Top balance Required for fresh cells?

[7Days_Late]

I am sure this question has been asked a million times already ? I'm sorry! but l just received 4 pristine factory fresh LF280K EVE cells.

They all show 3.295V on first check out of the box. After I build my 4S Pack and put the BMS on, should I top balance? The only charger I could think of to use is my PIP-LV-1012-MK MPP solar inverter/charger. I don't have a regular charger per say.

Thanks!

 

[sunshine_eggo]

You have two options:

1) do it wrong.
2) https://diysolarforum.com/resources...ls-using-a-low-cost-benchtop-power-supply.65/

 

[corndog]

I've heard it like this...

There are two ways to do things:
1. The right way.
2. Again.

 

[7Days_Late]

Cheers I'll give it a read!

Thanks,

 

[7Days_Late]

Will says in the video, you can passively balance them, I might just put them in parallel and leave them for a week while i wait for other parts to come in. could save me 70 bucks!

 

[joanna_lannie]

Of course this is possible. However, as the number of charging and discharging increases, imbalances will inevitably occur in the battery pack in the future. At that time, you may need to perform such a balancing method again, that is to say, in the future for a long time, you may Will repeat this work many times... As you mentioned, some BMS will have a certain function of balancing battery, but there is also a device called a balancer that can better help you maintain the balance of the battery.

 

[sunshine_eggo]

Will says in the video, you can passively balance them, I might just put them in parallel and leave them for a week while i wait for other parts to come in. could save me 70 bucks!

Unfortunately, I thought you were going to be easy. I see you're going to put up a fight. Gird your loins, bro.

Either you've misunderstood what @Will Prowse said, it was taken out of context because you want a shortcut, or Will's wrong.

I suspect it's one or both of the first two.

Here's why:

How does current flow? Current flows from high voltage to low voltage. If you have all cells at 3.295V, how will current flow between them to balance them? Answer: It won't.

Identical voltages in the flat portion of the operating range don't mean balanced. It simply means they are the same voltage.

Would you consider a 100Ah cell at 3.295V and a 280Ah cell at 3.295V as "balanced?" Answer: Hell no.

"BALANCE" refers to state of charge, not voltage. The only times you can be certain that cells are truly at the same state of charge is: 2.5V (bottom balance) and 3.65V (top balance).

Your 280Ah cells don't all have 280Ah capacity. They will vary. Maybe not by much, but even a 0.28Ah difference in capacity can cause a cell runner at full charge. I've seen it multiple times.

In other words, even if all of your cells at 3.295V are at the exact same state of charge - let's say 40%, they all don't have the same capacity, so when you charge them, one or more of them will hit full before the others. In almost all cases, this results in a shit show.

Use the guide.

 

[Will Prowse]

Yeah that does not work. Where did I say that?

 

[Will Prowse]

I am sure this question has been asked a million times already ? I'm sorry! but l just received 4 pristine factory fresh LF280K EVE cells.

They all show 3.295V on first check out of the box. After I build my 4S Pack and put the BMS on, should I top balance? The only charger I could think of to use is my PIP-LV-1012-MK MPP solar inverter/charger. I don't have a regular charger per say.

Thanks!

If they are brand new and matched, don't balance them. Build a pack with a BMS and run a capacity test. If you pull full capacity, leave it alone.

 

[sunshine_eggo]

I hate to disagree with the oracle, but too many people have been sold "matched" and/or "balanced" cells that just aren't - even if they are otherwise good cells.

I bought Eve cells from Amy back when they could be delivered in 7 weeks for $125 EA. No complaints, but none were grade A even though she said they were (none met rated capacity - about 272Ah) and total capacity varied by about 10Ah across 9 cells (1 extra ordered "just in case"). The 9th cell was quite the outlier compared to the other 8.

I've seen an imbalance as small as 0.1% cause a BMS shutdown trying to get to 28.8V on 8S. Top balancing avoids this.

 

[stok]

Unfortunately, I thought you were going to be easy. I see you're going to put up a fight. Gird your loins, bro.

Either you've misunderstood what @Will Prowse said, it was taken out of context because you want a shortcut, or Will's wrong.

I suspect it's one or both of the first two.

Here's why:

How does current flow? Current flows from high voltage to low voltage. If you have all cells at 3.295V, how will current flow between them to balance them? Answer: It won't.

Identical voltages in the flat portion of the operating range don't mean balanced. It simply means they are the same voltage.

