wvmarle:
That schematic does not protect against voltage reversals, so is a very bad one for use in automotive (where voltage reversals can and do happen - and one -50V spike is more than enough to destroy an Arduino).
As mentioned in my post first you need to know what you are monitoring before you can look at how and this was just one example not one to cover all.
But i am curious as i have worked on vehicles quiet a lot, what voltage reversals do you refer to? In my vehicle (Holden Rodeo 2005) i have 12v, ground and various voltages inbetween along with pulsing voltages. I have installed stereos, two ways, taxi equipment and camera systems and can not think of one vehicle that has negative voltages or reversals.
Not by any means saying you are wrong but i have not seen or heard of this other that the alternator before the rectifiers?
Ps the circuit with divider and a 5 volt zener is ment to stop any voltage above 5 volts from reaching the Arduino. It could use a bit more for added protection but in my experience a 50 volt spike would/should destroy nearly every circuit in the vehicle.
Based on personal experience BUT Never Say Never!!!!