Powering Arduino with car batteries
Use a switching regulator such as the LM2596. You can get a ready board from eBay for about US$1.



Just set the output voltage to 5&nbsp;V and feed the Vcc input of Arduino; this bypasses the onboard regulator.

The benefit is that unlike the linear onboard regulator, a switching regulator is very efficient which means less consumption and less generated heat on the regulator. An additional benefit is that it can withstand a higher input voltage (about 35&nbsp;V for the specified regulator), just in case the circuit is used in a vehicle that has 24&nbsp;V batteries.

The regulator has already several protections like for short circuiting, overheating, etc.