Most of the basic claims being made in this thread are true. Neither thermally-induced DCR change or BMS smarts alone explains all the observations made here, and they are valid observations. One does not cancel out the other or negate the fact that it exists.
Yes, performance suffers with cold temperatures because cell DCR, internal resistance--whatever you like to call "it"--increases (worsens) as temperatures get colder from room temperature. "It" is, more or less, the collective effects of electrochemical losses that we tend to model as pure resistances because that's what they look like to us as electrically-oriented thinkers. The symptoms we all see with this are sluggish performance (from the extra voltage drop imposed by the higher DCR) and reduced range (because more power is being lost as heat in the cell while DCR is higher.) And yes, cells will self-heat if discharge is agressive enough to overcome the cold, which will bring DCR back down and restore performance gradually back to what we're used to when it's warm outside. This is all during DISCHARGE.
During CHARGE, there are multiple things to consider. If you have a stone-dumb charger that will charge full blast no matter what, it will charge a cold battery at the rated current in all but perhaps the most extreme cases. If all else is equal, charging with this unit will take longer than using the same charger at room temp because 1) electrochemical heat losses across the cell (high DCR) mean less applied charge power is actually stored in the cell, and 2) the DCR shift slows the end portion of the charge process for the reasons stated earlier in the thread (by Luke?). However, if you were to use this dumb charger to charge a cold pack in this way, you will pay dearly in calendar life and capacity in a short period of time. This is because of loss of cyclable Li in the cell, as so eloquently explained by Luke in his first post in this thread. THAT is why a smart BMS limits current, whether it be from regen or the grid, into a battery that has cell temperatures below a certain threshold.
So, to recap, most performance losses in the cold during discharge are due to increased DCR in the cells imposed by the cold temperatures. Those same things WOULD limit charge times, but there is a bigger risk to long term cell health by allowing full rated charge current into a cold cell, so the BMS limits charge current to a level safe for those cells at those temperatures. This is usually going to be more noticeable than the electrochemical effects alone would have been, so you could technically say that in most systems the slower charging of a cold battery really doesn't have anything to do with elevated cell DCR....even though that elevated DCR is there and can slow charging all by itself.
Hope that clarifies.
