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Do I need to top balance ?

Loving plumber

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I bought 4- 280ah cells last weekend . This is my first and probably the last 12 volt batter pack built ,exciting! I’ve got a BMS on the way .and I bought a longwei dc power supply to top balance (on the way) . after measuring voltage here’s how they turned out . 2 of the cells came in at 3.303 volts and 2 came in at 3.301 volts . That’s really close ,do I still need to top balance ? The papers say max charge voltage is 3.65 . Thanks for your help !
 

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You cannot evaluate the SOC of the cells from resting voltage. They could be 5 or 10% different in SOC. Top balancing is the an easy way to get them in balance. Because of the size of the cells it could take a long time for a typical BMS to balance them.
 
I bought 4- 280ah cells last weekend . This is my first and probably the last 12 volt batter pack built ,exciting! I’ve got a BMS on the way .and I bought a longwei dc power supply to top balance (on the way) . after measuring voltage here’s how they turned out . 2 of the cells came in at 3.303 volts and 2 came in at 3.301 volts . That’s really close ,do I still need to top balance ? The papers say max charge voltage is 3.65 . Thanks for your help !
Cells can be really close at "shipping" voltage but still quite un-balanced at the top.
So the answer is, we don't know.
Its getting more popular to assemble the cells into a series pack complete with bms and charge them up to see if a top balance is required.
If it is you can top off the cells individually without breaking the pack down.

I suggest.
1. test the bms high cell disconnect at a safe voltage. I suggest 13.3(3.325 per cell) volts.
Don't leave the pack charging un-attended until you have verified that the disconnect works.

2. set the charge voltage to 14.4(3.6 per cell)volts for the first charge and see how close the cells are at the start of the high knee.
delta should be < 50mv

3. If the delta is acceptable low and no cell tripped then you are done If not then top off the cells individually.

If you have 1 cell that is quite a bit higher than the others you might want to bleed the high one down before topping the cells off.
 
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Anyways that is the simple version.
I left out some detail to make it easier to swallow.
Can fill in the blanks later.
 
high knee delta ?????
It seems my terminology has "5 question mark" confused you. :)

Delta refers to the voltage differential between the highest and lowest cell.
Knee refers to the extreme ends of the charge curve for lifepo4 cells.


MNXMe8Y9.png

I tried to link directly to the data at https://off-grid-garage.com/battery-data/
Had to download it so I could insert it as an image.
 
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You can skip the top balance. Assemble with the BMS and see how it goes.
Can always boost the low cells with the power supply even with the battery assembled.
An active balancer can also do the balance.
 
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