Batteries

Anatomy

Take a look at any battery, and you'll notice that it has two terminals. One terminal is marked (+), or positive, while the other is marked (-), or negative. In normal flashlight batteries, like AA, C or D cell, the terminals are located on the ends. On a 9-volt or car battery, however, the terminals are situated next to each other on the top of the unit. If you connect a wire between the two terminals, the electrons will flow from the negative end to the positive end as fast as they can. This will quickly wear out the battery and can also be dangerous, particularly on larger batteries. To properly harness the electric charge produced by a battery, you must connect it to a load. The load might be something like a light bulb, a motor or an electronic circuit like a radio.

The internal workings of a battery are typically housed within a metal or plastic case. Inside this case are a cathode, which connects to the positive terminal, and an anode, which connects to the negative terminal. These components, more generally known as electrodes, occupy most of the space in a battery and are the place where the chemical reactions occur. A separator creates a barrier between the cathode and anode, preventing the electrodes from touching while allowing electrical charge to flow freely between them. The medium that allows the electric charge to flow between the cathode and anode is known as the electrolyte. Finally, the collector conducts the charge to the outside of the battery and through the load.

When a load completes the circuit between the two terminals, the battery produces electricity through a series of electromagnetic reactions between the anode, cathode and electrolyte. The anode experiences an oxidation reaction in which two or more ions (electrically charged atoms or molecules) from the electrolyte combine with the anode, producing a compound and releasing one or more electrons. At the same time, the cathode goes through a reduction reaction in which the cathode substance, ions and free electrons also combine to form compounds. While this action may sound complicated, it's actually very simple: The reaction in the anode creates electrons, and the reaction in the cathode absorbs them. The net product is electricity. The battery will continue to produce electricity until one or both of the electrodes run out of the substance necessary for the reactions to occur.

Modern batteries use a variety of chemicals to power their reactions. Common battery chemistries include:

• Zinc-carbon battery: The zinc-carbon chemistry is common in many inexpensive AAA, AA, C and D dry cell batteries. The anode is zinc, the cathode is manganese dioxide, and the electrolyte is ammonium chloride or zinc chloride.
• Alkaline battery: This chemistry is also common in AA, C and D dry cell batteries. The cathode is composed of a manganese dioxide mixture, while the anode is a zinc powder. It gets its name from the potassium hydroxide electrolyte, which is an alkaline substance.
• Lithium-ion battery (rechargeable): Lithium chemistry is often used in high-performance devices, such as cell phones, digital cameras and even electric cars. A variety of substances are used in lithium batteries, but a common combination is a lithium cobalt oxide cathode and a carbon anode.
• Lead-acid battery (rechargeable): This is the chemistry used in a typical car battery. The electrodes are usually made of lead dioxide and metallic lead, while the electrolyte is a sulfuric acid solution.

Experiment

f you want to learn more about the electrochemical reactions that occur in batteries, you can actually build one yourself using simple household materials. One thing you should buy before you start is an inexpensive ($10 to $20) volt-ohm meter at your local electronics or hardware store. Make sure that the meter can read low voltages (in the one-volt range) and low currents (in the five-to-10 milliamp range). With this equipment on hand, you'll be able to see exactly how well your battery is performing.

You can create your own voltaic pile using quarters, foil, blotting paper, cider vinegar and salt. Cut the foil and blotting paper into circles, then soak the blotting paper in a mixture of the cider vinegar and salt. Using masking tape, attach a copper wire to one of the foil discs. Now stack the materials in this order: foil, paper, quarter, foil, paper, quarter, and so on until you have repeated the pattern 10 times. Once the last coin is on the stack, attach a wire to it with masking tape. Finally, attach the free ends of the two wires to an LED, which should light up. In this experiment, the copper in the quarter is the cathode, the foil is the anode, the cider vinegar-salt solution is the electrolyte, and the blotting paper is the separator.

A homemade battery can also be made from copper wire, a paper clip and a lemon. First, cut a short piece of copper wire and straighten out the paper clip. Use sandpaper to smooth out any rough parts on the ends of either piece of metal. Next, gently squeeze the lemon by rolling it on a table, but be careful not to break the skin. Push the copper wire and the paper clip into the lemon, ensuring that they are as close together as possible without actually touching. Finally, connect your volt-ohm meter to the ends of the paper clip and the copper wire, and see what kind of voltage and current your battery produces.

By now you should be well acquainted with the basic principles by which batteries discharge electricity. Read on to discover how some batteries can be recharged.