Burris states in its owners manual that this unit will use 18650 rechargeable batteries. The factory provided rechargeable battery will install fine, but after purchasing 2 Olight brand 18650 batteries I cannot close the battery compartment even with pressure applied. Upon a comparison with the factory battery I noted it appears approximately 1/8 shorter and will install easily. Although not mentioned in the owners manual a Burris employee in a promo Utube video also states it will take 2 cr123 batteries as an alternative. They also will install but requires much effort, though tech support seems to deny any knowledge of this as an alternative power source. (The unit powers on and operates fine with 123 batteries but now Im Leary of their use due to the aforementioned). So whats the answer guys ? User's recommendation: Hopefully Burris will address this issue and contact thermal owners.