Electric smoke alarms beep when the battery is nearly dead. The alarm usually beeps every few minutes until the battery is completely dead or until it is replaced.
A functional battery is crucial for a smoke detector, whether the unit operates solely on battery power or uses the battery only as a backup in case of electrical system failure. It is best to replace the battery as soon as the beeping starts or plan to replace the battery twice a year before it is drained. Most fire departments recommend changing smoke detector batteries when the time changes each fall and spring. Some fire departments offer free batteries to residents who live in areas that are at high risk for fire.