Can I ask what you're using the cells for? I was thinking about building my own power wall. Are they lithium? Is the quality good? Have you done any serious testing of them? What is the chemistry?
I rebuild electric bike batteries for people, and specialize in one particular type of older system, so they're matched to that. Yes, they're lithium - spinel LiMn. Define "serious testing" - I've capacity and IR tested them (they're consistent and fine), and verified that they're legitimate cells. I'm not ordering UltraFire or something, they're a very specific chemistry and model to work with these older pack designs. If you are using new cells, the sort of testing required for junk cells isn't needed - they've passed QC at the factory and all I do is verify voltage on the modules after I build them. I've yet to have a bad cell show up in thousands of cells (I've pulled a few that were slightly dented by the production machinery, but that's just me being cautious - they worked fine).
If you want to store serious amounts of power, especially at low C rates, 18650s aren't the right answer. Get prismatic cells and use those instead - I have some old electric car batteries that are something like 65Ah cells (give or take). Those are much, much easier to make significant storage packs out of. They're harder to heat and cool, but that's not an issue when you're running at C/10 or C/20 rates instead of C or 2C or 5C rates.
If you do insist on using 18650s, please invest in a spot welder. The majority of "DIY Powerwall" builders are soldering, which is explicitly mentioned in pretty much every data sheet as something you shouldn't do. It stresses the cells thermally - I can touch the end of a cell immediately after finishing spot welding, and generally let them cool between welds (I'll run a row of welds down the cells, then come back and do another pass, so there's less heat flux - you can't do that soldering).
I'm pretty happy with what I pay per kWh for large orders, but despite having an off grid office, I don't power it with lithium - flooded lead is still a great storage mechanism if you have enough panel to charge things, and since I do, lead is a lot happiest plodding along at 100% SoC for most of the day. That stresses lithium.
If you do insist on building something with 18650s, consider segment fuses, absolutely install a high quality balancing BMS with pack cutout capability (so the BMS can terminate charge or discharge if a bank gets out of voltage range), and don't be stupid. The bulk of "WOW DIY POWERWALL!" builds I see are a structure fire waiting to happen. They start with junk, abuse the junk more, and then spend a while explaining about how they don't need a proper BMS because they check it, at least, you know, for the first week or so.