System: Electrical and Electronic

System: Electrical and Electronic

·  Accessories

·  Battery

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·  Circuitry Basics - Switches and Fuses

·  Computer basics

·  Electronic Instrumentation

·  Lighting Circuits

·  Starting System

·  Wiring Diagrams

The Battery Cell

CAUTION: The acid in electrolyte is sulfuric acid, which is extremely dangerous.

CAUTION: Always wear eye protection and chemical protection when working around a battery.

The working parts of a battery are called a cell. A simple battery cell is made up of two plates made of different metals that are placed in a container of acid solution called electrolyte. The acid in the electrolyte reacts with the positive (+) plate to release electrons (- charges). They transfer to the other plate, giving it a negative (-) charge. When the battery is fully charged, the negative (-) plate has an excess of electrons. If this battery is connected into a simple circuit, the excess electrons are pulled away from the negative (-) plate and flow to the lightbulb. Chemical action is still going on inside the cell; but as this action continues, the electrolyte becomes weaker and weaker, and the negative (-) and positive (+) plates become more and more alike in chemical composition. Eventually, all the electrons are used up, the lightbulb goes out, and the battery is discharged.

In an automotive battery, the active material of sponge lead (Pb), and lead peroxide (PbO2) make up the negative (-) and positive (+) plates, respectively. The electrolyte is a mixture of sulfuric acid (H2SO4) and water (H2O). When a cell is discharging by completing an external electrical circuit (as occurs when the engine is cranked), the sulfuric acid acts on both positive (+) and negative (-) plates to form a chemical compound called lead sulfate (PbSO4). The electrolyte becomes weaker in direct proportion to the amount of electricity used. When a certain amount of the sulfuric acid has combined with the plates, the battery can no longer deliver current at a useful voltage and the battery is discharged. The chemical action during charging and discharging is shown below. .

When the battery is connected to an external circuit, current begins to flow from the battery. This is known as the discharge cycle. The lead peroxide (PbO2) in the positive (+) plate is a compound of lead (Pb) and oxygen (O2). Sulfuric acid is a compound of hydrogen (H2) and sulfate (SO4). Oxygen in the active material of the positive (+) plate combines with hydrogen from the sulfuric acid to form water (H2O). At the same time, lead in the active material of the positive (+) plate combines with the sulfate, forming lead sulfate (PbSO4).

A similar reaction takes place at the negative (-) plate while the electrolyte is reacting with the positive (+) plate. The lead (Pb) of the negative active material combines with the sulfate to form lead sulfate (PbSO4).

As the discharge continues, both plates tend to become more alike in composition. This accounts for the loss in voltage, because potential (electrical pressure) depends on the difference between the two materials plus the amount of acid concentration of the electrolyte.

The chemical reversibility, or the battery's ability to accept a charge, allows it to be restored to an almost new condition. The chemical reactions that take place within the battery during the charge cycle are basically the reverse of those that occur during the discharge cycle. The accumulated lead sulfate on the plates is converted back into electrolyte by the action of the charging device (alternator). The lead sulfate of both plates is split up into its original form of lead (Pb) and sulfate (SO4), and water is broken down into hydrogen (H2) and oxygen (O). As the sulfate leaves the plates, it combines with the hydrogen and is restored to sulfuric acid (H2SO4). During this process, the oxygen combines with the lead of the positive (+) plate to form lead peroxide (PbO2).

Battery Construction, Types and Sizes

An automotive battery is constructed of positive (+) and negative (-) plate groups. Each plate group is made by lead burning (welding) a number of plates to a lead casting called a plate strap. The plate strap includes a battery post. The two groups are assembled with alternate negative (-) and positive (+) plates. Negative (-) plate groups normally contain one more plate than the positive (+) plate groups. This means that negative (-) plates are on the outside of the assembly. A separator is placed between any two plates in the group. The separators in most batteries are thin sheets of insulation of microporous rubber, fiberglass, or purified wood. All separators have one ribbed side facing the positive (+) plate. The ribs are vertical to improve circulation of the electrolyte to the positive (+) plates and to channel any loosened particles downward.

The assembly of a positive (+) and a negative (-) plate with a separator forms an element. There may be elements of any number or size, depending on how much energy is to be stored. The greater the plate surface area per element, the higher the output during high rates of discharge, especially at low temperatures.

When the element is placed into a container with an acid solution, it becomes a cell, the basic component of the battery. No matter what the size of the cell or the number of plates in the element, the voltage of a fully charged cell is only a little over 2 volts.

When the negative terminal of one cell is connected to the positive terminal of another cell we have a series connection. With a series connection, the voltage is the sum of all the cells connected together. The battery voltage is the sum of the voltage of its cells. Six cells connected together provides 12 volts.

Automotive battery containers are one molded piece, usually made of hard rubber or plastic. They must withstand extremes of heat and cold, mechanical shock, and must resist acid. The bottom of each cell compartment has narrow, molded rests, or bridges, on which the element sits. This minimizes the danger of short circuits resulting from sediment that falls from the plates onto the bridges where the plates sit.

To connect battery cells in series, the elements are arranged with the negative (-) terminal of one cell next to the positive (+) terminal of the next cell. Cell connectors are then placed through the partition and welded together to connect the cells in series. There are five cell connectors in a 12-volt battery. A one-piece molded cover on the battery forms an acid-tight seal. The parts of a typical battery are shown.

