I started out using my modded T3i with batteries, lots of batteries. Then I purchased a AC to DC converter that fits in the battery well and works continuously provided I had access to 110 AC. Then I hacked my AC to DC converter by adding a voltage regulator. Now I can operate off my 12V battery at a dark site. Here is the completed system:
Here is a close up of the old AC to DC converter in a new slightly larger housing, along with the 12VDC to 7.5 VDC adjustable voltage converter.