I was under the impression that mixing and matching cells (by model or by age/number of charge cycles used) can be sketchy. For example, if you have a very heavily used battery in series with new battery, everything may look okay externally, but you may end up over-discharging the older battery.
What are your thoughts on this? Haven't had any issues? Or do you take steps to mitigate it?
Yes, I can confirm. I use them only in parallel, and they usually come from the same battery pack.
A series connection would require extra care to select them for identical characteristics, and the use of a BMS to keep them balanced, avoiding the strongest ones to discharge at reversed polarity through the weakest ones, which would be a recipe for disasters. BMS circuits however can't do miracles; too different cells shouldn't be used together anyway.
You'd be far better off matching them based on percent of original capacity and internal resistance. A 3200mAh cell worn to 2600mAh is in far worse shape than a 2800mAh cell worn to 2600mAh, though you'd probably also see a big difference in internal resistance between them. You might also consider checking IR at various states of charge, if you're being comprehensive.
... and at that point, short of massive automation in a fireproof area, you're almost certainly going to come out ahead buying new cells.
What are your thoughts on this? Haven't had any issues? Or do you take steps to mitigate it?