For many years..I have charged my laptops by turning them off, plugging an inverter it my DC outlet in the parked car, and letting it charge. It takes an hour or two, yes. But it hardly puts a dent in my battery. It takes very little current to charge a battery that is not in use. The inverter probably consumes more power than that battery takes.
(Note that your car battery needs to be in good condition..This bit me last year when my battery was at the end of it's life)
Most laptops require 18vdc at the power input, not 12v, (At least, all of mine do) so if you want to charge with DC only, you need a dc-dc convertor.