How does the cold affect 18650 cells?
How does colder temps affect the life cycle of these high capacity cells? I realize that there must be a bunch of variables at play, but boil it down so that a numbskull like me can understand. If one normally can go 2 hours before charging in warmer temps, will the cooler temps degrade that time by 15-30 minutes? Or are lithiums more tolerant of temp changes than some other chemistries?
I assume in normal use the cells would be charged at room temperature.
If by "life cycle" you mean the 'cycle life' which is the number of cycles to the point the cells are considered at the end of their life (80% capacity), then cool temperatures do nothing but good as the cycles will be shallower (as you discovered) and deterioration slower.
If by 'life cycle' you mean the energy or capacity that can be obtained from the cells on discharge, then this can be considerably less as you discovered. The chemical reaction slows down which increases the internal impedance and causes the voltage to drop much more under load. Using higher discharge rate cells can help a bit (lower capacity but lower impedance) if you can find them. I haven't found any 18650s yet that are fast-charge/discharge...
Other cell chemistries including other lithium formulations are less affected by low temperatures, but these have lower voltages than lithium ion which makes them unusable.