The ideal runtime profile

An honest runtime graph shows how the flashlight behaves - at least in this test, at this temperature, with this battery etc. There are the design objectives for different types of lights and different modes in it. Drivers, thermal regulation, LED specs, efficiency and more put limits on how the runtime may look like. There are not too many lights whose runtime graphs look alike.

My question is: completely theoretically and disregarding most of the limitations, how the ideal runtime graph should look like? Flat, than down at the end to lower flat again as the reserve (for how long?), then off? Or flat, then down gradually to off? Or something else?

I’ve mentioned it elsewhere: my micro-headlamp annoyed me by e.g. High mode decreasing gradually below medium, which is flat. But then it made sense recently (at least some sense) - I can switch it on in any of the modes and rest assured that it will produce light for at least 6 hours rather than switching off suddenly even if I forget that it was on high or turbo at the beginning, which I do often.

I’d say that it’s personal preference.

I prefer it when the output is constant (flat) and then it steps down to a lower output. I don’t want to get stranded without light. But I still want low voltage protection to kick in eventually.

IMALENT MS12 Mini almost has that runtime but it cuts off:

Cyansky P25 V2 is another:

A less than ideal runtime with PWM by the Sofirn IF23:

The gradual drop in output is annoying when hunting. It makes identifying things difficult.

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I’m with you on flat outputs with some reserve, but there is also another ‘school of thought’ which more or less states that since perceived brightness is non-linearly related to luminance, relatively small differences in light output seem even smaller to the eye. I wonder how drastic the gradual drop in output needs to be for it to be consequential in focused activities such as night hunting?

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a few more lights w regulated output
Emisar D2

Skilhunt lights such as the H150

also Emisar D3AA (cant find a runtime chart)

I’d say that a drop to 1/3rd or half the output would impact hunting. I normally use a torch with 2x the required beam distance so that I can clearly identify animals at half the distance.

Brinyte T28 review by zeroair. The output has dropped to about half after 2 hours.

Sofirn C8L. Turbo is less than ideal. High is pretty good.

Convoy L21B SFT40 review by 1lumen. You might as well use 35% instead of 100%.

Nitecore MH40S review by 1lumen. Turbo offers a slightly higher output but the perceived difference between Turbo and High might not be enough to justify selecting Turbo over High. It depends how far you’re shooting and whether you want the extra runtime.

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So for instance something like this shouldn’t be much of a problem, visually?

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If voltage = low( lets say 2.7V) then actual level - 1. It is implemented in Anduril long time ago.

Probably because of half regulated hardware, like Fet+1.
Hardware and software must work together. If Fet cannot sustain brightness at certain voltage, software should switch to regulated channel or increase PWM on Fet ;))

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Yeah. It’s okay at 60 minutes.

I want flat, with a significant reserve at 20-30% of my selected level. The reserve should be a hard stepdown so that it’s easily noticed rather than a gentle ramp. Zebralights usually behave this way.

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There are other options to warn about battery voltage. For example 3 blinks at 2.8V next at 2.7V and only at 2.6V start to step down -1 level

Indeed there are, but because of how voltage sag works, it’s usually sensible to have a reserve anyway, and a large stepdown makes the reserve last much longer.

My point of view is that user decide when to decrease brightness. For reserve you must have spare battery anyway.

That’s a good option for advanced users… which is most of us reading here. I’m often thinking about how more mainstream users will use lights though, and I find they often don’t remember battery management instructions very well.

For reserve you must have spare battery anyway.

Not necessarily. Reserve means “you have X minutes at this level to get to a place where you won’t need this light”.

What level ? 1, 100, or 500lm?
Again there is other options:
// C&H: momentary turbo/ emergency mode at any voltage!
if (event == EV_click2_hold) {
else if (event == EV_click2_hold_release) {

The ‘ideal’ runtime profile? Depending on the mode, for example, High mode, you want fully regulated output at 100% of startup output for x time, or however long the battery can supply 5 volts to the boost/buck controller IC. Longer the better. For max output like Turbo, you should have the highest output for as long as possible, ANSI happens at 30 seconds, so that’s fine, but longer is better if the host can handle it (heat wise). For LVP, the best lights ive tested step the output down to about 20-30% for a short time, like another 20 minutes or so, then step down again to 10% and camp there for a good while accompanied by some kind of obvious notifier (blinking switch or indicator LED, brief flashes from the main LED, or haptic like Olight uses) that shut down imminent. Aftrt it does shut down, you should be able to reactivate the light at low output, but some batteries have protection boards that cut the output off or the battery is so dicharged it can’t power the boost IC and that’s not possible.

LEDs are highly influenced by heat, and some more than others so you want to balance the drive current and voltage (thermal regulation) against the host’s ability to transfer and dissipate heat. Batteries have gotten so good lately that unless you’re working with the hottest hot rods voltage sag isn’t a huge issue if using the right cell on a well regulated driver.

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IMO: A perfect runtime curve will hold turbo for as long as possible, then drop to thermally sustainable without overcompensating (and increase output if the cooling of the light increases). When battery is critically low, drop to moon mode and run on moon for at least a couple of hours before powering off.

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