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Another Top Balance Screwup

Given the low internal resistant of LFP, and a 100AH plus cell size, you would need 500-1000A or more to get a cell at 80% to 3.65V immediately. One can estimate the exact number of Amps required by using the internal and bus bar resistance.

I will see if I can find my 80s textbook about estimating currents in big battery systems. Its an interesting read. Originally written for big telecom battery backup banks (whole rooms of lead), the current levels scale surprisingly well due to the low IR of LFP.
Absolutely! You'd brown out the neighborhood for a few seconds top balancing 4 280ah cells!! Youd also be exceeding the 0.2C recommended charge rate LOL
 
So, lets try to drill down a little further into this to make sure everyone is talking about the same thing and ensure the correct information is being spread.

I agree 100% with what I have quoted you as saying above as long as these two conditions are met:
  1. The power source you are using is set to provide 3.650v
  2. The power source is capable of providing enough current to actually maintain 3.650 volts
Those conditions are not ever met by anyone using a 10amp power supply.


I agree with you, but if you are charging LiFePO4 to 3.65V at less than 0.05C (14A for a 280Ah cell), you are creating more problems than just a prolonged lithiation cycle for the highest SOC cell.
 
I agree with you, but if you are charging LiFePO4 to 3.65V at less than 0.05C (14A for a 280Ah cell), you are creating more problems than just a prolonged lithiation cycle for the highest SOC cell.

Do you have any sources on this? I mean we are talking 10-20 hours above 3.5V max. That is maybe equivalent to 60 days with a 10 minutes absorb per day. Lots of batteries experience something like that with minimal degradation.

What exact problems are we creating? Hand waving about "problems" is has no usefulness in this discussion.

The cells in this thread spent a whole night at 4.5V+ and still delivered pretty much nominal capacity. (They just completed a second discharge cycle with the same results). I think LFP are a lot more durable than some folks believe.
 
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I agree with you, but if you are charging LiFePO4 to 3.65V at less than 0.05C (14A for a 280Ah cell), you are creating more problems than just a prolonged lithiation cycle for the highest SOC cell.
I cant argue that because quite frankly I simply dont have enough education/understanding to discuss intelligently. I would sincerely appreciate it if you were willing to link up some support for your argument so I could dive in and do some learning.
 
Good leads are a must, and pretty cheap to make yourself. These cheaper power supplies do have some internal voltage drop at their rated current. So the connectors on the front will be a fair bit lower voltage than the indicated voltage on the display.
 
The voltage should never be set over 3.65 on LFP. That is just a given. If your supply current starts falling off, it will just take a little longer. Turning up the voltage is not the correct fix. If you want more current when the voltage is getting close, then you need very low resistance wire and maybe a power supply with remote voltage sensing. Charging at 20 amps per cell is fine, but the voltage limit should never be raised above 3.65 volts. While it does seem true that LFP cells are quite durable and much safer than cobalt chemistries, they will still degrade very quickly when the safe voltage range is exceeded. Basically, you remove capacity and cycle life. If it removed 25% of the cycles, it just made the effective cost 25% more. So now your abused cell is 250 amp hour instead of 280, and gets 2,500 cycles instead of 4,000 cycles. Does not sound too bad, but it just made your cost per cycled kwh much worse. Treat your batteries nice, they are a large part of the system cost.
 
I cant argue that because quite frankly I simply dont have enough education/understanding to discuss intelligently. I would sincerely appreciate it if you were willing to link up some support for your argument so I could dive in and do some learning.

There is no single source i can point to that i’m aware of. Do some research into the lithiation process, as well as the intercalation process that occurs in the SEI.

My experience comes simply from over a decade of observation of LiFePO4 cells in operation, the involvement of a battery manufacturer and several BMS manufacturers, and the system parameters that have resulted in early failures.

I’m not too fussed if people don’t mind shaving lifespan off their cells, it’s not my money.

