I finally got a chance to characterize my light today. I like to see how much current and power my lights draw from a "perfect" power source. That is, a regulated laboratory power supply with a current read out that tells me the current draw. I was able to learn a bit more about how the low battery indicator works, where it cuts in, etc. The most interesting thing I found is that the light gets more efficient (less current draw) as the input voltage drops. So as batteries are depleted, the light gets more efficient. The battery current still goes up, but the total power drawn from the batteries decreases as compared to higher input voltages. The other interesting thing I learned is that in turbo mode, for voltages less than 1.1V/cell (4.4V stack), the light starts to step down in output. I could not notice it with my eyes, but I could see the current going into the light drop significantly, like 0.5A. And once this occurs, even if your battery recovers (not likely) the output stays reduced until you disconnect power from the driver. This happens even before the low battery indicator is illuminated. That happens all the way down at 3.7V, at which point you likely don't want to be using rechargeables anymore, so get out of turbo mode, and likely high mode also. Of course if you chose alkalines, then who cares about killing the batteries . . .
On all levels the low battery indicator turns red at 3.7V or lower, so the batteries are definitely low at that point. As someone mentioned to me in another thread, if you really want to use this light on turbo for longer duration, the lithium primaries are really the best bet. Too bad they are so $$$ for recurring use.