-off voltage.
Charging by Ah count is the best bet, but this will only work on repetitive charges if the meter can count in both directions, else you again never know where you are on SOC. I have never worked with the JLD404, but as I recall from Jack’s demonstrations, it can’t count Ah in both directions. Voltage cut-off in my experience the safest bet, but it has to be set above the reverse knee to be effective.
So higher voltage charger and cut-off at 3.5V (or even lower) average voltage per cell would be my suggestion.
Just a note about lithium battery temperature from American Chemical society http://www.sciencedaily.com/releases/2013/04/130410103921.htm
It sounds like nonsense on the face of it. But it coudl be the story instead of the paper. Without the source paper, I can’t make anything of it. LiFePo4 cells demonstrably work BETTER above 86 degrees, I strongly suspect they have longer cycle lives above 86F . That seems to be supported by Joots paper.
I would have to see the American Chemical Society’s paper to comment on it. A blog story describing it is almost always garbled.
Jack maybe they opine on a different chemistry? Most peoples Laptop batteries with pathetic lives run fairly warm and those Leafs losing capacity in hot climes. Maybe its in the choice of electrolyte. A123 that supplied the cells for the decade old Mars rovers would certainly need a different temperature regime to operate?
Just wondering. Can’t expect accuracy on news media.
I agree that you should not use an “Ahr” or capacity counter to trigger critical events (such as charge cutoff or start).
I worked on charge algorithms and “fuel gauges” for NiMH and early Lithium batteries in the early 90’s. The difficulty with them is that you can only count the energy in/out. You then need to make some assumptions on where that energy goes or comes from within the cell, and model the actual SOC by applying that model.
If you read Groot’s paper, you probably can see that the internal model for a cell is really quite complex, with various series and parallel resistances, capacitances, and inductances, and with all of those varying with frequency and temperature. And that doesn’t take into account any cell differences and the aging process, both of which change all those variables.
In our developments, we had multiple matrices of dependent variables programmed into a microprocessor, and measured energy, transient frequencies, and temperature, and then adjusted the capacity calculation based on those. We developed those matrices by testing the specific cells that we manufactured over many years. And yet our “fuel gauges” still “drifted” away from the “real” capacity over time & use.
The only way we could get it to work reliably was to require a voltage “end event” on a regular basis. The end event was either a fully charge voltage or a fully discharged voltage, since these are the most reliable events. We’d then reset the “fuel gauge” on that event, and start coulomb counting again. With the NiMH cells we recommended an “end event” every 20 cycles, and had an error counter which flagged the broadcast capacity as unreliable if this didn’t happen.
Anyway, the point of all this rambling is to hopefully give a relevant experience which illustrates why I agree with Anthony: don’t use a Coulomb counter as a charge/discharge trigger mechanism. It won’t be reliable enough over many cycles. By all means use it to track an approximation of SOC or capacity, but use a defined voltage event to trigger.
Carfully bottom balance to 2.75v with a long resting state (48-72hr) and undercharge to 3.5v (or a little less). Then enjoy the car….
CALB’s techy info states presumably for the Solar panel guys never hold a charge to 3.4V or greater per cell.
I can understand this “whack in the juice and knock it off” principle.
The BBC still pushing that deeply flawed Statoil survey: http://www.bbc.co.uk/news/magazine-22001356. There is a good dismantling of it here: https://plus.google.com/+RobertLlewellyn/posts/QcZd7My7tYF.
As a Brit I’m deeply embarrased
Misinformation and disinformation in all directions. Ergo the value of the “university research paper.”
You can’t pay people to think for you.
Which is more efficient, 20 million small engines or one big one?
What about an electric car would make it more environmentally or energy damaging than an ICE car? Aluminum. It is made with electricity often made with coal.
Of course the ICe car has a heavy steel motor. For steel you can use coal directly. What’s the difference.
Lb for lb, teh CAR is not going to be an advantage or a disadvantage.
The results on the fuel are both obvious AND easy to calculate. But if you are going to calculate the energy used to make electricity, you also have to do that same calculation on the energy used to make the gasoline – all the way back to the hole in the ground. And again the results are obvious in the extreme.
But it is relatively inexpensive to whore a bunch of study generators and if you can fool some of the people some of the time.
The current issue of Scientific American has an article on that topic- a look at the energy used to obtain energy. In one graph they show that if you invest 1GJ in energy production, it yields enough electricity to drive an electric car 6500 miles, but enough gas to go half that distance.
