Back in 2013 I designed a board to power a Raspberry Pi in a vehicle. It provides 5V regulated for the R-Pi and 12V switched for accessories.

The idea is to power up the R-Pi when the ignition was switched on and allow the R-Pi to power down after the ignition is turned off. This gives the R-Pi time to complete whatever is going on and gracefully shut down, or maybe to connect to the home network to upload/download before shutting down.

The board also supports battery backup of the R-Pi.

Shown here mounted to the top of a R-Pi plastic case on 5mm tall stand-offs.

More details and how it works: Here.