I am a travelling salesman. I sell VCDs and CDROMs from my van (mobile unit). I wish to connect my Dell Sempron laptop to an external battery in order to display the CDs to my customers at fairs and exhibitions. Will a regular car battery be able to power my laptop for a few hours each day? Will this supplement the battery that is already in the laptop? Can I connect the external battery to the van so that it recharges when the vehicle is running?

Your best option would be to buy a Power inverter.

What this does is plugs into your Cigarette lighter and converts the power from there into a standard power outlet.

From there, you can just plug your normal power adapter into that inverter and avoid messing with extra batteries and such.