Nine-volt battery

PP (Power Pack) battery series from left to right: PP1, PP3, PP4, PP6, PP7, PP8, PP9, PP10, PP11.

Historically, the now popular PP3 battery size was a member of the PP (Power Pack) battery series that was originally manufactured by Ever Ready in the United Kingdom and Eveready in the United States. The company claims that it introduced the PP3 battery in 1956,[6] then it was added as an ANSI standard in 1959, currently known as ANSI-1604A.[7] The PP (Power Pack) battery series consisted of PP1, PP3, PP4, PP6, PP7, PP8, PP9, PP10, PP11. The PP1 and PP8 were 6 volt, the PP11 was two 4.5 volt batteries, and other PP members were 9 volt, including the PP3. Today, only the PP3, PP6, PP7, PP9 sizes can still be purchased, with the PP3 being extremely common.

Before the mid-1950s, in the days of vacuum tube (valve) radios used batteries designed specifically for vacuum tubes, there was a nine-volt grid bias battery or (US) "C" battery, which had taps for various voltages between 1.5 volts and 9 volts.

Early transistorized radios and other equipment needed a suitable voltage miniature battery. Early transistor radios required a ​22 ¹⁄₂ volt battery. Although the transistors would theoretically operate from lower voltages, in 1954, the point contact transistors had to be operated very close to their V_CB0 limit in order to get the required frequency response. However, a suitable miniature battery was already marketed for (vacuum tube) hearing aids. As transistors rapidly improved, particularly when alloy transistors were introduced, radios were able operate from lower voltages and the battery manufacturers introduced suitable batteries as the demand arose.

The PP3 (physically identical to 6LR61 or 1604A) appeared when portable transistor radios became common, and is still called a "transistor" battery by some manufacturers.

Nine-volt battery snap

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 load device; the smaller one connects to the larger one and vice versa.[2] 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.[8] Because of this hazard, nine-volt batteries should be kept in the original packaging until they are going to be used.[9]

Collage of images showing the opening of a 9-volt battery to reveal six LR61 size cells, which are similar to the LR8D425 AAAA cells often used in medical equipment

Alkaline battery showing rectangular cell construction

Three different kinds of 9-volt primary battery internals: rectangular cell zinc-carbon, rectangular cell alkaline, and cylindrical cell alkaline

Rechargeable (NiMH) 9-volt battery internals

The most common type of nine-volt battery is often called a 9-volt, although there are less common nine-volt batteries of different sizes. Codes for the usual size include PP3 (for size and voltage, any technology), 6LR61 (IEC code for alkaline batteries), and in Japan 006P.

The PP3 size battery is 48.5 mm × 26.5 mm × 17.5 mm or 1.91 in × 1.04 in × 0.69 in. Both terminals are at one end and their centers are 0.5 inches (12.7 mm) apart.

Inside an alkaline or carbon-zinc 9-volt battery there are six cylindrical or flat cells 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 of nominal 9V rating have between six and eight 1.2 volt cells. Lithium ion versions typically use two cells (3.7–4.2 V nominal each). There are also lithium polymer and low self-discharge NiMH versions.

Mercury batteries were formerly made in this size. They had higher capacity than the then-standard carbon-zinc types, a nominal voltage of 8.4 volts, and very stable voltage. Once used in photographic and measuring instruments or long-life applications, they are no longer manufactured as mercury is highly toxic and accumulates in the ecosystem, posing a risk to humans and wildlife.

Most battery voltage testers and chargers that can also test nine-volt need another snap clip to hold the battery, while cylindrical batteries often share a holder that may be adjustable in size. Because of the proximity of the positive and negative terminals at the top of the battery and relatively low current of most common batteries, one informal method of testing voltage is to place the two terminals across a tongue. A strong tingle would indicate a battery with a strong charge, the absence, a discharged battery. While there have been stories circulating of unfortunate outcomes, the process is rarely dangerous under normal circumstances, though it may be unpleasant.[13]

Lithium 9-volt batteries are consumer-replaceable, disposable high-energy-density batteries. In the PP3 size they are typically rated at 0.8-1.2Ah (e.g., >1.2Ah @ 900 ohms to 5.4V @ 23 °C for one type),[14] about twice the capacity of alkaline batteries. Manufacturers claim "High energy density, up to 5x more than alkaline".[14] Common applications for lithium nine-volt batteries are smoke and carbon monoxide (CO) alarms, and electronic parking meters.