I wouldn't use a battery powered unit in your vehicle. You can just about guarantee it will have a dead battery when you need it most out in the middle of nowhere. One that lugs into a 12V accessory outlet (cigarette lighter for us old guys) or clips to the battery directly is much more reliable at any time IMO...
I keep a DC to AC power converter in each of my vehicles, so it the vehicle battery has enough power to start the engine and the alternater is charging the vehicle's battery I can charge the batteries for the air compresser, the jump box, my PC, my cell phone, etc. Proper prior planning dictates that you maintain your vehicle's emergency "stuff" periodically so that it works when you need it.
