What Is in a Lead Acid Battery?
Invented in 1859, the lead acid battery has proved to be a reliable, economical source of electric power. Though the materials used to make the battery have been improved over time, the basic design and principles are still at work. Inside the battery are three main parts--the plates, the separators and the electrolyte.
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Plates
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One of the main parts of a lead acid battery are the plates. They are rectangular grids made of lead, alloyed for strength with tin, antimony, calcium or selenium. The holes in the grid are filled with a paste made of lead, sulfuric acid and other compounds. The battery has two types of plates, positive and negative. The positive plates are lead dioxide, the negative plates are a gray so-called sponge lead. In the battery, the plates are packed in an alternating sandwich of positive and negative types with a separator in between. Usually, the battery has one more negative plate than positive, so the plates at both ends are negative. Each plate type is connected electrically to each other and to the corresponding post or terminal on the battery.
Separators
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The plates are kept apart by non-conductive sheets called separators. They are made of PVC, rubber or a number of other materials. Their main function is to keep the plates from shorting out, but they also help vent gas released by chemical reactions. They must be tough enough to hold up to the jolts a car battery takes, and they must not react to the electrolyte, a strong acid.
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Electrolyte
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A chemical battery needs an electrolyte to carry out reactions that make electric current flow. A lead acid battery uses a 33 percent (by weight) solution of sulfuric acid. The plates are immersed in it. As the battery is discharged, the ions in the acid combine with the lead, leaving water behind. This changes the solution's specific gravity, so the charge state of the battery can be measured with a float. This also changes the freezing point of the electrolyte--a battery that is dead will freeze sooner than a charged one. The ions flow back into solution when the battery is recharged, making sulfuric acid again. The ability to discharge and recharge thousands of times makes this kind of battery very useful.
Function
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A battery's job is to provide a reliable, portable source of electric current. For a lead acid battery, this includes being used and recharged many times. In cars, a lead acid battery provides the power to start the gasoline engine. In other vehicles, like golf carts and fork lifts, it provides all the power. The lead acid battery is also widely used as a power source for emergency lighting or to run computers when the main power goes out.
Warning
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A lead acid car battery is capable of delivering a lot of current, hundreds of amperes, if short-circuited. Under very heavy discharge conditions, it can emit hydrogen gas, which will explode if sparked. Since the lead acid battery contains both lead and sulfuric acid, it is recommended to take it to a hazardous waste collector, recycling center or auto battery dealer when it no longer works.
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