Nine-volt battery

Mercury batteries of this format, once common, have been banned in many countries due to their toxicity.

[4] A range of PP batteries was produced in the past, with voltages of 4.5, 6, and 9 volts and different capacities; the larger 9-volt PP6, PP7, and PP9 are still available.

Carbon-zinc types are made with six flat cells in a stack, enclosed in a moisture-resistant wrapper to prevent drying.

Modern batteries have higher capacities and lower internal resistances than early versions.

Early transistorized radios and other equipment needed a suitable voltage miniature battery.

However, a suitable miniature battery was already marketed for (vacuum tube) hearing aids.

A problem with this type of connector is that two loose batteries with terminals exposed can touch and short circuit, discharging them, and generating heat and possibly a fire.

[1] Keeping nine-volt batteries in their packaging until use helps to avoid accidental discharge.

Costlier industrial-grade batteries may use chemistries that provide higher power-to-size ratio, have lower self-discharge and hence longer life when not in use, more resistance to leakage and, for example, ability to handle the high temperature and humidity associated with medical autoclave sterilization.

[19] Common applications for lithium nine-volt batteries are smoke and carbon monoxide detectors.