Would you consider a 100Ah cell at 3.295V and a 280Ah cell at 3.295V as "balanced?" Answer: Hell no.

"BALANCE" refers to state of charge, not voltage. The only times you can be certain that cells are truly at the same state of charge is: 2.5V (bottom balance) and 3.65V (top balance).

Your 280Ah cells don't all have 280Ah capacity. They will vary. Maybe not by much, but even a 0.28Ah difference in capacity can cause a cell runner at full charge. I've seen it multiple times.

In other words, even if all of your cells at 3.295V are at the exact same state of charge - let's say 40%, they all don't have the same capacity, so when you charge them, one or more of them will hit full before the others. In almost all cases, this results in a shit show.

Use the guide.

this 100%

 

[Will Prowse]

I hate to disagree with the oracle, but too many people have been sold "matched" and/or "balanced" cells that just aren't - even if they are otherwise good cells.

I bought Eve cells from Amy back when they could be delivered in 7 weeks for $125 EA. No complaints, but none were grade A even though she said they were (none met rated capacity - about 272Ah) and total capacity varied by about 10Ah across 9 cells (1 extra ordered "just in case"). The 9th cell was quite the outlier compared to the other 8.

I've seen an imbalance as small as 0.1% cause a BMS shutdown trying to get to 28.8V on 8S. Top balancing avoids this.

I totally agree actually. About non grade a ces. But if they are matched grade A cells (which I assume they are. The original post states brand new factory fresh), over a few weeks, they will balance if close enough with the BMS passive dissipative balancer. That's what it's there for. If it's not balancing, you have a defective BMS (or balancing trigger threshold might be too high, and absorption set too low) or your cells are not matched/brand new.

If you are pulling full capacity and one cell is spiking, it's ok. Just cycle it 20 times or so and it will go away. I had a really nasty imbalance on a recent 16S pack, and after a month, they are equalized. It's fine.

A lifepo4 BMS, even with a modest balance current, can balance a large pack (if matched and healthy). You don't need a active balancer or top balance if the cells are good.

Now all of this goes out the window if you are using grade b cells. Or heavily used cells. They just won't balance. Need to top balance those suckers every couple months.

Paralleling lifepo4 cells in a static configuration simply does not work to balance them. They must all hit the last bit of absorption together. Anything below the knee of the curve is simply ineffective for balancing.

 

[Will Prowse]

I hate to disagree with the oracle, but too many people have been sold "matched" and/or "balanced" cells that just aren't - even if they are otherwise good cells.

I bought Eve cells from Amy back when they could be delivered in 7 weeks for $125 EA. No complaints, but none were grade A even though she said they were (none met rated capacity - about 272Ah) and total capacity varied by about 10Ah across 9 cells (1 extra ordered "just in case"). The 9th cell was quite the outlier compared to the other 8.

I've seen an imbalance as small as 0.1% cause a BMS shutdown trying to get to 28.8V on 8S. Top balancing avoids this.

And yes, your non-grade a cells require a solid top balance. No way around it.

 

[Will Prowse]

I hate to disagree with the oracle, but too many people have been sold "matched" and/or "balanced" cells that just aren't - even if they are otherwise good cells.

I bought Eve cells from Amy back when they could be delivered in 7 weeks for $125 EA. No complaints, but none were grade A even though she said they were (none met rated capacity - about 272Ah) and total capacity varied by about 10Ah across 9 cells (1 extra ordered "just in case"). The 9th cell was quite the outlier compared to the other 8.

I've seen an imbalance as small as 0.1% cause a BMS shutdown trying to get to 28.8V on 8S. Top balancing avoids this.

Imbalance of .1% referring to what? The voltage? What was your capacity test results? What was your absorption figure?

 

[upnorthandpersonal]

Since I had so much solar power these last few weeks, I did a test. I deliberately took a pack out of balance, and had a 2A active balancing JK BMS connected to it. The cells are back in balance after two weeks.

 

[RCinFLA]

The reason just passively paralleling cells will not balance them is due to the cell overpotential (also called polarization) voltage required to get current to flow to or from cell. It is overhead required to move lithium ions through cell to supply demanded cell current. The terminal voltage shift has a logarithmic relationship to cell current.

