What approximate voltage does a standard automobile starter battery usually supply?

A standard automobile starter battery is a lead-acid battery comprised of six cells connected in series. Since each lead-acid cell produces approximately 2.1 volts, the total nominal voltage is around 12 to 12.6 volts. This 12-volt standard is universally used in most passenger vehicles. It is the primary reason why mobile amateur radio equipment is designed to operate on a 13.8V DC power supply, which matches the voltage of a running car's alternator while it is charging a 12-volt battery. Distractors such as 24 volts (used in heavy commercial or military vehicles) or 9 volts (used in small household electronics) are incorrect for a standard passenger automobile.