Explainer: How batteries and capacitors differ | Science News for Students

Explainer: How batteries and capacitors differ

Each energy-storage device has its own advantages and disadvantages
Jan 18, 2017 — 12:58 pm EST
circuit board

Many electronic circuits (like the one shown) are powered by batteries. Increasingly, however, engineers are looking to capacitors as another option for providing energy as needed to all or parts of such circuits.

fotek/iStockphoto

Energy can be stored in a variety of ways. When you pull back on a slingshot, energy from your muscles is stored in its elastic bands. When you wind up a toy, energy gets stored in its spring. Water held behind a dam is, in a sense, stored energy. As that water flows downhill, it can power a water wheel. Or, it can move through a turbine to generate electricity.

When it comes to circuits and electronic devices, energy is typically stored in one of two places. The first, a battery, stores energy in chemicals. Capacitors  are a less common (and probably less familiar) alternative. They store energy in an electric field .

In either case, the stored energy creates an electric potential. (One common name for that potential is voltage.) Electric potential, as the name might suggest, can drive a flow of electrons. Such a flow is called an electric current. That current can be used to power electrical components within a circuit.

These circuits are found in a growing variety of everyday things, from smartphones to cars to toys. Engineers choose to use a battery or capacitor based on the circuit they’re designing and what they want that item to do. They may even use a combination of batteries and capacitors. The devices are not totally interchangeable, however. Here’s why.

Batteries

Batteries come in many different sizes. Some of the tiniest power small devices like hearing aids. Slightly larger ones go into watches and calculators. Still larger ones run flashlights, laptops and vehicles. Some, such as those used in smartphones, are specially designed to fit into only one specific device. Others, like AAA and 9-volt batteries, can power any of a broad variety of items. Some batteries are designed to be discarded the first time they lose power. Others are rechargeable and can discharge many, many times.

miscellaneous batteries
Batteries, one form of storing energy, are vital to many devices that won't get plugged into an electrical wall outlet.
scanrail/iStockphoto

A typical battery consists of a case and three main components. Two are electrodes. The third is an electrolyte. This is a gooey paste or liquid that fills the gap between the electrodes.

The electrolyte can be made from a variety of substances. But whatever its recipe, that substance must be able to conduct ions — charged atoms or molecules — without allowing electrons to pass. That forces electrons to leave the battery via terminals that connect the electrodes to a circuit.

When the circuit isn’t turned on, the electrons can’t move. This keeps chemical reactions from taking place on the electrodes. That, in turn, enables energy to be stored until it is needed.

The battery’s negative electrode is called the anode (ANN-ode). When a battery is connected into a live circuit (one that has been turned on), chemical reactions take place on the anode’s surface. In those reactions, neutral metal atoms give up one or more electrons. That turns them into positively charged atoms, or ions. Electrons flow out of the battery to do their work in the circuit. Meanwhile, the metal ions flow through the electrolyte to the positive electrode, called a cathode (KATH-ode). At the cathode, metal ions gain electrons as they flow back into the battery. This allows the metal ions to become electrically neutral (uncharged) atoms once again.

The anode and cathode are usually made of different materials. Typically, the cathode contains a material that gives up electrons very easily, such as lithium. Graphite, a form of carbon, holds onto electrons very strongly. This makes it a good material for a cathode. Why? The bigger the difference in the electron-gripping behavior between a battery’s anode and cathode, the more energy a battery can hold (and later share).

As smaller and smaller products have evolved, engineers have sought to make smaller, yet still powerful batteries. And that has meant packing more energy into smaller spaces. One measure of this trend is energy density. That’s calculated by dividing the amount of energy stored in the battery by the battery’s volume. A battery with high energy density helps to make electronic devices lighter and easier to carry. It also helps them last longer on a single charge.

flaming battery
Batteries can store a lot of energy in a small volume, sometimes with tragic consequences.
weerapatkiatdumrong/iStockphoto

In some cases, however, high energy density can also make devices more dangerous. News reports have highlighted a few examples. Some smartphones, for instance, have caught fire. On occasion, electronic cigarettes have blown up. Exploding batteries have been behind many of these events. Most batteries are perfectly safe. But sometimes there may be internal defects that cause energy to be released explosively inside the battery. The same destructive results can occur if a battery is overcharged. This is why engineers must be careful to design circuits that protect batteries. In particular, batteries must operate only within the range of voltages and currents for which they have been designed.

Over time, batteries can lose their ability to hold a charge. This happens even with some rechargeable batteries. Researchers are always looking for new designs to address this problem. But once a battery can’t be used, people usually discard it and buy a new one. Because some batteries contain chemicals that aren’t eco-friendly, they must be recycled. This is one reasons engineers have been looking for other ways to store energy. In many cases, they’ve begun looking at capacitors.

