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Help -LifePo4 cells voltage difference after first charge


CCC
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I just assembled a new 12V Lifepo4 battery with four cells, connected to a BMS. All new cells, measured 3.29V at the begining. Connected the battery and after my first charge last night, the cells  this morning read 3.41 / 3.40 / 3.34  / 3.34. 

Do I have a problem with the battery?

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Nope. It takes some time for the cells to settle. At what volt rate did you charge? Dont go try and charge them at full rate. Then the bms protection starts kicking in to prorect if n cell starts running away. 

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Thanks for the reply Gerlach. I am very glad to know it's not an issue. I am very much new to this. The inverter charger started of at 13.6,V, it was there for a long time, then it went to 13.7V. I did not monitor it much after that, it was taking a long time to charge. It stopped the charge automatically at 14.1V reading on the charger.  I connected at volt meter to the battery as well. That reading was always 0.01 Volt less than what the reading on the charger was.  I have no idea what the charge current was.  It took 5 hours to charge up.

 

I was expecting the BMS to manage the charge and they all have the same voltage afterwards?

 

Edited by CCC
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1 hour ago, CCC said:

I just assembled a new 12V Lifepo4 battery with four cells, connected to a BMS. All new cells, measured 3.29V at the begining. Connected the battery and after my first charge last night, the cells  this morning read 3.41 / 3.40 / 3.34  / 3.34. 

Do I have a problem with the battery?

You cannot tell if you have a problem yet since you didnt charge the batteries fully. Charge it up to 3,6 maybe even 3,65 depending on battery. When you reach this voltage or a little before you BMS should be kicking in and start bleeding the cell that reaches this voltage. What BMS do you have?

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That's a Daly. Here are the specs:

afbeelding.png.bf4c77ca41cae698aca028f75dc112b8.png

As you can see it will start balancing at 3,5 Volt but your charger will never reach that. This implicates your battery will get out of balance more and more. How fast this will go is depending on the cells. If they are (almost) the same in terms of SoH it could take a while but sooner or later the imbalance will be to much. What happens is that 1 cell get up to 3,5V but the other are way too low. This will not cause damage by definition but you will have a much lower capacity from your battery.

Think about a charger that you can adjust yourself and that will give 4 x 3,6 = 14,4 V. Then you can profit from your batteries the max.

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Thank you for your reply. It is very informative. I did not know that. I will keep my eyes open for a charger that can be manually adjusted. I am glad to find out what the problem is with my setup. Thanks again.

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@RikH what found with this sort of bms and with the lifepo4 batteries, if you charge to high volts, the battery protect kicks in, thats, 3.75v and when it switch off the protection 3.55v and the ballance is like 30mA, so it takes supper long on 120ah cell. 

So what happens is, cells charge, one starts getting close to the over charge protection, the bms cuts the charge, now it waits for the cell to drop, while yhe other ballance but they don't get to level out because just as the over protect switch off that over charge cell starts to run away again. And this keeps going on and on.

Why i know this, my bms got n digital display that shows me all the info on every sell and i can check this via bluetooth to and i can adjust all the settings on the bms

On my 16s setup i founded that if you bring the charge volts down a bit, in my case 54.6v boost and float at 53.6v, the cells start stable out because there is now cell running away. They say with the 16s i can charge up to 58v but found out quickly at 56v that it will not work. 

Edited by Gerlach
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Thank you Gerlach, your post was also very informative. Is it a poor choice of BMS?

I did a 2 hour test run with the inverter, very light load. The combined voltage read 13.2V when I stopped the test run.  During the run, I measured each cell and they all measured 13.30V . (I did not take note of the time I measured - so not sure how far into the load run it was taken) I then started  another charging cycle and the cells all measured 13.36V at 15 minutes into the charge cycle. This is already looking better.

 

Here's the information on my charger:

Charge current 25A

Constant voltage 14.4V

Float 13.5V

I will keep an eye on the system and see how it pans out. 

 

Edited by CCC
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49 minutes ago, Gerlach said:

what found with this sort of bms and with the lifepo4 batteries, if you charge to high volts, the battery protect kicks in, thats, 3.75v and when it switch off the protection 3.55v and the ballance is like 30mA, so it takes supper long on 120ah cell.

