Do this simple test: Fully charge, then unplug your laptop so it's running on just the battery. Put it into standby and leave it there over night or a couple days. Then bring it back up and see how much of your laptop's battery is drained. I'm pretty sure you'll still be at 99 or 98%.
I use standby on my system (you can check the specs) all the time. I do drive a lot and the longest I've parked my car since I've put the carputer in is a about a 20 hrs. There are power supplies that cut power if the battery gets below a certain voltage. or you can simply use a relay and cut the power to your laptop when the car's not running.