Not long ago I showed off some portable 12V / 5V USB battery packs that were able to power a variety of camera accessories. These battery packs worked so great, I picked up a few more, and there's always one in my backpack (in case my cell phone runs low). YouTube member LucaFurgiuele runs a time test powering up the 352 LED Ring light at Full Power.
It's not often you'll use a light like this on full power the entire time, but these batteries were still able to push about 85 minutes of run time. Definitely not a bad deal for these versatile little packs. You can find some additional information about the batteries in my old post here. [Thanks Luca]