Very interesting. It seems the 3 x D cell configuration is good for long run time and also for high output for those who don’t know and want lithium ion cells. This one seems a lot brighter than my Tmart auto-zoom, and also much easier to modify. D is the original flashlight battery size and deserves respect. If one really wants to use alkalines, D seems the only size that gives good output, and it also gives the most energy per price.
I wonder how it would do on NiMH D cells. The types I know of are Eneloops that are really three AAs in a D package (much better than adapters that use only one AA cell), ordinary white low self discharge Tenergys, grey high drain non-LSD Tenergys, and light and dark blue Tenergys that are too old and losing their capacity from long storage. I haven’t found any tests of D cells.
I am trying again to measure the internal resistance of alkalines by shorting them through my meter. My old D cells are down to 1.48 V, which I think means they still have significant charge left. With the three in series, I measure 0.43 A, and similar readings with only one cell, so it looks like around 3 Ohms internal resistance each. So I expect that the output of the Feit Electric light will drop rapidly as the cells wear down, as it did with my auto-zoomy. With a newer cell reading 1.56 V, the short circuit current is about 5 amps., so the internal resistance is much less and not distinguishable from that of the meter and leads.