How long can a flashlight run on an 18650 in an emergency?

I was extracting some laptop pulls that turned out to be toast, i could not get them to fully charge, they heated up instead at 4.1V. So i want to drain them before recycling. I put them in two lights and left them on, predictable full brightness then dimming, but one light has custom firmware and does not flash on low, but just keeps going. About 8 hours later its still going but on very moonlight output, i got maybe 3-4 hours of low but usable light.

It seems hard to completely drain the battery, though i know i will never actually be able to get to 0V, since the XM-L2 needs a basic amount of voltage to just turn on, but has anyone else tried this battery killing exercise?

I figure in an emergency its good to know how much we can push a light, and if we drain a battery below safe battery voltage replacing it is a small price to pay to get out of a bad situation.

Your thoughts?

Below 2.5v there is very little capacity left / you get very little out of killing the cell. You can see in HKJ’s charts. Just plan and drop down to moonlight sooner, before it gets that low. In normal situations never run your flashlight empty, keep a little in reserve in case you need it in an emergency.

This is the advantage of unregulated lights using alkalines. Increasing efficiency of LEDs at lower currents together with the internal resistance of cells results in long runtimes. The internal resistance of the old 18650s are high so will reduce current greatly. In an EMERGENCY, i’m definitely willing to sacrifice a few 18650s for the sake of light however, one must have an unregulated flashlight, or maybe one with a boost driver to use every last drop of energy and after the cell is dead, maybe you can take it apart and use the deposited lithium to light a fire! Hahahha! If your priority is to discharge the cells, dead shorting them at this stage would be harmless. The internal resistance is too high for the cell to heat up and become a hazard.

Many lights are rated at 1 month of 1lumen with a standard 18650.

As far as i know generally the capacity of the cells will drop about 10-12% per year. Also as a general rule of thumb, Li-on cells like to be stored at ~3.9v or 40% capacity. But Leaving them at 4.2v for a while is not terribly good for the cells. I have had cheap Chinese cells not last 18 cycles before, but I have had laptop packs last probably 350-450 cycles with Japanese cells. Quality always matter ;)

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I meant in an emergency where you have one light with little juice left.
In normal use i typically recharge at 40ish percent remaining, i keep my storage batteries at 3.8V but a few i fully charge and use as needed