A single AA alkaline battery has a standard nominal voltage of 1.5 volts.
The AA designation describes the battery’s physical size, not one particular chemistry. Alkaline AA cells use zinc and manganese dioxide chemistry and provide a nominal 1.5-volt output. Their actual voltage changes during use and depends on load, temperature, age, and measurement conditions.
Rechargeable nickel–metal hydride AA batteries are a common source of confusion because they typically have a nominal voltage of 1.2 volts. Lithium-based AA-sized batteries can have different electrical characteristics as well. Thus, “AA” alone identifies a size class rather than a universal voltage.
Connecting cells in series adds their voltages. For example, two nominal 1.5-volt alkaline cells provide about 3 volts, while parallel connections increase available capacity without adding voltage.