Well yes but that is assuming he has 120 Ah cells but that is nowhere mentioned. (@CCC, what type of cells are we talking about in Ah?) Once the pack is balanced, yes that will take time, it has enough with this 30 mA balancers to keep it balanced.

53 minutes ago, Gerlach said:

So what happens is, cells charge, one starts getting close to the over charge protection, the bms cuts the charge, now it waits for the cell to drop, while yhe other ballance but they don't get to level out because just as the over protect switch off that over charge cell starts to run away again. And this keeps going on and on.

Of course this will never happen with a well balanced pack.

29 minutes ago, CCC said:

Constant voltage 14.4V

What a coincidence, exactly what I advised you in my previous post 😉. So this is good news, no need for another charger. Now just let it charge till it switches to float, after that measure individual cell voltages again. You will see it is getting better all the time. If not, you cells aren't that equal that a 30 mA balancer can bring it in balance. But let's see first how it turns out.

 

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I monitored the charger during the charge session. It stopped charging at 14.2V. Cell one measured 3.71V.

Cell 2 3.57 Cell 3 3.35  Cell 4 3.35. 

 

Is there a problem with cell 1? Or should I connect all the cells in parallel to balance? 

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To add some more info to a lot of very informative info that is already here.

One cell will always get full first. That is normal. After that the balancer(s) kick in and start moving charge from that high cell to other cells, slowly pulling the others up until they are all the same voltage. Then when you discharge, one of them will be empty first. That's because the cells are not identical, and again, that is perfectly normal. When the first cell is empty, discharge stops (regardless of whether the others have charge left), and similarly, when the first cell is full, charge stops regardless of whether the other cells are full or not.

If the cells are well chosen to have very similar capacities, then after some time they will be aligned at the top, and at the bottom they will be pretty close (if you get my drift).

In a new pack there is almost always initial imbalances. This can be avoided by fully charging the cells before assembly, but it seems more often than not these days, this step is skipped. It usually takes about two weeks for the balancer(s) to sort it out.

This is where the high end BMS/inverter setups come into their own. The BMS can do a calculation. If one of the cells is at 3.6V, for example, then it can add up the voltages of all the cells, plus a small offset, and instruct the inverter/charger to charge at that voltage. This effectively holds that one cell at 3.6V while the balancer moves charge (slowly!) to the others. As the other cells catch up, the BMS will raise the charge voltage, and over time you'll get to a place where they all even out around 3.5V (which is what you want).

In he absence of such a BMS and/or charger system, you can do this manually. Raise the charge voltage slowly (assuming this is adjustable) while keeping at least one of the cells at 3.6V, and give the balancer time to work. With some batteries (those with passive balancers I think) I found that you have to also cycle the battery slightly, eg a recent BYD battery I worked with had to be cycled between 80% ad 100% a few times before the imbalance started to improve.  I suspect it depends on whether the BMS does active or passive balancing. With an active balancer, you just have to hold the voltage sufficiently high and wait, while with passive balancers it is better to cycle it. At least, that is my experience.

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Thank you Plonkster for your very good explanation. I was under the impression the cells should all be the same. It is all starting to make sense for me now. OK, so nothing wrong with the cells, nothing wrong with the BMS or the charger. Phew! The fact that I am OCD does not help. 😉

Edited by CCC
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4 minutes ago, CCC said:

the cells should all be the same

They may not have been charged to exactly the same level. LiFePO4 cells have a very flat voltage curve, so even if they measured similarly before assembly, they may have had widely different states of charge. You'd be amazed how often this question is asked these days. I have a theory, not sure how plausible, that the increased demand means that the battery makers spend less time making sure the cells are balanced, and essentially expect the balancing to happen later after commissioning. As a result it is very common for Pylontech banks to initially report different SOC levels on each module, and it is common for BYD batteries to raise high voltage alarms in the first two weeks.

Some BMSes have impressive balancing capabilities, eg I once saw an active balancer that could pass 2A between cells. The engineer working on it said to me that he thinks it is overkill: The only time that capacity is going to be used is in the first few days... after that it is essentially a waste of money.

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5 hours ago, CCC said:

I monitored the charger during the charge session. It stopped charging at 14.2V. Cell one measured 3.71V.

Cell 2 3.57 Cell 3 3.35  Cell 4 3.35. 

