A battery that can no longer power a device may not actually be empty. In a Wired.com article (full article available to subscribers), physics professor Rhett Allain explains how a simple circuit known as a joule thief can extract residual energy from seemingly dead batteries and use it to keep devices such as LEDs operating.
As a battery discharges, its voltage gradually falls. An incandescent bulb becomes progressively dimmer because less current flows through its filament. LEDs behave differently. A white LED typically requires about 3 volts, so once the battery voltage falls below that threshold, the LED stops producing light even though chemical energy remains in the batteries.
The joule thief addresses this problem using a transformer and transistor. A transformer consists of two separate coils of insulated wire wrapped around a common core. When current passing through one coil produces a changing magnetic field, Faraday’s law of induction causes voltage to appear in the second coil. Faster changes in the magnetic field can generate higher voltages.
A transistor provides the rapid switching needed to make the system work. It repeatedly turns the current on and off thousands of times per second, creating an oscillating current. The changing magnetic field in the transformer produces voltage spikes high enough to illuminate an LED. In Allain’s demonstration, a single 1.5-volt battery powers an LED that normally requires 3 volts. The light continued operating for several days.
The concept extends beyond a physics experiment. Some high-end flashlights use miniature versions of this circuitry, called boost converters, to operate from a single battery. Similar voltage-boosting techniques are also useful in solar-energy systems. If solar panels produce only 10 volts on a cloudy day, for example, a boost converter can raise the voltage sufficiently to charge a 12-volt battery.
The joule thief demonstrates that a supposedly dead battery can still contain usable energy. The challenge is creating circuitry capable of converting that remaining energy into the voltage a device needs.
