A ~ 850 lumen 18650 Uniquefire UF-2100 that costed me $14.30 shipped from Dinodirect group buy. A bit big at 120mm and 21 x 21mm, but still manageable with the mini lanyard hooked to the keys. Draws 3 amps and bright like hell on the lux meter! The good thing is that low mode is pretty low, like 70mA only and about 30 lumens so it can run like forever in this with the 18650. And PWM is at 1.7kHz measured, it is absolutely impossible to detect with the naked eyes even on fast moving objects, unless you are talking about fan blades on high mode. That's like 2X as fast as iTP.