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Does this come with a “know what you are doing” caveat?Yes, and I'll suggest that a disturbingly large fraction of the people doing this on YouTube don't know what they're doing, and are being brutally unsafe with the batteries, even if it works.
I've done battery pack rebuilding semi-professionally for a few years (I was rebuilding ebike packs for most of North America from about 2015 to 2018), and the people screwing around with the used/recycled cells scare the hell out of me. In no particular order:
* They don't care to invest in a spot welder and they solder directly to the terminals. Every datasheet out there for 18650s that mentions soldering says the same thing: "DO NOT solder directly to our terminals." You spot weld a strip on and then solder to that. Soldering to the terminal puts a ton of heat into the end of the battery which then flows into the cell windings. The separator is usually plastic. You really don't want to weaken it. With a spot welder, I can put my finger on the terminal immediately after spot welding, and I can spread out the welds in time. If I want three welds (six spots) on a terminal, I can do one weld on each cell, then come back for the rest, and keep the temperature "comfortable to a fingertip." I cannot do that soldering.
* They tend to take a single snapshot of cell capacity, maybe internal resistance, and then assign cells that way. Unless they organize them by wrapper color for aesthetics, ignoring the cell capacity and behaviors. Some cell chemistries will degrade far faster than others, and you'd ideally like to not have a mess of stuff in the same pack.
* They recharge dead cells with no idea how long they've been dead. Lithium cells are physically stressed at full charge and fully empty, and a cell that has been empty for a while may have very real internal physical damage, such that it will fail later. I won't charge anything under 2.5V unless I know exactly how and when it got there, and if it's much below 2.0V, I still won't put voltage to it. The risk of a lithium battery runaway and fire is too high for my tastes. Lithium battery runaways, beyond lighting stuff on fire, tend to emit things like HF, which you very definitely don't want to breathe. It doesn't hurt a bit while it's killing you, because it destroys the nerves first. Health-wise, you're actually better off if the battery venting is flaming, because the mixture of gases coming off that is somewhat less toxic. Again, it depends on the particular chemistry what you get. It's never anything friendly.
Unfortunately, "recycling" lithium batteries isn't really established yet, and most of what seems to pass for "recycling" is the cells getting shipped to China, having new ends put on to hide the spot welds, and then getting a shiny wrapper and sold with impossible capacities to vapers for $0.75 or something silly. The highest capacity 18650s are around 3600-3800mAh, and don't source an awful lot of amps in the deal. Anything claiming higher (again, in the 18650 form factor) is certainly a lie.
You can do it, if you're careful, but by the time you've properly characterized the batteries, I'm not convinced it's worth the time/effort. Those who claim it is tend to skip a lot of checks and be really, really casual with their packs. To their credit, they rarely catch fire, but the whole "DIY Powerwall" crowd using scavenged and abused cells is not a good example to follow.