Squeeze More Juice Out of a Dead Battery!

You’re walking the dog at night, and your little flashlight, which has been getting dimmer, conks out. Instead of stumbling home in the dark, wouldn't it be great if you could somehow eke a little more energy out of the batteries inside?

That's not as crazy as it sounds. When a battery-powered device stops working, we say the battery is “dead”—but it's not really. It still contains chemical energy, and the voltage isn't zero; it's just not high enough to run a current through the light bulb or LED (or whatever load you have).

But with a little physics chicanery, you can indeed get that light to run longer. Much longer! I'm going to show you how to build a simple electrical circuit, pairing a transformer and a transistor, to tap that residual energy. It's waggishly called a “joule thief” circuit. Get it?

It's not only fun to build and kind of mind-blowing, it's also a great illustration of Faraday's law of induction, the same principle that's used in electric generators—and induction cooking stoves, for that matter. Let's do this!

Basic Battery and a Light Bulb

I'll start with a very simple circuit. This is a 1.5-volt AA battery connected to a small incandescent light bulb using a single copper wire.

It's a complete circuit. Electric current comes out one end of the battery, runs through a bulb with a tungsten filament inside, and then returns to the other end of the battery. Because that filament is super thin, the current heats it up—to like 4,500 degrees Fahrenheit—so that it glows white-hot. (Luckily, tungsten has the highest melting point of any pure metal.)

As long as the circuit is complete—which in a flashlight means the switch is left on—current will continue to flow, gradually using up the battery’s chemical potential energy. As the voltage drops, it produces less current. At some point, there won't be enough to produce any light.

In a way, this circuit is also a joule thief. If you leave the switch on, current will continue to flow even after the light goes out, and it'll use up the battery's remaining energy. But who cares about a joule thief that drains a battery and doesn't produce light?

LEDs vs. Incandescents

Today, most devices use LEDs instead of incandescent bulbs. An LED doesn't produce light by making things hot; instead, it's a solid-state device with an energy gap. When electrons in the current fall to a lower energy level, they release the extra energy as light. It's way more efficient, because you don't have all that wasted thermal energy. The only downside is that a white LED requires 3 volts, so now we need two of those AA batteries:

As the batteries run down, they'll eventually drop below the 3-volt threshold. You might still have 2.8 volts, but you get zero light. This is where the magic of the joule thief comes into play. In fact, we can get a 3-volt LED to turn on with just a single 1.5-volt battery! But first, there are two things we need: a transformer and a transistor.

The Transformer

There's more than one way to make an electric current. Yes, a battery does the job, but you can also use a magnetic field. This is Faraday's law. It says that if a loop of wire sits in a region with a changing magnetic field, a voltage will be induced in that wire.

Well, that's exactly how a transformer works. It has two coils of wire wrapped around a common core. The wires are insulated so they aren't in contact—it's two separate circuits. But if you run a current through one coil, it generates a magnetic field, and when that field changes in any way, it creates a current in the second coil.

Here's the DIY transformer I'm going to use. (There are two sets of wires here, but they're both red—I should have used different colors.) The iron ring increases the strength of the magnetic field to make the transformer more effective.

Just to be clear: It's not the magnetic field per se that induces a voltage in the second coil. It's the change in the magnetic field. When you turn the current on or off in one coil, you get a voltage spike in the other coil. If you turn it on and leave it on, that voltage falls to zero.

In fact, the strength of the induced voltage depends in part on the rate of change. Reduce the magnetic field slowly and you get a low voltage; turn it off quickly and you get a high voltage. You can also get a higher voltage by having more turns of wire in the secondary coil.

That's the secret to this whole magic trick. With a transformer, you can get a higher voltage in the secondary circuit, which is attached to the LED, than you started with in the battery. So all we need now is a way to turn the primary circuit on and off repeatedly and quickly. That's where the transistor comes in!

The Transistor

At the most basic level, you can think of a transistor as a valve for electricity. It can turn the current off or let it pass through. And the transistor gate is controlled by … wait for it … an electric current. Yes, it's a little mind-bending. It's as if you need a stream of water to open a spigot and create a stream of water.

But that's pretty much all you need to know about how the joule thief works. The transistor switches the current on and off thousands of times a second, essentially creating an oscillating current so that the transformer produces a higher voltage.

The Joule Thief

Now we're ready for the full circuit. There are many tutorials online for building one of these, but I'm going to use a very simple version. Here's a diagram:

So the battery pushes a current through the primary (blue) coil in the transformer—creating a magnetic field and thus a burst of current in the secondary (red) coil. This induced current then goes into the transistor and switches off the primary current—but that again causes a jump in the magnetic field. Each time, there's a spike of 3 volts in the second coil, lighting up the LED. But the battery can't keep that up, because it only has 1.5 volts, and this switches the transistor back to the first loop. Et cetera, et cetera.

Below is a photo of my actual joule thief. As you can see, it has but a single 1.5-volt battery, which is half the voltage the LED needs. When I took this picture, the light had already been on continuously for several days. And it'll keep going till the battery really is dead.

Now, you're not going to carry around a contraption like this when you're walking the dog at night. But guess what? High-end pocket flashlights often have a joule thief built in so they can run on a single battery. It's all miniaturized onto a tiny circuit board called a boost converter.

The same method of creating an oscillation with a DC power source is used to step up voltage in other devices. Solar panels, for example, are often connected to home batteries to store electric power for use at night. But on a cloudy day, the panels might generate only a 10-volt potential, which won't charge a 12-volt battery. So these systems often contain a boost converter.

See? This isn't just a neat lab trick. Joule thiefs are out there, lurking in your own neighborhood. And it's a good thing!