This is what Fenix says:
Actually, we don't suggest you use 3.6/3.7 v battery in the LD10, though some customers has used, and you can use 1.5v or 3.0v battery in this model, you can buy the brand battery, such as the Sony, Sanyo, and so on.
This is what Mary@4Seven says:
Please do NOT use any battery over 3v with the PD20 or LD20, as this
will fry the circuitry in the head. The only light/head in this series
that can take a higher voltage than 3v is the PD30, which takes 2
rechargeable CR123A 3.7v batteries, and uses a different regulation
circuit in the head.
=)