This forum has a fixation on parallel top balancing, without too much thought on alternative methods and pros/cons.

@Luthj, i honestly can’t tell you the extent of the cell degradation for each minute a cell is held high in it’s knee - the results won’t be seen for five years or so.

I have posted in this thread as many people aren’t aware there are other ways to balance your pack that don’t involve holding a cell high in its voltage knee
 

That white paper cites no sources, and lists no experimental data. Nor does it qualify any of the details we seek here. Worst of all, it uses bolding for emphasis... They are also selling equipment (mild conflict of interest).


Overcharge is defined as charging the cell above the maximum voltage give by the manufacture. (4.0V for Winston cells, 3.9 for CALB cells, 3.7 to 3.9 for many other brands of LiFePO4 cells).
We are talking about 3.65V, so again this advice isn't applicable.

-----------------------------------------------

Let me lay out the case from a logical/debate standpoint. toms has a position that they support. However they have no specific data to support it, and when asked they make hand waving about experience or common knowledge. Or better yet, FUD about "problems". That's a big red flag when evaluating if someone is reliable. Whenever a specific fact of their claim is challenged, the goalposts are moved farther down the field, and new ad hoc hypothesis are created. This is typically what I would see with a weakly supported position, or someone who just hasn't thought about it much. This doesn't mean the position itself is untrue, but it does strongly support the null/default position.

As a skeptic I try to hold the default position until demonstrated otherwise. In this case the default position is that there has not been sufficient evidence to demonstrate that a 10 hour period at 3.65V will cause significant damage to a LFP cell. And in fact there may be decent evidence to the contrary. Note the 5V cells in this very thread.

The way I see it, we have knee jerk reactions to the early 4.2V charging regimes MFGs listed at the beginning of the big LFP cells years back. This combined with folks floating at 14V for months straight has resulted in a group who religiously preaches the benefits of never exceeding 13.8V. Despite scouring literature, I have found no significant data indicating there is any major gains from such extreme measures. Everything I have seen shows that degradation at high SOC is quite gradual. For example one test with cells kept at 3.65V for a year resulted in 5% capacity loss above control kept at 90% SOC.

I liken this argument to the faith based belief that one must change their automotive engine oil every 3,000 miles. There is lots of hand waving about unspecific "problems" to be avoided. Generic claims to special knowledge, goalpost moving, and references to outdated info (such as detergent free oils from 40 years ago). They just KNOW that a 10,000 mile change interval is too long, and thus a conspiracy.
 
Thank you Sir for posting something up that I could sink my teeth into.

This is what I got out of that document:
  • The are discussing routine charging of a battery with series cells.
  • They are not discussing one time or occasional/infrequent parallel connection and top balancing
  • Their proposed out of balance scenarios are wildly out of balance
  • They are trying promote themselves and sell products.
  • They state in the document as "their 6 tips": "Decrease the full charge voltage level for ordinary charging of the cells. You can make many cycles with the decreased full charge level before making the full charge. For most cells charging to 3.65V per cell is OK. Some user charge only to 3.60 per cell." And: "The cells should be balanced to full one by one before the assembly of the battery pack. This way the will have the same capacity and they will need only very little balancing"
It is IMO a somewhat relevant document and helps with understanding. It does make relevant points to the discussion. The main thing I got from it is that it is probably a good idea to have as high capacity power source as you can get to reduce the time it takes to creep up through the "knee". They suggest 1 at a time but that seems as though it has the possibility of introducing different errors into the balancing equation.

The paper does fail to head on address the actual topic that we are discussing here though.
 
There is no single source i can point to that i’m aware of. Do some research into the lithiation process, as well as the intercalation process that occurs in the SEI.
Then feel free to post several to help support your position. I really dont mind sifting through information.

My experience comes simply from over a decade of observation of LiFePO4 cells in operation, the involvement of a battery manufacturer and several BMS manufacturers, and the system parameters that have resulted in early failures.
These battery and BMS manufacturers had to have published something. Who were they?

