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BMS cut off , one unbalanced cell

maydayone

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Aug 16, 2021
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I've just purchased a brand new Sterling 120ah lifepo4 battery and one cell seems to be quite severely unbalanced. On charging (14.4v as advised by battery data sheet) i have 3 cells at about 3.4 and one going over 3.75v which causes the BMS to cut off charging (see the attached). The difference between highest and lowest cell is 0.37v which is huge as far as my understanding goes.

I'm charging it with solar victron mppt 30a charger, set absorption at 14.6v (charging voltage from battery data sheet) and float at 13.8v as stated in the above data sheet. The victron profile for lifepo4 had slightly lower settings -14.2 absorption and 13.5 float which I also tried with the same outcome.

The charger never switches to absorption because at about 13.9V ( 3x3.4V +3.75v) the Bms disconnect the charging. After this the voltage gradually drops over couple of hours to 3.5v and n the unbalanced cell, then the Bms enables charging and the whole cycle repeats.


Is this a faulty cell and i should return the battery or is there something in the settings I'm missing?


I don't know if it matters but the battery has not been cycled at all. Came 92% charged from which it got changed to 100%-all reported by BMS so could not be very accurate.


Thanks in advance
 

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Thanks guys, Makes sense, just thought I could balance it easy enough without going through the hassle of sending it back.
 
So I've contacted the sterling and they basically claim that everything is ok, and it's a normal behaviour of the BMS to cut out the highest cell when the battery is fully charged (apparently 3.4V is a full state of the cells, and one with even the smallest deviation in capacity will reach the top 3.75 and the BMS will cut in).

This is the bit which I don't get. If the say that you should charge with 14.4V and float with 13.8V, how is the battery supposed to get to 14.4V if the full state is 3.4V( 4 cells gives 13.6V). To me you either need lower charging voltage or all of the cells need to get to 3.6V(4x3.6=14.4V) and the charger will then switch from bulk to absorption stage.

Can someone help me understand the above?

Thanks.
 
The cell shouldn't be hitting more than 3.65v. Sounds like the BMS cell voltage limit is too high.

You could try cycling the battery a few times to allow the BMS more opportunity to balance the odd cell. It might come in line, but I have my doubts.

Sterling's response seems a little defensive to me.
 
The way it's set up in the BMS app(I can't change it without password which I don't know) is that the balancing starts when one of the cells goes over 3.5V. When this happens(on charging from anything less than 100%) it's a matter of minutes for that cell to go from 3.5 to over 3.75V and the BMS disconnects, so there is not much time for the BMS to actually balance. Then it takes an hour of two for that cell to go below 3.5V and the charging resumes(three cells 3.4 and one 3.5V = 13.7 in total so the charger is still trying to hit 14.4V . Then its a matter of seconds before the cell hits 3.75V again. Charger is always in bulk as it never gets to 14.4V before BMS cuts in.

So how does that work with charging it at 14.4V, surely you would need 3.6V per cell to get 14..4V to start the absorption stage?
 
I have tried that, the battery is in my campervan so I have two victron chargers, one DC to DC and one mppt solar on which i reduced the output initially to 1A. But then I realized the the balancing only works if it's more than 2A. So after settings it to 2A the same thing happened it just took a bit longer(that was my assumption that the BMS will have more time to balance it before the cell goes over 3.75v). So instead of seconds it took a few minutes before the cell went from the start balance threshold 3.5V ) to 3.75V.

As far i understand how the balancing works with the 120Ah cells it would take a "Considerable" amount of time?
 
BMS balancing is good for small-ish imbalances. .37v difference is big.

You don't HAVE to get to 14.4v. If you use 14.2v - or even 14.0v - instead, how long does it take for the runner cell to trip the high voltage cutoff?

For what it's worth, I've changed my charge profile to go up to 14.2v. I don't need my cells to be at 14.4v.
 
I agree i don't have to use14.4v but at the same time i paid a lot of money for a brand new battery and I'm using parameters which are explicitly stated by the manufacturer (14.4v charge and 13.8v float). But of course i understand what you're saying.
When it trips i have 13.9V ( 3x3.4 plus 3.75V) which is 0.8V below what they say to change it with.

When I'm charging it from let's say 95% it takes maybe about 3 minutes for the cell to get from 3.5V(balance start setting) to 3.75v when it trips this is with about 5 to 8A from solar. When I reduced it to 2A then it's maybe 5 minutes.

Just before it tripped with 3x3.4 and one 3.75v, when it came down to 3.5v and charging resumed it took less than a minute to trip it again.

So do your cells actually go to 3.55V before the absorption starts(14.2V)?
 
would you say something like 13.7v or 13.8v would be ok?
If you do not have any chance of warranty or return, this might be the reality of your charging profile. The good news is that you probably will have over 95% of your battery capacity.

HRTKD's suggestions are spot on as far as I know.

Here are some voltage vs state of charge charts that many use and respect:
LFP Voltage Chart.jpg
 
So I've contacted the sterling and they basically claim that everything is ok, and it's a normal behaviour of the BMS to cut out the highest cell when the battery is fully charged (apparently 3.4V is a full state of the cells, and one with even the smallest deviation in capacity will reach the top 3.75 and the BMS will cut in).

