Charging batteries in a car for Sony EX3

Hi there!

I will be shooting a little doco in a couple of weeks and I am researching how I could charge my batteries. I will be out in the field for several days and the only power source I have os the car power outlet (cigarette lighter).

Is there a charger out there which I could use to charge my batteries through the car power outlet? Or should I buy an dapter which comes first in the line?

Maybe one of you has dealt with this problem in the past and can give me some advice.

Thanks,
Bernhard