Idle Curiosity

June 4, 2021 6:58 AM

Why, when a flashlight will not turn on, does shaking or hitting it sometimes cause it to work?
posted by Hobgoblin to Technology (16 answers total)

The old D-cell flashlights were dependent on 4 batteries in a row having clean connections end-to-end, and they sometimes got misaligned. Jostling them sometimes got them back in alignment. (If you're seeing this with a modern LED flashlight then I don't know what to tell you.)
posted by restless_nomad at 7:03 AM on June 4, 2021 [15 favorites]

I've always assumed that the battery connection is loose (or potentially another connection like the bulb), and shaking or hitting it gets them to connect?

Basically seconding restless_nomad and adding that I currently have an LED flashlight that runs on two C batteries end-to-end.
posted by mskyle at 7:05 AM on June 4, 2021 [1 favorite]

I had one where the LED assembly gradually became unscrewed (presumably from vibration) and the electrical connection became unreliable. Since the power switch was also used to cycle modes (by momentarily turning the light off), it began to randomly turn on and off and switch modes. Opening it and screwing the internals tight fixed the problem.
posted by alexei at 7:14 AM on June 4, 2021

As per restless_nomad but corrosion at the contact sites between batteries and between batteries and the flashlight would create high resistances. Shaking the light would make the batteries move relative to each other thereby scratching the thin surface coat of corrosion and lowering resistance of the connection.
posted by Mitheral at 7:16 AM on June 4, 2021 [9 favorites]

The connection points can also get oxidized and oxides are good insulators. Mechanical agitation can knock a very thin layer off and make it work again. I've also seen this in TV/stereo remotes where just rotating the batteries in their cavity can make them work for a little longer. It only seems to happen when the batteries are close to running out.
posted by ArgentCorvid at 7:17 AM on June 4, 2021 [8 favorites]

I don't know, but back in the 60s and 70s (and probably 80s) my mother used to bang the side of the TV and it would suddenly improve the picture or reception. In my house, pretty much with any appliance, banging the side was the step before bringing it in for repair. My kids don't know from repairing an appliance. They just get a new one.
posted by AugustWest at 7:46 AM on June 4, 2021 [5 favorites]

Slight corrosion can stop the flow of electricity, rattling it may rub off the corrosion. If you have any old flashlights with incandescent bulbs, throw them out; don't even donate, as they are really useless. LEDs use way fewer batteries so you can pick up a flashlight and assume it will work.

When I worked in a school computer lab, we had some monitors with wonky color; a hard whack on the side (percussive maintenance) corrected it.
posted by theora55 at 7:56 AM on June 4, 2021 [2 favorites]

In my house, pretty much with any appliance, banging the side was the step before bringing it in for repair.

In the IT industry we call that "percussive maintenance".

The reason it works is that connectors all suck. It turns out to be surprisingly hard to design a way for two separate bits of metal to be pressed together in a way that allows the junction to remain electrically conductive over the long term, especially when that junction needs to be broken and re-made periodically, and sometimes all it takes to restore a connector's conductivity is to joggle it by the width of an atom or two.

Typical battery connections are particularly bad at this. Batteries are low-voltage power sources from the start, which means that any increase in connector resistance is going to impose quite substantial limits on their ability to deliver power. The round button top on a typical battery ends up pressed not terribly hard against a flat piece of metal in the battery holder, usually ending up with a tiny single point of contact; there's a spring contact for the battery's flat base that also usually has a round bump on it, or is made entirely from round springy wire, and also usually ends up making a really small contact zone.

Neither of these contacts is in any way gas-tight and the longer a battery holder or rechargeable battery has been in service the more likely it will be that there's a thin film of corrosion ready and able to make contact unreliable from the get-go.