Capacitors

Capacitors can serve a variety of functions. In a circuit, they can block the flow of direct current (a one-directional flow of electrons) but allow alternating current to pass. (Alternating currents, like those obtained from household electrical outlets, reverse direction many times each second.) In certain circuits, capacitors help tune a radio to a particular frequency. But more and more, engineers are also looking to use capacitors to store energy.

Capacitors have a pretty basic design.  The simplest ones are made from two components that can conduct electricity, which we’ll call the conductors. A gap that doesn’t conduct electricity usually separates these conductors. When connected to a live circuit, electrons flow in and out of the capacitor. Those electrons, which have a negative charge, are stored on one of the capacitor’s conductors. Electrons won’t flow across the gap between them. Still, the electric charge that builds up on one side of the gap affects the charge on the other side. Yet throughout, a capacitor remains electrically neutral. In other words, the conductors on each side of the gap develop equal but opposite charges (negative or positive). 

miscellaneous capacitors
Capacitors, several of which are shown above, are used to store energy in electronic devices and circuits.
yurazaga/iStockphoto

The amount of energy a capacitor can store depends on several factors. The larger the surface of each conductor, the more charge it can store. Also, the better the insulator in the gap between the two conductors, the more charge that can be stored.

In some early capacitor designs, the conductors were metal plates or disks separated by nothing but air. But those early designs couldn’t hold as much energy as engineers would have liked. In later designs, they began to add non-conducting materials in the gap between the conducting plates. Early examples of those materials included glass or paper. Sometimes a mineral known as mica (MY-kah) was used. Today, designers may choose ceramics or plastics as their nonconductors.

Advantages and disadvantages

A battery can store thousands of times more energy than a capacitor having the same volume. Batteries also can supply that energy in a steady, dependable stream. But sometimes they can’t provide energy as quickly as it is needed.

Take, for example, the flashbulb in a camera. It needs a lot of energy in a very short time to make a bright flash of light. So instead of a battery, the circuit in a flash attachment uses a capacitor to store energy. That capacitor gets its energy from batteries in a slow but steady flow. When the capacitor is fully charged, the flashbulb’s “ready” light comes on. When a picture is taken, that capacitor releases its energy quickly. Then, the capacitor begins to charge up again.

Since capacitors store their energy as an electric field rather than in chemicals that undergo reactions, they can be recharged over and over again. They don’t lose the capacity to hold a charge as batteries tend to do. Also, the materials used to make a simple capacitor usually aren’t toxic. That means most capacitors can be tossed into the trash when the devices they power are discarded. 

The hybrid

In recent years, engineers have come up with a component called a supercapacitor. It’s not merely some capacitor that is really, really good. Rather, it’s sort of some hybrid of capacitor and battery.

So, how does a supercapacitor differ from a battery? The supercapacitor has two conducting surfaces, like a capacitor. They’re called electrodes, as in batteries. But unlike a battery, the supercapacitor stores energy on the surface of each of these electrodes (as a capacitor would), not in chemicals. 

Meanwhile, a capacitor normally has a non-conducting gap between two conductors. In a supercapacitor, this gap is filled with an electrolyte. That would be similar to the gap between the electrodes in a battery.

Supercapacitors can store more energy than regular capacitors. Why? Their electrodes have a very large surface area. (And the larger the surface area, the more electrical charge they can hold.) Engineers create a large surface area by coating the electrode with a very large number of very tiny particles. Together, the particles produce a rugged surface that has much more area than a flat plate would. That lets this surface store far more energy than a regular capacitor can. Still, supercapacitors can't match the energy density of a battery.

Power Words

(for more about Power Words, click here)

alternating current     (in electricity) Often abbreviated AC, alternating current is a flow of electrons that reverses direction at regular intervals many times a second. Most household appliances run off of AC power. But many portable devices, like music players and flashlights, run off of the direct current (DC) power provided by batteries.

anode     The negative terminal of a battery, and the positively charged electrode in an electrolytic cell. It attracts negatively charged particles. The anode is the source of electrons for use outside the battery when it discharges.

atom     The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

battery     A device that can convert chemical energy into electrical energy.

capacitor     An electrical component used to store energy. Unlike batteries, which store energy chemically, capacitors store energy physically, in a form very much like static electricity.

carbon     The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.

cathode     The positive terminal of a battery, and the negatively charged electrode in an electrolytic cell. It attracts positively charged particles. During discharge, the cathode attracts electrons from outside the battery.

ceramic     A hard but brittle material made by firing clay or some other non-metal-based mineral at a high temperature. Bricks, porcelain and other types of earthenware are examples of ceramics. Many high-performance ceramics are used in industry where materials must withstand harsh conditions.

chemical     A substance formed from two or more atoms that unite (become bonded together) in a fixed proportion and structure. For example, water is a chemical made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O. Chemical can also be an adjective that describes properties of materials that are the result of various reactions between different compounds.

