I asked for suggestions about In-car iPad charger on Apple.com and someone recommended this (http://www.tinydeal.com/rig8355-p-7375.html). The following is his/her description.
I've registered here to ask if this one is eligible. I'm sorry the DC tension should be 24V.I'm using this in-car transformer (http://www.tinydeal.com/rig8355-p-7375.html), which converts DC 70V to USB 5V. The latter electric tension should be applicable to iPad. Besides, the transformer puts out AC 220V electricity through a through a three-jack socket, where two-pin plug is also compatible. so that other devices like laptop can be charged in car.