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Top Balance - JKBMS - drift voltage between pair and odd cells

What was your charge voltage?
I would say put the pack into service and see if the BMS can keep the cells top balanced.

Ok, so I will let them settle down, and give the final charge of 3.65*8 in series.
Can I assume after that, that they are top balanced?
 
One thing I saw and forgot to mention.

Change the setup for start of balancing. You presently have it set to 3.0v. Change it to 3.4v.

Balancing in the flat portion of discharge curve will likely misbalance cells so you have a cell overvoltage shutdown problem when attempting a full charge absorb cycle

When you have a moderate amount of inverter discharge current, slight cell impedance variations and variation in battery terminals connection resistance will cause more or less voltage drop, enough to send the balancer balancing in the wrong direction.

Somewhere in the setup screens there is a balance variance setup. No benefit to set it any lower than 5 millivolts as that is about the real accuracy of BMS cell voltage determination. Setting it lower just causes balancing to thrash on and off more.
 
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What is the expected in service charging voltage?
Time for the mad scientist part to end and put these cells to work.
 
One thing I saw and forgot to mention.

Change the setup for start of balancing. You presently have it set to 3.0v. Change it to 3.4v.

Balancing in the flat portion of discharge curve will likely misbalance cells so you have a cell overvoltage shutdown problem when attempting a full charge absorb cycle

When you have a moderate amount of inverter discharge current, slight cell impedance variations and variation in battery terminals connection resistance will cause more or less voltage drop, enough to send the balancer balancing in the wrong direction.

Somewhere in the setup screens there is a balance variance setup. No benefit to set it any lower than 5 millivolts as that is about the real accuracy of BMS cell voltage determination. Setting it lower just causes balancing to thrash on and off more.
I followed the previous advice of @smoothJoey and already change to 3.425.
 
Time for the mad scientist part to end and put these cells to work.
As a newbie I'm just following the experts advices: top balancing + capacity test before putting the cells at work.
I have more 8 cells to top balance, and then I will do a full 16 cell capacity test.
 
I’d say top balance in parallel if you want but I’d almost say the BMS is doing its job and put it in service.

Proper top balance may get you another what 2% capacity? But the BMS should get you there after the first dozen cycles or so.
 
Time to cycle your battery a few times to test your efforts and then throw a Heltec 5A balancer into the mix.
I think we fuss too much about balancing to the last mV. Cell delta under 50mV is quite acceptable.
Purists , shoot me now.
 
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Hello,

I'm building a 16 cell (48V) 272Ah battery bank, and yesterday I started to top balancing the first 8 cells.
I'm using JKBMS 24S 2A 200A BT and two benchtop power supplies (30V10A anda 60V5A) with 15A total.
I put the cells in series and the top balance procedure started, btw the "Balance" feature of the BMS was turned on during the top balance.
After aprox 14h the BMS was triggered since "Cell Over Voltage" 3.6V was reached.

And this is what I got:

Cellinitial state (V)4h later (V)
(charging)
14h later (V)
(stop charge)
17h later (V)
(w/Balance:On)
13.2953.5553.419
23.2963.3813.354
33.2953.5493.419
43.2923.3663.350
53.2933.3753.5513.419
63.2953.3803.354
73.2953.5443.419
83.2903.3923.3713.348
Delta Cell0.0060.0170.1890.071

As we can see 14h later there is a big diference between the cells voltage, but curiously the cells 1,3,5,7 are very close, and cells 2,4,6,8 also.
(the column "4h later" shows the lowest and highest voltage cells)

So, what could happen? Different SOC cells, but why exactly between pair and odd cells?
Should I leave the cells in series so the JKBMS could balance them? Should I connect them in parallel and let them balance?

Edit: add "17h later" column (the cells are settling with BMS "Balance" mode on)


Thanks
You top balance in parallel, not in series.
 
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What’s your “balance Trig, Volt” set to? .020?
I saw one screen where there was zero balance current but it should have been working. You can set that to .005
If you were balancing below 3.4 volts with a small charge current(too much time balancing under 3.4 vpc) it’ll be chasing its tail and it’ll be crazy.
 
Ok, so I will let them settle down, and give the final charge of 3.65*8 in series.
Can I assume after that, that they are top balanced?
No
It's like you got a rowboat with 16 people in it ... pushing the boat to 57 miles an hour (ok .. it's kind fast .. : :giggle: ) do not assure you that all those 16 men are equally good .. one will be the weakest and will exhaust first, while an other will be the strongest .. same with cells.
In serie you do not know who is working the most ... they are not working equally (different voltage, if it make sense) cause they are all 16, different.
One thing, the higher you go in charging voltage, the bigger the voltage delta between cells will be high... cause it's at their high state of charge that they show their diffrences (well .. let's simplify it like this)... in one word .. you got to take the weakest one and use it as the "standard".
So if your weaker cell can do 95% of the capacity, then charge your pack at 95% or lower of total capacity .... You do not need to push your cells to 100% SOC, better be at lower voltage and not "forcing" the weaker cell to work out of it capabilities.
And trying to go further then 95% .. will not work, well it may be .. but it will be forcing the system out of it's limits.

I would say that even a badly balanced pack should balance itself (well the bms will do) if you do not apply high C, AND do not try to charge the pack at 100%, you got near no capacity between 3.4V and 3.6V. The jkbms will do the jobif correctly set.
 
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