Hi BG.
This part bothers me a bit. I realize your saying "In general", but this statement assumes that the cells have always been properly charged and discharged and have never (not even once) been abused in anyway.
Correct, it assumes that the cells have never been abused. Basically, I am assuming that you, Lux, or another CPF member bought the cells new and know how to use them and store them correctly. Same could apply to cells that have never been used and have been sitting for 5+ years near room temperature between 3.6-3.8V.
All bets are off when you are talking about old cells from unknown sources. If you don't know the history of the cells or the pack, best not even bother with it.
My concern would be, what if the cell were over discharged/overcharged, or trickle charged in a manner that allowed copper shunts to form, or metallic lithium to have plated out on the anode? The effects of either of these conditions build up over time, after repeated abuse, not all at once, and can potentially cause a dead short, or violent chemical reaction internally within the cell. A cell that only retains even say, 50% of it's original capacity still would have enough potential energy to "vent with flame".
Your concerns are valid, especially with respect to the lithium plating. However, lithium plating is far, far more prevalent in prismatic and pouch cells. I rarely see it in cylindrical cells, and only when the cells have been charged at rates well above what is specified. Lithium plating does not appear to be a typical end of life failure mechanism in cylindrical lithium-ion cells, which is by far what is used most by CPF readers.
The 80% capacity rule of thumb is likely on the safe side, but may also have in mind some insurance against other possible degradation, caused by conditions other than those a cell encounters when properly maintained. Your statement suggests that Li-Ion cells are safe, regardless of their loss of original capacity. From what I've learned from SilverFox and others, is that in some situations, particularly LiCo cells, can become unstable when capacity drops below 80% of original, due to other factors not covered in AW's comments.
You are absolutely correct. I can certainly devise an abuse regimen that will make a <80% capacity lithium-ion cell far less safe than when it was new. However, my interpretation of LL's original post was that he wanted to know about inherent degradation that would occur during normal use and/or storage.
So, to set the record straight, let me be clear that just because a cell has less than 80% of its original discharge capacity does
not mean that it is inherently safe. The usage (and misusage) history of the cell is very important.
I realize some members are using cells that retain less than 80% of their original capacity. A lot of these cells are pulls from battery packs. These cells were likely treated in a proper manner and may be safe to use at a diminished capacity that is below 80% of original. On the other hand, loose cells used in flashlights/torches can be subjected to many adverse handling conditions outside those experienced by cells in laptop battery packs, for example.
Yeah, I really cringe when I read posts by people who are pulling cells from used battery packs of unknown origin. I don't have much of a problem pulling cells from new packs, and indeed this is the only way to get access to certain manufacturer's cells.
Use of loose Li-Ion cells in flashlights is not an industrial application, as these cells were intended for. Therefore, I would suggest that cells used in this manner, should fall under stricter usage guidelines than cells in commercial packs and would suggest that the 80% rule should be followed. Better safe than sorry.
I subscribe to the better safe than sorry rule as well. And I totally agree that they should fall under the stricter usage guidelines. That is why I emphasized that older cells are more prone to self-discharge, and that one needs to keep a closer watch on them.
However, let me say this. The cells harvested from power tool and laptop computer packs are almost certainly manufactured by top tier companies, and have gone through fairly extensive testing prior to being used in that application. When you buy lithium-ion cells online, you often have no idea who the original manufacturer is or what level of quality you are getting. I can tell you from personal experience that many of the Chinese manufacturers screen and grade their cells based on capacity, self-discharge and impedance. The lowest quality cells get sold and rebranded for individual sale to distributors.
So, in a way, harvesting cells from new, unused commercial packs is a pretty good way to insure that you are getting high quality cells since there is a very good chance that those cells and that manufacturing process has been heavily scrutinized by the company whose name is printed on the pack.
I think we are both on the same page here. Hopefully I clarified some of my points.
Cheers!
BG