chemical reaction     A process that involves the rearrangement of the molecules or structure of a substance, as opposed to a change in physical form (as from a solid to a gas).

circuit     A network of that transmits electrical signals. In the body, nerve cells create circuits that relay electrical signals to the brain. In electronics, wires typically route those signals to activate some mechanical, computational or other function.

component     An item that is part of something else, such as pieces that go on an electronic circuit board.

conductor     (in physics and engineering) A material through which an electrical current can flow.

current     A fluid body — such as of water or air — that moves in a recognizable direction. (in electricity) The flow of electricity or the amount of electricity moving through some point over a particular period of time.

density     The measure how condensed an object is, found by dividing the mass by the volume.

direct current     (in electricity) Often abbreviated DC, direct current is a one-way flow of electrons. DC power is generated by devices such as batteries, capacitors and solar cells. When a circuit needs DC power, certain electronic devices can convert alternating current (AC) power into a direct current.

electronic cigarette     Battery-powered device that disperses nicotine and other chemicals as tiny airborne particles that users can inhale. They were originally developed as a safer alternative to cigarettes that users could use as they tried to slowly break their addiction to the nicotine in tobacco products. These devices heat up a flavored liquid until it evaporates, producing vapors. People use these devices are known as vapers.

electric charge    The physical property responsible for electric force; it can be negative or positive.

electric current     A flow of electric charge, called electricity, usually from the movement of negatively charged particles, called electrons.

electric field     A region around a charged particle or object within which a force would be exerted on other charged particles or objects.

electricity     A flow of charge, usually from the movement of negatively charged particles, called electrons.

electric potential    Commonly known as voltage, electric potential is the driving force for an electrical current (or flow of electrons) in a circuit. In scientific terms, electric potential is a measure of the potential energy per unit charge (such as electron or proton) stored in an electric field.

electrolyte     A non-metallic liquid or solid that conducts ions — electrically charged atoms or molecules — to carry electrical charges. (Certain minerals in blood or other bodily fluids can serve as the ions that move to carry a charge.) Electrolytes also can serve as the ions that move positive charges within a battery.

electron     A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.

energy density     The amount of energy stored in a battery, capacitor or other storage device, divided by its volume.

engineer     A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.

factor     Something that plays a role in a particular condition or event; a contributor.

field     (in physics) A region in space where certain physical effects operate, such as magnetism (created by a magnetic field), gravity (by a gravitational field), mass (by a Higgs field) or electricity (by an electrical field).

frequency     The number of times a specified periodic phenomenon occurs within a specified time interval. (In physics) The number of wavelengths that occurs over a particular interval of time.

graphite     Like diamond, graphite — the substance found in pencil lead — is a form of pure carbon. Unlike diamond, graphite is very soft. The main difference between these two forms of carbon is the number and type of chemical bonds between carbon atoms in each substance.

hybrid     An organism produced by interbreeding of two animals or plants of different species or of genetically distinct populations within a species. Such offspring often possess genes passed on by each parent, yielding a combination of traits not known in previous generations. The term is also used in reference to any object that is a mix of two or more things.

insulator     A substance or device that does not readily conduct electricity.

ion     An atom or molecule with an electric charge due to the loss or gain of one or more electrons.

lithium     A soft, silvery metallic element. It’s the lightest of all metals and very reactive. It is used in batteries and ceramics.

mica     A family of minerals, many of which readily break into small, glittering flakes.

mineral     The crystal-forming substances, such as quartz, apatite, or various carbonates, that make up rock. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in certain regular three-dimensional patterns).

range     The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists. (in math or for measurements) The extent to which variation in values is possible. Also, the distance within which something can be reached or perceived.

smartphone     A cell (or mobile) phone that can perform a host of functions, including search for information on the internet.

supercapacitor     A capacitor with two conducting surfaces, or electrodes (like other capacitors), on which a charge of energy is stored. Unlike ordinary capacitors (but like batteries), an electrolyte separates the two electrodes. In this sense, a supercapacitor is essentially a battery-capacitor hybrid.

surface area     The area of some material’s surface. In general, smaller materials and ones with rougher or more convoluted surfaces have a greater exterior surface area — per unit mass — than larger items or ones with smoother exteriors. That becomes important when chemical, biological or physical processes occur on a surface.

terminal     The end point or last station in some system, network or process. The end of the line.

toxic     Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.

tune     (in engineering) Adjust to the right level.

turbine     A device with extended arm-like blades (often curved) to catch a moving fluid — anything from a gas or steam to water — and then convert the energy in that movement into rotary motion. Often that rotary motion will drive a system to generate electricity.

voltage     A force associated with an electric current that is measured in units known as volts. Power companies use high-voltage to move electric power over long distances.