MausBerry Circuits' on-the-go power supply: This product allows the Pi to be used on the go and powered by the car's power supply. The unique feature of this solution is that it recognizes when the car ignition is turned off or on and automatically shuts down or starts the Pi. The power supply includes a 12/14V to 5V buck converter that communicates with the Pi via two GPIO pins and sends commands to the processor based on the ignition being on or off (this requires tapping the appropriate ground and ignition wires). The power supply can be maintained for up to 20 minutes even when the ignition is switched off, so that frequent stops do not cause the Pi to shut down every time. You can order the on-the-go power supply at http://mausberry-circuits.myshopify.com for $21 plus $5 shipping.