I wish to connect my laptop to a battery. How can I do this? What is the capacity of the battery?

I wish to connect my laptop to a battery. How can I do this? What is the capacity of the battery?

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?