The LED illumination module uses a 3v CR2032 battery that can be quickly and easily replaced by the user. To replace the battery:
Remove the battery cover by turning it counter-clockwise (lefty-loosey).
Inspect the o-ring to ensure that that it is present and not damaged.
Remove the old battery.
Insert a new CR2032 battery with the positive (+) side UP.
Replace the battery cap by turning it clockwise until snug (rightey-tightey).
Note that the illuminator is very dim by design. This allows for use in operational environments without producing a signature visible to enemy forces.