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There are a number of different types of portable air compressors, some of which are designed to operate on 12 volt direct current (DC). These devices are designed primarily to inflate low pressure objects, such as sports balls and air mattresses, though many are also capable of inflating car tires. Unlike most air compressors, these units typically lack any type of pressurized vessel to accumulate compressed air. Instead, a 12v compressor will typically pump air directly into an object. These compressors are typically less powerful than portable units that operate on alternating current (AC), though the fact that they can run on the 12 volt electrical systems found in cars and boats often makes them more useful when traveling.
Air compressors are devices that use various methods to pressurize air. Most air compressors pump air into some type of sealed container in order to pressurize it, though others are capable of pumping air directly into an inflatable object. The 12v compressor is a prominent example, as most of those devices lack any type of pressurized vessel. A motor inside the 12v compressor draws air in and then pumps it directly into an object. That can place a limit on the effectiveness of some pumps, since car tires and other similar objects often require a very large volume of air in order to reach the necessary pressure.
Compressors that have tanks only need to operate until the pressure vessel has been filled. The motor will then turn back on automatically when the pressure drops below a certain level. Since 12v compressors typically lack any sort of tanks, they need to run continuously. That makes it important to shut them off from time to time during usage, especially when filling large volume objects such as car tires. Extended usage without a break can burn out a 12v compressor motor, so most manufacturers list a time limit for continuous operation.
Aside from lacking a pressure vessel to accumulate compressed air, the other main difference between a 12v compressor and other portable units is the power source. Most compressors run on AC power, while 12v compressors are designed to operate on the 12v DC systems found in most automotive and marine electrical systems. These units typically come equipped with plugs that are designed to be connected into cigarette lighter sockets, though others have alligator clips that can be connected directly to a car battery. That can make these units very useful on the road, allowing them to fill up tires, and other inflatable objects, in situations where no AC power is available.