I have one of those on the back of my helmet for a backup. I don't know how long the battery in it lasts because it's always dead when I reach home and I can't see the back of my head when riding. Not sure when or where I got it. Probably a GoodWill bin thing. Oh wait, now I remember. They used to sell them for a buck at Cash King stores and my great grand kid picked it up for me because he got a couple for his trike and thought gramps should be cool too. See, kinda obligated to use it haha.