A fire detector has a detection unit working on the optical scattered-light principle, as well as an electronic evaluation unit. An acoustic and/or optical warning device outputs an acoustic and/or optical alarm when a fire is detected. The fire detector has an energy storage device, in particular a battery, for the autarkic supply of electrical power to the fire detector as well as a voltage measuring unit for measuring a battery voltage present at the energy storage device. A warning device outputs an acoustic warning if the battery voltage falls below a first voltage threshold. The fire detector also has a sensor sensitive to ambient light and a device for suppressing the output of the acoustic warning if the ambient light falls below a predefinable brightness threshold. The detection unit is an open scattered-light detection unit with a detection space outside the fire detector and the sensor sensitive to ambient light is at the same time an optical receiver of the scattered-light detection unit.