When is cell imbalance considered serious for new cells?

[Dzl]

And how does one go about balancing capacity, given that generally each has a unique capacity?

With the caveat that I can't speak for @toms am not positive what he means by capacity balanced, and the theory behind balancing is one of the weakest parts of my understanding/knowledge, this is what I am envisioning:



Red lines equal max bandwidth of balanced pack, green lines equal reduced bandwidth of unbalanced pack (?)

In all three scenarios, mismatched cells will leave some capacity on the table, as the lowest capacity cell (or weakest cell?) would limit the entire pack. But with a top or bottom balance, SOC is at least aligned at one or the other end of the SOC range, so that all cells theoretically hit either full or empty at the same time. At the opposite end of the range, cell differences will become apparent and the smallest or weakest cell would hit the cutoff threshold first.

With an unbalanced pack I imagine it being like the left hand example. If SOC is never aligned you could have cells (limiting the bandwidth at both the top and the bottom of the SOC range.

As I said, balancing theory is one of my weakest points, by a long shot, so if anyone sees an error in my thinking, or a fundamental misunderstanding anywhere, or even just questions any part of it, please point it out.

 

[snoobler]

capacity balanced means the supplier provided you with cells that are all the same capacity.

 

[Dzl]

capacity balanced means the supplier provided you with cells that are all the same capacity.

Sounds like @toms means something different than.


edit: and technically I would think that would be called 'capacity matched' as in 'matched by capacity and internal resistance'
I think of:
Matching = process of cell selection
Balancing = processes to keep cells aligned/balanced/equalized after cell selection

 

[toms]

Dzl has explained it how i see it also.

For clarification, when i say capacity balanced i mean that the cells will all be the same capacity as a percentage from the top and have the same voltage.

ie 95% SOC 3.55V

The cells could be all exactly 3.5V but anywhere from 80 - 90% SOC.

They are voltage equalised, but not at the same capacity (not capacity balanced). It isn’t much of a problem if you are using a BMS or balancer, but the OP said they weren’t using one.

edit: the cell capacity is a percentage from full in this instance, it doesn’t mean that all cells will reach zero percentage SOC. Maybe that is where the confusion lies.

Dzl’s illustration explains it very well

 

[toms]

And how does one go about balancing capacity, given that generally each has a unique capacity?

As described, the SOC of each cell is the same percentage from full and the voltages are equal. The SOC of the weakest cell is now the zero SOC reference for every cell.

I guess that is how you were operating your mismatched pack?

 

[Ampster]

I guess that is how you were operating your mismatched pack

Exactly, I was staying away from the green lines in the visual that @Dzl posted at post #21. That is such a great visualization that it should be in the Wiki or a Resource. Maybe @FilterGuy knows how to get that done.

 

[snoobler]

Exactly, I was staying away from the green lines in the visual that @Dzl posted at post #21. That is such a great visualization that it should be in the Wiki or a Resource. Maybe @FilterGuy knows how to get that done.

I "reported" it for "make sticky". Let's see what happens.

As described, the SOC of each cell is the same percentage from full and the voltages are equal. The SOC of the weakest cell is now the zero SOC reference for every cell.

I guess that is how you were operating your mismatched pack?

The flatness of the LFP curve essentially makes this concept of "capacity balanced" as presented meaningless.

 

[Zil]

I thought every thing about balancing cellos has been posted by now. Top balance to 3.65 per cell. then discharge down to 3.2 or 3.3.

 

[snoobler]

I thought every thing about balancing cellos has been posted by now. Top balance to 3.65 per cell. then discharge down to 3.2 or 3.3.

It will never end as long as the first instinct is to ask instead of search. Even then, time will pass, memories will fade, and it will all start over again.

 

[Ampster]

The flatness of the LFP curve essentially makes this concept of "capacity balanced" as presented meaningless

I agree there could be a better term. There is an ambiguity in that term. It is not the same as saying "equal in capacity. That was the point I was trying to make in post #19 when asking @toms the question, " how does one go about balancing capacity, given that generally each has a unique capacity? I also don't think there is clarity around the terms, "cell drift" and normal voltage "settling" after a charge.

 

[Ampster]

It will never end as long as the first instinct is to ask instead of search. Even then, time will pass, memories will fade, and it will all start over again.

Life goes on and there is a reason that the acronym RTFM has become commonplace. I am still learning from these discussions.

 

[Luthj]

The big challenge is that many folks just want a youtube video with step by step instructions. They either don't want to understand the subject in depth, or don't realize its beyond the simple instructional depth.

So we supply great source material to read and understand, but it just gets skipped over for various reasons. Not the least is because its quite complicated.

 

[Ampster]

The big challenge is that many folks just want a youtube video with step by step instructions. They either don't want to understand the subject in depth, or

I think this site and Will's videos provide a cross section of options. That goes from those that want to DIY a battery pack for $125 per kWh to those that are happy paying $900 per kWh for a Battleborn.

 

[Luthj]

I think this site and Will's videos provide a cross section of options.

Sure.

I bet many folks have zero issues, and we just never hear from them. But there is a fair number who start skipping steps cause its slow or expensive. Thats where the bloated cells and magic smoke loss start. I guess the caveat is: If you don't fully understand what your doing, Don't deviate form the instructions!

 

[Ampster]

I think that phenomema of those that have no issues not reporting may be known as selection bias or some similar concept.

 

[Dzl]

Exactly, I was staying away from the green lines in the visual that @Dzl posted at post #21. That is such a great visualization that it should be in the Wiki or a Resource. Maybe @FilterGuy knows how to get that done.

I "reported" it for "make sticky". Let's see what happens.

The flatness of the LFP curve essentially makes this concept of "capacity balanced" as presented meaningless.

Just to be clear, I made that visual myself, and am not 100% confident I understand the theory behind balancing well enough to know if the visual is correct. Approach it with a critical eye, if it looks correct, I'm happy to add it as a resource or add it to a Wiki page, or make improvements/correctiosn if needed. But definitely want to make sure its accurate first.

 

[Luthj]

I think the bar chart is accurate and useful. The importance of using a balance method which ensures the cells are all at the same SOC, cannot be understated. For Lithium this means either a top or bottom balance, where you are in the knee area. Everything else is too flat for accurate SOC balancing.

 

[snoobler]

Just to be clear, I made that visual myself, and am not 100% confident I understand the theory behind balancing well enough to know if the visual is correct. Approach it with a critical eye, if it looks correct, I'm happy to add it as a resource or add it to a Wiki page, or make improvements/correctiosn if needed. But definitely want to make sure its accurate first.

It's correct. You nailed it nicely.

 

[Zil]

While my cellos are balanced , I was talking about cells.

 

[Burke]

That was a balance, just at a lower V. Nothing wrong with that.

When balancing, you are balancing the charge, not the voltage. Voltage is only an indicator of SOC when near the top of the voltage range, or at the bottom. What kind of charger are you using? If you don't have a charger, you should get one. There are many good hobby chargers that will individually fully charge your cells to 3.6v, so they are all up at the 3.5V range after settling out and have full capacity. Then you can use the same charger to fully balance using a balance charge and balance leads.