You could try something like this:
Amazon.co.uk User Recommendation
It's excellent. It means I can use the MB-Air for 15 hours or more without needing to re-charge, so don't need to worry about conserving battery.
If you want to use the MB-Air as a passenger, then the battery is a good way to get extended use without mains power. On the other hand, it's a lot more expensive than the little car charger above.
If you just want to charge the laptop while driving, or have it play music on a long drive, then the small car charger is probably the right way to go.
Good luck