So I started using 18650s for externals on a couple Panasonic builds...using the cheapest batteries I found and now after doing some more research I found these sites.  I could be getting much more time in the field it seems by just purchasing TrustFire vs the UltraFire that I currently have...and these are 2 of the cheapest and poorest performing(comparatively) out of the bunch.  Don't get me wrong....I seem to be getting more time in the field using the 2 Ultrafire 3100maH batts than I would get out of 3 Energizer C cells for externals which take up approximately the same amount of space though shallower profile(so will fit where sometimes the Cs will not).  There are other sizes available as well for more options though sometimes it is harder to find holders for them.  18650 is probably the most common size and holders available on Ebay cheap.  
*anyway here are a couple links for tests/reviews and another forum to join to find out everything you need to know about flashlights and power :bleh
another forum(useful info, discussion and links) I may have to join :Hammer
this next is cool for everything you want to learn about battery basics and the technology etc:
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 Nice info. thanks
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