BEWARE…..you can start a fire when you discard a 9-volt battery. These are the size of battery you would normally find in smoke detectors. Because the positive and negative posts are so close to each other, all it takes is a little bit of conductive metal for disaster to strike. In fact, just touching a piece of steel wool to a 9-volt battery will instantly make the steel wool erupt in flames.
In order to properly keep a 9-volt battery safe for disposal, the National Fire Prevention Association recommends covering the posts of the battery in electrical tape. NFPA also insists that you never throw away 9-volt batteries, but instead take them to a disposal site near you.