Unfortunately 4X18A’s 3V driver (the one used in the SBT90.2 model which the user “4X18A” used) is not fully regulated on Turbo (see my regulation testing below). It’s fully regulated on all the other modes though, which L8 is not AFAIK.
L7 is fully regulated on all modes (see my regulation testing below), which is very impressive for the price. Not even Thrunite’s $250 TN42 V2 could manage that.
I modified my driver to 25A output. TacGriz, did you use included Liitokalas on your testing? It could be that it is regulated, but battery voltage just sags enough for it to drop out of regulation. In my tests those Liitokalas give little over 7A in direct drive lights and 18A/4 is 5.5A. Maybe enough to cause voltage to sag.
Just by lowering current sense resistor value. If you remove the driver it has 2 big resistors with R020 written on them. So 10mOhm for 18A. From there you can calculate sensing voltage. Knowing the voltage you just change the amperage to your desired value in U=RI equation.
U=10mOhm*18A-> U=180mV.
R=U/I-> R=180mV/25A-> R=7.2mOhm.
Edit: I have tested it to function with 25A, but there’s always risk for something to break.
I used NCR18650GA’s for my testing but I think your explanation is correct. As the cells are depleted, the voltage will sag lower, and eventually it sags lower than the LED’s Vf so you cannot get full brightness.
That’s been my experience with all the SBT90.2 lights I’ve tested that have one cell, or multiple cells in parallel. I think SBT90.2 is best driven my multiple cells in series with a high current buck driver like in Convoy L7.
The L8 is definitely my favorite single cell sbt90 Light. I like everything about it, but the biggest drawback is the limited compatibility of a single 26800 / 26900. They're just not great cells for high amp sbt90 lights. I'd much rather the L8 head / Reflector with single 21700, 21700 in series or even 26800 in series if need be, and the appropriate buck driver of course (Side switch only). But, i still use the L8 daily and own two of them. One for each hand, of course.
As someone mentioned above, at 18A, it should be pulling about 4.5A per cell, which may cause sag in an NCR18650GA, but should be well within its capabilities. Probably less at most voltages since it’s a buck converter and the vF is close to 3v, so at higher cell voltages, it should be pulling less than 4.5A for the driver to deliver 18A @ 3v.
Even at 25A it’s only 6.25A per battery, which should also be ok for Sanyos. Sbt90.2 is about 3.6V on 25A if extrapolating from diagram. And GA’s drop to 3,8V at 7A current draw.
I didn’t know the regulator was linear, touché. Still, worst case is 4 - 6A draw per cell. Voltage sag may still be the culprit, but well within the cells’ capabilities. Only way to know for sure would be to test with different cells and compare I suppose.