I’m not too fussed if people don’t mind shaving lifespan off their cells, it’s not my money.
You shouldnt be, especially when it is such a marginal reduction in lifespan. I fully acknowledge that there is the potential of reduced life span by holding cell voltages at the 3.650 suggested voltage for an extended period of time, but what is the penalty?

I have posted in this thread as many people aren’t aware there are other ways to balance your pack that don’t involve holding a cell high in its voltage knee

Someone please correct me if I am wrong here but it seems as though there is a conceptual misunderstanding here. The view above seems to be similar to this: if you have 4 containers that need to be filled with water and each has a hose entering the top of container and each hose is supplied from a header and water enters the header and is equally distributed to each container. If each container is at a different level when they start, the one that was the fullest will overflow before the one that started with the lowest level is full. Pretty basic and simple

My view is the same as above however instead of a hose into the top of the container, the hose is connected at the bottom. The head pressure in the individual containers reduces flow into the container with the highest level and each container slowly equalizes out and then they proceed together to full. Pretty simple too but with a huge functional difference.

I never recommend this method of top balancing - it is so much safer to always use cell level voltage protection.

The vast majority of cell damage i have seen is due to top balancing errors, and it is so unnecessary.

If your cells are very unbalanced when you get them, charge them one at a time to 3.65V 0.05C. (i use a modified EVworks 3.65V balancer board as a backup disconnect for my charger).
What is the penalty you end up paying by charging at a 0.05C rate to 3.65V and then leaving the cell sitting at that SOC while you do the next 3 or 7 or 15 or 31 cells and then connecting all in series? The first one has sat at a higher SOC for a lot longer period of time than the last one. Is there no penalty for that?

Im sorry my friend there are just too many holes and unverifiable suggestions for me to try to plug. I acknowledge that what may be happening here is the difference between common sense and good sense. I fully acknowledge I lean heavily towards the common sense spectrum on this subject and need more education to be in the good sense camp. That is why Im arguing this. This is education for me but you seem to be resistant to offering it up. You just seem to want me to accept you word as gospel and that will never happen in 2021 when it takes 5 minutes to offer up support for your argument.
 
I made a similar mistake top balancing. I hooked up my 4 280Ah cells in series to charge my pack with 60A. I do have a new Progressive Dynamics lifepo4 12V 60A charger, and I walked away for 90 minutes or so, when my cells were charging at 3.4V. I returned to find the charger had terminated charging, and one of my cells was still at 4.5V. I discharged the pack and the overcharged cell dropped down to the same level as the other 3.

I'm not sure yet if my capacity is compromised. I am top balancing the cells in parallel now, but it's really slow as my bench supply can only source 2A. Cells are at 3.35V now, could be a few days more. ?
 
One of the reasons I would recommend using a relatively inexpensive hobby type charger to charge cells, they will automatically shut down when cells are fully charged. What was the voltage on the power supply?
 
Several on this thread have been asking for more information about how over voltage can damage cells. Here are two web site links that talk about cell over voltage hurting the capacity and cycle life of the batteries, but they don't go into a lot of detail, other than to say, don't do it.


I also attached 2 PDF papers that go into great detail about the chemistry of batteries, and more on the science of what is actually happening in the cell. I am still trying to read it all, as it can be quite tedious. On the second paper "TVJOB..." around page 45 they start talking about the aging process and cell damage in detail. This is still in Chapter one, the introduction. The whole document is 272 pages. It will take me a few days to truly understand all the data in this paper. If you are a data geek like me, it is a good read, but wow, just wow! I never knew how much I didn't know about lithium batteries.

In short, the higher the cell voltage, the faster things degrade. Even stored at no current, having a high voltage on the cell is causing the electrolyte to degrade. Lithium metal will start plating onto the negative electrode (cathode). In extreme cases, this can lead to a short of metallic lithium across the anode and cathode. Even in small cases, the plated out lithium is lithium that can never actually hold charge again. This obviously reduces the cell capacity. Even if the cell recovers and keeps working, the loss of capacity, and reduction of usable cycles will never come back.

