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Old 17-09-2018, 15:40   #1
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Over-discharge of LiFePO4 cells

Two more papers to take a look at. These explore over-discharge failures.

Failure Study of Commercial LiFePO4Cells in over-Discharge Conditions Using Electrochemical Impedance Spectroscopy

Failure Study of Commercial LiFePO4 Cells in over-Discharge Conditions Using Electrochemical Impedance Spectroscopy

And

Failure Investigation of LiFePO4 Cells in Over-Discharge Conditions

Failure Investigation of LiFePO4 Cells in Over-Discharge Conditions

In these papers the authors are testing A124 1.1AH 18650 cells in over-discharge conditions. They define over-discharge in terms of amp hours removed from the cell. 100% SOC is defined as charging to 3.6 VPC (CC 1C rate) until a tail (CV) current of 0.02 amp was reached. 100% DOD (0% SOC) was defined at 2.0 VPC with the resulting delivered AH defining the capacity of the cell.
Over-discharge was defined as the percentage of charge removed from the cells past 100%DOD. Thus 105% DOD (5% over-discharge) defined as the point when 105 % of the capacity was removed from the cell. At a constant current discharge of 1C it would take 1 hour to take a cell from 100%SOC to 100% DOD (2.0 vVPC). Thus 105% DOD would be achieved in 1.05 hours, 110% DOD at 1.1 hours and 120% DOD 1.2 hours. A key here is to remember that this is capacity taken beyond when the cell reached 2.0 VPC which is quite a bit deeper than we normally take our LiFePO4 cells. My BMS gets excited at 2.9 VPC (at 40 degrees C).
We are used to measuring SOC in terms of VPC. 100% DOD as defined here (2.0 VPC) is very deep into the knee. 102% DOD has a 1.0 VPC, 104% DOD has a VPC of 0 volts, 105% DOD has a VPC of -1.0 VPC, 107% DOD has a VPC of -1.4 VPC and 110% DOD has a VPC of -0.6 VPC. Increasing DOD causes a cell voltage reversal.
In any case, 2.0 VPC is really low to us.
In any case I’ll just jump to the results. For this study a cell has failed is defined as when the cell cannot be charged or discharged.
Discharging to 100 DOD results in cells that can cycle thousands of times. 105% DOD results in 49 cycles until failure, 110% DOD results in 7 cycles until failure, 115% DOD results in 3 cycles until failure and 120% DOD results in 1 cycle until failure.
It should be noted that the 49 cycles until failure for 105% DOD means 49 cycles to 105%. Each cycle below 100% DOD results in some loss of capacity and that it appears that should you only over-discharge once then continue using your cells but not go below 100% DOD then you will get many, perhaps thousands of cycles at a reduced capacity.
Discharge below 100% DOD does result in reduced cell capacity and reduces cell cycle count (permanent damage) but does not result in catastrophic cell failure until you reach 120% DOD. Of course reducing the cycle count from thousands to tens is very significant.
The second paper looked into the cause of the cells failure. The anode of the cell is composed of graphite which is deposited on a copper foil current collector. Under conditions of over-discharge the atoms of the copper foil were oxidized to Cu+ and Cu++ ions which then diffused to the cathode side where they were reduced to metallic copper (Cu++ to Cu+ to Cu). This metallic copper formed dendrites that reduced the active area of the cathode (capacity loss) and continued to grow with both the level of over-discharge and number of time over-discharged until they penetrated the separator and created a cathode to anode micro short. These dendrites do not grow in normal DOD ranges (0% DOD to 100% DOD). It should be noted that the cells have very little temperature change in normal discharge ranges but experience significant increases in cell temperature when over-discharged. This is due to the micro shorts.

