Comment by userbinator 3 days ago
Cells in parallel will stay balanced and act as one big cell even if they're different capacities although they do need to be the same voltage first, but in series they need to be matched in capacity.
It doesn’t magically morph into one big battery, suppose you have 5 different batteries in parallel, 1 new and 4 used different model, and you discharge this assembly at 50A: a large proportion of the current will flow through the lowest IR cell, say 45A of it. But the problem is that cell is only rated for 10A.
So yes, the net current is supposed to be okay, but it’s not, and rather than responding individually to all the other comments I may as well mention that no amount of temperature monitoring will fix this. Temperature sensors don’t detect internal cell stresses that cause spontaneous failures.
Unless they're wildly different chemistries, they'll have an IR that's inversely proportional to SoC and capacity, so you won't get that situation of 1 cell supplying 90% of the current unless it also has 90% of the capacity of the pack. Any cell that discharges a little more will have its IR increase, reducing its share of the current.
Slight mismatches in capacity can be OK in series too, as long as you have a BMS watching the cell voltages to make sure none get too low when discharging and none get too high when charging. The risk is that the lower capacity cells get too low or too high while the overall pack voltage seems fine, causing dendrites to form inside those cells, leading to a short and a scary fire. It really comes down to how well the BMS is designed / programmed and how much you want to trust it.
Most people aren't aware of how dangerous it is to try to "revive" a cell that got too low so this technology definitely comes with significant risks for user error.