Passive Top Balancing ? How/Why does it work ?

[2Big2B]

The answer is no.

Paralleling LFP cells at rest to "balance" them

Hypothesis: Paralleling cells at rest does almost nothing beneficial. While voltages may be equalized, their states of charge remain dramatically far apart. I decided to do an experiment. Take two CALB 40Ah LFP cells. Fully charge to 3.65V Fully discharge to 2.5V Charge ONE to 3.65V Let them...

diysolarforum.com

BMS passive balancing applies a resistor to a cell to burn off excess capacity - not as you describe.

BMS active balancing involves circuitry that actually transfers charge from high to low.

Lastly, balancing at any SoC less than 100% will result in frustration.

Non-Sequitur. That is not what I am asking.

 

[Bob B]

Again, Non-Sequitur.

The only "connection" shared by the cells is the fact that their common poles are glued together. I think the problem is conceptual. I think what you are saying is that by doing that the entire bank becomes one big communal cell that somehow shares its chemistry. If that is what you are telling me, then that is some dimension worth exploring that I must be unaware of. Do tell...

OK .... I'm starting to think that you are just having fun creating a problem that doesn't exist.
If you have actually read all the explanations that have been given .... and are have studied and used electronics ... not understanding what has been explained is not possible.

 

[noenegdod]

Thank you. I appreciate your effort to explain it to me.

However, everyone seems to be missing the point. I am being treated like a small child asking how immaculate conception works. You can carefully and politely explain how human reproduction works until you are blue in the face but I will continue to probe with my simple question about how reproduction can take place without intercourse. It becomes a circular argument because that bible story isn't about REPRODUCTION - it is about fervent religious folklore. It is about magic. It is understood to be SUPERNATURAL. In that fable Christ was not a reproduction, but a supernatural new creation. Excuse me for this unfortunate analogy. I don't mean to drag religion into this or offend anyone. That isn't my intent. I am just getting frustrated by some apparent miscommunication.

The old saying is "You can't argue with ignorance".

I know all about the facts of life and don't need it explained to me. That isn't my point. I also know all about how electricity works, or would be dead many times over - and don't need that explained to me either.

Somehow my simple question about how something that seems to be supernatural has turned into a some weird dogmatic argument.

Last night I was chastised and "banned for two weeks" because I was not "interested in learning" but in arguing and being annoying - sort of like a small child who responds to each explanation with "but why? ... but why?? ... but why ???" I thought the spirit of this forum was to learn from each other and nurture the hobby.

I was reprimanded with an announced two week suspended membership by "Snoobler" without warning or recourse- immediately gagged for persisting to ask for clarification to my simple question. Nobody seems to understand what I am trying to say, regardless of how I try to explain what my question is. Look at how I write. I am more literate and grammatically diligent than most, am I not?

Perhaps it is because the question to even be a question is so ridiculous that it just goes over everyones' head. It is exposing some apparent common myth that seemingly is accepted as reality.

The myth is that by some kind of spontaneous regeneration balancing between cells can occur without a closed circuit. That myth is being propagated by the innocent omission of a detail when there is an instruction about top balancing. You get to see the setup, but not the actual process, and I think that detail is that there must be some kind of closed circuit, beit a BMS connected using passive balancing or else a power supply. Some of seem to believe that the stand alone construction of a parallel configuration will result in balancing - without a closed circuit of any kind between positive and negative. THAT IS ABSURD. I think that there seems to be some mysticism about LiFeP04 battery science that is driving the myth. Else, I am an idiot. Yet I have managed to live to be 68 years, have a four year Bachelor of Science degree (as well as British A Levels in the social sciences), am a licenced radio technician, a retired computer engineer, am widely regarded as intellectually gifted and very much a self made man. How can I have survived and have been successful all these years and have some black hole in something as basic and simple as this.

I am not alone in questioning this. "Boondock Saint" seemed to agree with me last night, and at that point it seems to have become personal to "Snoobler", the acting moderator who decided to go all fascist about it.

I give up.

You all need to work it out between yourselves. I am not welcome here apparently. Believe whatever mysticism of the immaculate ionic transfiguration you want to, if that is what this really is, if it suits you. Apparently my out of the box curiosity is not appreciated here.

I believe in the principal of "minimum necessary force". I believe this is the minimum required:

Your self made widely regarded intellectually gifted self is being a little inconsiderate ATM. Slow the hell down and just contemplate what people are saying to you. It really isnt that hard, especially for someone that has the resume you claim to, unless of course cognitive decline is becoming an issue.

