Not all LED flashlights are created equal. Some LED flashlights will drain most of the power out of their batteries while other LED flashlights will hardly touch the power available in the batteries. You can think of these batteries as disposable water bottles that have only had a few swallows of water drunk form them. So how can you tell if your flashlight is a picky eater when it comes to batteries or if it literally sucks all the life out of them? At this point you may think “Why does this matter, I only use rechargeable batteries anyway” well please let me explain.

If your LED flashlight requires 3 cells you may have noticed that the intensity of the light starts to diminish almost as soon as you start using the flashlight (especially if you use rechargeable batteries). But surprisingly LED flashlights that require only one or two cells maintain their brightness for quite awhile. If you follow this simple rule you will always get the most out of your batteries “LED flashlights that use less than three batteries will be more effective at draining batteries than three cell flashlights”

This fact actually saves batteries in three ways:

  1. Two cell flashlights simply uses less batteries to begin with.
  2. You can get hours and hours of consistent light output from a one or two cell flashlight, because they use the available power very effectively.
  3. One cell flashlights, in particular, are a great way of draining almost all of the power out of “nearly dead” batteries that were previously used somewhere else. I only use used batteries in my LED flashlights. This means your running your flashlight essentially free.

So what determines how many batteries the flashlight needs? This has everything to do with the LED itself. First let’s look at how LEDs make light and then how they use electricity.

Each basic LED element can only make one color of light. LEDs like the ones in giant video screens (jumbotrons) have three basic LED elements in one housing (a Red, a Green and a Blue) and various ratios of the brightness for each of the three colors can appear as millions of colors to the human eye. But most of the time the white LEDs used in lighting applications are actually blue LEDs with a phosphor placed just in front of the blue LED.

Phosphor is a substance that has the ability to absorb light and reemit it as a different color. As an example, things that glow in the dark use phosphor to absorb light and reemit it not only as a different color but also over a long period of time. If you look into a white LED when it’s unlit you’ll notice that it’s yellow, (especially in sunlight) or perhaps you’ve noticed that some LED “tungsten replacement” light bulbs have yellow globe but when they are powered on they appear white. That yellow color is the phosphor reemiting yellow light. (for white LEDs this phosphor is typically Cerium doped Yttrium Aluminum Garnet or YAG:Ce) Some of the original blue light leaks through the yellow phosphor and this combination of yellow and blue light actually produces a spectrum of light broad enough that our eyes perceive this as white light. The blue LED under the phosphor is actually brighter than the blue that enters your eye, because approximately 66% of the blue is converted into yellow. Figure 1 shows the spectral response of the human eye. The yellow graph is the output of a white LED. you can see that the combination of the yellow phosphor (larger curve on the left) and the blue LED (steep curve on the right) cover almost the entire range of human spectral response. This broad coverage makes the LED appear as white.

spectral response curves for color

Figure 1

Now that we understand; that a basic LED element can only make a single color, that we see white when our red, green, and blue color receptors are equally stimulated, and that most white LEDs only have a blue “basic” LED element inside. We need to understand what determines the number of batteries a flashlight uses.

Inside the LED itself, light is produced when an electron crosses from one layer of the LED to the another layer. LEDs are solid state devices this means that they have no moving parts and can be thought of as a solid piece of material like a block of glass. (Figure 2) The electron is crossing what is known as a Band Gap. This is a region in the solid LED where electrons do not normally inhabit. What we think of as electricity is actually the movement of electrons. When the electricity is turned on, electrons start to pile up on one side of the band gap. After enough electrons pile up, the ones near the gap are pushed into the gap.  Once in the gap they are attracted to the other side. As they arrive on the other layer they emit a photon (a particle of light). Picture it like this: electrons build up at the edge of a cliff, as the crowd of electrons thickens, eventually some get pushed over, as they fall they hit the water and make a splash, the higher they fall the bigger the splash. Blue light is a “big splash” and red light is a “smaller splash”, this is because red light has lower energy than blue light. So the larger the band gap the bluer the light. The smaller the band gap the redder the light. Voltage can be thought of as the height of the cliff. Red LEDs need about 1.5 volts and blue LEDs need about 4.5 volts. White LEDs also need about 4.5 volts because they use blue LED elements. As the voltage drops there comes a point where no more electrons get pushed off the cliff, the splashes stop and no more light comes out.

LED pn junction with yellow phosphor diagram

Figure 2

A simple way to get 4.5 volts is to use three 1.5-volt batteries added together in series. But as soon as the voltage in the batteries drops the brightness will be reduced. Using three cells is a cheap way to get 4.5 volts. Slightly more expensive flashlights that use only one or two cells employ what is known as a “Boost Converter” or “Up Converter” to boost the 1.5 or 3 volt battery voltage to 4.5 volts for the LED.

A boost converter is an electronic circuit that boosts the voltage powering the LED. Picturing our “cliff” again, the boost converter is like stairs leading up to the back of the cliff. Think of it like this, smaller voltages are stacked on top of each other until finally reaching the voltage desired. Boost converters are great, because as the battery voltage drops they just work harder, “stacking voltages”, and keep delivering 4.5 volts to the LED. Eventually all the usable energy has been drawn from the battery because even the stacked voltage is too small to reach the desired voltage. This is why a one cell flashlight is so effective at draining a battery completely. On e cell flashlights are even effective at using batteries that are too dead to work other devices.(Figure 3) One word of caution, completely depleted batteries have a tendency to leak so remove them from the flashlight as soon as they are dead.

disassembled and labelled single cell LED flash light with boost converter

Figure 3 – Close up of LED Flashlight

John P. Rogers

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