A chest strap is on when it's on your chest. The battery is good for a year or two of use, when you wear it it picks up the electrical activity of your heart, that's what wakes it up. I think the Garmin watches in broadcast HR mode work differently, where they'll automatically turn off after a while of not being connected to anything. To save battery. The Edges I've had always picked my chest straps back up when I left range and came back.