I bought one of these cheap XML zoomies that can take 3xAAA in an adapter or 1x18650; it came with an XPE instead of an XML so I intend to mod it, i.e. better LED (XML2) and while I’m at it better driver.
To this end I’ve bought a nanjing 105C, must have been late at night because in hindsight 2.8 A may be a bit too much for AAA cells. I know that using these cells in series will give me the capacity of one cell at 3x the voltage, but what about the max. draw they can endure? Will the circuit draw less, will the cells overheat, etc. They’re 850mA LSD NiMH similar to eneloops. Don’t have any lithium cells.
It wouldn’t be dangerous to do so, they might heat up slightly. The only concern would be the voltage drop. You could be hitting less than 3V in a mere 2-3 minutes and then the light falls out of regulation. You could take out some AMC7135 chips, you probably won’t notice the brightness difference too much and it will last much longer.
900mAh cells:
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/Eneloop%20AAA%20BK-4HCC%20900mAh%20(Black)-CapacityTime.png)
They should be fine…will be pushing them for sure but NiMH batteries can push quite a bit more current than say alkalines (internal resistance is high so current drop the harder you push them is very bad)
Don’t put Li Ion in 3x because you will end up with 9-12~Vdc(3.7-4.2 per batt x3 in series) vs 4.5vdc(1.5 per batt x3 in series), you CAN use a single Li Ion 14500 provided you use 2 dummy batteries in the other two slots, and a good 14500 can do 2.8A without much issue.
ryansoh3 is right though, 2.8A will suck those batteries down pretty quick, nice you have lower modes than wide open thankfully 
Will probably be like direct drive, which is fine except the brightness depends on the state of charge.
For a zoomie a XP-E2 may be better than a XM-L2, and then you don’t want to run it hotter then 2 amps, and that’s with a direct thermal path star.