I used an UN-Protected 18650 cell,
which BTW, is not such a great Idea with the L5, it can take the cell down too low.
Unless you swap out the cell right after you see it dropping out of regulation.
You shouldnt take an unprotected cell down much past 2.8V
I am still using that cell today, seems to be working just fine.
I have another cell that was taken down to 2.2V in another light. (dont try this at home kids!)
Remember, 18650 cells do NOT fit in a stock SF body, mine was bored out to fit them.
~John