That wasn't intended to be my implication. The replies to the original comment answered my question. Generally if you have a dead 12V you know it because you

That wasn't intended to be my implication. The replies to the original comment answered my question. Generally if you have a dead 12V you know it because you need a boost to get started, and you don't drive like that for long. I'm not worried about a cascade failure, I was worried about the 12V dying while somebody was driving and then having no brakes. That concern has been addressed.