What is the bottom line for LiFePO4 house banks?
Over-discharge (below 2.0 VPC) results in permanent damage to LiFePO4 cells. The deeper the over-discharge the more damage with a catastrophic failure happening at 120% DOD.
2.0 VPC is way below the point in which most if not all BMS signal alarms and disconnect the LiFePO4 bank. Clean Power Auto cell boards request disconnect at 2.5 VPC (at 0 degree C), 2.75 VPC (at 25 degree C) and 2.9 VOC (at 40 degree C). The BMS has its LVC at 11.6 volts (2.9 VPC in a 4 cell configuration).
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Old 17-09-2018, 19:38   #2
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Re: Over-discharge of LiFePO4 cells

Problem is, both the 100% starting point, and the AH (mAh) ratings are completely arbitrary for each vendor.

So the specific results will vary not just by model / brand but by production date.

I've had real-life variation from under 100% to over 115% of rated capacity for just CALBs, same model, all brand new.

Anyway, from a practical POV there is no reason to **ever** allow your bank to go below say 11.9V in daily cycling.

This is why bottom balancing makes me nervous.
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Old 17-09-2018, 20:06   #3
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Re: Over-discharge of LiFePO4 cells

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Originally Posted by john61ct View Post
Problem is, both the 100% starting point, and the AH (mAh) ratings are completely arbitrary for each vendor.

So the specific results will vary not just by model / brand but by production date.

I've had real-life variation from under 100% to over 115% of rated capacity for just CALBs, same model, all brand new.

Anyway, from a practical POV there is no reason to **ever** allow your bank to go below say 11.9V in daily cycling.

This is why bottom balancing makes me nervous.
What you say is true as far as it goes. However, the paper does define both the definition of the 100% SOC point of the specific cell and its 100% DOD point. Which gives us the actual capacity of that specific cell (by those definitions).

Further it appears (other papers etc) that they selected those upper and lower limits to be near the points where damage will happen to the cells but firmly within the limits of no-damage operation.

Not a bad selection - and rooted in reality.

A point can be made that 100% full and 100% discharged can be arbitrarily selected but this is a scientific paper and you just do not get to do arbitrary. (with rare exception)

For a layman/operator/driver you could consider the battery of your Tesla to be full when charging shuts off and empty when the car shuts down (If that is what it does, never drove one).

But we are engineering LiFePO4 systems for our boats and I think that knowing the engineering definitions of 100% SOC and 100% DOD are important.

As for never letting you pack reach 11.9V (2.975 VPC), Sure why not. 11.9 is just as good as 12.0 or 11.8 or.... All in your level of risk adversion if you are doing the engineering or just a good number if you don't do engineering.

It is interesting to note that the LiFePO4 cells tested in the paper suffered no damage at 8 volts! (2.0 VPC). Don't go there! It is only a short step further to damage your cells.

But knowing that they can go to 8 volts may keep you from having a heart attack if some day you discover that your pack is sitting at 9 volts.
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Old 17-09-2018, 22:22   #4
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Re: Over-discharge of LiFePO4 cells

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Originally Posted by evm1024 View Post
What you say is true as far as it goes. However, the paper does define both the definition of the 100% SOC point of the specific cell and its 100% DOD point. Which gives us the actual capacity of that specific cell (by those definitions).
Tautology there.

They, or you, or the maker or anyone can define 0% and 100% wherever you like.

Same with choosing how you measure AH capacity.

How do you define "damage"? For me, losing thousands of cycles off the back end is certainly within coooeee distance, just because you may not see it for many years. . .



> For a layman/operator/driver you could consider the battery of your Tesla to be full when charging shuts off and empty when the car shuts down (If that is what it does, never drove one).

Which are determined by setpoints programmed by humans making arbitrary choices.

By arbitrary I do not mean "without reason", I just mean "could be higher or lower without significantly affecting capacity"

But my bigger point is the industry is not putting bank longevity top of its priority list.

All vendors and industry / academic researchers far as I've seen are following the mfg definitions, which lead to these relatively short lifespans compared to what is possible when avoiding the shoulders.

> knowing the engineering definitions of 100% SOC and 100% DOD are important

Yes knowing what to avoid.

> As for never letting you pack reach 11.9V (2.975 VPC), Sure why not. 11.9 is just as good as 12.0 or 11.8 or

Yes exactly, as I said arbitrary, perfect example. Decide for yourself.


> It is interesting to note that the LiFePO4 cells tested in the paper suffered no damage at 8 volts! (2.0 VPC).

