Text b charge and discharge cycles

There's one important feature of a car battery that distinguishes it from a common dry-cell battery, such as those used in flashlights: a car battery can be discharged and recharged many times. Standard dry-cell batteries can’t be recharged because the chemical materials are depleted in the process of making electricity.

The discharge process works like this: let's begin with a fully charged automotive battery. True to Volta’s principle, it contains two unlike metals, in this case lead and lead peroxide. These are suspended in an electrolyte solution of water and sulfuric acid. When a load is connected to the battery terminals, a chemical reaction inside the battery causes current to flow, discharging the battery. Eventually, both metals will become lead sulphate and the electrolyte plain water. At this point, no more electricity can be made, so current flow stops. The battery is now fully discharged.

The charging process reverses these conditions. Running on electrical current through the battery causes the metals to become dissimilar again, and changes the electrolyte back to sulfuric acid. When the charging process is complete, the battery is again capable of providing electricity. In practice, however, a car battery never becomes fully discharged under normal operating conditions. It loses part of its charge when called upon to start the car or operate electrical devices. This loss is replaced by the generator when the engine is running.

With proper care, a car battery can undergo continuous discharge-charge cycles over many years before wearing out.

Text c battery construction

An automotive battery consists of several individual compartments, called cells. There are three cells in a 6-volt car battery, six cells in a 12-volt car battery. Each cell provides slightly more than 2 volts, regardless of size.

Each cell contains the two dissimilar metals mentioned earlier — lead and lead peroxide. These metals are formed into flat, porous plates that stand vertically in the cell. The number of

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plates and the surface area of each determine a battery's capacity to generate electrical current, which is rated in ampere-hours. The more plates, or the larger their size, or both, the more capacity will be provided. But, remember that each cell delivers only 2 volts, regardless of size.

Each cell starts with a negative plate, followed by a positive plate, followed by another negative plate, and so on, finally ending with a negative plate. This means each cell has one more negative plate than it does positive plates.

At the top of the battery, all negative plates are welded to a common plate strap: all positive plates are welded lo another plate strap. Perforated separators prevent the plates from touching each other, but allow electrolyte to pass easily between them. Individual cells are separated by a solid partition. At the top of the battery, the plate straps are joined in series from one cell to the next. At the bottom, the plates sit on ribs that provide space below the plates for sediment to collect the plates without shorting. At each end of the battery, the final plate strap is formed into a post that protrudes through the top or side of the case, forming positive and negative terminals.

On post-type batteries, t- he terminal posts are tapered, with the positive post being larger than the negative — 11/16 in. and 5/8 in., respectively. The size difference makes it less likely for you to attach the cables to the wrong posts. Tapered posts require a clamp-on cable terminal. Good-quality cable clamps are sized accordingly: cheap clamps are not.

Many battery manufacturers are offering batteries with the terminals at the side of the case, rather than on top. Side terminals are internally threaded to accept a mating cable with a built-in bolt.

Regardless of terminal construction, terminal polarity is always identified, either on the terminal or the battery case. Positive terminals are identified with P, POS, (+), or red paint. Negative terminals are N, NEG, (-) or paint of some colour other than red — often black.

Some batteries have removable cups to permit periodic checking of electrolyte level. Distilled water is usually recommended for topping off low cells. Never add electrolyte.

Many new batteries are "sealed for life." The chemical composition inside the battery is such that very little hydrogen gas is given off at normal charging voltages, so little of the electrolyte is consumed over the life of the battery. Sealed batteries are preferred because they eliminate the need for periodic maintenance. The types with side terminals are designed so the terminals and battery cables are located below the battery’s vent hole, reducing terminal corrosion.

Gel cell batteries are specially constructed to withstand severe shock, and are good for such applications as off-road racing. The plates on conventional batteries can short out when subjected to excessive vibration or shock.