@mabvs
So, the best way to run an Arduino is via a three-terminal regulator of some sort
<--Anachrocomputer is right.
Supply the "12" V line into a 7809 and the 7809 feed the Arduino. And don't forget to place capacitors filter ( at V In & at V out ) of the regulator, also may be an inductor ( C - L - C <-- a PI configuration ) at at the Vin of the 7809. Because the "nice" & "cool" classic 69 El Camino <-- a GM car produce a lot of noises & spikes in the "12" V line.
A car's charging system can go up to 14.8V or so when the battery's fully charged, and the voltage will drop below 12V when the engine's not running and the battery's low.
Yes it does. Wost case : going low to 8 V ( yes, it happen to me in the winter, when I start a diesel Ford E-350 School Bus at - 20 C ) when it fully charge, it about 13 V -14 V. <--- That is Normal operation. At start-up, the battary can go low 9V to 10 V. Well to be accurated, mesurments of the battery with a DVM. In general, It 12 V +/- 2 V for normal operation.