you mean like something that takes a 12v cigarette lighter thing to your laptop? I think my laptop takes 18v in, so you theoretically just need a good step up circuit. The problem might be that laptops are fairly sensitve to input power (at least I think so, or they wouldn't have the expensive feedback circuit in the power adapters) so you probably have to do some kind of good control. Theoretically I don't think it would be that hard, buy my analog circuit skills aren't good enough to help you out.