What to do if one cell out of 16 is faulty (Delta V is 0.532v)?

[SandyMcC]

I'm looking for advice
I have a problem with one cell (#6) in a 16 cell LIFEPO4 pack. It is a brand new pack so I hope Liitokala will replace it - but so far no response..
On arrival this cell had a slightly damaged corner, which I reported to LiitoKala. After testing, all the cells were exactly the same at 3.30v so we agreed to ignore the damage and hope there would be no adverse effects.

Suppose I remove cell #6 and replace it with cell #16.
I 'tell' the BMS (DALY) that there are now 15 cells and the pack cut-off voltages have changed accordingly.
I hope that I can persuade my Growatt SPF5000ES to run at these lower voltages.
Can this work OK?
Is there a better alternative?

Background information

As things stand now, I get less than 75% capacity, probably because of the voltage sag on the original cell #6; the BMS cuts out.
Average voltage per cell 3.068v
Delta V is 0.532v
Cell #6 is 2.629v


It may take 2 to 3 months to get a replacement.
This way I can get 15/16 = 93% capacity while I wait for a replacement cell to come from China.

I have watched lots of your videos on YouTube and get great joy from the help and inspiration they give me - thank you so much
It must take a big chunk of your time!


At 25% SOC... Big voltage difference 0.532v Highest-Lowest Delta V

At 78% SOC... Low Delta 0.029v

 

[Ellcon123]

I'm looking for advice
I have a problem with one cell (#6) in a 16 cell LIFEPO4 pack. It is a brand new pack so I hope Liitokala will replace it - but so far no response..
On arrival this cell had a slightly damaged corner, which I reported to LiitoKala. After testing, all the cells were exactly the same at 3.30v so we agreed to ignore the damage and hope there would be no adverse effects.

Suppose I remove cell #6 and replace it with cell #16.
I 'tell' the BMS (DALY) that there are now 15 cells and the pack cut-off voltages have changed accordingly.
I hope that I can persuade my Growatt SPF5000ES to run at these lower voltages.
Can this work OK?
Is there a better alternative?

Background information

As things stand now, I get less than 75% capacity, probably because of the voltage sag on the original cell #6; the BMS cuts out.
Average voltage per cell 3.068v
Delta V is 0.532v
Cell #6 is 2.629v


It may take 2 to 3 months to get a replacement.
This way I can get 15/16 = 93% capacity while I wait for a replacement cell to come from China.

I have watched lots of your videos on YouTube and get great joy from the help and inspiration they give me - thank you so much
It must take a big chunk of your time!


At 25% SOC... Big voltage difference 0.532v Highest-Lowest Delta V

At 78% SOC... Low Delta 0.029v

Have you fully charged and/or top ballanced the pack?

 

[SandyMcC]

Yup, fully charged several times.
I wonder if anyone has tried the idea of simply removing the faulty cell and adjusting the inverter settings by multiplying by 15/16. e.g. new cut-off becomes 56v x .93 = 52.5v

 

[SparkyJJO]

If the BMS and inverter are OK with the lower voltage, yes it'll work, but you'll have lost capacity and slightly higher running amperage due to the lower voltage.

 

[John Frum]

Yup, fully charged several times.

You don't explicitly mention top balance.
Did you top balance or not?

 

[SandyMcC]

You don't explicitly mention top balance.
Did you top balance or not?

I did not top balance rigorously, but I left the inverter and battery connected at full SOC balancing for 2 days.
The difference between the cells was tiny, and the lowest cell was not #6.
I will do it again if you think it might solve the problem. I might use a lab-power supply to adjust #6.

 

[SandyMcC]

If the BMS and inverter are OK with the lower voltage, yes it'll work, but you'll have lost capacity and slightly higher running amperage due to the lower voltage.

Thanks. The Growatt Spec says Low DC volt cut-off is settable between 40v - 48v And the Upper cut-off voltage is 48.0v to 58.4v. The BMS can cope with <200A discharge and the batteries are rated at 320AH. I aim for maximum charging and discharging current of 95A.

 

[John Frum]

I did not top balance rigorously, but I left the inverter and battery connected at full SOC balancing for 2 days.
The difference between the cells was tiny, and the lowest cell was not #6.

At what voltage was the delta tiny?
For the BMS to balance effectively the charge voltage has to be well into the high knee which is naturally above full resting voltage.
Some BMSs only balance while charging at greater than ~2 amps.

I will do it again if you think it might solve the problem. I might use a lab-power supply to adjust #6.

Big blue 320ah cells don't exist.

The resources section have data sheets for the popular cells.
You may be able to find out which you have by looking at the data sheets.
First thing to do is post a picture of the top of the cells so that some hawk eyed forum member can identify the manufacturer.
Its likely EVE but it is good to be sure.

 

[John Frum]

Liitokala has a bad repuation on the forum.

 

[SandyMcC]

Liitokala has a bad repuation on the forum.

I mentioned this bad-reputation to the contact at Liitokala in a light-hearted way at the time of ordering - in the hope that they would know that I was not a total novice. I realize, despite the Liitokala label saying 320Ah, that these are probably just good 280 Ah that on their first test got to 319.6Ah they seem OK except for the damaged one. The foam at the bottom of the box containing #6 was 20% thinner than for the other boxes (!)
Interestingly I scanned the QR-code and was diverted to bob.com which said my IP was blocked. When I VPNed to Taiwan and tried again, it was a junk site (Football, Wales, Finance)

 

[SandyMcC]

At what voltage was the delta tiny?

I have fully charged the battery and have taken a series of screenshots showing the states of batteries. Top left shows the time.
Blue indicates lowest voltage and red the highest. Can you draw any conclusions of
v (resting) and Delta_V is 0.004 at 2:16pm and cell #6 was marginallyhigher than the average (3rd place of decimals) .

 

[Ellcon123]

I did not top balance rigorously, but I left the inverter and battery connected at full SOC balancing for 2 days.
The difference between the cells was tiny, and the lowest cell was not #6.
I will do it again if you think it might solve the problem. I might use a lab-power supply to adjust #6.

What do you mean by full SOC? Voltage determines this so you need to be around 3.6V per cell or 57.6V for the pack. If your delta was low in this situation then I'd say you were balanced. This is only at the top and doesn't mean they will stay balanced at the bottom.

 

[Ellcon123]

I posted without seeing your latest post. 3.65V is full or 100% SOC. The BMS may be saying 100% but your voltages are still in the flat part of the curve. The BMS just counts amps in & out but has errors which accumulate over time. Every now and then you need to charge to full voltage to recalibrate the SOC.

What is the balance current of the BMS? Most are <200mA which will take some time on these large cells.

 

[Ellcon123]

Have a look at the latest youtube from off-grid garage,

.

 

[John Frum]

I have fully charged the battery and have taken a series of screenshots showing the states of batteries. Top left shows the time.
Blue indicates lowest voltage and red the highest. Can you draw any conclusions of
v (resting) and Delta_V is 0.004 at 2:16pm and cell #6 was marginallyhigher than the average (3rd place of decimals) .

Top balance voltage is 3.65 volts per cell.
The high knee starts at ~3.45.
The Daly BMS only balances during charge.

You need to top balance.