9 Volt Battery
A nine-volt battery, the most common of which (and the one referred to here unless otherwise stated) is designated a PP3 battery, is shaped as a rounded rectangular prism. 9-volt batteries are commonly used in pocket transistor radios, smoke detectors, carbon monoxide alarms, guitar effect units, and radio-controlled vehicle controllers. They are also used as backup power to keep the time in digital clocks and alarm clocks.
Nine-volt alkaline batteries are constructed of six individual 1.5V LR61 cells enclosed in a wrapper. These cells are slightly smaller than standard LR8D425 AAAA cells and can be used in their place for some devices, even though they are 3.5 mm shorter.