I had the same question after reading this warning at :
COMBO: Ultrafire WF-139 Lithium Ion Charger / 2 Ultrafire 18500 1600mah 3.7v Batteries
"These fully protected cells simplify usage with their integrated PCB. Perfect for your single cell application these cells are not for pack building and should not be welded or soldered on which may defeat pr daamage the PCB protection. If you are building a pack please choose one of our other cells and the appropriate PCB."
Note this combo kit is for 18500's but I have seen similar warnings for 18650's.
Does wiring 2 18500's in parallel (using a battery holder) constitute a pack?
What exactly is the concern here? Can anyone point to some reference that explains the problem?
I have noticed that a number of mods isolate the batteries from everything else. Is that done because of heat? Safety? Some other reason?
Thanks for the pointers!