PP3 battery
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A PP3 battery, commonly referred to simply as a nine-volt battery, is shaped as a rectangular prism and has a nominal output of nine volts. Its dimensions are 48 mm × 25 mm × 15 mm (ANSI standard 1604A). It is widely used in smoke detectors and as backup batteries for digital clocks or for personal alarms.
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. Inside a PP3 there are six 1.5 volt 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.
Trivia
- When the contacts are rubbed against steel wool, sparks are emitted by the steel wool, which might be able to start a fire if matches are not available (it's probably much less reliable than matches; don't count on it working; practice at home first).
- Battery life can be tested by pressing the contacts against your tongue. If the battery contains any useful charge, a mildly unpleasant but relatively harmless sensation occurs due to the passage of current through the skin of the tongue.
History
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 [link].
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