I assume you are going to plug this into an inverter that is plugged into a cigarette lighter/power port ... One of two things are going to happen ... it will charge your laptop or it will blow the fuse on your inverter or car. Ok, there is a small chance something will catch fire, but that's pretty small and assumes a fuse has failed to do it's job.
I will relay one story ... my uncle plugged a European MacBook (220v) into a 110v->220v transformer and it fried his MacBook ... I was kinda surprised that happened, but that kinda stuff can happen.