It really depends on how much you are planning on using the car battery. The typical battery produces 12 volts for 40 amp hours. Figure out the amperage of your devices, add the time you plan to use them for and you have a (very, very) rough guide to what you can get out of it.
Always remember that the car is your way out of the wilderness, so try not to drain the battery! If a full back up generator is going to take too much room, you could consider investing in UV panels to harvest some energy from the sun. That may not be enough to power a heater or cooker, but it could charge your devices like tablets and smart phones. In that way it lessens the burden on the car battery.