The more I’ve learned about switch-mode power supplies, the more amazing it seems to me. So a voltage source across a wide range of values can be converted into another voltage, just by pulsing current though an inductor. Of course, switch-mode power supplies are even easier to set up with ICs like this one:
IC REG BUCK BOOST, 296-17764-5-ND
So here is my first implementation of a 2A 12V-to-5V buck converter. The finished device hooks up to the automobile 12V adapter and outputs to four USB terminals. Now you and all your passengers can charge your phones on the go and at the same time!