KL27x's quote
You have not thought this all the way through, C. This a red herring. Computing the internal resistance allows you to calculate the energy available at a given load. But it is not necessary to include that into cutoff limits. On the low end, the cell with higher internal resistance will sag in output more than the others. And you will still safely cut off the output when it reaches the lower limit. Maybe it cuts out a little earlier than it could have, but at least it's not a safety issue.
Looks to me that "You have not thought this all the way through"
Yes a low voltage cutoff works for safety, but what is the result?
Simple example
panasonic ncr18650b
https://na.industrial.panasonic.com/sites/default/pidsa/files/ncr18650b.pdfIf you pick a protect the cell from low voltage at 3 volts,
you are stopping the output at around
2250 mAh for higher current case for use of about 69% of capacity.
3150 mAh for a lower current case for a use of about 96% capacity.
And this is from a graph that is not complete, this is just different discharge rates.
If you pick a lower voltage and then you have to hope that your voltage sense is up to task and some other variable does not go out of range.
You can't read the charge from the voltage of a Li-ion battery. There is a huge section of the charge/discharge curve which is flat. This is the reason why you need a coulomb counter calibrated for the specific chemistry to determine the charge in a battery pack. The only thing the voltage indicates is whether you can charge or discharge a Li-ion battery any further.
Yes, absolutely right. I believe this is part of the reason why traditional “ top balancing” of series strings of large LiFePO4 so often fails.
Even this is not complete.
To use a coulomb counter you need it calibrated to the specific pack & it needs to be re-calibrated as the pack changes.
One way to do this calibration is to sense the rate of voltage change with a constant current as you get close to lower limit.
You are sensing the curve.
But again if you have a series pack, each battery in series will have a different capacity and change over time.
So use little of battery an pick very safe top and bottom voltages where any other change can not take you out of safe zone.
Or get more facts by measuring them an making a better decision allowing more use of packs capacity.
C