A second problem is that nominally ''twelve-volt'' power in cars fluctuates widely. The actual voltage will be approximately 12.5 volts when dormant (less in cold conditions), approximately 14.5 volts when the engine and the alternator/generator are operating (more when cold), and may briefly drop as low as 5--6 volts during engine start. When used, DC to DC converters will usually compensate for small fluctuations, but reliable power may not be available without an independent battery-powered uninterruptible power supply.
Answer this question, making sure that the answer is supposed by the text: does the car have to be on for the cigarette lighter to work?
no