Unbalanced

[Kirill]

Hi everyone!

I need an balancer for my LiFePO4 16s1P battery but I’m got confused in the searching.

Is this balancer suitable for LiFePO4 50Ah cells?

https://sl.aliexpress.ru/p?key=oVtYU60

Model number 0855A.


Heltec has two more models for the same money, but I don’t know what the difference is:

https://sl.aliexpress.ru/p?key=8JtYUhO

AND

https://sl.aliexpress.ru/p?key=oMtYURd

 

[abagofcells]

The first model uses capacitors and is probably the better choice. LFP in the datasheet refers to LiFePo4. The other two, I'm not sure about.

 

[Kirill]

Lifepo4 Battery 50Ah is unbalanced. In 25 minutes drive in city mode, the most charged cell turned the most discharged. But this doesn't always happen.
Basically, the minimal difference is 3-7 mV.

I don’t have a balancer. Only BMS.

I was never able to fully charge the battery.
After stopping the charger 53.3 - 53.5V max.

What's wrong? Bad cells or just needs connected balancer?

After buying cells looks fine. Minimal resistance about 37-41 mOm at all cells. And the same voltage.

 

[amberwolf]

At those voltages, you can't tell what their SoC is--it's in the "flat" section of the discharge curve, so regardless of what their actual charged-up-capacity is, they all "look" the same.

You'd need to fully charge the battery first, to 3.4-3.65v per cell, or fully discharge it, to 2.8v to 2.5v per cell, to get them all at the same state, and then do the opposite, to see how much capacity each cell actually has relative to the others. You're probably going to have to manually charge or discharge cells during the first stage of this to make them all exactly equal in voltage before you do the second stage.

The ones that end up different from the rest at the end of the two-stage test have different capacity.

When charging, cells that fill up first are lower capacity.

When discharging, cells that empty first are lower capacity.


Regarding resistance, that amount of variation between them means they are very different from each other, and will only become worse over time. Without figuring out the formula and doing the math, my guesstimate is that the percentage of difference from 38-41 is something like 8-9%, which is very high. They probably also have equally significantly different capacities. They will never stay balanced, and the pack will only ever deliver, at best, the capacity and performance of the worst cell.

You can top or bottom balance them, and they will be at the same state of charge (balanced) at that moment, but as soon as you do anything to them or with them, they will no longer be balanced, and will never have the same capacity as each other.

So, rather than trying to keep them balanced, it would be better to first bring them all down to equal minimum voltage, and then charge them up to only about 3.3v, which is around 70% capacity. 3.35-3.4v is about 100% capacity resting, so you don't want to go that high, as it will then unbalance them again; if you can precisely charge to say, 3.31v you'll get a little more capacity, but it's tough to do manually. Then you never fully discharge them, staying within the "flat" range of the discharge capacity, staying up at 3.0-3.1v at minimum.

If you use a programmable BMS like JBD, you can set the LVC and HVC points for cell level shutodwn of charge and discharge, and d oing things this way will minimize the amount of balancing you ahve to do. Depending on the actual worst-cel-capacity, you won't really lose much if any usable capacity, dependign on the specific voltage you pick for HVC and LVC to keep them in the flat part of the curve.


Well-matched cells that are identical in characteristics will remain equal to each other, and balanced until they age enough to become different in characteristics. Cells like yours that are probably all different from each other will never be equal to each other and so they will never remain balanced.


This is one SoC charge for a LiFePO4 cell found via google

3.2V LiFePO4 Cell Voltage Chart​

Voltage	Capacity
3.65V	100% (charging)
3.4V	100% (resting)
3.35V	99%
3.33V	90%
3.3V	70%
3.28V	40%
3.25V	30%
3.23V	20%
3.2V	17%
3.13V	14%
3.0V	9%
2.5V	0%

 

[Kirill]

Yes, I’ve JBD smart bms. Ok, I'll have to get the capacity of each cell.
But sometimes there is a minimal unbalance.

Should I buy add balancer?

 

[amberwolf]

You already have a balancer in your BMS, from what I can see in the images of the app (it has a button to turn it on and indicators on each cell for state).

If you have to buy an additional balancer just to keep the pack usable, it's pretty far gone.

An ok pack will only need a little balancing and only after it has been used hard all the way down to empty. A pack that regularly needs balancing even after light use is fairly worn out.

In your case, the cells are poorly matched (wide range of internal resistance, probably wide range of capacity too), so my previous recommendations stand.

