Passive Top Balancing ? How/Why does it work ?

[2Big2B]

My question is about Top Balancing logic.

If I simply take a set of LiFePo4 batteries, line them up and connect them in parallel for top balancing without connecting the plus and minus poles to a my variable 20v/10A bench power supply, will they equalize and balance all by themselves, or does there need to be a current flow between the poles?

Is this a stupid question? Intuitively I cannot understand how cells that do not have a current flowing from negative to positive terminals in some manner would interact with each other - just because all their poles are commonly connected together with nothing else attached. What would cause them to equalize without some external load or power applied to the combined poles?

I have already top balanced my 8 cells both independently and in parallel with my bench power supply to 3.65v on all cells. I am about ready to hook up my BMS and move forward. Do I actually need to remove the power supply and just leave them all connected to each other in parallel overnight before proceeding? Or is that some meaningless ritual?

I am presently doing a few experiments though with some initial confusing results. If I try testing for current between the first negative terminal and the last in my string of 8 cells set up for parallel top balancing I seem to be seeing a slight voltage difference on my Fluke meter - consistently. That seems to suggest an argument that I am wrong in my thinking. Energy appears to be transmitted between the cells somehow. But How? Why? Am I operating with some basic misconception with a blind spot in my logic? Is it me?

I have never heard any discussion in any videos about such a phenomenon. Yet talking to others I have been assured that this weird osmosis occurs and seems to be commonly accepted. Am I missing some basic logic here, or is there some general misconception that all you need to do is hook up the cells in parallel and leave them alone after having brought them all up to 3.65v ?

Looking at one discussion I find a couple of curious terms too (from Energy Storage/DYI LiFeP04 Battery Banks forum:

"Top balancing, recommendations (fastest or best practices)"​

"PLAN A: 1. Parallel them and "balance" them au naturel overnight - not really balancing, just getting the voltages in the same ballpark."

And ... "active balancing " (i.e, with a BMS or Bench supply)

(Active vs Passive "au naturel overnight" Top Balancing) ? Really?

I am confused.

Or am I just having a senior moment?

 

[toms]

LiFePO4 cells can have the same voltage and be at different states of charge, so they will not balance if they are connected in parallel.

LiFePO4 cells can continue to absorb low current without increasing voltage when they are fully charged, the energy is absorbed by lithium plating.

Best practice for top balancing is to individually charge each cell to the same knee voltage/trailing current, while using a cell voltage monitor capable of disconnecting the charge source.

 

[2Big2B]

From everything I read and have seen, Passive Balancing is what the BMS does - it applies resistance to individual cells to pull down their voltage to do the balancing. Active Balancing does the opposite - less common, individual cell voltage is pulled up.

That is not my question though. There appears to be an unspoken ambiguity that only by virtue of a string of batteries having each side of the cells terminals physically connected in a common row, without any load or voltage charge applied to the terminals, that magically he cells will equalize. That makes no sense to me. without some externally coonected load or charge, what would cause any ionic/electron exchange between the cells, even if their respective terminals are connected together? Is there some basic concept I am missing, or am i right. If I am right, I suspect there ius some general misconception that balancing will occur that way. It should be clarified that balancing must involve an interaction between positive and negative terminals. I suspect that it is considered common sense and never actually specified.

People have corrected me who seem to think that just having a string of cells hooked up in parallel - not connected to anything else - will cause them to equalize. Are they correct? I see nothing written or illustrated that tells me that.

 

[Lt.Dan]

I believe it is because the internal resistances changes throughout the charge/discharge curve. The difference in internal resistance causes electrons to flow in the path of least resistance, thus causing the cells to balance. Albeit extremely slow. When the batteries voltages are at a higher SOC (ie 100% being 3.65v and 95% being 3.5v) there is a greater difference in internal resistance making the balancing between cells faster, which is also why we parallel top balance.

Or atleast this is how I understand it. I could be wrong.

 

[2Big2B]

That I understand. But that doesn't address my question. Can battery cells set up in parallel, sitting idle without anything connecting to the main terminals (e.g. "Rail") balance into a common SoC. If so, by what process. I don't think so. I think some others do though. Please explain.

 

[snoobler]

That I understand. But that doesn't adress my question. Can battery cells set up in parallel, sitting idle without anything connecting to the main terminals balance into a common SoC. If so, by what process. I don't think so. I think some others do. Please explain.

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.