Easiest temporary fix is to take an ink eraser (the rubber one with a tiny bit of abrasive distributed through the material) and give both the battery contacts and the battery holder contacts a good rubbing-over. This will usually give you a few months of not needing to bash your torch to make it work. If it doesn't, it's probably because the contacts inside the torch's on-off switch or those between the battery holder spring and the torch's lamp assembly are every bit as badly designed as those built into the battery.

There's a reason why the rechargeable cells inside a laptop or power tool battery pack have connector tabs welded to the cells and soldered to the circuitry.
posted by flabdablet at 8:07 AM on June 4, 2021 [24 favorites]

The best connector designs, by the way, all have mating conductors that slide into place against each other rather than merely being pressed together, and connectors that are plated with highly corrosion-resistant surface coatings like gold alloys. Torch battery connections and typical torch on/off switches have none of these features.
posted by flabdablet at 8:12 AM on June 4, 2021

(If you're seeing this with a modern LED flashlight then I don't know what to tell you.)

I have absolutely experienced this with the cheap single A cell LED flashlights, particularly if they'd been exposed to anything corrosive like water or dust.
posted by Candleman at 8:37 AM on June 4, 2021 [1 favorite]

The term of art is "contact resistance" if you'd like to dig into the physics of it. The excellent summary by flabdablet hits all the, uh, high points.
posted by drdanger at 8:43 AM on June 4, 2021 [1 favorite]

Response by poster: Well answered! I want to mark all answers as Best, and may do so. Thank you, everyone.
posted by Hobgoblin at 8:55 AM on June 4, 2021

I have absolutely experienced this with the cheap single A cell LED flashlights

Also standard operating procedure for the even cheaper three AAA LED flashlights that don't even have the voltage boost converter that a 1.3 volt single-cell design requires to run its white LED(s), the inherent physics of which yield a typical operating voltage of between 3 and 5 volts.

My cheapo three-cell flashlight has eleven shit-grade contacts in series - two for each AAA cell inside the removable three-cell caddy, plus two more at the ends of the caddy, plus the other end of the caddy spring pressed against the inside of the back cap, plus the screw thread between the back cap and the torch body (both aluminium alloy), plus whatever piece of crap is inside the switch. Poor contact in any one of these is enough to dim or extinguish the light.
posted by flabdablet at 9:01 AM on June 4, 2021 [3 favorites]

Tangentially related but fun - some ghost hunters will “communicate” with ghosts by asking questions in the dark, with the responses from the spirits coming in as flashes from the still, set aside flashlight.

So how does that work?

Radiolab figured it out - a nifty trick, and an explanation that dives into the workings of a flashlight
posted by glaucon at 9:19 AM on June 4, 2021 [3 favorites]

I don't know, but back in the 60s and 70s (and probably 80s) my mother used to bang the side of the TV and it would suddenly improve the picture or reception.

That TV probably had vacuum tubes which have a series of pins that plug into a socket. The pin-socket connection can have the discussed corrosion/oxidation problem that can again be fixed with a little wiggle.
posted by Mitheral at 10:12 AM on June 4, 2021 [2 favorites]

Still works occasionally on computers with cards connected to motherboards via edge connectors that plug into slots, and on modern TVs that have flex cables that plug into ultraminiature slot-descended sockets.

All connectors suck. Some just suck sooner than others.

There are only two ways to make a good long-lasting gas-tight metal-to-metal connection that I know of. The obvious one is soldering. The other involves actually damaging the connecting pieces to some extent, whether that be by crushing them together with bolts or crimping, or by scoring them heavily as happens inside insulation-displacement punch-down blocks or at the sharp corners of square-section wire-wrap posts.

In my experience, any connector designed to tolerate repeated disconnection and reconnection will eventually require disconnection and reconnection and/or explicit cleaning. It's just staggering how many intermittent PC faults can be fixed just by re-seating expansion cards and/or RAM and/or CMOS batteries and/or socketed CPUs and/or SATA cables.
posted by flabdablet at 2:36 PM on June 4, 2021 [3 favorites]