 

[Kirill]

Balance current at BMS is too low. While I was assembling the battery, I heard from everyone around me “why you won’t have a balancer?”.
Now, with a difference of 30 mV, balancing does not turn on itself.
Therefore, I decided to purchase an additional balancer.
Just it's fu....ing B grade.
I booght nearest me 16 cells.
I would be glad to buy new one cells but it’s hardest quest.

 

[Kirill]

Hi everyone!

The last 15th cell has the lowest voltage.



When the charger is connected, the voltage of 15 cell increases by 0.09 V. And it becomes higher than all the others.



Towards the end of charging, some cells stop charging, while others charge much more.



Could the BMS be faulty?

Or low quality cells?

I don't have an active balancer.



Thanks for attention.

 

[amberwolf]

A cell that has a higher internal resistance (and usually lower capacity) will have higher voltage drop during discharge, and more voltage rise during charge.


When your charging stops (0w) in one of the pics, you can see the cell delta (difference) is over a tenth of a volt. Some BMS that have advanced functions and safeties will shut down charge (and usually discharge) to prevent serious problems when cells are that different.



Cells that aren't identical in their characteristics behave differently from each other during charge and discharge, and require a balancer to let you use all of what capacity they do have. Without the balancer the differences cause the voltages to diverge more and more the older the pack gets and the more cycles it has.

And the more different the cells are from each other, the worse the problems, like those you're seeing.

FWIW, your screen says they are "balancing", so that means the app thinks you have a balancer. if you don't, you should change whatever settings the app has (if any) to tell it you don't have one.

If you do have a balancer, then you can leave the charger connected and the balancers turned on, and they will eventually make all the cells the same full voltage.

It doesn't change their differences, just masks them until over time the worst ones become unusable (too much voltage sag under load, and/or too little capacity, since the worst cells limit the pack's abilities).

The best way to fix this type of problem is to replace all the cells with perfectly matched new ones with better capabilities than you need, but it's very hard to find anyone that sells matched cells. Usually to get matched cells you'd have to buy a lot more than you actually need, and then sort them all to find enough identical ones to build a pack from, and sell the rest (or build spare packs from them with less-well-matched sets, etc). Very few people would want to go to this expense and trouble....

The way most people would fix it is to just balance it every charge cycle and use it until it just doesn't work well enough for them anymore, then replace the whole pack with a new one.

The way most battery DIYers would usually fix it is to just replace the worst cells (groups) with new ones, but if you don't have experience building packs, I wouldn't recommend this route. You'll have to buy repair and test equipment and learn how to use it on other parts successfully before you use it on your actual pack, or you can turn the whole pack into junk. It's probably more expensive to do it this way than to just buy a whole new pack (depending on the pack size and how good a quality it is).

 

[Kirill]

Thanks for the detailed answer.
When on screen says balancing it is the BMS itself balances with a low current.
I wasn't sure which specific balancer to buy.
And sometimes the BMS and the balancer do not work together correctly.

I have already disconnected cell #16 due to the same problems. And for some time everything was fine. But the imbalance of the last cell began again.


OK. I will buy a separate balancer. The best one I found was from Heltec.

 

[amberwolf]

You don't need a separate balancer if your BMS already has one. The BMS balancer will eventually make all the cells the same full voltage, just like any other balancer.

No balancer will actually fix the problem though.

You have a cell problem that requires replacing all of the cells that are different from each other to actually repair the problem.

That's why a BMS stops charging when the cells are so different in voltage--it detects that there is a severe problem with the cells, and is trying to protect you against the possiblity of damaged cells causing a problem that could lead to a fire.

 

[Kirill]

At the BMS is too little balance current(20 mV).
Separate balancer up to 4V.

It will be impossible to find replacements for the worst cells. I could buy new ones. But those cells that were the best in my package will become the worst.
You have already talked about the difficulties of selecting cells. These are too drastic measures. Easier to assemble a new one.

Therefore, now I have set the task of using this package as much as possible.

There is a large capacity (50Ah) By charging the package as much as possible, I can use quite a lot of energy.

I need to have an adjustable charger. Set the correct voltage. Of course less than it could be.

 

[amberwolf]

At the BMS is too little balance current(20 mV).
Separate balancer up to 4V.

I don't really understand what you are saying. Current is not measured in V (volts), it's measured in A (amps).

Assuming you just swapped the letters by accident, then if your BMS only has 20mA balancing capability, but the separate one can do 4A of balancing per cell (or group), then at least the balancing will be 4 / 0.02 = 200 times faster with the separate balancer.

