9V batteries are often found in devices that aren’t used very often. If you use a NiCd rechargeable battery you may find it completely discharged by the time you need it. Capacitors on the other ...

Generally, the best 9v battery for a smoke detector may last anywhere from 6 months to 2 years. Meanwhile, 9V lithium batteries typically have a longer lifespan and may last up to 5 years or more.