I have to wonder how many millions of "partly used" batteries get needlessly tossed simply because of digital (LED) flashlights.
For the kids in the audience who don't know about 'bulbs'... as a battery starts to get low, an incandescent bulb simply gets dimmer. At some point, it's too dim to be useful, so you replace the battery.
Digital, on the other hand is either "on" or "off"... there is no "dim" middle ground.
So, not hypothetically speaking, suppose you are riding your bike through a looooong 1.66 mile pitch-black train tunnel on the gorgeous Hiawatha trail near the Montana/Idaho border, and about half-way your LED flashlight's batteries hit the roughly 60% full mark. Do the LEDs get dim? No. They go out.
Naturally, you have a spare flashlight and you use that.
But when you're done, you toss a still-half-full set of batteries you mistakenly assume are dead because your digital flashlight told you so.