 

Is there a problem with cell 1? Or should I connect all the cells in parallel to balance? 

Yes. Because 3,71V is too high. Connect them in parallel when they are in the state you described above and leave them connected for 12 hours or so. Ideally you should charge them up to 3,6V but not everyone has a charger that can do that. But don't worry, after this you'va done what you could and have given the cells a top balance.

I guess that your charger didn't come to 14,4V because the BMS cut off right?

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10 hours ago, RikH said:

Because 3,71V is too high

I would not say that it is really too high (as in dangerously so), but many BMSes certainly consider 3.7V to be too high (for good reason too). If I remember correctly, the danger point is 4.2V per cell, and many BMSes will give you leeway up to 4V per cell. Of the BMSes I am familiar with, I've seen 3.6V (Pylontech), 3.75V (Victron VE.Bus BMS), 3.8V (BYD), which are safe points that give you a nice large margin. Besides, very little additional energy is stored once you go above 3.5V, which makes this a good strategy.

The trick with proper balancing is to operate the battery in the margin between the balancing and switch-off voltages, so you want to hold that one high cell between 3.5V and 3.7V until the lower cells catch up. Not really the easiest thing in the world if the BMS cannot do it for you.

Edited by plonkster
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I actually came up with an algorithm to determine the charge voltage using only the maximum cell voltage (if your battery provides that). This is for a 16-cell battery. There are some fudge numbers in there, eg I assume that at least half the cells are close to 3.6V.

# mcv is max cell voltage
# bms.voltage is current battery voltage 
if mcv > 3.6:
    offset = 3.6 - mcv
    cv = max(bms.voltage - 1, min(56.8, bms.voltage + 8 * offset))
    return round(cv, 1)
return 57.6 # 3.6V per cell

This algorithm causes the charge voltage to scale according to how far over 3.6V the highest cell is by subtracting 8 times the offset from the current battery voltage (essentially stopping charge).

I found it useful to put the calculated charge voltage through an exponential decay filter to smooth it out a bit, but to still favour new values, ie large changes filter in quickly.

Of course, if you actually had the voltages to all the cells, it becomes significantly less hacky to do this...

Edited by plonkster
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3 hours ago, plonkster said:

I would not say that it is really too high (as in dangerously so), but many BMSes certainly consider 3.7V to be too high (for good reason too). If I remember correctly, the danger point is 4.2V per cell, and many BMSes will give you leeway up to 4V per cell. Of the BMSes I am familiar with, I've seen 3.6V (Pylontech), 3.75V (Victron VE.Bus BMS), 3.8V (BYD), which are safe points that give you a nice large margin. Besides, very little additional energy is stored once you go above 3.5V, which makes this a good strategy.

Now wait, you're mixing up 2 chemistries. We are talking about LiFePO4 cells here and the data you are talking about belong to NCM chemistry. We should compare apples....

3 hours ago, plonkster said:

The trick with proper balancing is to operate the battery in the margin between the balancing and switch-off voltages, so you want to hold that one high cell between 3.5V and 3.7V until the lower cells catch up. Not really the easiest thing in the world if the BMS cannot do it for you.

Very much agreed with this.

 

3 hours ago, plonkster said:

I actually came up with an algorithm to determine the charge voltage using only the maximum cell voltage (if your battery provides that). This is for a 16-cell battery. There are some fudge numbers in there, eg I assume that at least half the cells are close to 3.6V.

# mcv is max cell voltage
# bms.voltage is current battery voltage 
if mcv > 3.6:
    offset = 3.6 - mcv
    cv = max(bms.voltage - 1, min(56.8, bms.voltage + 8 * offset))
    return round(cv, 1)
return 57.6 # 3.6V per cell

This algorithm causes the charge voltage to scale according to how far over 3.6V the highest cell is by subtracting 8 times the offset from the current battery voltage (essentially stopping charge).

I found it useful to put the calculated charge voltage through an exponential decay filter to smooth it out a bit, but to still favour new values, ie large changes filter in quickly.

Of course, if you actually had the voltages to all the cells, it becomes significantly less hacky to do this...

This is interesting indeed but only usable when you are able to control the charge voltage. @CCC is not but given the equipment he has best he can do is as I described above IMHO. And yes, now you're talking LFP voltages! 😄

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