JK BMS seems to discharge lowest cell during balance of fully charged battery

[nvbt]

I use JK-B2A8S20P with 8s eve lf280k (from Docan) battery bank installed couple of month ago.
I see a strange behavior during balancing at the high SOC (balance is on after 3.40).
Charging current is 1-2A or lower(battery is charged). All others cells are around 3.5 V but one was 3.38 and constantly going lower and lower. During around 20 minutes of balance it decreased to 3.36 and seems to go further.
There is no significant difference in cells IR. Bus bars and sense terminals are tightened well.
I will measure balance leads current when current clamp will be at hand.
Anyone encounters such weird behavior with JK?
Or it can be cell fault?

 

[Steve_S]

When the BMS is Balancing, you will see the cells indicated in Blue & Red will blink. The Blue blinks when it is discharging to capacitors for transfer of energy, the Red blinks when it is receiving charge from the capacitors. You have set balancing to begin at 3.400 which will start the balancing process when one or more cells cross the 3.400V threshold. The Balancing function will only transfer energy from Hi cells to Lo cells which is what it is doing per your screenshot. Just observe the Blinking indicators and the Balance Current line and you'll see how that is working.

You are also charging Above the LFP Working Voltage Range which is from 3.000V per cell to 3.400 with nominal being at 3.200 (50% SOC) You are better off charging to 3.450Vpc (27.6V) and terminating Bulk/Absorb charge @ 14A which is your Endamps/Tailcurrent. LFP will always settle when no charge current is being applied and that tends to take the cells to around 3.400... it IS the chemistry.

The one cell: The only way to know for sure the state of that cell compared to the others is to test it with a proper cell tester such as a Yaorea YR1035+ Cell Tester, these are also available on Amazon/EBay - pay attention to where it ships from, you can get one pretty fast if shipped from "local" warehouse. These testers give you the cell voltage & Internal Resistance of that cell. If the cell has a different Internal Resistance as compared to the others in the pack, that will affect the amount of charge it can take and will hold and it appears you have one weak cell but that is not certain. NOTE that the BMS does NOT read Cell IR, that is just the Cell Sense Lead resistance.

Troubleshooting this:
Rule 1, Assume NOTHING ! Assumptions are great for making asses out of oneself. ;-)
Start at the basics;
- disconnect the BMS (unplug Harness from BMS). Your Cell Wire Resistance shows good but good to verify resistance independently.
- remove the busbars from cells and clean all contact surfaces (bars & cell post/pad) with 99% pure rubbing alcohol. Same with ring terminals used for the BMS sense leads.
-- Now is the time to test Cell IR with Tester, remember to ZERO the tester by touching the tips together till it zeros.
- IF there are any Burrs or Ridges on the busbar holes, file them down, wipe clean again, place "ridge/burr side" Face Up so as to not interfere with cell contact.
- Reinstall busbars and Sense Leads on the (+) Terminal of the cells, torque to 5psi
- Reconnect BMS Sense Harness to BMS

Now charge the battery pack to 27.6V (3.450Vpc) and ensure that it is saturated to where the pack is taking 10A or less current. The cells should be close IF all of them have the same IR +/- 0.02, any greater difference, the further out of synch they will be.

Please Note:
Properly Matched & Batched cells will have identical IR throughout the "Working Voltage Range" of 3.000-3.400 which ensures that the cells will be pretty much at an equal Delta (0.005) within that range whether under load or charge. Once outside of the Working Range the cells will deviate farther apart the more you get from the range (high or low). This is normal for LFP.

Strongly Suggested:
-Do not charge above 3.450, set Bulk/Absorb to charge to 27.6V Your EndAmps/TailCurrent is 14A at which point the charger should transition to Float. (Constant Voltage/Variable Current)
- Do FLOAT charging @ 27.6 or 27.5V(3.437Vpc) (some gear requires a 0.1 lower float rate) and you will see Amps taken continues to decrease as the cells saturate to Full. At some point the BMS will just stop taking charge and drop to storage mode, allowing float to service any demand that exists if it has the power available from the solar array.

Allow the BMS to do at least 2 full cycles before its SOC reading is accurate.

Calibrating the BMS !
- Charge your battery pack to full and allowing it be saturated at float voltage.
- Allow the pack at least 1 hour after charging input has stopped to completely settle.
- Turn OFF Charge, Discharge & Balancing on the BMS.
- Take the Pack Voltage Reading at the CELLS (+) & (-) terminals and NOT at the battery pack posts. The cells where the wires to the BMS (-) & (+) post are taken from.
- Enter that voltage on your Settings Page for Voltage.
!! be Certain you are using a Good DMM/DVOM which is at least 2 decimal place accurate ! Also remember to ZERO the DMM by touching the probes together and waiting for the readout to be zeros. IF you see Voltage bouncing around a bit, Magnetic / electrical fields around you can do that. It's okay, just be aware of it. IF IN DOUBT apply Rule of 3, and take 3 separate measurements waiting each time for the reading to settle.

Once Calibration Voltage is entered, then Reenable Charge, Discharge & Balancing functions.

Hope it helps, Good Luck.
Steve

 

[uksa007]

Anyone encounters such weird behavior with JK?
Or it can be cell fault?

