Note: There are two versions of the led collar.1.USB Charging VersionCharging once the light gets weaker will help to improve the service life of the battery2.Button Battery VersionReplace the button battery when power offCR2016 Button battery x 2 pcsPlease choose the version you want.specifications Width Neck Circumference XS 2.0 cm 28 - 40 cm/11 -15 inch S 2.5 cm 35 - 43 cm/13 -16 inch M 2.5 cm 40 - 48 cm/16 - 18 inch L 2.5 cm 45 - 52 cm/18 - 20 inch XL 2.5 cm 52 - 60 cm/20 - 23 inchFeature: Three flash ModeFirst Click_Quick FlashSecond Click_Slow FlashThird Click_Steady Light