Cloud Defensive 18350 unprotected, flat-top cell. Note that the other common brand weapon lights are not rated for un-protected cells such as this battery.
Specifications:

Nominal Voltage	3.6V
Rated Capacity mAh	1000-1100
Configuration
Charge Time
Style
Includes
ANSI NEDA
Chemical Composition
Size	18350
Rechargeable	Yes
Quantity