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Difference of Cell Voltage @300mv+ and I do not know how to stop it without from climbing!

GamingGringo

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Jan 4, 2021
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2
Hey guys,
So I've been using my 266Ah, 24v battery for a few nights now and I'm noticing some issues with the cells not balancing correctly.

Every night after I lose the solar power the battery starts to drain specific cells faster then others and the difference is shutting my battery off every few seconds.
To prevent this so i can sleep comfortably knowing my fridge and fan won't shut off, I've shut off the "power off" feature and just let it flash it's warning that the difference is higher than what its set.
Is that a problem?

Also, how do I prevent this without re-balancing? It's a pretty big battery that I plan on using frequently. Does anyone know if there are some settings that need to be tweaked? It seems to constantly use #5 @ #6 the most. Idk why...

Any help would be appreciated! thanks
 

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Disabling low voltage cut-off may have destroyed your battery if the cells have drifted lower than 2.5V.

Assuming you haven't destroyed your battery...

Have you triple checked that all connections, bus bars, BMS sensing terminals/leads are properly tightened and of high quality?
Did you top balance it?
Did you put it into service immediately after top balancing?
Is your BMS set to balance only during charge, above 3.40V and > 20mV difference?

If the answer to any of the above is "NO," then you need to resolve it by correcting the connection issues or top balancing it.

If you did not source quality cells from a reputable supplier, you may have bad cells.
 
Take a look at These Chargery Settings (mine) adjusted for your 266AH cells.
The Chargery only has a light Passive Balance which is fine for smaller cells below 100AH but when you get to the big cells, it's terribly slow AND only burns OFF the high Voltage and does not transfer it to low volt cells. An Active Balancer of 1A or 2A is capable of shifting the voltage & balancing up a pack fairly well, as long as it starts out that way with a good top balance. If the BMS is "internally" powered it will have foibles which is why the external power adapter was made available which you can drive from the battery packs main terminals.
ALSO check and ensure that each cell connection is Clean, no ridges/burrs on busbars and that it is all installed nice & tight 4-5 newton meter force is all that is required.

I added QNBBM-8S Active Balancers onto my fleet of packs with teh Chargery's and they worked out. But as things evolved & changed "big" I am replacing the whole fleet of packs including utility ones with JKBMS' with 2A Active Balancer & better comms (RS485/CanBus) and BlueTooth of course.|

SettingDefaultMy settingsNOTES
Over charge P Voltage3.653.65
Over charge R Voltage3.553.55
Over charge Current50130A0.5C rate 50% of AH Capacity.
Over Discharge P Voltage3.002.65CUTOFF Trigger
Over Discharge R Voltage2.002.75Release @ this V.
Over Discharge Current300266A1.0C Rate
Low SOC cutoff20%0%forces cutoff @ % (faulty) DISABLE THIS - It WILL clobber you repeatedly.
High Temp cutoff50C50C122F
Diff of Batt Temp1015C
Diff of cell Voltage30mv200mvUnmatched cells drift, at the top during charge. This can be up to 1mv per AH capacity
Temp UnitCCCelsius - Hey, I'm Canadian F is too weird.
Key BeeperONON
LCD Backlight1010min
Cut off Delay Time1010S
Current Calibration-SET-IF Factory supplied shunt No Need.
Temp AlarmONON
Cell Empty Voltage2.502.50
Cell Full Voltage4.20?3.65
Default SettingEnable
Balance Parameter-SET- (OFF)*Passive is ON charge & Storage on, start at 3.40V, allow for 30mv diff.
Battery Capacity AH1266Label Value of cells
Battery Power WH10006809FORMULA (NominalVolts * #ofCells * RatedAH) (3.2*8*266=6809)
Low Temp cut off in Charge2C2C
Low Temp cut off in Discharge-10-10C

LOW SOC CUTOFF is directly using the Chargery BMS "SOC" State which is "Terrible" as it depends on the lowest cell and the math is pooched. Jason knows this, was supposed to fix it in a patch update, dunno what he did as he was bringing out the newer versions of the hardware.

Good Luck, Hope it helps.
PS: Once everything is switched & tested all Chargery, DCC-300's, QNBBM-8S and other brands, JBD & Heltec BMS will goto Garage Sale.
 
Disabling low voltage cut-off may have destroyed your battery if the cells have drifted lower than 2.5V.

Assuming you haven't destroyed your battery...

Have you triple checked that all connections, bus bars, BMS sensing terminals/leads are properly tightened and of high quality?
Did you top balance it?
Did you put it into service immediately after top balancing?
Is your BMS set to balance only during charge, above 3.40V and > 20mV difference?

If the answer to any of the above is "NO," then you need to resolve it by correcting the connection issues or top balancing it.

If you did not source quality cells from a reputable supplier, you may have bad cells.
Sorry, should have specified, It's LiFePo4 not lead acid so it's fine.
Thanks for the info
 
At 3.08V and no load your batteries are fully discharged. They will not all discharge to same voltage at same time due to slight variations of cell capacities and internal resistance. Your BMS is performing as expected. You may want to increase cell voltage difference alarm threshold to avoid nuisance trips. I would probably disable that feature. You should be covered by cell low voltage disconnect to protect each cell against overdischarge.
 
I'm repeating the questions but did you top balance your cells? How does your BMS balance the cells? What are the cell voltages when your SCC goes in float mode?

Something is terribly off with your setup. If cells are similar they should drain equally. With properly top balanced cells the difference will be observed near the low SOC levels. But you are saying, that once there is no power coming from the sun the cells are starting to drift.

I'll give you an example of my 4S LiFePo4 battery that I'm testing currently. Cells are all top balanced. Once the SCC goes into float mode the cell difference is close 0.001V (3.409 to 3.410). Once I've drained half of the capacity (~50Ah) cell difference is 0.002V (3.316V to 3.318V). They all go down the same way. This is how a LiFePo4 battery is expected to work.
 
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