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Battery Basics Richard Perez
©1991 Richard Perez
Abattery stores electrical energy. Batteries are chemical machines. In the battery, chemical energy is converted into electrical energy. Electricity is stored within the battery as potential chemical bonding between the battery's active materials. Batteries are simply chemical engines used to push electrons around.
Primary and Secondary Batteries
As a battery is charged or discharged, its chemical composition changes. In some batteries the chemical reaction is not reversible. This type may only be discharged. It cannot be recharged. Batteries which cannot be recharged are known as "primary" batteries. One example of a primary battery is the disposable zinc-carbon cell used in flashlights. Other types of batteries are rechargeable. The chemical reaction within a rechargeable battery is reversible. Rechargeable batteries are known as "secondary" batteries. They may be emptied and refilled many times. An example of a secondary battery is the lead-acid battery used to start an automobile.
How Batteries Store and Transfer Energy
A battery converts chemical energy into electrical energy. In rechargeable batteries the conversion process is reversible. Rechargeable batteries can also convert electrical energy into chemical energy.
The conversion and storage processes take place in the basic building block of all batteries – the cell. The cell contains the active materials and the electrolyte. Most batteries are composed of many cells because the voltage potential of each chemical cell is quite low (a few volts at most). The electrical storage capacity of a cell is roughly proportional to its physical size. The larger the cell, the more capacity it has. A battery is composed of cells which are assembled together to increase the voltage or to increase the capacity of the battery.
30 Home Power #27 • February / March 1992
The cell's negative pole
The cell's positive pole
THE ELECTROCHEMICAL CELL
A liquid medium for electron transfer between the anode and cathode
The cell contains two active materials which can react chemically to release free electrons (electrical energy). Such materials are known as "electrochemical couples." The active materials are usually solid. The cell also contains an electrolyte which transfers the electrons between the electrochemical couple. The electrolyte is usually a liquid, a jelly, or a paste. Electrolytes may be either acids or bases (alkaline). In some cells such as lead-acid cells, the electrolyte participates in the chemical reaction in addition to acting as a path for electrons. In other cases, such as nickel-cadmium or nickel-iron cells, the electrolyte does not participate in the cell's chemical reaction, but merely acts as a transfer medium for electrons.
During the discharge of a cell, the active materials undergo chemical reactions which release free electrons. During this reaction, the chemical compositions of the active materials are changed. The reactants actually become different chemical compounds. When all the original active materials have undergone reaction, the cell will produce no more free electrons. The cell is "dead."
In the rechargeable secondary cell, the chemical process is reversible. By forcing electrons through the cell in the opposite direction, the active materials can be restored to their original chemical composition. This is known as "recharging" the cell.
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