The problem some would have with that device is with cars with a dead battery that don't connect the socket until the ignition sw is on. Turning on the switch in the dead car could be draining away most of the current the device is providing. My Honda truck has three accessory outlets and all of them were disabled with the switch off. I jumpered around a relay that connected one of them, so it is now hot all the time. I did that so I could recharge small devices even when I was away from my truck.
Trikester