BMS not properly balancing a battery

[sviatpizdat]

Hi everyone!

I have a battery 16s5p of Samsung 30q.

After half a year of occasional usage (about 20-25 full cycles) I found that, when the pack is fully charged, the 11th parallel is 0.25V lower than the highest one. When balancing turn-on, it balances to 0.2V (at about 4.1V of the highest cell) but then balance only gets worse! To about 0.3V (at about 3.9V of the highest cell). I've repeated a few attempts to balance it, but the picture did not change!

I also found, that when the balance is on, the 11th loses voltage (but it is the lowest cell, and should stay as is). When the balance is off it sometimes loses voltage, sometimes stays as is (but I had only few observations, so not sure about self-disharge).
My BMS has passive balancer. When cell is loaded by balancer, the (B) sign is shown near the cell in the BMS's mobile app.

When discharging, the 11th parallel seems to discharge slightly faster than other ones, but that's probably explainable by an initially lower voltage. If it'll have less capacity, I guess it would also charge faster then the others.

I've disassembled the battery and un-soldered the 11th from the (-) side, so I can measure individual cells, while (+) is still connected.
By discharging in the range 3.42-2.8 the capacity of individual cells is 0.407-0.416Ah, and resistance is 12.84-13.17 mOhm. The resistance of cells in nearby totally healthy parallel is also +-13mOhm.

The measurements seem totally fine, so I guess it may be a BMS balancer problem.
The BMS is this one https://www.lithiumbatterypcb.com/product/16s-17s-18s-19s-20s-lithium-or-lifepo4-smart-bluetooth-battery-bms-with-50a-constant-current/
It seems to be connected properly.

Where do you think the problem may be and how to resolve it?

 

[e-beach]

When you are balancing your cells, how long do you leave it connect to the charger?

 

[sviatpizdat]

I usually disconnecting charger immediately when see that the battery is charged. It keeps balancing when disconnected, if that's the issue.

 

[fechter]

It may balance better if you leave the charger connected for a day after it reaches full charge.

Another test is to charge it up, then disconnect the cells from the BMS and watch the voltage on group 11 to see if it is dropping. There is some chance you have a bad cell that is draining the group. More likely the BMS is causing the drain though.

Based on the specs for that BMS, it seems like the cells should eventually all get to 4.15v after charging and resting for a day or so.

 

[sviatpizdat]

It may balance better if you leave the charger connected for a day after it reaches full charge.

Noted. But could you explain mechanism around this feature?

Another test is to charge it up, then disconnect the cells from the BMS and watch the voltage on group 11 to see if it is dropping. There is some chance you have a bad cell that is draining the group. More likely the BMS is causing the drain though.

Should I charge it completely, or voltage drop will be observable at any voltage? Currently 11th parallel is disconnected and at ~3.5V, so could I observe from here?

More likely the BMS is causing the drain though.

What the problem may be in that BMS?

 

[sviatpizdat]

I think I have found the cause. At a voltage level of about 3.5V overnight four of five cells discharged 0-0.003V, but the last one discharged 0.016V. That cell draining the whole parallel.

On Monday I will check the discharge rate again, just to be sure. The battery shop is not working at the weekend anyway. Then will replace the faulty cell and then charge and manually top-balance the battery.