According to their definition of "no damage". Compared to the same bank lasting 3x longer, I'd call it damaged.

> if some day you discover that your pack is sitting at 9 volts

Allowing them to **sit** there would be foolish.

When destructively testing like this, essential to get recharging **immediately** after hitting the bottom setpoint.

As when load-testing lead to the standard 10.5V bottom.
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Old 17-09-2018, 22:52   #5
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Re: Over-discharge of LiFePO4 cells

No tautology there, just science.

If we take a fuel tank and fill it up we can say it is full when the gauge reads 100%. We can say it is full when adding more fuel will cause a spill at STP. We could even say that it is full when adding more will cause fuel to spill out at the temp where the fuel is most dense.

Which is the right full? If we were in an argumentative mood we could argue the merits of each for hours.

The volume that fills this tank is not the same as the volume that fills that tank. Who is to say what is true.

And there we have it. The paper defines 100%SOC and 100% DOD in a measurable way that is predictable and repeatable by other researchers.

I'm not sure that you were saying that the authors were in the pockets of the manufactures. I doubt that is true.

As for defining damage, Hmmm. I think that you are doing your argument a disservice by claiming that operating the cells as they do and getting 2500 cycles to 80% capacity retention is damage and compared to operating them in such a way as to get (oh say) 10,000 cycles.

It is the difference between wear and tear vs ripping apart.

Can we agree that their use limits represent hard use?

From a scientific standpoint we could say that the point start of damage is the DOD where Cu dendrites start to form. SEM (scanning electron microscope) images show no dendrite formation at 2.0 VPC.

Also, in regards to any increase in cycle count (which I take on faith to exist) with less hard use - Can you point me to a few papers that characterize the increase in cycle count?

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Old 18-09-2018, 00:51   #6
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Re: Over-discharge of LiFePO4 cells

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Originally Posted by evm1024 View Post
The paper defines 100%SOC and 100% DOD in a measurable way that is predictable and repeatable by other researchers
As is any similarly objective definition at other voltage points or capacity measurements.

And no conspiracy, just they follow the makers' specs, which are not conducive to maximising longevity, whatever the reason may be.

I think everyone understands what we're saying, further arguing over semantics isn't productive.

For me, avoiding the shoulders is my goal in daily use, not so interested in the internal processes at a molecular level myself.

And no my whole point here is that no formal industry / academic research is conducted to document much less explain what Maine Sail and others using the gentler care specs are seeing.

But it does apparently largely eliminate the need for live balancing or even cell level monitoring, which complex gear has often caused problems more than solving them.

The original monster thread is of course the motherlode source for those reports, as well as MS' HowTo.
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Old 18-09-2018, 01:43   #7
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Re: Over-discharge of LiFePO4 cells

Hi John, this is absolutely true.

It is like buying an expensive potentially classic car.

Put it in your garage for 50 years and do not use it to the specs.
The longevity will pay off when re-selling later, no wear and tear - all parts original.

Same for toys. Just collect them in the original un-opened enclosure.
No wear and tear and a much higher resale price later.

But you have to admit, that some people buy their toys to play with them, and yes, they wear out during usage. Nothing wrong with it. If someone buys a battery, that has a life expectation of 5000 cycles when using to the specs (full charge to 3.65V, DOD 80%) and he is happy with the advertised 5000 cycles - nothing wrong with this use pattern. That the battery can survive 10000 cycles when charging only to 80% and discharging only to 50% simply makes it LESS usable than using her full potential to the specs. While some like collect cells end enjoy them for decades stored, other like to use them heavily end enjoy them to their full potential.

I know, you advocate no BMS, no balancing and no full charge. Keep the battery below 50% all the time for longevity. That is a collectors approach, not a user approach. So yes, if you are a LFP collector, keep the cell in their original box at 50% SOC all the time.

