Self limiting, if the opamp's power supply comes from the car battery. It takes the car battery voltage between 6 and 16V and converts it to an inverted 0.2 to 4.9 volts analog signal, to be fed into an analog Arduino port. Some math is needed to convert it into the car voltage.
Only measuring between 6 and 16V, because beyond that, your car battery is toast anyway. And it is good to spend the Arduino's AD-converter's bandwidth to accuracy, instead of measuring the whole range between 0 and 16V.
The AC supply represents the fluctuating voltage of the car battery; the DC can be taken from the Arduino's board.
