Your home is just not safe without one. In the event of a power outage, the battery back-up will ensure that your detector will always work. When smoke enters the sensing chamber a light beam is broken up or deflected, causing the alarm to sound.
Photoelectric devices detect smoldering fires before they reach a flaming condition.
They can also sense the smoke from a flaming fire. Smoke entering the chamber reduces the electric charge, causing the alarm to sound. Ionization detectors are designed to sense superheated gases of flaming fires in their earliest stages. Ionization detectors are inexpensive, but are very sensitive to most types of airborne particles, making them prone to nuisance alarms. And ionization detectors are not effective in sensing smoldering, smoking fires.