 

[2Big2B]

This is a couple of google results

There are two basic approaches to balancing: Passive balancing drains charge from cells having too much charge and dissipates drained energy as heat. Active balancing moves charge from “high cells” to “low cells,” attempting to conserve energy in the battery pack.

Passive balancing allows the stack to look like every cell has the same capacity as the weakest cell. Using a relatively low current, it drains a small amount of energy from high SoC cells during the charging cycle so that all cells charge to their maximum SoC. ... Passive cell balancer with bleed resistor.

There seems to be an implied belief that cells will balance without any such interaction. I believe that is a myth - which I am trying to either affirm or bust. I have found nothing in any video that explicitly says so one way or the other which maybe leading to a popular gross misunderstanding. I would like to see Will explicitly point that out as there seem to be some folks that expect that osmosis to work.

 

[snoobler]

This is a couple of google results

There are two basic approaches to balancing: Passive balancing drains charge from cells having too much charge and dissipates drained energy as heat. Active balancing moves charge from “high cells” to “low cells,” attempting to conserve energy in the battery pack.

Passive balancing allows the stack to look like every cell has the same capacity as the weakest cell. Using a relatively low current, it drains a small amount of energy from high SoC cells during the charging cycle so that all cells charge to their maximum SoC. ... Passive cell balancer with bleed resistor.

There seems to be an implied belief that cells will balance without any such interaction. I believe that is a myth - which i am trying to either affirm or bust. I have found nothing in any video that explicitly says so one way or the other which maybe leading to a popular gross misunderstanding. I would like to see Will explicitly point that out as there seem to be some folks that expect that osmosis to work.

See the post above yours.

 

[2Big2B]

To that post I have just replied as follows:

I still don't get it. The suggestion here is that there will be some intercourse between the paralleled cells, but not very significantly.

Why should there be any interaction between them if no charge or load is applied to the terminals? I might expect a slow degradation of SoC independently that might be interpreted as a result of interaction, but that would be incidental / coincidental to the observation. I believe that in this situation there is NO interaction of equalizing/balancing whatsoever.

Yet this discussion seems to imply that there is, but in a limited way. Please explain.

 

[snoobler]

Current flows from high voltage to low voltage.

 

[2Big2B]

I am not talking about a BMS and how it passively balances. I am talking about the case of two cells, one fully charged (Cell A) , the other in a discharged state (Cell B). Assume both battery cells are healthy. Check voltages with an accurate VOM, like a Fluke meter. Then set them up in parallel - having both of their terminals of same polarity joined as parallel. No BMS. No charger. Leave them to sit for a day. Disconnect them and then test voltages. I would expect both cells A & B to have a slightly dissipated voltage. I would not expect the discharge battery cell B to now have a higher voltage than before their pairing. If cell B does end up with a higher voltage than before, I would like to know why. Magic?

 

[Bob B]

Yes ... the lower cell would be higher after sitting for an extended period of time parallel to a cell with a higher voltage.

Current will flow from the higher cell to the lower .... charging the lower cell.

 

[snoobler]

I am not talking about a BMS and how it passively balances. I am talking about the case of two cells, one fully charged (Cell A) , the other in a discharged state (Cell B). Assume both battery cells are healthy. Check voltages with an accurate VOM, like a Fluke meter. Then set them up in parallel - having both of their terminals of same polarity joined as parallel. No BMS. No charger. Leave them to sit for a day. Disconnect them and then test voltages. I would expect both cells A & B to have a slightly dissipated voltage. I would not expect the discharge battery cell B to now have a higher voltage than before their pairing. If cell B does end up with a higher voltage than before, I would like to know why. Magic?

Did you bother to read the other thread?

You're describing EXACTLY what was done EXCEPT voltage was not used to approximate SoC. Capacity testing was. I did it for 24 hours and again after 7 days.

You may be missing the point that voltage is a horrible way to approximate SoC.

 

[Boondock Saint]

It doesn't matter the number of the calls, be they 2 or 32. What matters is this: if you tie allll the negatives together and then tie alll of the positives together, will these cells somehow (being in parallel) balance the electrons and come to a common voltage and a common SOC passively?

So I pose, why then would electrons flow along a common conductor in some redistribution whereas everything becomes equalized?

I dunno, a sense of fairness? Magic dust? Gravity waves?

.

 

[2Big2B]

Thank you. That is my question exactly. I agree that voltage is a lousy way to measure and test such a phenomenon. The premise though, no matter what method is used to measure, is that if Cell A has a high SoC and Cell B has a low Soc, that by combining their poles in parallel will accommodate a transfer of Soc between them. How? What is the process?