Chapter 2 starts on page 57 (of the pdf) and is titled "Capacity Fade".

On page 64 (page 66 in the pdf) there is a good comparison of graphs between the charge/discharge curves of LFP and NMC cells. The curves are completely different, and I can see why balancing LFP cells in the middle of their charge curve is basically impossible. These examples are at low charge and discharge rates, similar to what we use in solar storage. At a full 1C rate, they do swing a bit more.

The NMC cells (like my LG Chem ones) are only flat between 20% and 50% SoC, where the voltage only changes from 3.8 to 3.85 volts while charging. From 5% to 20% the voltage goes from 3.4 volts up to 3.8 volts. And from 50% up to 100% the voltage goes from 3.85 to 4.25 volts.

During charge, the LFP cells are nearly flat from 10% up to 85% SoC where the voltage only changed 125 mv from 3.25 to 3.375. Discharging the battery from 3.25 down to 2.0 volts per cell is only getting 10% of the capacity. And at the top, charging up from 3.375 to 3.65 volts, is only pushing 15% more in. And once at 3.65 volts, removing the charge current and letting the cell rest, the voltage will settle back down to about 3.35 volts when any load is applied. So under discharge, the cells start at 3.35 volts. After you use 85% of the capacity, the cell is still at 3.15 volts. That is incredibly flat. At a C/5 rate, it is a little easier to see the cut off point as it does start to roll down and hits 2.7 volts at a little less than 20% remaining.

If (when) I replace my batteries with an LFP pack, I will certainly want a good coulomb counting battery state of charge meter to tell the system how much is left. My BMS will do it, but it can't send that information to the Schneider inverter/charger. For now, I am only using battery voltage control, which works just fine on the NMC cells, but it is clearly not near ass accurate for use on LFP cells.

I know I went a bit off the original topic, but I wanted to show that in normal use, there just is no reason to really push the cell voltages up there. The initial top balance should just get all of the cells to hit that top knee area. You don't want to hold the voltage up there too long, but a full day sitting at 3.65 is not too bad. But I think holding them all at just 3.5 for longer would top balance just as well, and not put as much stress on the internal structure of the cells.
 

Attachments

  • LFP-battery_damage_study.pdf
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  • TVJOB1de1BatteryDiscussion.pdf
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You should think yourself lucky i took the time to offer you any information!

Hubris, look it up. (How dare we ask for verification of a factual statement! :p)

We can do leg work, just list the names of the companies, paper authors, books, etc. If you have done experiments yourself feel free to summarize, we don't need you to write a book. If you don't have any sources, just opinions, that's fine too, but you might consider if the positions you hold are based in fact or not.

We are genuinely not trying to trap you, or perform some logical wizardry. We all offer are time for free, and because we enjoy it. I could spend time interacting with others to whom my knowledge is far superior, but instead I try to interact with individuals who can challenge me and my ideas. Robust discussion in beneficial to all. If that isn't the type of interaction you want, you are free to post or not post as you desire. Posting in a public forum means your ideas may be challenged. That is the essence of truth finding. Take it or leave it.

I do appreciate your contributions. However I won't spend any time assuaging your ego. If your position can't stand on its own merit, its not my responsibility.
 
Fair enough, i deserved that :)

My personality doesn’t alter the chemistry of LiFePO4 though, and won’t give you back the cell life you lose from holding a cell high in its upper knee voltage.

That is completely different from charging to a high knee voltage at a >0.05C current then removing the charger.

Others have linked supporting documents from various sources (and yes you should also be thankful for their advice). I think you would gain more from researching the chemistry involved.

Also ask yourself how the manufacturer charges cells for a system that requires matching cells.
 
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