An automotive battery is an electrochemical device capable of storing and producing electrical energy. Electrochemical refers to the chemical reaction of two dissimilar materials in a chemical solution that results in electrical current. 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.
FIGURE. Typical automotive 12-volt battery.
The automotive battery has several important functions, including:
- It operates the starting motor, ignition system, electronic fuel injection, and other electrical devices for the engine during cranking and starting.
- It supplies all the electrical power for the vehicle accessories whenever the engine is not running or when the vehicle’s charging system is not working.
- It furnishes current for a limited time whenever electrical demands exceed charging system output.
- It acts as a stabilizer of voltage for the entire automotive electrical system.
- It stores energy for extended periods of time.
NOTE! The battery does not store energy in electrical form. The battery stores energy in chemical form.
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.
After the engine is started, the vehicle’s charging system works to recharge the battery and to provide the current to run the electrical systems. Most AC generators have a maximum output of 60 to 150 amperes. This is usually enough to operate all of the vehicle’s electrical systems and meet the demands of these systems. However, under some conditions (such as the engine running at idle) generator output is below its maximum rating. If there are enough electrical accessories turned on during this time (heater, wipers, headlights, and radio) the demand may exceed the AC generator output. During this time, the battery must supply the additional current.
Even with the ignition switch turned oft’, there are electrical demands placed on the battery. Clocks, memory seats, engine computer memory, body computer memory, and electronic sound system memory are all examples of key-offloads. The total current draw of key-offloads is usually less than 50 milliamperes.
In the event that the vehicle’s charging system fails, the battery must supply all of the current necessary to run the vehicle. The amount of time a battery can be discharged at a certain current rate until the voltage drops below a specified value is referred to as reserve capacity.
Most batteries will supply a reserve capacity of 25 amperes for approximately 120 minutes before discharging too low to keep the engine running.
The amount of electrical energy that a battery is capable of producing depends on the size, weight, active area of the plates, and the amount of sulfuric acid in the electrolyte solution.
In this chapter, you will study the design and operation of different types of batteries currently used in automobiles. These include conventional batteries, maintenance-free batteries, hybrid batteries, and recombination batteries.