I don't know about other EVs, but in a Tesla, a failing 12V battery will be detected and the car presents a warning on your screen about it. Mine failed after

I don't know about other EVs, but in a Tesla, a failing 12V battery will be detected and the car presents a warning on your screen about it. Mine failed after ~5 years. Replacement was inexpensive ($128) and Tesla service drove to my house to install it.