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.
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". :-)
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