You might with some(all?) LFP even find a slight hysteresis in pack voltage, at 
exactly the same SOC..

 (most visible if you are in the 30-70% SOC-zone) 
depending on ..if you have had a regen or a discharge pulse as your last event, 
then the no load voltage seems not to be exactly the same, at the same SOC. 
A higher rest voltage if you did a charge/regen pulse compared to if you just 
did a very short discharge.  

I agree with the others, count Ah is the way ot go to know the SOC % in the 
flat part of the discharge curve, 

Also my experience was, that decent cells dont have any / a lot of self 
discharge to balance out when in normal use, only milliamps might be needed 
over time, so if they are well (top)balanced once they seems to stay well 
balanced. But if the cells are damaged / have mfg problems from the begining 
then it might be a different situation,

Regarding balancers maximum current:
 we had a 5 Amp as the charger minimum current, so we did a pulse charge 
instead of use large balance currents, 

So if one cell reach the "balancing" voltage then we can just stop the charger, 
and wait for that cell to reach its lower voltage, with only 100mA or so as 
balancer discharge current, then we re-enable the charger(5Amp) until any 
cell(s) again reach the balance-start voltage. 

If you dont have any cell voltage monitoring , or any kind of signal / feedback 
from the balancers, then it might be tricky to do this, I dont have any good 
solution to shut of the charger in time if we dont know when we have a problem. 
(other than to use a lower charge current than your balancers can handle, but 
if one balancer do fail, then you will probably overcharge that cell later)

 I would prefer to use some kind of good cell voltage monitoring so you can get 
a warning in time if some cell go to low or to high, and also use it to shut 
of/cut down the charger, or cut back on the trottle if some of the cells get to 
low when driving.

in my opinion that should be a minimum when charging a large expensive pack..of 
more than 4 cells in series. :-)  

If we only use the full pack voltage for the charger to decide whan to go in to 
constant voltage mode, then we can get in troubles, for example if one cell in 
the pack reach "full" and lift off almost like a capacitor, long before all the 
others have start to climb up faster in the end, so if all the other cells that 
still are the flat and lower voltage region the charger will give the pack and 
the already full cell its maximum current. Not good.
For example:
if we use 3.60 V as the chargers maximum cell voltage * 25 cells = 90 volt
what now if one cell is full and the others are still at 3.45 V each?

3.45 * 24 cells = 82.8 volt  
Minus..say..89.8 Volt from the charger? 
 = 1 cell will now try to reach up to about 7 Volt, and maybe still at full 
charger current...if so, that can probably be "bad".  :-)

/ John

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