Bleed down is of more primary importance, IMO.
Any cell with noticeable self-discharge will continually unbalance the entire pack. If you constantly balance charge ... you might be OK for a while. Balance charging typically involves the painfully slow process of bleeding down any bank that is charged higher than the others. With one cell of excessive bleed down, it's entire bank will bleed down, forcing a balancing by bleeding down all other banks, via balance charger or BMS. And, self-discharge seems to get progressively worse ... so, I prefer to get rid of these "pack spoilers" right at the get go.
I agree with you mostly except I reiterate that the "Bleed Down" test is a poor way to test cells. Here why: Cells that have reached their useful life don't always bleed down.
Ok, after reading this thread I made an impulse buy of what turned out to be about 325 18650 cells. They cost me $70.00 USA and were mostly from used dell laptop battery packs. I have separated all the cells and I am just finishing the study of potentially usable Samsung cells (just over 100 pink, 2600 mAh cells of various dates and chemistry) and my data show that cells beyond their useful life can be nearly fully charged and won't always loose that charge once put on the shelf. This is regardless of the fact that they won't even last 15 minutes under a .38C discharge. So the question becomes how does one separate the cells that don't
bleed down and are good, from the duds that don't
bleed down and are no-good.
One could make a guess....and that is what most who use the "Bleed Down Test" are doing, guessing. And one can even get very good at guessing...Or one can just charge the cells in a smart charger then discharge them through a watt meter and skip the time wasted of putting them on the shelf for weeks.
DrkAngel wrote:As for discharging "to 2.65V" ... I would classify that as needlessly and wastefully damaging!
LiCo cells, (18650 & LiPo), have minimal useable energy-capacity below ~3.6-3.7V.
(Varies by specific formulation)
I agree with you that a continuous and repeated discharge of lithium cells to a low voltage will shorten their life span. A lot of good science is out there to support that statement. (Never fully charge, Never fully discharge, Never store fully charged, The fastest charge possible without heating the cells because heat is the killer of lithium cells...) I believe, all things you have posted in this thread at some point, but lets look at a real world situation....
I roll around on a 36v front DD hub motor. The LVC on my controller, cuts out at about 31.5v. So if I were to build a 12s pack the LVC would kick in at 2.625v per cell (31.5/12=2.625v) Now, I don't get to LVC often, but it can happen. It has happened to me only once in 18 months when I unexpectedly needed to extend a days ride for business.
So, since LVC happens, used cells need to be tested to their full capacity under controlled conditions to fully understand the characteristics of that cell. So by testing a cell from full charge to deep discharge, through a watt meter, we can see what the full capacity of that cell is.... their-by making it mush easier to group cells for optimum pack balance and keeping the weak or expended cells out of the pack in the first place, no "Bleed-down" guessing needed.
Favorite Quote: "This is L.A., sugar. There is no 'over the top." --- Chris Erskine
Current build: Liahona w/ cheap front suspension and suspension seat post. Yescomusa 36v 800w generic front hub motor. 15ah Headway triangle mounted pack. Tronsung 30 amp,
Previous Build:1992 Trek Antelope 800 - Bone Crusher (no suspension) - Yescomusa 800 watt 36 volt front wheel kit. Don't do it! Get suspension!!!