As people have tried to explain to you, 2 cells in parallel is a complete circuit. As in the jumpstart example above but with a little more negative reinforcement, try taking one lifepo4 cell and draining it down to 2.5V, take an other and charge it up to 3.65v. Connect the negatives together with a bus bar and then holding onto a piece of stainless steel (and keeping you hand on it), connect the positives. Your education will soon be complete

Edit:
Edited by moderator because it was a bit offensive/disrespectful and then again by noenegdod because moderator misspelled one of the words they substituted for my offensive/disrespectful word.

 

[2Big2B]

Let's start fresh. The below image shows the equivalent circuit model of a Lithium Ion battery:
View attachment 54362

Source: https://www.researchgate.net/figure...del-of-the-lithium-ion-battery_fig1_276035953

I quote from the publication: "The equivalent circuit model is shown and consists of three parts: (1) the equivalent ohmic internal resistor R s ; (2) the resistor-capacitor (RC) parallel network C p // R p (where R p is the equivalent polarization resistance and C p is the equivalent polarization capacitance), which is used to simulate transient responses of the battery during charging-discharging transients; and (3) the OCV v oc (h(t)), which is a nonlinear function of SoC h(t). The equivalent circuit model considers the current as the model control input and the terminal voltage as the measured output."

Let's put two of these cells, at different state of charge (and let's assume a noticeably different terminal voltage), in parallel with one another. This forms closed circuit (agreed?) whereby an imbalance of voc means that one cell will have a Vb (terminal voltage) higher than the other will lead to a current to flow from the higher potential to the lower until the two terminal voltages equalize - essentially charge from the cell with the higher potential moves to the cell with the lower potential, increasing voc in the cell that was at lowest, and decreasing voc in the cell that was at highest potential.

The issue with LiFePO4 is that while theoretically there is a difference in voc at low and high state of charge, in practice this is not the case across the majority of the state of charge except in the knees - meaning either at low state of charge, or high state of charge. See diagram below:

View attachment 54363

While there is a small difference in voltage between say 40% and 60% state of charge, the delta between these is so small that if we were to take a cell at 40% SoC and 60% SoC and we were to put these in parallel, the resulting current flowing from the cell at higher potential to the one at lower potential is a) very small and b) because of that any imbalance in SoC (20% in this case) would take a very long time (impractically so) to balance out.

However, the same can not be said for cells at the 'upper knee' in the chart. where large deltas in voltage actually occur at very low deltas in SoC. This is where you balance with a BMS, this is where you top balance with a power supply in the mix.

In summary, putting LiFePO4 cells in parallel balances them, in some cases, but not at random states of charge somewhere on the 'flat part' of the curve. This is not the same for all chemistries: LTO for example has a curve that is not flat like this and there paralleling cells without additional power supply has more effect. Same is true for lead acid. For LiFePO4, you balance in the knees where you have a clear voltage indicating state of charge, and where relatively large voltage deltas correspond to small percentages of state of charge.

"Let's put two of these cells, at different state of charge (and let's assume a noticeably different terminal voltage), in parallel with one another. This forms closed circuit (agreed?)"

Huh? This is exactly where I am stuck. The implication is that by slapping a pair busbars across their respective same polarity poles it creates a circuit. Really?

Everything else you so exhaustively explain is just more abstraction. You seem to be telling me that by attaching positive to positive on one rail, and negative to negative on the other rail, it creates some kind of electrical membrane that allows for a circuit between the cells resulting in active balancing at some internal level. Really?

 

[upnorthandpersonal]

Huh? This is exactly where I am stuck. The implication is that by slapping a pair busbars across their respective same polarity poles it creates a circuit. Really?

Of course. Take a 9V battery and a 1.5V battery, and connect the + of both and - of both together. Check what happens. It's not that just because it's another cell that there is no circuit.

electrical membrane

Not a membrane. It's just a circuit. A cell is just, simplified, a voltage source with an internal series resistor. Putting two of those, with different internal resistors and voltages in parallel will have current flowing.

 

[Bob B]

Of course. Take a 9V battery and a 1.5V battery, and connect the + of both and - of both together. Check what happens. It's not that just because it's another cell that there is no circuit.

Not a membrane. It's just a circuit. A cell is just, simplified, a voltage source with an internal series resistor.

He's just jerking us around .... this can't possible be a serious discussion.

 

[2Big2B]

I believe in the principal of "minimum necessary force". I believe this is the minimum required:

Your self made widely regarded intellectually gifted a$$ is being a little stupid ATM. Slow the hell down and just contemplate what people are saying to you. It really isnt that hard, especially for someone that has the resume you claim to, unless of course cognitive decline is becoming an issue.

