A 9-volt battery is a small, rectangular-shaped battery typically used in portable electronic devices such as smoke detectors, remote controls, and guitar pedals. It has a nominal voltage of 9 volts and is typically composed of six cylindrical or rectangular cells connected in series. The cells are usually either alkaline or carbon-zinc.

The 9-volt battery have a wide range of applications in portable electronic devices that require a relatively low amount of power.

Smoke detectors: Many residential and commercial smoke detectors use 9-volt batteries as a backup power source in case of a power outage.
Remote controls: Many TV remote controls and other household appliances are powered by 9-volt batteries.
Toys: Many battery-operated toys, such as remote-controlled cars and robots, use 9-volt batteries to power the device.