Nine-volt battery

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Nine-volt batteries are small power sources that usually provide about 9 volts when fresh. In practice, their voltage can range from roughly 7.2 to 9.6 V depending on the chemistry and how full the battery is.

The best-known size is PP3 (also called IEC 6LR61 or NEDA 1604A). It is a cuboid with rounded edges and two snap connectors on the top. The smaller circular contact is positive and the larger hexagonal/oval contact is negative. The battery and the device have matching connectors, so they connect in only one way.

Inside most PP3 batteries are six small cells stacked together. There are several common chemistries:

- Zinc–carbon and alkaline (the most common disposable types)
- Lithium (lithium manganese dioxide or lithium iron disulfide; sometimes labeled CRV9)
- Rechargeable options include nickel–cadmium (Ni–Cd), nickel–metal hydride (Ni–MH), and some lithium-based rechargeable variants

Mercury batteries were used in the past but are banned in many places due to toxicity.

In the PP family, PP3 is the standard 9-volt size. Larger but still 9-volt options such as PP6, PP7, and PP9 exist, as do a few other sizes like A10 and A29. The PP3 is the one you’ll see most often today and is common in many household devices.

History and use: The PP3 battery was introduced around 1956 by Ever Ready (UK) and Eveready (US). It became a widely used “transistor radio” and general-purpose 9-volt battery. Today, you’ll find PP3 batteries in smoke detectors, gas detectors, clocks, toys, and some other small devices.

Safety tips: The two exposed ends can touch and cause a short circuit, which can heat the battery or start a fire. Keep batteries in their packaging until you’re ready to use them, and avoid leaving loose batteries where their contacts could touch.

Testing and charging: You can test a nine-volt battery with a multimeter by measuring the voltage across the two terminals. Most disposable PP3 batteries are not designed to be recharged; only certain rechargeable chemistries (Ni–Cd, Ni–MH, some Li-based types) should be charged with an appropriate charger.

Lithium 9-volt batteries are a popular rechargeable option and generally store more energy than alkaline versions, making them useful in devices like smoke detectors. When replacing a battery, choose the type specified by the device and avoid mixing different chemistries or brands in the same device.