It's not the BMS, the BMS is trying to charge the low cell.
The cell is currently reading too low and balancer may not be able to balance it.

You either have a bad connection in BMS balance wires, bus bar, or it needs top balancing.
Measure the cell with a multimeter and see if it's voltage matches BMS reading.
Did you top balance the cells? Put them all in parallel and charge them to 3.65V, leave them in parallel for a day or so to equalise.
If cell still doesn't hold charge then it maybe faulty, but you need to top balance it first to determine.

 

[nvbt]

It's not the BMS, the BMS is trying to charge the low cell.
The cell is currently reading too low and balancer may not be able to balance it.

You either have a bad connection in BMS balance wires, bus bar, or it needs top balancing.
Measure the cell with a multimeter and see if it's voltage matches BMS reading.
Did you top balance the cells? Put them all in parallel and charge them to 3.65V, leave them in parallel for a day or so to equalise.
If cell still doesn't hold charge then it maybe faulty, but you need to top balance it first to determine.

I have a solid understanding of how balancing works in general and in such bms in particular. BMS clearly states what cell is discharged (with highest voltage) and what cell is charged (lowest one). But this lowest cells is going lower and lower during such "balancing" (in time range of tens of minutes). No current is drawing from the battery at this time (0 to +couple of Amps charging). That is why it looks quite strange for me.
The voltage is same on battery leads below the busbars as in BMS screen. IR was also measured on the battery terminals with YR1035 before assembly and today - no significant difference between cells.
I have not done top balance yet. First of all I assembled the battery with BMS to charge it faster than in parallel. After charging that there was no significant difference in voltage, so I keep it assembled for some time going to make top balance in future.
Sure, I am going to perform top balance, assemble again and check all wiring. Also, before that, I will check current flow during balancing to confirm that bms is trying to charge a cell in similar conditions.

 

[upnorthandpersonal]

I have a solid understanding of how balancing works in general and in such bms in particular. BMS clearly states what cell is discharged (with highest voltage) and what cell is charged (lowest one). But this lowest cells is going lower and lower during such "balancing" (in time range of tens of minutes).

If the state of charge of that cell is well below the others, you will see the voltage drop. LiFePO4 settles, and 2A from the other cells is not enough to keep its voltage up. Charge the cell on its own, and see if it holds the charge. If it does, plug it back in and you're good to go. If it doesn't the cell is bad.

 

[nvbt]

If the state of charge of that cell is well below the others, you will see the voltage drop. LiFePO4 settles, and 2A from the other cells is not enough to keep its voltage up. Charge the cell on its own, and see if it holds the charge. If it does, plug it back in and you're good to go. If it doesn't the cell is bad.

I see. This could be the reason. I have topped-up this cell individually for a short period of time and it didn't fall to so low voltage again (but with balance turned off). I'll check today more thoroughly.

 

[uksa007]

I have a solid understanding of how balancing works in general and in such bms in particular

Then why haven't you top balanced the pack?

Cell 3 is at about 90% the rest are almost full.
So it's missing about 28Ah which is alot, not something that a 2A balancer is going to fix.

 

[nvbt]

Then why haven't you top balanced the pack?

Cell 3 is at about 90% the rest are almost full.
So it's missing about 28Ah which is alot, not something that a 2A balancer is going to fix.

I had set the battery to the system in order to charge it first in the system, not in parallel (way too long). Then I had seen a very small difference in cells voltage and decided to keep it for some time. At this point I had a confusion. Now I realize that I should measure difference not in the middle, but only on the top (or bottom) SoC to determine an imbalance.

I have top balanced cells. Now they are almost perfectly balanced for 2-3 weeks even with balance turned off. Turning balance in BMS near top charge (after around 3.45) eliminated minor difference in minutes to several millivolts.

Thank you guys for suggestions!
PS after balancing my batteries I have encounter several highly unbalanced packs in neighboring systems (used without top balance) and helped to balance them. So, I have got a lot of insights and how-to's during this process .

 

[Delphinus]

I had set the battery to the system in order to charge it first in the system, not in parallel (way too long)

Top balance is necessary it takes too long but less them the time you waste to fix a problem the don't exist.

I only use grade A cells and assumed that don't need to top balance I was wrong and luckily someone told me to do it and save me time, when at top and bottom you will see some deviation but that is normal.

 

[nvbt]

Top balance is necessary it takes too long but less them the time you waste to fix a problem the don't exist.

I only use grade A cells and assumed that don't need to top balance I was wrong and luckily someone told me to do it and save me time, when at top and bottom you will see some deviation but that is normal.

I gained a lot of experience with balancing lifepo batteries last weeks helping neighbours. I would suggest that top bapance is necessary for all batteries regardless of grade . It can be much difference in SOC doesnt matter there is absolutely no difference in voltage after shipment. Cells can have be 1 mV deviation as received, but tens of Ah difference to complete charge. If there is no much difference, I do top balance applying small charging current and giving BMS with 2A balance do the trick, helping it by working like passive balancer - manually attaching powerful ceramic resistors to topping cells. And manually charging by 3.65 charger cells which are behind all others.
It takes several hours to balance 8s 280 Ah.