The battery has two terminals. One terminal is positive (+) and the other is negative (-). Three common styles of terminals are used as shown below. . Tapered post-type terminals are common. The two tapered terminals are different diameters. The positive terminal is slightly larger to prevent installing the battery cables on the wrong terminal. Stud-type terminals are threaded rather than tapered. Side terminals are located on the side of the battery instead of the top.

Battery Terminals

All automotive batteries have two terminals. One terminal is a positive connection, the other is a negative connection. The battery terminals extend through the cover or the side of the battery case. The following are the most common types of battery terminals :

1.  Post or top terminals: Used on most automotive batteries. The positive post will be larger than the negative post to prevent connecting the battery in reverse polarity.

1. Side terminals: Positioned in the side of the container near the top. These terminals are threaded and require a special bolt to connect the cables. Polarity identification is by positive and negative symbols.

3.  L terminals: Used on specialty batteries and some imports.

Battery Cables

Battery cables must be of a sufficient capacity to carry the current required to meet all demands. Normal 12-volt cable size is usually 4 or 6 gauge. Various forms of clamps and terminals are used to assure a good electrical connection at each end of the cable. Connections must be clean and tight to prevent arcing, corrosion, and high voltage resistance.

The positive cable is usually red (but not always) and the negative cable is usually black. The positive cable will fasten to the starter solenoid or relay. The negative cable fastens to ground on the engine block. Some manufacturers use a negative cable with no insulation. Sometimes the negative battery cable may have a body grounding wire to help assure that the vehicle body is properly grounded.

CAUTION: Connecting the battery cables in reverse polarity can damage many of the vehicle's computer systems.

Battery Types and Sizes

Several different types of batteries are in use: the conventional vented cell battery, the low maintenance battery, and the sealed maintenance-free battery.

The conventional vented cell battery is the oldest design. These use a vent cap over each cell. The vent cap fits in threaded or tapered holes and can be removed for inspection and for adding electrolyte or water when necessary. A hole in the vent cap allows hydrogen gas to escape as the battery is charged. The problem with the conventional vented cap battery is that the gas venting causes the cells to lose water. The loss of water from the cells can eventually lead to early battery failure. These batteries require regular inspection and water replacement. The low maintenance and maintenance-free batteries are designed to eliminate this problem.

The low maintenance battery does not have cell vent plugs. The cells are not allowed to vent to the atmosphere. Because there are no cell vents, the hydrogen and oxygen are routed into an expansion area within the battery where the gas condenses into water. The water then runs back into the cells. This design does not require frequent inspection and water filling. The vent cover on top of the battery can be pried off for access to the cells.

Many batteries are maintenance free and thus are completely sealed. They use materials in the plates that reduce the amount of internal heat in the battery and the amount of gassing during charging. This, in turn, reduces the amount of water usage in the cells.

Battery Size

When you replace a battery in a car, you must be careful to get one that matches the car's electrical system. You must also be careful to get the correct size and one that has the correct type of terminals. Batteries are identified by a group number and often by a catalog or part number. The owner's manual will list the correct battery for the car.

The group number of a battery is usually located on the battery top. It is used to find out the physical size of the battery with a battery size chart. A battery size chart will tell you, for example, that a battery marked as a group 24 has a length of 10 1/4 inches; a width of 6 7/8 inches; and a height of 8 5/8 inches.

The group numbers starting with a 2 indicate a top terminal battery. A group number starting with a 7 indicates side terminal design. The letter F indicates batteries for Ford vehicles that use terminal posts in a reverse position from other batteries.

Battery charts also list an electrical rating for the battery. This is as important as the size because a battery that is not matched electrically to the car's electrical system can lead to electrical problems.

Battery charts also rate the battery's the cold cranking power capacity. Capacity is a measure of how long a battery can supply current. It is determined by the amount of active material and electrolyte in each cell. A battery with a few small plates will become discharged very quickly when supplying a high current demand. On the other hand, one with many large plates can supply current for a longer period of time.

In the cold cranking power test, the current that can be drawn from the battery for 30 seconds at 0'F is measured. The higher the current, the higher the battery capacity. A single group number may have many different cold cranking power ratings. Always be sure to use a battery with the correct capacity rating.

Automotive and Conventional Batteries

An automotive battery is an electrochemical device that provides for and stores electrical energy. When the battery is connected to an external load, such as a starter motor, an energy conversion occurs that results in an electrical current flowing through the circuit. Electrical energy is produced in the battery by the chemical reaction that occurs between two dissimilar plates that are immersed in an electrolyte solution. The automotive battery produces direct current (DC) electricity that flows in only one direction.

When discharging the battery (current flowing from the battery), the battery changes chemical energy into electrical energy. It is through this change that the battery releases stored energy. During charging (current flowing through the battery from the charging system), electrical energy is converted into chemical energy. As a result, the battery can store energy until it is needed.

The automotive battery has several important functions, including:

1.  It operates the starting motor, ignition system, electronic fuel injection, and other electrical devices for the engine during cranking and starting.

2.  It supplies all the electrical power for the vehicle accessories whenever the engine is not running or at low idle.

3.  It furnishes current for a limited time whenever electrical demands exceed charging system output.

4.  It acts as a stabilizer of voltage for the entire automotive electrical system.

5.  It stores energy for extended periods of time.

The largest demand placed on the battery occurs when it must supply current to operate the starter motor. The amperage requirements of a starter motor may be over several hundred amperes. This requirement is also affected by temperatures, engine size, and engine condition.