A bit of history
The trope used to be Truth in Television in the first decades after the invention of the flashlight, that back then featured a carbon filament bulb and zinc-carbon batteries – the reason it has this name is exactly because it could only be "flashed" in short bursts, or the batteries would die at about the rate a safety match burns out. Technology Marches On, however, and with it came less power-hungry tungsten filament bulbs and batteries with better chemistries and capacity, that could sustain a low-powered incandescent model for a few good hours... provided the bulb held (a regular one would hardly last a full battery charge before frying; more durable halogen ones were expensive, and fluorescent lamps couldn't be miniaturized enough to fit anything smaller than a big, heavy searchlight). It all changed to aversion levels in The New '10s with the introduction of extremely efficient light-emitting diodes (LEDs) as the emitter of choice for pocketable torches. Nowadays, a good rule of thumb is that, in equivalent brightness levels, you can expect an old incandescent torch to have the same battery life as a finger-long LED keychain light loaded with an AAA battery.
Not a Dead Horse Trope
Even with the technological advancements listed above, the trope can still apply for more diverse reasons than "it's what writers are used to". Now good luck finding a single work of fiction that takes any of that into account, or a writer that's ever heard of this.
- Compared to incandescent lamps, LED bulbs are much more flexible as to how much light they provide depending on the current fed into them, regulated by the driver circuitry. As such, flashlights with multiple brightness levels are ubiquitous. Typically, while lower modes that put out about as many lumens as the dim incandescent lights we're used to are almost efficient to Infinite Flashlight levels, LEDs can reach much higher brightness levels when driven at high currents, and in that mode they tend to last only about an hour or so, even less if the light is small with an equally small battery (a 10180 keychain micro-light as of 2018 can't sustain 100 lumens for more than 10 minutes; an AAA light can remain on that level for around 20 to 30 minutes).
- Extremely potent models with multiple thousands of lumens, commonly called "blasters" among collectors and enthusiasts, can actually be ten-second for one reason: overheating. LEDs are more efficient than filament bulbs, but they still waste large amounts of energy as heat, and that can damage the flashlight by burning up the emitter or even melting the solders. As such, manufacturers usually include a safety measure like a thermal sensor or timer that triggers a step-down function that drops the output to less hot levels, or turns the flashlight off entirelynote . The exact same effect applies to "pocket rockets", very small flashlights able to spew the same lumens as bigger models, due to the reduced heatsinking of their tiny bodies.
- Mechanically-powered (crank or dynamo AKA "squeeze") LED flashlights, once their batteries have worn out sufficiently, tend to only last about a minute before needing a quick recharge. If you crank them fast enough, they will indeed recharge in considerably less time than the light lasts and can even be used continuously, but it is easier to do so with them off and not pointed in front of you, meaning they function almost exactly like video game flashlights, with the added drawback of being quite noisy and, in the case of crank ones, needing both your hands free to recharge them. The Faraday "shake" light tends more towards the ten-second end – you're not going to be able to use one effectively while shaking it to power the capacitor.
- One more piece of the puzzle to complicate things further is the battery chemistry.
- As far as 1.5v batteries go, nickel-cadmium (NiCd) cells work well in high-drain devices but has low capacity, while nickel-metal hydride (NiMH) are the inverse. A light fed a NiCd cell will shine brighter for a shorter time than if it was running on NiMH, in which case it wouldn't be as bright right off the bat, but it'd sustain its max brightness for longer.
- Much the same goes with lithium-ion cells. The IMR (lithium-manganese oxide) chemistry can sustain much higher current draw than ICR (lithium-cobalt oxide) without battery sag, but its capacity is lowernote . The newer hybrid INR (lithium-manganese-nickel) like the Samsung 30Q muddies the waters, having capacity almost as good as ICR and current draw tolerances on par with IMR.
Data Pads and light sources
An interesting reconstruction of the trope is the main light source most people carry with them today: their smartphones. By cranking the camera flash's LED (or turning the screen white at max brightness if it's a tablet or an older device that doesn't have a flash), the device generates enough light to see by. However, smartphones are not designed to be used as flashlights, and if the phone is on, it's bound to have a number of programs and apps running in the background. As such, it's best used in quick bursts to flash a light when necessary, like flashlights of old; using the light function too long or too often, especially in an emergency situation, will suck the battery dead very quickly, likely leaving the user dead in the water and as blind as a bat, and worse, deprived of a vital communication device.
