Re: car circuits
For sensitive electronic device, you had better put some protection device and voltage regulator.
But, most car accessories/equipment (e.g. car CD/DVD player is usually manufactured with all constraints in mind .... they have already incorporate voltage regulation, current/voltage protection, mechanical vibration protection, etc). Problem will arise when you try to connect home AV devices to car electrical system. They have different mechanical and electrical environment criteria.
And for Cherrytart, I think the appearance of 18 volt in a 12 volt environment means problem in car battery charging system. The voltage regulator should regulate the voltage to a maximum of 14.5 volt. I experienced this incident, my VR got broken and the voltage reach 18 volt in high engine RPM.
aaNg