Anyway evm1024, thanks for the research posting. It is interesting, what wrecks the cells and why a BMS is necessary and a good idea to prevent this from happening automatically. It also show the "reserve capacity" evangelists on the FLA side, that in a life threatening emergency, the LFP will not let you down below the low voltage protection point and there is no real advantage of the FLA over LFP.
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Old 18-09-2018, 04:37   #8
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Re: Over-discharge of LiFePO4 cells

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Originally Posted by john61ct View Post

How do you define "damage"? For me, losing thousands of cycles off the back end is certainly within coooeee distance, just because you may not see it for many years. . .

Just curious.. what research is there that shows thousands of more cycles when using lower depth of discharge?
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Old 18-09-2018, 05:24   #9
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Re: Over-discharge of LiFePO4 cells

http://agronomy.emu.ee/wp-content/up...1_Papezova.pdf

Hope this is what you are looking for...

Damage from Overcharging is not considered (Charging 100% = 4V / cell)

Here is a generic diagram from GWL Power.
http://gwl-power.tumblr.com/post/140...y-cells-versus

Some more (abusive) research pushing the charge voltage to 4.2V:
http://www.uc.pt/en/efs/research/EES...irinhaEtAl.pdf
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Old 18-09-2018, 05:59   #10
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Re: Over-discharge of LiFePO4 cells

Here is a research for batteries and possible DIY BMS circuit design pattern (unfortunately in German only)
https://www.esa.informatik.tu-darmst...13_MA_Denk.pdf

Some Battery - research and comparison.
https://openprairie.sdstate.edu/cgi/...83356927502499

http://researchrepository.murdoch.ed...21/1/whole.pdf
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Old 18-09-2018, 06:36   #11
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Re: Over-discharge of LiFePO4 cells

Quote:
Originally Posted by CatNewBee View Post
http://agronomy.emu.ee/wp-content/up...1_Papezova.pdf

Hope this is what you are looking for...

Damage from Overcharging is not considered (Charging 100% = 4V / cell)
The Papazova paper is a good one, I have it on my hard drive and they used basically the same power supply for charging that I do in my own little lab.

This study was charge to 4.0VPC or 3.8VPC and stop charging immediately upon hitting the target voltage and the discharge load turns on. This is simply not how most existing LA charge equipment works and many tend to confuse;

Charge to 3.6VPC & stop
vs.
Charge to 3.6VPC and then hold it there for hours

IIRC there is a graph in that paper that shows the vertical charge knee and discharge knee, on the charge / discharge transition point.

My questions are always:

* Why push into the vertical when it's not needed for capacity?

* What benefit is there by pushing into that area where there is really no quantifiable extra capacity?

* What would the outcome on cycle life be if you did not push into that knee every cycle, even with a "stop charge" protocol?

If you find a paper that answers any of these questions please let me know.

Regarding the original post, and over discharge, I can only mention that the difference between a lab, and the pin point accuracy they have, and the real world is on the order of comparing apples to centipedes. The lower knee becomes so vertical below about 2.9VPC that the drop will occur really quickly and damage can be done, if the pack is not protected with LVC.

Also the A123 LiFePO4 nanophosphate cells (now owned by Lithium Werks) and the K2 cells are some of the best and most robust 18650 cells I have tested here. I simply can't seem to kill them but, building a DIY pack from them without the proper tooling is just not very feasible.

There really is no effective capacity to go after below about 3.0VPC. I have actually raised all my testing here to 3.0V as we are talking a minutia of capacity left and not worth risking even with test cells.
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Old 18-09-2018, 06:55   #12
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Re: Over-discharge of LiFePO4 cells

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Originally Posted by CatNewBee View Post
Put it in your garage for 50 years and do not use it to the specs.
Ridiculous straw man

I don't advocate going bareback, just point out that many do, and many more prefer to go without cell-level live balancing.

Presenting options to consider never advocated by vendors, fo obvious reasons.

I also never advocate taking the DoD / cycle lifetime into account, in fact specifically say not to, just above

use them as you like, some people want reserve capacity above normal daily use anyway.

> If someone buys a battery, that has a life expectation of 5000 cycles when using to the specs (full charge to 3.65V, DOD 80%) and he is happy with the advertised 5000 cycles - nothing wrong with this use pattern.

