I must be missing something? I have a Fenix PD35 and have been running it on cr123 batteries. I’d like to start using it with rechargables. Fenix recommends the 18650 li-ion cell but not the rechargable cr123. What I don’t get is 2 cr123 cells in series gives you 6v nominal and an 18650 only has 3v. How does this work? Is it the built in regulation on the flashlight?