The 9-volt battery is one of the most common types of battery available. Lifehacker.com reports that 9-volt batteries are composed of six individual 1.5-volt AAAA batteries wrapped in the rectangular housing of the standard 9-volt battery. (Safety note: Even though 9-volt batteries may be constructed this way, it is inadvisable to cut them open to access the smaller batteries inside.) Although not as pervasively used as AA or AAA batteries, 9-volt batteries do have several uses.
Possibly the most well-known use of the 9-volt battery is in smoke alarms. Smoke alarms or, in some cases, heat-sensitive fire alarms use 9-volt batteries as the power source. It is generally recommended that the batteries in a smoke alarm be changed about every six months to keep the batteries fresh, as the charge can dissipate over time. Some homes have the alarms wired into the electrical system. However, because of the possibility of power outages, it is good safety to maintain at least one or two battery-operated alarms.
Some clocks and clock radios use 9-volt batteries as a backup power source. In theory, the battery could be used to maintain the clock display. It is more common for the battery to serve as a means for the internal circuits to maintain enough power to keep the time. This allows the display to be correct after a power outage.
Radio controllers will often make use of 9-volt batteries, even if the object being controlled does not. This is common among radio-controlled cars and trucks, in particular. The battery is used solely to power the transmission of the radio signal to the toy car or truck.
A specialized type of flashlight, called a Pak-Lite, uses 9-volt batteries as the power source, reports 9voltlight.com. The Pak-Lite flashlight has no housing for the battery. Instead, the light itself is designed to snap directly onto the battery connectors. The 9-volt battery serves as the light handle. Pak-Light flashlights use light-emitting diodes to produce light rather than an incandescent bulb with a reflector, as many standard flashlights do.
Walkie-talkies, particular those intended for children, frequently require the use of 9-volt batteries. The battery, much as with an RC controller, is used to power the transmission of a signal between the individual devices (or any device on the same frequency nearby). Unlike the RC controller, the battery is also used to power the microphone and speaker to pick up or generate the audio input (voices and background noise generally).