So I will be using two maybe three arduino boards in a car for various temperature sensing, rpm, speed, fluid levels...etc. etc. I am curious what is the cheapest and most efficient way to supply a constant voltage of 9 or 12v. I'm assuming hooking them straight to the battery is a bad idea due to the large voltage spikes/drops and high amperage. With the alternator car batteries make about 14-15v which would be above the recommended 7-12 volts anyway.
I could do DC12V>AC120V>DC9V but that seems like a big hassle, lots of space taken up, and probably not too efficient for such a small load. Is there a good 12(15volt) DC to 9/12 volt power supply? Or do I need to build one.