septillion:
Or just grab a USB car charger and a USB cable between that and the Arduino. Then there is no heat at all in the onboard regulatorAnd those chargers are dirt cheap.
That is the route I would choose. I have a third battery in my truck with an isolation relay on it that we use for our CPAP machines when we are sleeping in the truck. It also has a decent volt meter on it. When I first stop, the battery voltage is typically 12.8 or so and even after running 2 CPAP machines all night, the battery voltage is still in the 12.5v range (it isolates from the main system when the engine is not running). Battery voltage when the engine is running is more like 13.9 to about 14.4v. You should also fuse anything you are powering off a 12v battery - those things have a very low internal resistance and if something bad happens, it is YOUR circuit that will "let the magic smoke out" to protect the battery 