As people have tried to explain to you, 2 cells in parallel is a complete circuit. As in the jumpstart example above but with a little more negative reinforcement, try taking one lifepo4 cell and draining it down to 2.5V, take an other and charge it up to 3.65v. Connect the negatives together with a bus bar and then holding onto a piece of stainless steel (and keeping you hand on it), connect the positives. Your education will soon be complete

Hmmm... Ok, I'll bite. That certainly is not intuitive, but its an experiment that might be worthwhile.

Maybe I will learn something. That's what I am here for.

Thank you.

 

[Bob B]

Hmmm... Ok, I'll bite. That certainly is not intuitive, but its an experiment that might be worthwhile.

Maybe I will learn something. That's what I am here for.

Thank you.

Better wear gloves and safety glasses, because it will be a very rude awakening.

 

[noenegdod]

Better wear gloves and safety glasses, because it will be a very rude awakening.

Wear safety glasses for sure. Unfortunately wearing gloves reduces the negative reinforcement

 

[2Big2B]

Interesting. I guess I never really thought about it that way.

So what I think you are saying is that in my way of thinking, to the left are two cells, like containers of fluid, self contained.
A pipe, eg "rail" connects the (+) poles together on top and (-) poles at the bottom. In this case, nothing flows.

In your way of thinking, in the illustration on the right, that pipe, eg "rail" is actually a gate that allows a current to flow between them. If these were containers of water, gravity would create the pressure to equalize. If electronic cells, potential difference creates the pressure to equalize.

Really? This is the crux of my confusion. I am thinking that there is no circuit, hence no flow because of a barrier that doesn't actually exist. Eureka! (?)

I need to prove that to myself. I might have just learned something new about batteries! (As Homer Simpson would say, "Doh ...!")

Put another way, if I were to take two car batteries, one fully charged and the other flat and jumpered them together for a while and then later removed the jumper cables I would end up with two half flat batteries? Hmmm... I never really thought about that. Maybe you are right.

 

[Substrate]

@2Big2B - maybe we can simplify it. I also performed a test you may be interested in.

If you get your nearly half-discharged cells freshly shipped out of the box, and put them together in parallel, you'll be "mid-balanced" after the small amount of current flows between each cell. But as we know, in the flat part of the curve, this means nothing. From a voltage standpoint, one cell could be at 80% capacity, and another at 30% despite the voltages appearing to be the same when you take them apart and measure again! (unless you have lab equipment)

Practically - mid balanced by paralleling means nearly nothing useful in the real world.

What has to happen is to get each cell charged up well into the upper knee individually, and THEN place them in parallel. Now a more realistic, albeit not exact, balance is achieved. The miniscule differences in differential voltage between the cells creates miniscule amounts of charge current flowing to each other. This only works however because when we are well into the high side of the charge knee, there is very little capacity, so tiny amounts of current can seemingly balance. We are still ballpark though, but certainly better off than mid-balance!

However, as snoobler pointed out, voltage is not an indicator of usable capacity!

To prove that to myself, I created a 4S pack made out of three 40ah cells, and the last cell was only 20ah.

I put them all into the upper knee, and connected in parallel. Yup - a certain amount of balancing took place, but I wasn't fooled. I left it like that for a month.

Lo and behold, I had minimal balance issues when cycling - not great - like no more than 0.075v delta between the cells ON CHARGE when they got into the upper knee. At-rest voltages are not useful for balance determinations.

But of course, as a goof - the 4S test bank was of course limited to that 20ah cell. Just to prove that balance does not mean health!

So I don't know if this helps or hurts.

 

[Substrate]

Ah, I see your diagram. So imagine you start out with one cell at 3.60050v. And the other at 3.6010v.

The differential voltage is so small, the only very small amounts of current will flow between them. Might take a month of sitting in parallel to do anything useful.

 

[2Big2B]

I guess that is how this works - although it is rather subtle. Lead Acid batteries of course have a different profile, so the effect would be more dramatic. My thinking all along has been that each cell is a closed system. However, depending upon SoC, the connections at each common terminal become gateways.

Wow. Really? I thought about that.

 

[Substrate]

OH, if only LFP would recombine at CV! Would make our lives a lot simpler.

I think the point I'm trying to make is to not drive yourself absolutely crazy about balance (yes, one needs to achieve some semblence of balance using proper methods) based solely on voltage.

