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gasses inside 18650 - (self)discharged below 2,6 volts
#1
Shocked 
A local battery refurbisher says: 
- Lithium-ION (18650) cells generate gasses inside (chemical reaction) when voltage sinks below 2,6 / 2,7 Volts. That's why these low (self)discharged ones become dangerous after a while of further use; charged again.   Sad

- It's importent to group cells with a so close as possible value (delta) of internal resistance.

Is this like it is?

ben teker
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#2
(05-01-2019, 08:11 AM)ben teker Wrote: A local battery refurbisher says: 
- Lithium-ION (18650) cells generate gasses inside (chemical reaction) when voltage sinks below 2,6 / 2,7 Volts. That's why these low (self)discharged ones become dangerous after a while of further use; charged again.   Sad

- It's importent to group cells with a so close as possible value (delta) of internal resistance.

Is this like it is?

ben teker

@ben teker

I partially agree with that to a point. There are many cells that have a max discharge cutoff that is in the 2V range. Several examples include the LG LGDBHG21865, LGDBHE21865, the Samsung INR18650-20Q, and the Sony US18650FT among others.
There are also many with a 2.5V cutoff. Including but not limited to Panasonic NCR18650, NCR18650A, Samsung INR18650-25R, INR18650-29E, and Sony US18650VTC6  etc. Too many to mention. Now I agree that the majority of 18650 Li-ion cells have a 2.75 to 3V cut off but I don't believe a  discharge to <2.5V on most cells is that destructive. I agree that anything <2V can have an impact on the cells health and these should be treated with caution. Very slow recovery recharge at ≤50mA and of course an IR check to see if the cell has abnormally high IR.
Usually >100mΩ will indicate an unhealthy cell.

I have done many an experiment with low V cells as in the <2V all the way to 0.5V and even some of them below that with good IR readings and have recovered them. With cells like these I always run them through at least 5 cycles of C/D/C at manufactures specs of "Standard" charge rate and "Standard" discharge rate. Many of them test very well and once exercised the mAh readings stabilize to an acceptable level.

As far as grouping cell with similar IR values together I totally agree with that. My personal goal is to keep all my cells in a pack to an isometric window of 35mΩ to 75mΩ. I find that most cells that fall into those values are generally healthy. We would all like to have the same IR on all of our cells and them to be all of the same manufacturer which would be ideal but we live in the real world. At least I am not going to buy 3000 new cells. As we are harvesting cells from all kind of manufactures and different model numbers some variations have to be taken into account and built into the formula.
There are many powerwalls out there performing just fine with no attention paid to some of these parameters. In some ways I envy them as I am spending a lot of time recording, sorting and checking my cells. BUT in the long run I will know every cell in my powerwall has been checked and rechecked before use and if there is a failure I can trace that back to the origin of that cell and its testing parameters and results.

Note: All measurement for IR are taken with a 4 wire kelvin 1kHz AC mΩ meter.

Wolf
If 18 X 650 = 2200+mAh then we have power! 
May all your Cells have an IR of 75mΩ or less Smile
Last count as of 5/23/2019
Total Number of Cells                          5354
Cells  >80% of Capacity                      3801
Cells <80% of Capacity                       1553
Cells ≥2200mAh & ≥ 80% & ≤75mΩ    2645 (155) to go
For Info Google Drive
Not your average Wolf       
            Cool



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