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The nine-volt battery, or 9-volt battery, in its most common form was introduced for the early transistor radios. It has a rectangular prism shape with rounded edges and a polarized snap connector at the top. This type is commonly used in walkie talkies, clocks and smoke detectors. They are also used as backup power to keep the time in certain electronic clocks. This format is commonly available in primary carbon-zinc and alkaline chemistry, in primary lithium iron disulfide, and in rechargeable form in nickel-cadmium, nickel-metal hydride and lithium-ion. Mercury oxide batteries in this form have not been manufactured in many years due to their mercury content. This type is designated NEDA 1604, IEC 6F22 and "Ever Ready" type PP3 (zinc-carbon) or MN1604 6LR61 (alkaline).
Most nine-volt alkaline batteries are constructed of six individual 1.5V LR61 cells enclosed in a wrapper. These cells are slightly smaller than LR8D425 AAAA cells and can be used in their place for some devices, even though they are 3.5 mm shorter. Carbon-zinc types are made with six flat cells in a stack, enclosed in a moisture-resistant wrapper to prevent drying.
As of 2007, 9-volt batteries accounted for 4% of alkaline primary battery sales in the US. In Switzerland as of 2008, 9-volt batteries totalled 2% of primary battery sales and 2% of secondary battery sales.
Other nine-volt batteries of different sizes exist, such as the British "Ever Ready" PP series and certain lantern batteries.
The battery has both terminals in a snap connector on one end. The smaller circular (male) terminal is positive, and the larger hexagonal or octagonal (female) terminal is the negative contact. 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 same snap style connector is used on other battery types in the Power Pack (PP) series. Battery polarization is normally obvious since mechanical connection is usually only possible in one configuration. A problem with this style of connector 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. An advantage is that several nine-volt batteries can be connected to each other in series to provide higher voltages.
The PP3 appeared when portable transistorized radio receivers became common, and is still called a "transistor" battery by some manufacturers. The Eveready company claims that it introduced this battery type in 1956.  Early transistorized radios and other equipment needed a suitable voltage miniature battery, but the commonly available small batteries at that time was either too low voltage or too high. The 22.5V battery made for vacuum tube/thermionic valve hearing aids and for photo flash bulbs was at the upper limit of the transistor voltage ratings, for instance.
These batteries are commonly named 9-volt, and also colloquially named PP3, Radio battery, Square battery, and Japan "006P".
They all have a rectangular shape; the dimensions are height 48.5 mm, length 26.5 mm, width 17.5 mm (or 1.9"x1.0"x0.68"). Both terminals are at one end and their centers are 1/2 inch (12.7 mm) apart.
Inside an alkaline or carbon-zinc 9-volt battery there are six cells, either cylindrical or flat type, connected in series. Some brands use welded tabs internally to attach to the cells, others press foil strips against the ends of the cells.
Rechargeable nickel–cadmium (NiCd) and Nickel-metal hydride (NiMH) batteries have between six and eight 1.2 volt cells. Lithium ion versions typically use two cells (3.7V nominal each). Lithium polymer and low self-discharge NiMH versions can also be found.
Formerly, mercury batteries were made in this size. They had higher capacity than carbon-zinc types, a nominal voltage of 8.4 volts, and very stable voltage output. Once used in photographic and measuring instruments or long-life applications, they are now unavailable due to environmental restrictions.
Devices that use these batteries are generally designed to work properly over the operating voltage range of the battery, from fully charged (typically up to 9.6 V) to nearly dead (typically 5.0 V).
Alkaline batteries drain very slowly over time, even when not in use. An alkaline battery that is unused or used with extremely low power consumption devices (transistor leak current, etc.) can be expected to last approximately for 6 years.
Lithium 9-volt batteries are consumer-replaceable, high energy density batteries designed to last up to 5 times longer than alkaline 9-volt batteries and up to 10 times longer than carbon-zinc 9-volt batteries in many applications. In addition, lithium PP3 batteries have a long shelf life of up to 10 years. Common applications for lithium 9-volt batteries are smoke / CO (Carbon Monoxide) alarms, and electronic parking meters.
Other nine volt batteries
There were several other sizes of nine volt batteries. Two are still available, the PP7 and PP9. These date from the days of early transistor radios and are now less common than PP3. The PP4, PP5, PP6, PP10 and PP11 (this last battery was actually a 4.5+4.5 volt battery) sizes have long been obsolete. There was no PP2 size, and the PP1 and PP8 were 6 volt (the latter being designed for electric fence use and is still available). In the days of valve radios which used batteries designed specifically for vacuum tubes, there was a nine volt grid bias battery or (US) 'C' battery which had tappings for various voltages between 1.5 and 9.
- Duracell Catalogue or IEC
- IEC 60086-2-2011 §184.108.40.206
- Life Cycle Impacts of Alkaline Batteries with a Focus on End-of-Life - EPBA-EU
-  INOBAT 2008 statistics.
- IEC 60086-2-2011 §220.127.116.11
- 9-volt Battery Safety, NFPA, retrieved 13 May 2014
- "Battery History". Energizer.com. Retrieved 2010-11-27.
- IEC 60086-2011 pt2-§18.104.22.168
- ANSI C18.1M Part 1 and C18.3M Part 1
- Duracell. "9V (6LR61) datasheet"
- A Look Inside a 9 Volt Battery
- What charger should I use?
- Your guide to types of household batteries (AAA, AA, C, D, and 9V)
- Brand-neutral drawing of a 9 volt alkaline battery based on ANSI specifications
- Brand-neutral drawing of a 9 volt carbon–zinc battery based on ANSI specifications
- Brand-neutral drawing of a 9 volt lithium battery based on ANSI specifications
- Brand-neutral drawing of a 9 volt rechargeable battery based on ANSI specifications