Should you get close to each other? Yes, but as my goof-pack of a 4S with three cells rated at 40ah and one cell rated at 20ah, if I spent weeks trying to make all the cell voltages under charge exactly equal to each other with 6 decimal points of precision, and succeed - I'm still left with a bank that's a total joke! And I spent far too much time in the upper knee balancing trying to get my bms voltmeters to line up like soldiers and plating with degradation.

I mean, one can play with all the differential voltage equations as much as they want. But real-world manufacturing tolerances (like bms bleeders), cell characteristics, and my own wiring infrastructure, can all contribute to lining up the voltages exactly, but when measured in a lab, they have UNbalanced the system!

So just to save some gray hair, we can play games on paper, but unless other real world factors are taken into account, one may spend too much time trying to achieve the impossible. Strive for it? Certainly. Just watch the clock and make a judgement call on how exacting one needs to be in regards to balance, since it may not make a practical difference in your application.

 

[Supervstech]

Interesting. I guess I never really thought about it that way.

So what I think you are saying is that in my way of thinking, to the left are two cells, like containers of fluid, self contained.
A pipe, eg "rail" connects the (+) poles together on top and (-) poles at the bottom. In this case, nothing flows.

In your way of thinking, in the illustration on the right, that pipe, eg "rail" is actually a gate that allows a current to flow between them. If these were containers of water, gravity would create the pressure to equalize. If electronic cells, potential difference creates the pressure to equalize.

Really? This is the crux of my confusion. I am thinking that there is no circuit, hence no flow because of a barrier that doesn't actually exist. Eureka! (?)

I need to prove that to myself. I might have just learned something new about batteries! (As Homer Simpson would say, "Doh ...!")

Put another way, if I were to take two car batteries, one fully charged and the other flat and jumpered them together for a while and then later removed the jumper cables I would end up with two half flat batteries? Hmmm... I never really thought about that. Maybe you are right.

View attachment 54371

Think about it like this…

A battery is a well of electrons that flow out the terminals when connected to a drain, and flow in when connected to a source. Batteries fully charged will send out the charge to any drain… even another battery.

And a battery has internal resistance, so some types of batteries flow the electrons more freely. Car batteries sip current in, but present the current rapidly.

Lithium ion cells have low internal resistance, so they are closer to a dead short when low state of charge, if connected to a full cell.

 

[noenegdod]

Interesting. I guess I never really thought about it that way.

So what I think you are saying is that in my way of thinking, to the left are two cells, like containers of fluid, self contained.
A pipe, eg "rail" connects the (+) poles together on top and (-) poles at the bottom. In this case, nothing flows.

In your way of thinking, in the illustration on the right, that pipe, eg "rail" is actually a gate that allows a current to flow between them. If these were containers of water, gravity would create the pressure to equalize. If electronic cells, potential difference creates the pressure to equalize.

Really? This is the crux of my confusion. I am thinking that there is no circuit, hence no flow because of a barrier that doesn't actually exist. Eureka! (?)

I need to prove that to myself. I might have just learned something new about batteries! (As Homer Simpson would say, "Doh ...!")

Put another way, if I were to take two car batteries, one fully charged and the other flat and jumpered them together for a while and then later removed the jumper cables I would end up with two half flat batteries? Hmmm... I never really thought about that. Maybe you are right.

View attachment 54371

Congratulations Brother!

I wasnt sure if you were going to stay stuck and double down or if you had the capacity to step out of a fixed mind set and expand. Really glad to see it and I actually feel quite happy for you. Not because you got this figured out but because you have the ability to back up, have another look, consider new information and adjust your position, even after what was admittedly a bit of a smack in the face (for which I do now apologize)

Huge

 

[Hedges]

Better wear gloves and safety glasses, because it will be a very rude awakening.

Hilarious, and true.

But in all seriousness, to avoid the possibility of losing eyesight and even fingers, take all the precautions such as removing jewelry.
And instead of a busbar between the batteries for both positive and negative, how about using a 1-cell flashlight bulb in place of the positive busbar?
This won't be nearly as dramatic or memorable. But if the light bulb lights, you'll know that a complete circuit existed where you thought there was none.

 

[svetz]

First off, let me offer you an apology for the drama that's occurred in the thread.

Somehow my simple question about how something that seems to be supernatural has turned into a some weird dogmatic argument.

I get that you didn't get it.
The reason why it became dogmatic is because a lot of folks think it's elementary that current flows due to a voltage difference. Somewhere in trying to convey what aspects of that you didn't understand and their explanations to help you "get it" just didn't mesh. I think a part of that was you believed it to be a myth while asking the question if it was real.

