I have one (NUSTE brand on Amazon) that has a built-in rechargeable battery. It takes up much less room than some I've had.
The ones that plug in to a lighter socket (which sometimes require the car to be running or the ignition switch in ACC position), or have to be clipped to the car battery are not as convenient.
The ones that plug in to a lighter socket (which sometimes require the car to be running or the ignition switch in ACC position), or have to be clipped to the car battery are not as convenient.