I have - yet another - dumb question.
If there was some kind of advance warning - would it do any good to disconnect the car battery?
I read an article that said disconnecting the battery will not help. The EMP will fry the computer chips. The only way to protect a vehicle would be to turn your garage into a faraday cage. The article debated whether the car would still function and decided it would (if you can start it) run badly until you ran out of gas. Gas would not be available for a time because the pumps at gas stations would not run, they are electric and computerized. Gas when available would be rationed for electric generators. Since they are just engines and a basic generator they could still function. However, generators do use some solid state devices which if fried would make the output inconsistent. But they would work. Since that article, newer cars are even more computer dependent. I doubt they will run, anything from the early 90s and older would still run based on the article.
The article was in a military trade magazine, not a survival type magazine. They were not trying to sell anything. They wanted to analyze what would occur during an solar EMP event. Solar EMP burst have happened before, but not since the world has become so computer dependent. If I remember correctly, the article compared the solar EMP event results to terrorists taking out the power grid or the explosion of an EMP device over the USA. The conclusion being that a solar event, which is likely in the future, would be far more devastating as it would be on a global scale. 2020 is not over yet, there is still time for an EMP event.