Battery components

All batteries used in modern automotive applications are of the lead-acid storage type. Essentially, a lead-acid storage battery is an electro-chemical device for storing energy in chemical form so that this energy can be released as electricity when connected to an outside circuit. A battery can perform this operation repeatedly.

Plate grids
See Figure 12

The plate grids are the vital elements of the battery, for they support the active material, and although they are not an active part in the production of electricity, they must be a good conductor to support the flow of electricity. There are two types of plates -- positive plates and negative plates. The positive plates consist of a grid over which active lead peroxide is placed. This dark brown crystalline material has a high degree of porosity in order to allow the electrolyte to penetrate the plate freely. Negative plates are grids pasted with a type of lead referred to as sponge lead, which is simply finely ground lead. Grinding the lead allows the electrolyte to penetrate the grid.

There may be any number of plates used in a battery; it all depends on how much energy you want to store. The more plates (or the larger the plates), the more energy the battery can store and release. The negative plates will always outnumber the positive plates by one for reasons of improved performance.

Figure 12 Compound battery element or cell showing positive plates, negative plates, and separators.

Separators

No positive plate may touch a negative plate, or all the plates in the cell will lose their stored energy. This is called a short (or short circuit). To prevent the plates from touching, thin sheets of non-conductive porous material called separators are used. These are placed between every positive and negative plate.

Battery elements
See Figure 13, 14 and 15

An element is the desired number of positive and negative plates placed together with a separator between each plate. The simplest unit you could construct would be a single positive plate and a single negative plate, kept apart by a porous separator. This would be a single element. If this element is put in a solution of sulfuric acid and water (electrolyte), a simple two-volt cell is formed. Electricity will flow if these plates are connected to an electrical load. When six of these cells are connected in series, a group or battery of cells is formed. This battery of cells will produce six times as much electrical pressure as a simple two-volt cell, or 12 volts.

Figure 13 A simple battery element.

Figure 14 A two-volt cell connected to a load.

Figure 15 Typical 12-volt battery cell arrangement.

Electrolyte

Electrolyte is a mixture of sulfuric acid and water. Ordinarily, the electrolyte used in a fully charged battery contains about 25% sulfuric acid and 75% water. The strength or percentage of the sulfuric acid in the solution is measured by its specific gravity, that is, the density of the electrolyte versus the density of pure water. The specific gravity or electrolyte strength of a fully charged battery is in the range of 1.260-1.275. This means that its electrolyte is at least 1.260 times heavier than pure water. This is only true at 80°F (27°C) however. Above or below that temperature, the reading must be corrected to allow for the temperature. See the section on checking electrolyte level.

Containers and terminals

The battery terminals are the external electrical connections. They are connected inside the battery to the positive plates (+ terminal) and the negative plates (- terminal). For years, the terminals were located on the top of the battery, and in many cases, still are. However, side terminal batteries have been developed to minimize or eliminate the problem of dirt, acid spray, or moisture corroding the terminals or cables.

Covers and vent caps

Most "maintenance-free" or "lifetime" batteries were designed to eliminate the need for periodic checking and addition of electrolyte. For this reason, most of these batteries are sealed (meaning they do not have removable covers or vent caps). Vent plugs or covers are used on older, conventional style batteries and on some newer "maintenance-free" units for a number of reasons. In addition to keeping impurities out of the battery, the vent plugs provide a convenient way to check and/or add electrolyte.

Every storage battery used in an automobile has three essential functions:

1. To provide current for the starter and ignition system when cranking.
2. To provide current (in addition to alternator current) to operate the radio, lights, etc.
3. To act as a voltage stabilizer or reservoir in the electrical system.

While the first two functions are obvious, the third may require some explanation. To understand it, first consider the battery and alternator (or generator) as opposing forces. Current will flow from the greater force to the lesser force. For example, after running the starter motor, the battery will be discharged since some of the acid has been absorbed into the plates. If the vehicle is driven immediately, which is usually the case, current will flow back into the battery from the alternator. The voltage regulator will cut off the current when the battery is recharged.

The most important attribute of a lead - acid storage battery is its chemical reversibility. This means that unlike a dry cell battery, a storage battery is capable of being recharged by passing an electric current through it in the opposite direction of discharge. Through a chemical reaction, the battery's active chemicals will be restored to a state of charge.

To understand the charging process, you first have to understand how a battery is discharged.

The discharge process in a battery is begun as soon as an electrical circuit is completed, such as turning on the vehicle lights. Current flows from the battery through the positive terminal. During the time that there is a drain on the battery (it is discharging), sulfuric acid in the battery works on both the positive and negative plates' active material, normally lead peroxide and sponge lead respectively. Hydrogen in the sulfuric acid combines with oxygen available at the positive plate to form water, which reduces the concentration of acid in the electrolyte. This is why the state of charge can be determined by measuring the strength (specific gravity) of the electrolyte. The more dense the solution, the higher the charge. The more like water the solution becomes, the lower the charge.

The amount of acid consumed by the plates is in direct proportion to the amount of energy removed from the cell. When the acid is used up to the point where it can no longer deliver electricity t a useful voltage, the battery is effectively discharged.

To recharge the battery, it is only necessary to reverse the flow of current provided by the alternator through the positive terminal and out the negative battery terminal. The sulfate that formed on the plates during discharge is changed back to sponge lead, and the sulfur returns to the electrolyte, forming sulfuric acid again. At the positive plate, the lead sulfate changes to lead peroxide and returns even more sulfuric acid to the electrolyte.

Figure 17 The discharge process.

Figure 18 The charging process.

Battery rating system

Under the battery rating system, there are two standards used to determine battery power. When purchasing a battery for a cold climate, Cold Cranking Amps (CCA) is the most important factor. In a warm climate, look for a higher Reserve Capacity (RC) rating.

The CCA rating is used for measuring battery - starting performance, and provides an approximate relationship between battery size and engine size. The most important factor is sizing the battery's CCA rating to meet or exceed, depending on the climate, the vehicle's OEM cranking requirement. As batteries age, they are less capable of producing cold cranking amps.

When replacing a battery, you should always choose a battery with AT LEAST as high a CCA rating. It is usually a good idea to purchase a battery with a higher rating to assure proper vehicle starting.

The reserve capacity rating is used for measuring electrical capacity. It shows how long (in minutes) the battery will operate the vehicle's electrical system in case of a charging system failure. The reserve capacity is the number of minutes a fully charged battery at 80°F (27°C) can be discharged at 25 amps until the voltage falls below 10.5 volts. For example, if your battery has a reserve capacity rating of 135, this means you have approximately 2 hours and 15 minutes to get to a service station before the battery "dies." If more reserve capacity is required, two or more 12-volt batteries can be connected in parallel.

Continue to page 3 of The electrical system

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