Found this page on the numbers behind it, but the article itself obviously requires subscription:
I’ve looked at the numbers and if I am understanding things correctly they are making assumptions about the EV drivetrain which are hilarious; for example their baseline for a motor controller/inverter is an industrial unit. They estimate the material content (no doubt correctly for that unit) at about 75kg. The Curtis I am using in the Civic weighs 6.4 kg on my bathroom scales: rather less than the coolant radiator and fans from the original car (7 kg) which I have taken out. The same applies to the motor (138 kgs in the paper, 63 in my car) and the charger (74 versus 10.3). They have thrown in 60kg of aluminium and 20kg of steel for a “battery cooling system”. The cells in my pack only weigh 118 and as any fule knwo, these cells need warming rather than cooling.
It would be charitable to assume ignorance rather than malfeasance. I am working on it
Yes, of course. Your charity extends to a professional research organization under the supervision of a University professor actually believing inadvertantly that an electric car caries a 150 lb inverter?
Or a 1000 lb motor?
You are indeed charitable.
For the graph that shows an EV going twice as far as an ICE (for a given energy used for production) it seems it was a pretty simple calculation:
“For electric cars, I also used EPA estimates of the miles they can go per kilowatt-hour of electricity input, multiplied by the EROI for average U.S. electricity”
For ICE cars it was based on 30 mpg.
The subtlety here is that the article is all about the EROI (Energy Return on Investment) of different fuels, and the 1GJ of energy the graph was based on is the energy used to get the fuel to the car, not the energy content of the fuel itself. The way I interpret that is that for a gas car to go 3000 miles, it would take 100 gallons of fuel which required 1GJ of energy to extract, refine, and deliver. For the same 1GJ of energy investment, you can obtain fuel, deliver it to a power plant, use it to generate electricity, and transport it into an EV, then drive twice as far.
There’s nothing British about the BBC.
HI Jack !
Maybe you have seen this thesis
You can file it in the section you like.
I found it interesting.
This article was referenced in the Irish ev owners forum and caused quite a panic amongst the members :
I would value your opinion. Here is the response I posted in that forum :
“I’m going to put my neck on the line a little bit here and call that utter hogwash.The most relevant sentence in the article is “Ours is the first study that has specifically looked for a memory effect in lithium-ion batteries.” It’s the proverbial “don’t think about elephants” that is so pervasive in the battery community.We want memory effect therefore we will find it! It is almost as if they try to make li-ion behave like older chemistries and if it won’t then we’ll damn well stuff it with electronics until it does. The top balancing BMS is the most typical example. Li-ion does not need float or absorbtion charge like lead acid but with a bms now you can! Now we have memory effect? Sweet so now we can stuff in more electronics to remove this harmful effect. Leaf owners can relax even if I am completely and utterly wrong as you guys dont use lithium iron phosphate cells. I do however and have quite a bit of testing under my belt. I can also say that the lifepo4 cells in my car refuse to pay any attention to even the most world renowned university papers telling them how they should behave. I think its very arrogant of them but hey , that’s me”
Well, there is certainly no reason to panic.
The “memory effect” they allude to is so small I hardly think it can be a factor in operation of the batteries. They had to know to look and then look very closely to find it. It is more a minor distortion in the charge curve than anything you need to deal with in a practical sense, and it requires rather extreme magnification to detect at all.
It is new information. And very appropos to what I just wrote about how grudgingly we gain these little nuggets of information. Their explanation of lagging chemical potential with the removal of lithium ions sounds a bit majestic and magical to me. What causes this lag? What barrier? That some ions have a more tortuous path in and out is rather known. Beyond that,the hypothesis of what causes it sounds a little contrived. But the effect should be measurable. In fact, our little Cell Lab 8 should make this overvoltage hump evident in a rather quick experiment if it can be reproduced and it should certainly be reproducible from the information given.
I found it most confusing the discussion of an idle time between partial charge and full discharge as a palliative.
This article referenced in the above links is in my view pure BMS. That would be BS with an added M to the acronym for magnified!
To which article do you refer Anthony?
“Memory effect in a lithium-ion battery” from Nature Materials(2013).
I have seen a german version of it and if I got it correctly then you absolutely need a BMS to see that memory effect. Might be a bug in the BMS. Most severe bug was to implement a BMS in the first place.
Self discharge of Lithium Iron Phosphate has been proven too. It is dependent on the DC resistance of the BMS modules again.
But the best story I have seen is about the power hunger of mobile devices and their servers in particular. They need more power than our little EVs and will bring the grid down for sure. I am going to build an USB adapter for our little i-MiEV so I can charge it from my laptop should I ever get stranded on a highway. I always thought there must be something like that little gas canister for ICE cars for EVs too. My laptop is about the right size and weight. Hope I get that adapter ready for April 1st.
Confirmation bias based on bad data.
OT but interesting about Missouri gun owners who have sought or are using meds. Isn’t there a better use for Lithium?: