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Panasonic PP3 (9 volt) battery

A PP3 battery, commonly referred to simply as a nine-volt battery, is shaped as a rounded rectangular prism and has a nominal output of nine volts. Its nominal dimensions are 48 mm × 25 mm × 15 mm (ANSI standard 1604A). It is widely used in smoke detectors, guitar effect units, pocket radios, and as backup batteries for digital clocks and alarm clocks. However PP3 refers to the type of connection that is on top of the battery or snap. The PP3 connector (snap) consists of two connectors: one smaller circular (male) and one larger, typically either hexagonal or octagonal (female). The connectors on the battery are the same as on the connector itself -- the smaller one connects to the larger one and vice versa.

The PP3 appeared when portable transistorised radio receivers became common, and is still called a "transistor" battery by some manufacturers. The Energizer company claims that it introduced this battery type in 1956 [1]. It is very widely used in smoke alarms and carbon monoxide alarms.

Placing one's tongue across the terminals will short circuit the battery and may cause a mild shock, and as such may be used as a prank or a crude test of the battery's charge. This can produce tiny amounts of chlorine gas^{[citation needed]}, which is toxic.

Additionally, placing steel wool across the terminals will cause the steel to ignite. This method of fire starting is often taught from a wilderness survival perspective, as it will catch flame even while wet.

The clips on the 9-volt battery can also be used to connect lots of 9-volt batteries in series. At least one death has been attributed to electrocution from too many 9-volt batteries connected together^{[citation needed]}. Alternatively, one can connect two 9-volt batteries together in a short circuit. The high current and heat may lead to a fire.

Sometimes cutting open the battery yields 6 AAAA batteries. [2] ^{(note that the comments correct the article).}

An alternative construction uses 6 oval shaped button cells, or 7 cells in a rechargeable for 8.4V nomimal.

The battery has both the positive and negative terminals on one end. The negative terminal is fashioned into a snap fitting which mechanically and electrically connects to a mating terminal on the power connector. The power connector has a similar snap fitting on its positive terminal which mates to the battery. This makes battery polarization obvious since mechanical connection is only possible in one configuration. One problem with this style of connection is that it is very easy to connect two batteries together in a short circuit, which quickly discharges both batteries, generating heat and possibly a fire. The wiring usually uses black and red wires, red for positive.

Inside a PP3 there are ordinarily six alkaline or carbon-zinc 1.5 volt (nominal) cells arranged in series. These are either AAAA cells, or special flat, rectangular cells. The exact size of the constituent cells varies from brand to brand -- some brands are slightly longer than others -- as does the manner in which they are joined together. Some brands use soldered tabs on the battery, others press foil strips against the ends of the cells.

Very cheap versions may contain only five 1.5 volt cells. Rechargeable NiCd and NiMH batteries have various numbers of 1.2 volt cells. Lithium versions use three 3 V cells - there is a rechargeable lithium polymer version.