The 12V socket, also known variously as a cigarette lighter receptacle or 12V auxiliary power outlet, is the primary method by which power is delivered to portable electronics in cars, trucks, boats, and a handful of other contexts. While these sockets were originally designed to heat up cigarette lighters, they quickly gained popularity as a de facto automotive electrical outlet.
Today, it is possible to power anything from a GPS navigation unit to a tire compressor with a 12V socket in a car. Some vehicles come with multiple sockets for the express purpose of powering multiple accessory devices, although it is uncommon for more than one to be capable of accepting a cigarette lighter. Accordingly, the specifications for these power sockets are contained in ANSI/SAE J563 includes two variants: one that works with cigarette lighters and one that doesn’t.
The History of Automotive Accessory Power
When the first automobiles hit the road, they didn’t include electrical systems of any kind. Since they used magnetos to provide spark, just like your lawnmower does today, and lighting (if any was included at all) was provided by either gas or kerosene lamps, an electrical system simply wasn’t required.
The first automotive electrical systems made use of DC generators, which (unlike modern alternators) didn’t require any voltage input to operate. These generators were belt-driven (just like modern alternators), and they provided the necessary DC power to run accessories like lights. With the addition of lead acid batteries, it suddenly became possible to add other “accessories” that we take for granted today — like electric starter motors.
Although electrical systems that included a DC generator and a lead acid battery technically made electrical accessories possible, the widely variable voltage produced by these generators created issues. Mechanical devices were used to regulate the voltage, but automotive electrical systems didn’t arrive in the modern era until the introduction of alternators. Unlike generators, these alternators produced alternating current, which was then converted into direct current to charge the battery and provide accessory power. Although these modern electrical systems still don’t provide entirely uniform voltages, they are capable of keeping the voltage output relatively steady regardless of how fast the alternator is spinning.
The Smoking Gun
Although people had been powering accessory devices with their automotive electrical systems ever since automotive electrical systems were first invented, accessories had to be wired in manually. The appearance of a 12V automotive electrical socket was almost accidental, as it was co-opted from a completely different initial purpose.
Cigarette lighters (along with lights and radios) were among the first accessories to take advantage of early automotive electrical systems, and they started to appear as OEM options by about 1925. These early cigarette lighters used a “coil and reel” system, but it was the so-called “wireless” cigarette lighter that would eventually become the de facto automotive (and marine) power socket.
These “wireless” cigarette lighters consist of two parts: a cylindrical receptacle that's typically located in the dash of a car, and a removable plug. The receptacle is connected to power and ground, and the plug contains a coiled, bi-metallic strip. When the plug is pushed into the receptacle, the coiled strip completes an electrical circuit and subsequently becomes red hot. When the plug is removed from the receptacle, the red hot coil can be used to light a cigar or cigarette.
Easy DC: Introducing the 12V Socket
Although they weren’t originally designed with this purpose in mind, cigarette lighter receptacles provided an opportunity that was simply too good to pass up. Since the actual lighter portion was removable, the receptacle itself provided easy access to power and ground. That allowed for the development of a power plug that could be inserted and removed with no need to permanently wire an accessory into the electrical system of a car.
The ANSI/SAE J563 specification was developed to ensure compatibility between cigarette lighter receptacles and 12V power plugs made by different manufacturers. According to the specification, the cylinder portion of a 12V socket has to be connected to negative (which is battery ground in most automotive systems), while the center contact point is connected to positive.
Problems with Using an Automotive 12V Socket
Since cigarette lighter receptacles weren’t originally intended for use as accessory sockets, there are a few inherent issues with using them in that capacity. Accordingly, devices that are designed to use a 12V socket have to be capable of working around these shortcomings.
The biggest issue with using a cigarette lighter receptacle as a 12V socket is the size (inner diameter and depth) of the receptacle itself. Since there is some variation in the size of a receptacle (sometimes referred to as a can), 12V power plugs typically have spring-loaded contacts. That allows them to maintain electrical contact within a given range of tolerances, but it also means that the plug may lose electrical contact from time to time.
Another issue with using an automotive 12V socket is related to the way that automotive electrical systems work. Although modern alternators are capable of maintaining a relatively uniform voltage output, normal operation does allow for a range of output voltages. With that in mind, all automotive electrical accessories have to be capable of running on roughly 9-14V DC. In many cases, a built-in DC to DC converter is used to convert the variable input voltage to a steady output voltage on the fly.