Comment by aetherspawn
1 day ago
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.