An auto DC adapter has a cigarette lighter input plug on one end that is plugged into the cigarette lighter receptacle and on the other a special output connector that is plugged into the laptop's power receptacle. Although recent P4 laptops have higher and higher power requirements, auto batteries have the capacity to supply the necessary output for the PC.

IBM
    IBM ThinkPad 300 Series
        ThinkPad 340, ThinkPad 340-2610, ThinkPad 345,
        ThinkPad 355, ThinkPad 360, ThinkPad 360PE-2620,
        ThinkPad 370, ThinkPad 370-9545, ThinkPad 380CE,
        ThinkPad 380CS, ThinkPad 380CSE

    IBM ThinkPad 700 Series
        ThinkPad 700, ThinkPad 700-9552, ThinkPad 720,
        ThinkPad 720-9552, ThinkPad 750, ThinkPad 750-9545,       
        ThinkPad 765, ThinkPad 790

    IBM ThinkPad 755 Series
        ThinkPad 755, ThinkPad 755C, ThinkPad 755C-9545,
        ThinkPad 755CD, ThinkPad 755CDV, ThinkPad 755CE,
        ThinkPad 755CS, ThinkPad 755CSE, ThinkPad 755CV,
        ThinkPad 755CX

    IBM ThinkPad 760 Series
        ThinkPad 760, ThinkPad 760-9546, ThinkPad 760-9547,
        ThinkPad 760C-9546, ThinkPad 760C-9547, ThinkPad 760CD-9546,
        ThinkPad 760CD-9547, ThinkPad 760E-9546, ThinkPad 760E-9547,
        ThinkPad 760ED-9546, ThinkPad 760ED-9547, ThinkPad 760EL-9546,
        ThinkPad 760EL-9547, ThinkPad 760ELD-9546, ThinkPad 760ELD-9547,
        ThinkPad 760L-9546, ThinkPad 760L-9547, ThinkPad 760XD-9546,
        ThinkPad 760XD-9547, ThinkPad 760XL-9546, ThinkPad 760XL-9547