Hey guys..
I have some 2-3 18650 flashlights... As I know, multicell flashlights always have a higher risk and you always should use same batteries with same voltage/charge level...
My questions is.. What can be the difference between the individual voltages, without taking any risk? 0.1V ? 0.2V ? ... ?
In my experience, a matching set of balance charged LICO cells will be within 0.01V - 0.02V of each other after being discharged in multi-cell flashlights. Never 0.1V - 0.2V
Never mix cells of different conditions like age, number of cycles, amount of wear or charge state. Use matched cells and always balance them to 0.02V difference or less. Protected cells help some but is no 100% guarantee, its generally better to observe proper battery safety as well.
0.1V diff is really not all that big. I somewhat doubt a cell only 0.1V above flat can force much current through its compatriot. I suppose >0.2V might be bad in some of the more aggressive driver like the one in the 3800 3xml since it keeps lowering the load to maintain power. Keeping it <0.1v should be pretty easy though for two same cell of decent quality (not crap "ultrafires"), what you really want to avoid is one full cell and one dead one.