12 vdc to 15 vdc circuit
HI
You need a DC/DC converter to convert the approx 12 V from your car battery to the 15 V DC that is required by your laptop. You will need to know how much current is required by your laptop in order to determine the correct DC/DC converter circuit. The laptop most likely has the battery charging circuitry built in to it so you don't have to be concerned with correctly charging the batteries in the laptop. Many companies have excellent application notes to describe how to build these types of circuits. Linear Technology, TI, Microchip, National Semiconductor, etc. are some of the many companies that have these circuits.