I don't believe it. Either that is a myth or else I am ignorant.

Which is it?

 

[snoobler]

Thank you. That is my question exactly. I agree that voltage is a lousy way to measure and test such a phenomenon. The premise though, no matter what method is used to measure, is that if Cell A has a high SoC and Cell B has a low Soc, that by combining their poles in parallel will accommodate a transfer of Soc between them. How? What is the process?

I don't believe it. Either that is a myth or else I am ignorant.

Which is it?

Ignorant.

By connecting the two cells in parallel with conductors, you establish a circuit and current can flow from the high SoC cell to the lower SoC cell.

 

[Boondock Saint]

If you read along this rail (from negative to positive) with a voltmeter from the first cell to the last cell, even that value with change. If all else (ideally) is equal, why would there be variation in the reading?

There is no pressure or guidance or compelling of electrons to auto balance along a rail because there is no potential there. It's a common rail.

A common rail is not a circuit. It never has been. Where is the potential in a common rail?

The battery chemistry makes electrons flow from pole to pole, plate to plate, not evenly distribute like some woke culture fairness fantasy.

So the choice is then to use a device or means to burn off and redistribute electrons from one cell (or pair of cells) to another of the same.

This is not a set of interconnected buckets of water obeying the laws of gravity. I hope everyone has better fantasies.

If someone take a set of batteries will all different measurable voltages and SOC and then binds on the negatives to negatives and also the positives to positives, and there is no other type of connection or commonality, and can show over the course of time that all these cells with end up the mean or median value of all of these cells I'd like to see it.

3.25 + 3.5 + 3.75 does not all end up at 3.5 after time just by binding rails, not that I've veer seen.

Please cite an example of this and the electrical theory that states this is what happens based on chemistry.

Do not taunt Happy Lucky Fun Ball.

So if your goal is to "balance" your cells by wiring them in parallel and letting them sit, it's not likely that you'll achieve anything meaningful.

.

 

[2Big2B]

Thank you. I was wondering if I stutter.

We share the same ignorance I guess.

Yet it seems that a lot of people seem to believe it works that way and present it as truth.

But, I guess I am just ignorant and shouldn't ask stupid questions. Meanwhile I have my string of 8 cells wired up in parallel without a charger connected with the faith that it will all resolve itself by morning through some magic voodoos that I cannot understand.

I note that when I measured voltage between first and last terminal on the negative rail I seemed to see a tiny non-zero difference. I am inclined to think that is just an environmental/internal wandering of the Fluke meter at zero and is insignificant.

 

[Bob B]

3.25 + 3.5 + 3.75 does not all end up at 3.5 after time just by binding rails, not that I've veer seen.



.

Yes it will .... Well, maybe not exactly, but current will flow from the highest voltage potential to the lowest voltage potential if connected by a conductive material ....until a difference of voltage potential no longer exists ..... That's just basic current flow theory.

 

[2Big2B]

If you read along this rail (from negative to positive) with a voltmeter from the first cell to the last cell, even that value with change. If all else (ideally) is equal, why would there be variation in the reading?

There is no pressure or guidance or compelling of electrons to auto balance along a rail because there is no potential there. It's a common rail.

A common rail is not a circuit. It never has been. Where is the potential in a common rail?

The battery chemistry makes electrons flow from pole to pole, plate to plate, not evenly distribute like some woke culture fairness fantasy.

So the choice is then to use a device or means to burn off and redistribute electrons from one cell (or pair of cells) to another of the same.

This is not a set of interconnected buckets of water obeying the laws of gravity. I hope everyone has better fantasies.

If someone take a set of batteries will all different measurable voltages and SOC and then binds on the negatives to negatives and also the positives to positives, and there is no other type of connection or commonality, and can show over the course of time that all these cells with end up the mean or median value of all of these cells I'd like to see it.

3.25 + 3.5 + 3.75 does not all end up at 3.5 after time just by binding rails, not that I've veer seen.

Please cite an example of this and the electrical theory that states this is what happens based on chemistry.

Do not taunt Happy Lucky Fun Ball.




.

Yes it will .... Well, maybe not exactly, but current will flow from the highest voltage potential to the lowest voltage potential if connected by a conductive material ....until a difference of voltage potential no longer exists ..... That's just basic current flow theory.

Huh? by what path?