am about to build a 54v battery pack for an ebike out of recovered laptop cells. It is not difficult to arrange the cell groups by the same voltage but the capacity of these cells may test a lot different even though their voltage is the same. You say in your book that the cell groups should be made up of cells that have close to the same voltage but I have not seen anything about how to deal with cells with different capacities. Is that an important issue, do I need to add more cells in parallel to insure that I have enough capacity? I understand that a cell with lower capacity will not produce the rated voltage as long as one that test up to rated capacity but how will that affect the other cells within the cell group? I will greatly appreciate any and all comments or recommendations.
Its best to try to make sure the capacity of each parallel group is as close as possible to the rest. For example, if you have a 3s battery, it'd be better to have the groups be 9.8 Ah, 9.9Ah and 10 Ah. You want to avoid the groups being something like 8 Ah, 10Ah, and 12 Ah. Big differences between the total capacity of each parallel cell group will mean that the pack will end up much more unbalanced.
Opus battery capacity testers are a good option. They can handle 4 cells at once. The speed is going to be proportional to the cost. Some people buy many testers so they can test more cells at once. 5 testers would give you 20 cells testing at once, but at the cost of 4 extra testers. If you can wait, one tester can probably test all your cells in about 2.5 weeks assuming you can do 2 batches of 4 each day.