Completely agree, but again if you slavishly follow only vendor recs you would never know about the greater possible longevity. Again, presenting options to consider.

And avoiding the voltage shoulders is using daily to 80% even 90% DoD, **nothing** to do with not using them fully, least of all storage, the way you put that is specious and deceptive.

In this case yes more advocating, since there is **nothing practical to be gained** by shortening lifespan by pushing into the shoulders.

> no real advantage of the FLA over LFP

What, not even cost?

You think your pricey setup should be installed on the 31' boat that cost under $3000 all up has been fulltime liveaboard cruising over 3+ years?

Context is relevant.
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Old 18-09-2018, 07:09   #13
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Re: Over-discharge of LiFePO4 cells

Quote:
Originally Posted by Maine Sail View Post
The Papazova paper is a good one, I have it on my hard drive and they used basically the same power supply for charging that I do in my own little lab.

This study was charge to 4.0VPC or 3.8VPC and stop charging immediately upon hitting the target voltage and the discharge load turns on. This is simply not how most existing LA charge equipment works and many tend to confuse;

Charge to 3.6VPC & stop
vs.
Charge to 3.6VPC and then hold it there for hours

IIRC there is a graph in that paper that shows the vertical charge knee and discharge knee, on the charge / discharge transition point.

My questions are always:

* Why push into the vertical when it's not needed for capacity?

* What benefit is there by pushing into that area where there is really no quantifiable extra capacity?

* What would the outcome on cycle life be if you did not push into that knee every cycle, even with a "stop charge" protocol?

If you find a paper that answers any of these questions please let me know.

Regarding the original post, and over discharge, I can only mention that the difference between a lab, and the pin point accuracy they have, and the real world is on the order of comparing apples to centipedes. The lower knee becomes so vertical below about 2.9VPC that the drop will occur really quickly and damage can be done, if the pack is not protected with LVC.

Also the A123 LiFePO4 nanophosphate cells (now owned by Lithium Werks) and the K2 cells are some of the best and most robust 18650 cells I have tested here. I simply can't seem to kill them but, building a DIY pack from them without the proper tooling is just not very feasible.

There really is no effective capacity to go after below about 3.0VPC. I have actually raised all my testing here to 3.0V as we are talking a minutia of capacity left and not worth risking even with test cells.
I have just such a system .

I charge to 14.2 volts usually with no current tail off . When I hit this voltage I simply turn off my generator .

When my system hits 12.8 volts with no load I start charging again . I have found this method keeps me within the knees and I have yet to experience any cell drift. Three years in of full time use. My system generates on average 200 amps when charging and the generator now has 200hrs on it. I have 600 amp hrs of Sinopoly cells on board .

Regards John.
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Old 18-09-2018, 07:12   #14
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Re: Over-discharge of LiFePO4 cells

Well, you are right, for this size a 100Ah LFP will do the trick, they very likely wont have a large FLA bank either - I guess in the range of 2 120Ah Equipmet GEL or even open 200Ah FLA. Its not really expensive if you have to replace the FLA on a regular base.

100Ah cells sell around 100..160€, so the battery is 600€, 2 quality AGM cost between 300€ and 500€. The "high-end" 75Ah Optima spiral cell sells for 245€. The cost advantage for FLA is not that big even upfront if you search for batteries with high cycle expectations (in FLA that means 600).
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Old 18-09-2018, 07:48   #15
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Re: Over-discharge of LiFePO4 cells

In your market. Here Deka FLA - true deep cycling, last much longer than the priciest Optima - are ~$1 / AH @12V.

Also little to do with size in itself,

I know plenty of very cheap smaller liveaboards that blast stereos, run freezer + fridge getting opened frequently, watch lots of movies, play big gaming rigs many hours per day,

can use 400+AH per cycle, and are perfectly happy with their FLA banks despite the extra weight.

Just a matter of preference and budget, when the market value of their whole boat + contents is a small fraction of what you spent just on electrickery, proposing an LFP system of any sort would just elicit laughter.

They also have zero interest in home style kitchen appliances, washing machines, heat water on the Coleman, what you would maybe call camping.
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