This is not recommended because car batteries are not designed to be discharged by more than 30%, and their operational life may be significantly reduced. Instead, consider obtaining a deep cycle battery or a marine battery if you are travelling in an area where there is no mains power. A deep cycle battery can discharge by as much as 80%, and a marine battery can discharge as much as 60%.
A car battery can be used in an emergency, preferably without a humidifier. Using a humidifier will drain the battery to a point where it may be unable to start the car.