The IRAY-AC37 BOLT Series Battery Extender threads directly onto the 18500 battery compartment of a BOLT Series allowing the installation of an 18650 battery which extends battery life by more than 75%.The IRAY-AC37 is compatible with BOLT TL35 TL35V2 TH35C TH50C TH50CV2 and TD50L models (not compatible with BOLT SE Series models).The IRAY-AC37 is designed for use with flat-top 18650 batteries only (not compatible with button-top 18650 batteries).