Flashlight runs even when off

I have a very bright small flashlight that after some months would not fully power off, it gave a faint gleam. It is of the type that pressing the power button makes it cycle between several output intensities. It can even flash SOS in morse.

Soon after not fully powering off, it developed other problems, like not switching on. I complained on Amazon, because it was an expensive thing, and got it replaced for free. I hope the replacement doesn't fail.

El 12/03/2026 a las 22:47, micky escribió:

Modern hi-efficiency LEDs generate enough light to be visible with just a few microamperes. The eye can see light with just a handful of photons (think of viewing a star).

An open switch must have infinite resistance; humidity or external dirt may got inside, or metallic powder fallen from the contacts may create a path for the current. I have seen both cases.

Amazing. Does all this mean that in my case, I can leave the batteries in the flashlight and it will still take months to run down the batteries? Right now I feel obliged to stop using this light to not waste decent batteries.

El 14/03/2026 a las 2:59, micky escribió:

Yes, the leakage current is probably lower than batteries autodischarge.

I would clean or change the switch, though, as the problem will grow with time.

Maybe. Most of today's electronic gadgets have electronic on/off switches instead of mechanical on/off switches. The mechanical on/off switch does not allow any leakage current to flow in the off position. However, that's not always the case with electronic on/off switches, which might draw some small current in the off position. In example of this problem are the common commodity digital calipers.

Flashlights have the same problem but to a much lesser degree. Leakage current from the coin cell found in digital calipers is a fairly large percentage of the battery capacity. It doesn't take long to totally discharge the coin cell. However, all my flashlights now use Li-Ion batteries (18650 or larger). The same leakage current is a tiny fraction of the much larger battery capacity. You won't notice a few microamps of discharge current from a battery rated at 2 amp-hrs.

Also, Li-Ion cells have a very small self-discharge current. I charge my Li-Ion flashlights and individual batteries to 100% SoC (state of charge). They will self-discharge to about 95% in a few hours, 90% in a month, and then stay at about 85% SoC essentially forever.

If you want to use a high availability flashlight with an electronic on/off switch, *MEASURE* the leakage current and calculate how long the flashlight will be useable ( >75% SoC). If the switch discharge the battery in less time you consider acceptable, either buy a better battery (Lithium, Li-Ion, Li-Iron-Phosphate) or recharge (or rotate) the battery on a schedule that insures you will always have a functional flashlight.

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