thunderstorm80 said:
john61ct said:
If you want both maximum range and good safety without a BMS cutoff, then balance at the bottom, and use cell level to to HVC when charging, accepting imbalance at the top.
This I don't understand. Let's assume one cell has 0.4Ah less than the others.
If I balance everything at the bottom, then even if I use 3.4V charge @ cell, then this single cell will reach 3.6V first, while the others are still at around 3.37V or so (the voltage during final charge but still not topped-out).
To repeat, charging "3.4V charge @ cell" means stopping when the first cell reaches 3.4V. Should be completely impossible for any cell to go higher.
If you are talking about using the "average" cell voltage, based on the pack voltage divided by S-count
that would be stupid, and I have nothing more to say about that.
> In my example, close to finishing the charge, the total pack voltage can be 3.37*23 + 3.9V ~= 81.4V, when I charge to to 3.4V. (finish at 81.6V)
Where the heck is that added 3.9V coming from?
john61ct said:
Charging to 3.40 means it is impossible for any cell to go higher.
> But due to imbalance issues - some cells can be well above it, some below.
No, see above again if you still think that way.
And please FFR, be more precise in your writing.
The setpoint setting on the charger
must be distinguished from what you measure during charging at the pack (or cell) terminals.
And neither has anything to do with the "true cell voltage" after resting isolated.
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> Since I have 24S battery, then this voltage spike of that single cell won't be as noticeable to the charger as if I had, for example, 12 cells in series.
You should not be allowing any such spike. If not basing whole-pack charge termination directly off the weakest (highest) cell level voltage
then **you** must calibrate manually the whole-pack setpoints in order to get the same result.
> Of course that suffering cell will gradually lose that surface voltage and so the others will slowly rise until the are full at rest, but this creates an accumulative damage to that cell.
Nothing will rise after charger is off, why would you think that?
> This is why I don't think it's a good idea to bottom-balance as it requires me to balance-charge every time...
Wut??
"Balance charge" means "trying to balance while charging".
Above I was not talking about that at all.
Terminating charge based on the state of the first / weakest / highest cell is just called "properly regulated charging" in my book
nothing to do with the act of balancing.
By balancing at the bottom, the delta between cell/groups at the top will be maximum.
By balancing at the top, the delta between cell/groups at the bottom will be maximum.
Middle balancing minimizes the delta at both ends.
> I still find the opposite extreme state from the balanced start a place I should never reach. With discharging it's easy. I just stop according to the Ah.
Wow, so that answers the above question begged earlier.
I do not know of **any** coulomb counting SoC meter I would trust to protect a pack at the bottom.
I sure hope you are using per-cell voltage at least as a failsafe backup
> With charging, it's hard NOT to charge the cells fully... Unless you want a very very very very slow charge.
Yes overcharging is the risk factor needing avoiding, and very low current rates make that **much** harder.
Undercharging is very healthy, not something to be avoided unless in practice depriving you of significant range.