"Keeping a pi running won't matter to a car battery."
don't be so sure; the capacity of a car battery is only a few tens of amp-hours.
I recently bought a battery for my car that's rated at 55AH at 20 amps.
Drawing an average of 200ma for a few days can drain it completely, like maybe an RPi "left running". Yeah. This makes a few assumptions about how good your power conversion is (a switching 5V power supply will be more efficient than a linear regulator) but you get the idea:
200ma * 24 = 4.8AH ==> ~12 days for 55AHAlso you're really not supposed to go below 50% capacity on a car battery if you don't want to damage it. So make that "less than a week". Plus there's the "can I start my car when the capacity is that low" factor.