This is the bit which I don't get. If the say that you should charge with 14.4V and float with 13.8V, how is the battery supposed to get to 14.4V if the full state is 3.4V( 4 cells gives 13.6V). To me you either need lower charging voltage or all of the cells need to get to 3.6V(4x3.6=14.4V) and the charger will then switch from bulk to absorption stage.

Can someone help me understand the above?

Thanks.
You’re absolutely right that the imbalance is way too much.

From your screenshot it seems they are using a Daly bms. That bms balances with 35 mA maximum. So it will take many cycles to repair that imbalance, if ever.

On the other side, if the battery can achieve the promised 120 Ah capacity you are good. And a perfectly balanced battery would gain another few amp-hours.

However I doubt because your low cell is still at 3.76. And from 3.76 to 3.40 is a long way to charge for LiFePo4.
When approaching full State Of Charge, all battteries always have one runner. That one cell that reaches the 3.65 within seconds. But normally the runner will start it’s sprint when most cells are at 3.50 or so.
 
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Thanks guys, so what do you suggest then? The battery is brand new bought from a supplier not from Sterling directly and it is covered by 5 years warranty. I've contacted Sterling directly and they claim that the battery is well balanced and when the cells are at full capacity an one of them will very quick go over the limit. They were surprised that it doesn't reach 14.4V on charging, as always they say it has never happened before... First tried to blame my charger, but gave up that idea after i sent them screenshot of the BMS with disabled charging. I tried to reason with them that you need cells at 3.6V to add it all to 14.4v but I don't seem to be getting anywhere.

What should be the basis for returning it if the Sterling doesn't see anything wrong with it? It's not like the battery doesn't work at all. I'm not entirely sure if i have any way of arguing the warranty here?

Thanks
 
The battery is brand new bought from a supplier not from Sterling directly and it is covered by 5 years warranty.
I'd use the warranty with the supplier because its clearly not up to their specs.

If that is too inconvenient (shipping and or supplier hassle), then i'd find the max battery voltage that your battery will accept and live within that range (you'd probably be closer to 100% capacity than your are imagining).

I'm not entirely sure if i have any way of arguing the warranty here?
It won't charge to their recommend charge voltage without the BMS cutting out long before that. That should be enough.
 
Thanks, I've discharged it today to about 20% and charging it back up - it's the first substantial discharge cycle as it came at92% and was charged from there to 100%. Then I did couple of ones down to about 95%(I don't have much connected in my camper yet so it takes time unfortunately). If this does not change anything or at least substantially reduces the difference between that cell and the rest of them, then I'll contact the place I bought if from.

Would you say reducing the start voltage for balancing from 3.5V to something like 3.4 or even lower so the BMS has more time to act would make any sense?

Also the guy from Sterling mentioned something about that cell going over because the static charge :" One of the battery cells receives a static charge from the BMS that almost immediately comes back into balance with the other strings of cells once there has been any discharge on the battery. The battery appears to me to be full and correctly balanced, the imbalance stems from the static voltage as mentioned."

If my the battery gets charged with a relatively low current from solar -something like 5A, would that mean that this trickle current is causing this imbalance? , and if the battery was charges with higher constant current we wouldn't have this issue?


Thanks
 
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Would you say reducing the start voltage for balancing from 3.5V to something like 3.4 or even lower so the BMS has more time to act would make any sense?
Yes but its likely balancing in the milliamp range which is minuscule compared to the amps of charging and amp hours of your battery. I wouldn't expect to see it change much if at all. Do the math on how fast its balancing relative to the size of your battery and the amount of charge/discharge ... its a big disparity.
If my the battery gets charged with a relatively low current from solar -something like 5A, would that mean that this trickle current is causing this imbalance?
My understanding is that a relatively low charge current is better. High C charging and discharging will cause greater battery imbalances.

According to them the battery is full when the cells are at 3.4V, which I have no way of disproving but according to your charts that is clearly not the case.
If its any consolation, that is more inline with how MY cells behave. My cells all settle to 3.35V within minutes of charging to 3.4V or 3.65V. Its uncanny for this to happen with 20Ah cells all the way up to 206Ah cells but that's how MY cells behave. Others have cells that hold around 3.5V when charged to 3.65V.
I cannot explain the difference other than perhaps how we charge them or how are cells were made.

I consider 3.35V to be my 100% charge voltage. I pretty much evaluate the state of charge based on voltage (i have a Victron shunt that i don't use). For me, if my battery is 13.3V (3.325Vpc) or higher, i think its pretty full. At 13.0V i think its getting low. I have a huge battery and do not need to squeeze every bit out of it so this is a bit of a luxury to manage the SoC in a simplified way.

Here is a SoC chart for the quick glance estimates (YMMV):
LiFePO4 SoC chart.png
 
Thanks again for the extensive answer.

If you don't mind me asking what are your settings for charging(absorption) and float? Do you see any advantage of having higher charging voltage / float voltages? . As far as I know a "standard" settings for LiFePo4 batteries is 14.2V absorption and 13.5V float. I just wonder why would Sterling recommend higher values for both 14.4V absorption and 13.8V float.
 
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