Easiest way is to get a Laptop power supply from Maplins. plugged into fag lighter and you turn it to whatever voltage ya lappy runs at Also has a set of different plugs to suit different lappies..
£30 cheaper to buy nuther lappy battery
£27 fer mine..well ermm... hersfooking int they want £60 for a noo battery for me HP lappy
Aint sure if lappys are ok off power invertors but ive got a couple of invertors if you need one