Recycled batteries. In order for that to make even a little sense, diagnosis needs to be performed on each and every cell to discard sufficiently damaged and/or ruined ones.
I prefer a good set of new, quality cheap cells. In order to maximize lifespan, I'd restrict charging voltage to 14/15th of maximum to dispel voltage related stresses (3.92/4.2V, for example). In such case I'd need ≈50% additional cells for the same capacity/energy, with a fourfold lifespan (or more so). Source: BU-808: How to Prolong Lithium-based Batteries @ Battery University
Charge balancing, proper charging algorithm?
I just skimmed the article but ebay circuitry and mismatched batteries is an explosion waiting to happen.
What i would love is a per battery independent circuitry and infinitely expandable, with selectable charge and discharge points (ie set for 4.0V at fully charged) and we could buy batteries as we feel like it (or recycled) and keep expanding with no risk from over charging/discharging.
When I looked at doing something not quite this large, I decided it was worth reading out the BMS information so I could avoid the trouble of tearing apart and testing cells from packs that were unlikely to produce a good yield.
My preliminary results were very promising. For about 30s effort/pack, I could identify most packs that had cells that were low-voltage and/or low capacity without ever cracking the pack. In the end though I decided that it didn’t make sense to proceed given the increasing availability of packs from totaled EVs, not to mention declining prices for large, new, commercially produced packs. Some details in my sig.
Pretty sure this was only a money saver if his time is worth nothing, but hey, if he enjoyed himself, who am I to judge?