Why Does My Flashlight Get Dim So Quickly?

LED Flashlights – General Info
1

Hi everyone! I recently started learning more about flashlights and noticed that some lights lose brightness after a short time, even when the battery is fully charged.

Is this usually caused by overheating, battery capacity, or the flashlight’s driver? I’d love to understand how to choose a light that maintains steady brightness and offers good runtime. Any advice from experienced flashlight enthusiasts would be appreciated. Thanks!

1 Thank
2

Generally you do this by not using the high or turbo Mode.

Turbo and High modes are usually not sustainable.
for a small poet size single cell flashlight you can get around 500-1000Lumen of constant output.

Sure some can do 10x this. but not for long.

It’s like a Electric car saying it has 1000Hp or something… for a 1/4 mile or so. but don’t compare it to a tractor.

Another reason is the stupid way “runtime” is defined and how manufacturers try to game/cheat the system.

The brightness is measured 30 seconds after turn-on, and the runtime is how long it takes for the brightness to drop to just 50% of its initial value.

If you now build a light that outputs 1,000 lm for 31 seconds before dropping to 501 lumens, you can advertise a runtime of 2 hours at 1,000 lm (even though it’s actually running at around 500 lm for 99% of the time).

If you built the same light but it didn’t drop to 500 lm and instead maintained 1,000 lm, you could only advertise a runtime of 1 hour at 1,000 lm.

2 Thanks
3

We’re very glad to have you here, Isla!

08 Will Smith
08 Will Smith2000×1284 173 KB

2 Thanks
4

I’d add that giving the light a few minutes to cool down between high-mode bursts helps a lot with heat-related stepdowns. For most everyday tasks, a steady medium mode actually ends up feeling brighter than turbo that fades after a minute.

5

Sometimes drivers are installed poorly, no heath path, and they overheat fast, multiple AMC drivers would shut off individual chips, when overheated, I build 2 lights with same drivers, same leds, one works as it should, other drops from high mode to low in minutes, I’m still trying to figure why it does it, so far, I can’t pinpoint at anything other then a defective driver.

1 Thank
6

A lot of flashlights have thermal protection built into the firmware that will step the power down as the driver/mcu heats up.
Some firmware is even configurable for thermal stepdown/protection.
Anduril 2 is highly configurable in this regard.

2 Thanks
7

It’s all about temperature.

Almost all modern flashlights are capable of much greater output than their heatsinking can absorb.

  • Modern flashlights are typically designed to only run at full brightness for a brief period of time.
    • Most higher end lights today have a temperature sensor that will automatically ramp the output down as it gets hot.
    • Some lights instead have timers that automatically turn down the output after a fixed period of time instead of a temperature sensor. This method is relatively uncommon on today’s lights, but was quite common 15 years ago.
    • Some manufacturers refer to max output “Turbo”. However, you can’t trust the specs. Many lights will still need to ramp down even from “High” or “Medium” as well as “Turbo”.
  • As a flashlight gets hot, it is programmed to ramp down to a safe sustained level. Sometimes this may mean 10 seconds or less of maximum output before ramping down.
    • Different lights handle this rampdown better than others. Some of my lights ramp down drastically almost instantly, while others can run at close to maximum brightness for around 5 minutes.
    • In some lights, the maximum sustained output can be as low as 10 or 20% of the maximum output.
    • Typically, the only lights whose maximum brightness is also their sustained brightness are very low output budget lights.
2 Thanks
8

What light, and what battery?

If it’s a crappy 3AAA light, it’ll be nice and bright with new cells then get irreversibly dimmer with each minute of use.

AAs same, but tend to hold up a bit longer because of their extra capacity.

Li cells will get dimmer except for those with regulated drivers. Otherwise it’s a gradual fade until recharged.

Others will throttle down on a timer or because of heat.

That’s the nutshell.

1 Thank