Doesn't get much simpler than this, you pop in 10 AA batteries, you get a 12v DC power battery. Handy!
AA batteries are available everywhere, and having the ability to power your camera or accessories with them is a great backup plan. This one i got takes 10 batteries, so with my 1.2v NiMH rechargables, i get a 12v battery backup. Regular Alkaline AA batteries will get you 18v with this clip.
There are several versions of these clips, 4x, 8x, and 10x, so get which you need depending on your needs and what type of AA's you have handy or want to plan for.
NOTE: be sure you buy one that's wired in SERIES, which sums the voltage, if you get a Parallel clip, you'll just be getting a big 1.5v battery.