I found out the mysterious reason why E07 drops in output overtime. The spring flattens out after being compressed and doesn’t return to the original length, which means it won’t push as hard on the cells. The weaker contact results in more resistance and lower output. I took some measurements with these fully charged cells. The 18650 cell uses a 21700 adapter, which makes them a good bit longer than the 21700 cells so the spring pushes harder on the 18650 cells.
Molicel P26A 18650: 6640 lumens
Molicel P28A 18650: 6300 lumens
Sony VTC5D 18650: 6300 lumens
Sony VTC6 18650: 5650 lumens
Molicel P42A 21700: 5600 lumens
Samsung 40T 21700: 5770 lumens
Samsung 30T 21700: 6500 lumens
This same E07 used to measure around 7,570 lumens with the same 30T and 7,017 lumens with the 40T. I swapped the tail cap with another E07 tail cap that is used less and the 40T measurements jumped from 5770 to 6000 lumens. There’s a possibility that I might have tried a 21700 protected cell which compressed the springs too hard causing permanent deformation, but I don’t remember. But now I know to never put protected cells in a flashlight not designed to accept protected cells.
These springs are not bad and all springs flatten out over time to a certain extent, but if Fireflies ever find any springs with better mechanical properties, they can sell them separately as an upgrade. There’s nothing we can do with the spring on the driver side but if Fireflies provide a longer tail cap spring, that will make up for the loss compression.