dougmc

When you say 1.5v alkaline and 1.2v NiMH ... you're comparing apples and oranges.

The reality is that alkaline batteries start at 1.5v and go down from there, and NiMH batteries start from 1.4v and go down from there. I don't know why they rate the voltages differently for primary vs. rechargeable cells, but they do. Your alkaline cell only hits 1.5 volts when brand new, but when a NiMH cell hits 1.2 volts ... it's around 75% of the way to dead.

In any event, for most lights, the difference in brightness between alkaline and rechargeables is negligible. Of course, the advantage is that when the light starts to get dim ... you recharge the rechargeable and throw away the alkaline, so it's more practical to keep the rechargeables near full charge and therefore near full brightness -- just charge them every ride or every few rides rather than replacing them when they get dim like you do with alkalines.

As for your Li-ion lights, Li-ion cells have a discharge curve just like alkaline and NiMH cells do -- they generally (there are some different chemistries out there) start at 4.2 volts, but their "nominal" voltage (i.e. the voltage that the package says) is 3.6 volts. The only reasons you don't notice the difference in brightness are that 1) many of them have regulators that make sure the LED gets a constant voltage, and 2) Li-ion cells are ruined by discharging below 3.0 volts/cell or so, so they usually have a cutoff circuit that just shuts it off completely rather than letting it get dimmer and dimmer like a NiMH or alkaline cell will.

Many of the single cell 18650 (li-ion) flashlights have no regulator, and you can clearly see them getting dimmer as the battery discharges ... and then they shut off completely when the cutoff circuit is activated. But most dual cell (starting at 8.4 volts and then going down from there) lights have a regulator so you don't notice any changes until it shuts off (with no warning as you said, unless it has a seperate warning light.)