Are my cells OK? Charger recommendations.

[akorchuk]

Hi community!
I've ordered 4 pcs LF280 Grade A Cells from Docan Tech (based on forum info).
It arrived with exact the same voltage of 3.290V and IR = 0.20mOm.
I'm using 200A JK BMS B2A8S20P (2A balance charge) and a Heltec 10A Active balancer + 38A CC/CV AC/DC power supply for charging.
I've charged the battery to 13.8V and the max voltage difference was 30mV so I decided that there is no need for Top Balancing the cells.

We have few scheduled electricity shutdowns per day and I'm using about 20-30% of battery energy per cycle. All this time the cells was balanced.
But after 2 weeks I've managed to connect the BMS to ESPHome and received more detailed data on the cells.

Spoiler: Voltage charts

I see that Cell3 and Cell4 performs the same
the Cell 1 is every time goes up when charging and down when discharging.
Cell 2 is in between.

Should I worry about this?
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And the second part is about the charger.
I'm using Eltek SMPS1000 10-30V DC power supply. It has Current limit circuit but I'm using it on max current it can provide (up to 39A)
I settled on it the 13.8V without the battery connected. On the charts you can see how the charging goes on. Is this ok for the battery or should I use more smart charger to be able to set higher voltage to achieve CC stage and then drop it to 13.8V?
Is it safe to leave the charger always connected with 13.8V when the current drops to 0A?

Thanks in advance for your suggestions.

 

[MisterSandals]

I see that Cell3 and Cell4 performs the same

To me, at least here, it looks like 1 and 3 are similar, and 2 and 4 are similar.

This is pretty disturbing:


What does your wiring look like? Happen to have a pic?

 

[Alkaline]

I believe you have what we call a RUNNER, a cell that shoots to HVC and screws up charging, this is probably the cell that will hit LVC (low voltage cut-off the fastest).

When did you buy your cells, did you perform a capacity test on them?

 

[akorchuk]

To me, at least here, it looks like 1 and 3 are similar, and 2 and 4 are similar.
View attachment 124876
This is pretty disturbing:
View attachment 124877

What does your wiring look like? Happen to have a pic?

Second chart is similar to to the first one, but the difference is when I turned charger off. On first picture it was On up to the red line and all cells were balanced during this time.
On the second picture I turned charger Off at the middle of the "running curve". This caused all cells voltage to slowly drop down. And they were balanced too after approx 30 min.
------
On the first picture the cells 2 and 4 were combined into the "top" curve and Cell1 and Cell3 were combined into the "bottom". And both this curves were balanced to the one at the end.
But on the second chart the Cell2 and Cell3 goes high and were "stitched" together.
I think this is due to the balancing logic of the BMS.
------
My wires are Ok I think. I have the WIP picture of the battery. It looks like + and - wires are dangerously close but it was under control. And it was only during the test run.

Spoiler: wiring:

Later I've added the insulation sheets between the cells and the balancer wires and packed the battery into draft plywood case.
Also I've added 250A fuse on "+" wire.
Now I'm waiting for custom case arrival to pack the batteries and use it in rack mount system.

 

[akorchuk]

I believe you have what we call a RUNNER, a cell that shoots to HVC and screws up charging, this is probably the cell that will hit LVC (low voltage cut-off the fastest).

When did you buy your cells, did you perform a capacity test on them?

I didn't make the capacity test because I don't have the electronic load. The only test I made was full charge/discharge cycle to calibrate the BMS.
I did only IR measurements and they looked good - all the cells were at 0.20mOms and exactly the same voltage. So I decided that cells are ok and to use them as is without Top Balancing etc.
As I understand the Cell1 should have greater IR to be the "Runner". What can cause the IR difference in 2 weeks?
I will order the IR tester and test again.

 

[akorchuk]

I've found the issue.
It's the poor quality connections between the batteries.
I have 70mv voltage drop on one connection of the Cell1 and 35mV on Cell2 when current is 70A. All other connections have about 10mV drops. I'm using busbars received with the cells but they sent only 4 of them and I have connected negative wire directly to the battery pin - this is the main voltage drop.
Actual cells voltages are the same.
I will fix this and post the update