I've currently got my Malkoff M60 in a SF C2 host, and have been thinking of picking up a Leef 1x18650 C-C body from Lighthound. My question has to do with regulation. Gene's website says that input voltage is 3.8 - 9.0 volts, and below 3.8 it drops out of regulation and runs direct drive.
Keep in mind that the 3.8V bottom end for regulation is a ballpark figure and will ultimately be determined by the LED itself. There are variations in LEDs from one to the next, some have higher Vfs than others to get the same forward current. So it's theoretically possible to have an unusually high Vf LED in a module, that will drop out of regulation at say, 4V, instead of 3.8V, on the flip side, it's also possible to have one that runs at 3.6V for the given forward current and would stay in regulation much longer on a single li-ion cell.
Hot off the Pila IBC charger, my cells are metering at about 4.17 - 4.18 volts.
Voltage off the charger is semi-irrelevant as it doesn't paint any picture about how the cell will behave under the load of the module. Most 18650s will probably start off around 4V into the load of an M60, and diminish steadily to about 3.5V. In the case of a module dropping out of regulation as this discharge happens, the load becomes lower and lower on the cell as the voltage drops, (less voltage to the LED results in less current flowing across it). So you might get say, 30 minutes, or maybe a hour of regulated output where the cell is holding above the ~3.8V mark, (depending on the behavior of the cell and the luck of the LED lottery as is relates to what Vf you actually get).
Now, regardless of whether I'm running a protected or unprotected cell, from what I understand a Li-Ion is basically dead at around 3.2 volts, someone correct me if I'm wrong. So, once the 18650 hits 3.8 volts, then all the way down to 3.2 volts, what is happening with the Malkoff?
It will just get dimmer and dimmer as the cell voltage drops, less and less current across the LED will continue to improve the efficiency of the emitter at the same time, so as it dims down, the lumen/watt performance improves. Runtime is extended WAY out as the LED will be drawing very little current when the cell gets down below ~3.5V under the load. Ideally, you should try to recharge the cell at this point rather than continue draining it down.
Is it brighter being direct driven than when it's regulated? Is the opposite true? Obviously when direct driven, it's a little harder on the LED, as well as the cell. But what I'm trying to ask is this; am I better off just sticking with my stock C2 body and running two 123 primaries, or is a 1x18650 option just as reliable?
Actually, when the module falls out of regulation, it's easier on the LED as the current flowing is lower. The LED runs cooler and more efficiently this way. (but still dimmer overall compared to running at maximum regulated output).
The 18650 option is fine, but I suggest protected cells to prevent extreme over-discharge which could be possible if accidentally left on.
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On a separate note: I've studied the output chart comparisons of MANY LED lights and modules that are buck-only regulated, like the MalkOff and found that most of them seem to even have slightly higher initial brightness when driven with 2xCR123 or 2xRCR123, and will obviously have more steady regulation on that setup....
This can make it a tough call, because if you have a higher Vf than 3.8V, then the actual regulated runtime of the module on an 18650 may be non-existent, and the light may always be in a state of steadily diminishing output through the entire run. (probably around 4 hours).
Using a pair of 3.7V RCR123s will guarantee that the module is running at maximum output through the duration of the discharge, but the total runtime will be less (I'm guessing around an hour, give or take). IIRC I think I read in another thread recently that it was tested having a runtime of about 50 something minutes till the cells shut off. Ideally, one should charge the cells before getting to that point to reduce unnecessary wear on the cells.
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18650 Advantages:
Longer runtime when you need it (great for emergency situation where any amount of light is better than no light).
You'll see dimming before the cell is over-discharged, making it easier to know when it's time to "top up."
18650 Disadvantages:
Potentially short run in regulation, followed by dimming output (varies by module Vf, out of your control)
RCR123 Advantages:
Entire run will be regulated maximum output
(and in your case, no need to splurge on a 18650 body)
RCR123 Disadvantages:
Less total runtime compared to other options
CR123 primary advantages:
Most of the run is in regulation, followed by some semi-useful emergency "dim" *moon* lighting after the cells are nearly depleted.
Better regulated runtime than RCR123s, (probably around double).
CR123 Disadvantages:
Cost of operation for frequent use adds up to more than a rechargeable solution.
Having to carry spares when you think the cells are getting low- (with rechargeable, you can always know you have a fresh top up whenever you leave the house).
Eric
My Wife's hair dryer broke last night, so she used the M6 to dry her hair, I'd say it's got some decent oomf 
