A pill (or driver) that is designed for use with 1x18650, and is actually optimized for 1x18650, uses a boost/buck setup that keeps the LED in regulation regardless of whether the input voltage is slightly above or slightly below the Vf of the LED at the drive level that the driver is setup for. Modules that do this, are almost exclusively only compatible with 1 li-ion cell, or 3x alk/NIMH/NiCD cells. Applying more voltage will fry the module. The 1S, and 3SD are examples of this type of regulation. This is, IMO, the best type of module to select if you are planning to use rechargeable 18650s exclusively.
A pill (or driver) that uses a buck-only regulation, is generally compatible with higher input voltages. These types of drivers need an input voltage higher than or equal to the Vf of the LED (sometimes as much as a full volt or more higher depending on the design) in order to maintain "regulated" current across the LED. When the voltage drops too low, the current across the LED drops as well. A module like this can be driven by a single li-ion cell, but the result is steadily diminishing output through the run, rather than a flat constant brightness. The advantage is increased runtime (usually around double), the disadvantage is that the module only operates close to full brightness when the cell has a full charge. This is like the 1SM-2, and the 3SM. If you were to compare a 1x18650 and 2x18650 setup on one of these modules, the difference WOULD be in brightness, as both configurations would be about the same ~4 hours. But the 2x18650 setup would maintain constant brightness, while 1x18650 would dim down as it ran.
Those buck-only regulated drivers, can handle input voltages of multiple cells without frying as mentioned. In the case of the 1SM-2, it can even handle the voltage of 4xCR123s, or even 5, if you wanted to for some reason. But I think the concept here, is a 4xCR123 length host, with the option to run either 2x18650s or 4xCR123s as needed. The output remains the same regardless of whether you use 2x18650 or 4xCR123s. The runtime is pretty similar both ways as well I would imagine.
Personally, I would not be too concerned about the different drive levels between the modules. The difference between driving an XRE at 1 amp, compared with 1.2A, is probably only something that can be measured. I highly doubt it would make any noteworthy difference in usefulness. Keep in mind, that it takes about 30% difference in output to really see. Increasing the drive level to an LED always reduces efficiency, so that extra 20% drive level probably only results in maybe 10-15% more light, hardly noticeable. IMO, if you are adding an extra cell, with the intention of having increased runtime (which is the main purpose IMO!), then having that slightly lower drive level is important, as it will keep the light cooler for those extended runs, and at the same time, extend that runtime out even further.
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Anyways, that's enough rambling. The point in the end, is that, if you want a longer running light, use a buck regulated module and 2x18650 host setup. If you want a more compact light, use a 1x18650 host, and a boost/buck module. If you want both, invent a better battery
Eric
