Most battery packs that die tend to only have a single cell that is actually bad. A lot of the times the issue is the IC and not all the cells are sill useable. So I went to a good will and asked offered a crazy low price for a whole box of batteries. They agreed and I got to cracking them open.
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After separating the 18650 cells, they each go through three tests independently. The tests determines the cells internal resistance, how much capacity it hold, and if there are any internal shorts.
Each cell is labeled with the date, the capacity rating, and it’s current charge. To be extra safe I let them sit for a month and discarded any that dropped more than 0.1 volt.
I used https://www.repackr.com/ to make sure each of the sets of ten 18650s in parallel are balanced in capacity.