12vdc adapter for 40D

I just got my 40D. I use it mostly for astrophotography. Most of this work is done in the field so I use large 12 volt batteries to run everything. I would like to run the camera from the same source. Notice Canon have a plethera of adapters but none for 12v reduction into the camera. Would appreciate it if someone knows if such an animal exists and I am just missing it. thanks

You can use the ACK-E2 AC adapter and a DC to AC inverter to convert the 12VDC to 120VAC to power the ACK-E2.

It seems redundant to go from 12VDC to 120VAC to 8.4VDC but that is what I do to charge my batteries in my car.

This one uses a cigarette lighter and alligator clip leads to connect directly to the battery.
http://www.buy.com …ingid=1345707&d​caid=17902