Your wiring from the cells to the balancer also have to be able to handle that 4A; it's easily possible that the existing wires to the BMS for this aren't thick enough since they don't have to be. (but you would still be leaving those existing wires in place to the BMS so it can still use them to do it's job of cell monitoring and protection).

I need to have an adjustable charger. Set the correct voltage. Of course less than it could be.

If you use an adjustable charger, you also have to use an adjustable balancer. Otherwise it will never activate if you set the charger low enough that the cells are not brought up to the voltage the balancer activates at.


BTW, since the problem and the project are the same, as are the possible solutions, I merged this thread with your original "unbalanced" thread about this problem to keep all the info and help in one place.

 

[Kirill]

Oh, sorry. Amps of course.

So a regular thin balance wire goes to each cell. There is voltage of about 3V. The manufacturer supplies the active balancer with such wires.



Yes, sure my BMS and this active balancer are completely adjustable.

Now I'm trying to find the maximum possible threshold for the highest voltage for all cells.
And set the values in the BMS settings.

Regarding the active balancer, I also thought about its necessity. That's why I haven't bought it yet. For good cells, just BMS is enough.
But I often founded information that with a large capacity, the separate balancer is simply necessary for the long life of the cells.
And of course, having received problems with the package, I thought that the whole point was that I did not have the separate active balancer.

 

[amberwolf]

So a regular thin balance wire goes to each cell. There is voltage of about 3V.

Voltage isn't relevant for what the balance wires can handle (they'll never see voltage high enough to exceed their insulation rating). Only current (amps) thru them matters.

For the BMS you could have a single strand of thin copper if it's only 20mA.

The manufacturer supplies the active balancer with such wires.

As long as they're thick enough to carry 4A without any voltage drop across them, and without any heating. If there's voltage drop, then the balancer can't read the voltages correctly and will stop balancing sooner, or will keep turning on and off all the time because the voltage it sees keeps going below the threshold for balancing, whenever the current is high enough. Depends on how much voltage drop there is, the actual threshold, etc.

Now I'm trying to find the maximum possible threshold for the highest voltage for all cells.
And set the values in the BMS settings.

Not sure what you mean. The highest voltage is whatever the cell spec sheet says is the cell max charge voltage. If these are LiFePO4 / LFP, then that's typically 3.65v max.

Balancing is meant to make all the cells the same voltage, either at full voltage (top balancing, most common) or at empty voltage (bottom balancing, rare). Either one works to do that job.

It doesn't balance the capacity of the cells, though that could actually be done with a very smart BMS (which doesn't presently exist) that monitored current into and out of each cell, all the time, in a pack specially designed to do this (since a pack built the usual way can't do this). It would keep track of the actual capacity of each cell, and stop charging of that cell as soon as it reached whatever the limit of the *lowest capacity* cell was. Then it would stop discharging of the pack as a whole as soon as that limit was reached in discharge. (it would also monitor cell voltages and stop if any went too high or low, etc).

Regarding the active balancer, I also thought about its necessity. That's why I haven't bought it yet. For good cells, just BMS is enough.
But I often founded information that with a large capacity, the separate balancer is simply necessary for the long life of the cells.
And of course, having received problems with the package, I thought that the whole point was that I did not have the separate active balancer.

If the cells are good, and matched, then it doesn't matter how high a current the balancing is, because it won't be being used much, if at all.

Balancing doesn't make the life of the cells better or worse. All it does is allow you to keep using failing cells longer, and/or use more of the available capacity of unmatched cells.

If a pack isn't balanced, then it has less available capacity, and if there is no balancing function at all, then a pack can become unusable because of the differences in cell properties leading to some being full while others are completely empty. So balancing affects the lifespan of the *pack*, but not the cells.

If the BMS fails to protect the cells by keeping them within their voltage limits because they're imbalanced, that's not because of the balancing...it's because the BMS is defective or incorrectly designed (and because the cells weren't matched or were failing).


There's plenty of misinformation about all things battery-related out there, mostly from people that don't understand how they work, or from companies that have a product to sell that they couldn't sell if things were built correctly of the right parts of high quality.

 

[Kirill]

I mean the maximum possible voltage for my cells. To use the maximum possible capacity.
Since my cells are not perfect.
The entire assembly can no longer be fully charged. So I want to charge as much as possible.
At the moment the highest possible voltage for each cell is about 3.4V.