Last night I was chastised and "banned for two weeks" because I was not "interested in learning"

Did you notice your ban ended early two weeks early?
That's because the moderators and Will reviewed the thread and ultimately decided you were interested in learning and reversed it.

To the moderator who banned you, it looked like you were trolling to create drama. We do ban trolls and a number of other bad behaviors that are spelled out in the terms and conditions of membership.

Being a moderator is hard, we have to make judgment calls. We do this without pay, after a long day at work, when the kids are acting up, on the weekends, and probably worse. We do it because we're trying to help people.

We're not always right, but we try to keep things civil so people get the help they need.

...Nobody seems to understand what I am trying to say, regardless of how I try to explain what my question is. Look at how I write. I am more literate and grammatically diligent than most, am I not?

Regardless of skill, if no one is understanding you it's often best to wait a day and think about what they're saying and what you're saying to try to figure out where the mismatch is and how to resolve the misunderstanding. In my experience, while I'm thinking about that, someone else usually comes along and gets it, they might even provide a response that provides that eureka moment of clarity where everything gels. I think this happened for you with @upnorthandpersonal's post and thank him for his post.

Perhaps it is because the question to even be a question is so ridiculous that it just goes over everyones' head. It is exposing some apparent common myth that seemingly is accepted as reality.

Most questions are not ridiculous. If you don't understand then you don't understand.
But, extraordinary claims (e.g., it's a myth two batteries in parallel constitute a closed circuit and current won't flow from high-to-low voltage) require extraordinary proof, and those were not offered.

...have a four year Bachelor of Science degree (as well as British A Levels in the social sciences), am a licenced radio technician, a retired computer engineer, am widely regarded as intellectually gifted...

That you didn't get "being in parallel" constitutes a "closed circuit" escaped most members' thoughts as they consider that elementary. That you are gifted just increased the incredulity and solidified the idea that you were trolling.

We are trusting that you weren't trolling, but sadly there are plenty of anarchists that do try to do nothing more with their intelligence than to stir up trouble. I hope that you do accept my apology and join the community as there is always a lot we can learn from one another.

...at that point it seems to have become personal to "Snoobler", the acting moderator who decided to go all *** about it.

Being disrespectful is against the rules, and your post was edited to remove the rude remark. If someone is disrespectful of you then report the post and it'll be dealt with.

Hopefully you will in time come to appreciate the moderators. We try to do a good job, but we're only human too.

 

[RyanNicholas]

I completely understood the initial question in this post, because I I was told this "passive" method would work - but I don't think it ever actually did. When I built my 280ah lifepo4 battery from 4 EVE cells back in February, I didn't have access to a power supply that wouldnt take weeks to charge/balance the cells (which was unrealistic while traveling)... So I did the "passive" method I was told would work - and even though I had a feeling it didn't, I let that suffice and have been using the battery for the last few months.

I hope no one minds if I squeeze in this question here, but - my 4 cells are currently all very low and uneven (I've been in cloudy/rainy weather for a while), and right now I do have access to a power supply with high amperage and I have the cells connected in parallel, ready to hook them up to the power supply at 3.65V. Am I correct in understanding that doing this will both a) fully charge, and b) successfully top balance all 4 cells?

Thanks for any answers here, and for the side-bar ??

 

[Bob B]

I completely understood the initial question in this post, because I I was told this "passive" method would work - but I don't think it ever actually did. When I built my 280ah lifepo4 battery from 4 EVE cells back in February, I didn't have access to a power supply that wouldnt take weeks to charge/balance the cells (which was unrealistic while traveling)... So I did the "passive" method I was told would work - and even though I had a feeling it didn't, I let that suffice and have been using the battery for the last few months.

I hope no one minds if I squeeze in this question here, but - my 4 cells are currently all very low and uneven (I've been in cloudy/rainy weather for a while), and right now I do have access to a power supply with high amperage and I have the cells connected in parallel, ready to hook them up to the power supply at 3.65V. Am I correct in understanding that doing this will both a) fully charge, and b) successfully top balance all 4 cells?

Thanks for any answers here, and for the side-bar ??

Yes .... just make absolutely sure you set the voltage while not connected to the battery .... and verify the voltage with a DVM ... Then connect it to the cells and don't touch anything on the power supply after that. If there is a current adjustment, I would set it before the final voltage adjustment.

Disconnect the power supply and Leave them in parallel for 24 hrs or so after the current has decreased to near zero.