I think you're making it too complicated... unless your battery is no good, then the laptop should be able to run for a couple hours on it's own battery. when you park, just leave the computer on, and you'll be able to transfer everything you want. when the battery gets low , the laptop will shut itself down.
NOTE: make sure you have the laptop power supply wired with a relay, so it doesn't get power from the car when the engine is off (otherwise you'll kill your car battery).
...or you could use a USB stick to transfer files instead of transferring over the network