Well, I replied to the YT post asking for such ideas but i'm especially curious as to how prevalent repackaged used cells are in supposedly new prebuilt batteries, such as the TimeUSB battery you took apart. I think you've basically already established that most of the cheap batteries that DO use new cells don't particularly suck, and depending on how you look at it pretty much range from 90% good to 100% good to the point that you've combined multiple battery dissections into one video lately because they're.. predictably just fine.
But used cells in 'new' batteries.. What known instances are out there? Would some things to look out for be, odd aH sizes that don't correlate with multiples of common cell sizes, suspiciously small BMS current ratings, unusually pessimistic cycle life claims (2000 or less vs typically higher claims), etc?
And couch this in, when does it stop mattering? How steep is the 'non-linearity' of capacity drop off after lifepo4 hits 80% of original capacity anyway? How cheap does something like that have to be to still be better than a spanking new lead-acid, for example? Is it even realistic to care about claimed cycles past 2000 etc when the battery has multiple cells and they ALL have to make it to 4000 for a battery that claims 4000 to actually get there, with daily cycling, in 10+ years!? Does 'sudden cell death' which might be more pragmatic of a concern (is it?) than the difference between 2000 and 4000 cycles, become more common after that 80% capacity mark, or are used cells likely to be essentially as 'reliable' as new but with quicker capacity deterioration? And what kind of uses would likely NEVER find the difference between 80% of 120 and 100% of 100 because they'll likely never cycle them often or deeply enough to find drastic differences, and is that a surprisingly large umbrella that covers a whole lot of people who would say they would not consider used cells but in all likelihood would still be happy with them?
Should we be crying wolf to the whole idea of derated used cells being sold 'retail' in new batts, or should we just be hounding them down to a lower price point, or forgoing the shamelessly easy Amazons return policy on 'pre-assembled' batteries and buying used raw cells cheaper through some other marketplace which would likely offer NO realistic chance of returns/support?
Sorry, as a car mechanic one of my main hobbies is finding out (on my own stuff) just how far and how long parts will go beyond their design intent. With lifepo4 I don't have the patience to wait all the years to figure any of this stuff out with my own testing, but would sure like to hear someone who's been watching and using them since closer to 'the beginning' give highlights of their accumulated knowledge, even if half of what i've asked for here might be considered vague generalizations and conjecture. I understand that it's easier to receive and find the value of such things, than it is to put yourself up to spreading it when you hold yourself to high standards. Or..maybe you've been JUST WAITING to rile up half of everyone by making any assertions along any of these lines.