Ok, thus you already posted the link but I was looking for a blog
I looked the curves, and personnally I am unable to conclude anything :
On the 2 first curves, Y axis are not the same, we cannot easily compare voltages, and on which criteria is based the 0% SOC !? It seems to be based on the global pack voltage for an average cell voltage of approx 2.1V.
On the first curve, we can deduct that the weakest cell (with the lower capacity) is the cell 4 : it reach the theoretical 2.1V LVC before other.
On the second curve, we can deduct the same thing, cell 4 is the weakest cell, because at the begin of the decharge it has the higher voltage, and thus it's this cell which triggered the HVC and stopped the charge.
Thus in the 2 cases (bottom balancing and top balancing), it's always the same cell which is completely charged AND completely discharged (voltage level range from HVC to LVC).
It's totally logical, because it's the cell with the lower capacity.
Because of this, if the pack is discharged in the same condition for the 2 tests, the curve of this cell number 4
cannot be different between top or bottom balancing curve in the range of HVC/LVC (3.65/2.1V range).
But we can see that the cell 4 curve is completely different in the 2 graph, there is a mistake somewhere !
And the Y axis should be time and no SOC, because which is interesting is to calculate the Wh that the pack output (knowing the continuous current discharge).
For the 2 other curves, cannot conclude anything, because the same discharge measurement is needed after a bottom balancing charge procedure, and there is not ! From this curve we can just conclude that I say above is true : the weakest cell is alway used on its full HVC/LVC voltage range in the 2 cases, thus the discharge curve of this cell
cannot be different in same discharge rate condition.
Quote:
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In any case the whole matter can be resumed in one sentence: Don't let the cells go over or under and you'll be fine. That's all.
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Absolutely, it's what I alway said, it's the primary goal of a BMS. It's why bottom balancing or top balancing during pack assembly is really not a primordial question, the final result will be exactly the same in term of Ah capacity because the global Ah of the whole pack will alway be the Ah of the weakest cell.
I think this bottom balancing debate only concern guys which want to use their pack without BMS during discharging and do the LVC cutoff on global pack voltage only.
But imo it's an error to think the pack will be bottom balanced all its life, because cell are not completely identical (temperature, internal resistance, ...), with peukert effect and thus charge after charge and discharge after discharge, voltage variation will appear on individual cells when cutoff on global cell voltage will trig.