Hello all,
I've been reading this thread since I ordered this light from DX about 3 weeks ago. I finally received it yesterday and been playing with it using the different battery options. I noticed it is a lot brighter with 4x RCR123 vs. 2x 18650. So I decided to run it through the bench DC power supply to see what its V-I curve looks like. Below is a graph that reveals some interesting aspects of this light's performance.
From the graph, the take-away is:
1) To get the full brightness, you need to feed it 10V or higher. Above 10V the regulation is pretty flat at about 7.7W.
2) Below 10V the brightness takes a huge dip. The current went from 0.77A to 0.12A !!!
3) Between 10V and 8V this light not so hot, with the worst brightness at just below 10V. This explains why the brightness is disappointing with 2 fully charged 18650's at 8.2V.
4) At 8V the light flickers.
5) Below 8V there's no regulation at all and the brightness drops off linearly with decreasing voltage.
Hope this helps to shed some light on this topic.