For the first 5-20 millivolts of overpotential voltage delta from rested, no-load, open circuit cell, there is very little cell current created. This means after a long time of paralleling cells, the cell receiving minor amount of current is 5-20 millivolt above its no-load rested voltage and cell supplying the small amount of current is 5-20 millivolt below its no load rested voltage. At this very low transfer current rate the cell self-leakage rate can actually be greater than actual charge current transferred so even if you leave them paralleled for months, they will never achieve balance.

With the delta voltage between cells caused by overvoltage overhead, the closest you can get open circuit unloaded voltage between cells is 10-40 mV. If you look up that amount of delta voltage on a no-load rested open circuit voltage LFP cell state of charge curve you can easily have 10% to 30% difference in state of charge.

It takes less than a 1% difference in series connected cells state of charge to have problems with BMS cell overvoltage shutdown when attempting a full battery charge.

 

[sunshine_eggo]

If you are pulling full capacity and one cell is spiking, it's ok. Just cycle it 20 times or so and it will go away. I had a really nasty imbalance on a recent 16S pack, and after a month, they are equalized. It's fine.

Most new builders aren't prepared for this situation. You might have missed the dozens of posts about it.

And yes, your non-grade a cells require a solid top balance. No way around it.

Note that I was told by Amy, a trusted seller here, that they were Grade A. MANY have posted results of capacity tests of "grade A" cells purchased from trusted sellers that fall 4-16Ah shy of rating. Again, this is not a complaint as I believe I got a good value.

Imbalance of .1% referring to what? The voltage? What was your capacity test results? What was your absorption figure?

As I mentioned above:
Voltage isn't a reliable measure.
Around 272Ah in a 10Ah range across all cells.
28.8V

0.1% means if one cell is at 100% SoC and tripping BMS OVP, the other cells will be at 99.9% SoC, i.e., 280mAh away from being fully charged.

These particular cells were the 272Ah Lishen and 0.1% imbalance occurred on two separate 8S batteries - different from my own 8S Eve battery and the spare cell.

 

[Will Prowse]

Most new builders aren't prepared for this situation. You might have missed the dozens of posts about it.



Note that I was told by Amy, a trusted seller here, that they were Grade A. MANY have posted results of capacity tests of "grade A" cells purchased from trusted sellers that fall 4-16Ah shy of rating. Again, this is not a complaint as I believe I got a good value.



As I mentioned above:
Voltage isn't a reliable measure.
Around 272Ah in a 10Ah range across all cells.
28.8V

0.1% means if one cell is at 100% SoC and tripping BMS OVP, the other cells will be at 99.9% SoC, i.e., 280mAh away from being fully charged.

These particular cells were the 272Ah Lishen and 0.1% imbalance occurred on two separate 8S batteries - different from my own 8S Eve battery and the spare cell.

Then we should tell them it's fine. I don't see the issue. If you're a beginner and you have a new cell pack and it's pulling full capacity (which I just said in the past post), then leave it alone. The BMS will do the rest.

Ok, so you did not pull full capacity. So you should do a top balance. That's what I said earlier as well. I don't see any disagreement here. I said exactly this.

 

[Will Prowse]

Most new builders aren't prepared for this situation. You might have missed the dozens of posts about it.



Note that I was told by Amy, a trusted seller here, that they were Grade A. MANY have posted results of capacity tests of "grade A" cells purchased from trusted sellers that fall 4-16Ah shy of rating. Again, this is not a complaint as I believe I got a good value.



As I mentioned above:
Voltage isn't a reliable measure.
Around 272Ah in a 10Ah range across all cells.
28.8V

0.1% means if one cell is at 100% SoC and tripping BMS OVP, the other cells will be at 99.9% SoC, i.e., 280mAh away from being fully charged.

These particular cells were the 272Ah Lishen and 0.1% imbalance occurred on two separate 8S batteries - different from my own 8S Eve battery and the spare cell.

If they do not pull full capacity out of the box from a trusted seller, they got scammed. Plain and simple. Did Amy sell bad cells? Who is a trusted seller selling bad cells? They are not considered a trusted seller if they are not selling brand new cells. This has nothing to do with top balancing, and everything to do with getting ripped off.

 

[toms]

Most new builders aren't prepared for this situation. You might have missed the dozens of posts about it.

It makes more sense to set up your system so that it is capable of bringing cells into balance.

If your system is incapable of doing that - there is your problem.

Manual top balancing is a bandaid fix for a poorly designed system.

If your system is shutting down because a cell is going overvoltage - it’s not a cell SOC variance issue - it’s a poor design issue.