Battery: Description and Operation
OVERVIEWThe battery, starting, and charging systems operate with one another, and must be tested as a complete system. In order for the vehicle to start and charge properly, all of the components involved in these systems must perform within specifications.
However, when attempting to diagnose any of these systems, it is important that you keep their interdependency in mind.
The diagnostic procedures used include the most basic conventional diagnostic methods, to the more sophisticated On-Board Diagnostics (OBD) built into the Powertrain Control Module (PCM). Use of a induction milliampere ammeter, volt/ohmmeter, battery charger, carbon pile rheostat (load tester), and 12-volt test lamp may be required.
All OBD-sensed systems are monitored by the PCM. Each monitored circuit is assigned a Diagnostic Trouble Code (DTC). The PCM will store a DTC in electronic memory for any failure it detects. See the On-Board Diagnostics Test in Starting and Charging/Charging System for more procedures.
INTRODUCTION
While battery charging can be considered a maintenance procedure, battery charging information is located here. This was done because the battery must be fully-charged before any diagnosis can be performed.
Maintenance-Free Battery:
The factory-installed maintenance-free battery has non-removable battery vent caps. Water cannot be added to this battery. The chemical composition within the maintenance-free battery reduces battery gassing and water loss, at normal charge and discharge rates. Therefore, the battery should not require additional water in normal service.
If the battery electrolyte level becomes low, the battery must be replaced. However, rapid loss of electrolyte can be caused by an overcharging condition. Be certain to diagnose the charging system before returning the vehicle to service. Refer to Starting and Charging/Charging System for more information.
The factory-installed battery also has a built-in test indicator (hydrometer). The color visible in the sight glass of the indicator will reveal the battery condition. See Built-In Test Indicator for more information.
It is important that the battery, starting, and charging systems be thoroughly tested and inspected any time a battery needs to be charged or replaced. The cause of abnormal discharge, overcharging, or early battery failure must be diagnosed and corrected before a battery is replaced or returned to service.
SYSTEM OPERATION
Battery
The storage battery is a device used to store electrical energy potential in a chemical form. When an electrical load is applied to the battery terminals, an electrochemical reaction occurs within the battery. This reaction causes the battery to discharge electrical current.
The battery is made up of six individual cells that are connected in series. Each cell contains positively charged plate groups made of lead oxide, and negatively charged plate groups made of sponge lead. These dissimilar metal plates are submerged in a sulfuric acid and water solution called an electrolyte.
As the battery discharges, a gradual chemical change takes place within each cell. The sulfuric acid in the electrolyte combines with the plate materials, causing both plates to slowly change to lead sulfate. At the same time, oxygen from the positive plate material combines with hydrogen from the sulfuric acid, causing the electrolyte to become mainly water.
The chemical changes within the battery are caused by the movement of excess or free electrons between the positive and negative plate groups. This movement of electrons produces a flow of electrical current through the load device attached to the battery terminals.
As the plate materials become more similar chemically, and the electrolyte becomes less acid, the voltage potential of each cell is reduced. However, by charging the battery with a voltage higher than that of the battery, the battery discharging process is reversed.
Charging the battery gradually changes the sulfated lead plates back into sponge lead and lead oxide, and the water back into sulfuric acid. This action restores the difference in the electron charges deposited on the plates, and the voltage potential of the battery cells.
For a battery to remain useful, it must be able to produce high-amperage current over an extended period. A battery must also be able to accept a charge, so that its voltage potential may be restored.
In addition to producing and storing electrical energy, the battery serves as a capacitor, or voltage stabilizer, for a vehicle's electrical system. It absorbs most abnormal or transient voltages caused by the switching of any of the electrical components in the vehicle.
The battery is vented to release excess hydrogen gas that is created when the battery is being charged or discharged. However, even with these vents, the hydrogen gas can collect in or around the battery. If hydrogen gas is exposed to flame or sparks, it may ignite.
If the electrolyte level is low, the battery may arc internally and explode. If the battery is equipped with removable cell caps, add distilled water whenever the electrolyte level is below the top of the plates. If the battery cell caps cannot be removed, the battery must be replaced if the electrolyte level becomes low.
Battery Size And Ratings
The battery Group Size number, the Cold Cranking Amperage (CCA) rating, and the Reserve Capacity (RC) rating or Ampere-Hours (AH) rating can be found on the original equipment battery label. Be certain that a replacement battery has the correct Group Size number, as well as CCA, and RC or AH ratings that equal or exceed the original equipment specification for the vehicle being serviced.
See the Battery Classifications and Ratings chart in Specifications for more information. Battery sizes and ratings are discussed in more detail below.
Groupsize
The outside dimensions and terminal placement of the battery conform to standards established by the Battery Council International (BCI). Each battery is assigned a BCI Group Size number to help identify a correctly-sized replacement.
Cold Cranking Amperage
The Cold Cranking Amperage (CCA) rating specifies how much current (in amperes) the battery can deliver for thirty seconds at -18° C (0° F). Terminal voltage must not fall below 7.2 volts during or after the thirty second discharge period. The CCA required is generally higher as engine displacement increases, depending also upon the starter current draw requirements.
Reserve Capacity
The Reserve Capacity (RC) rating specifies the time (in minutes) it takes for battery terminal voltage to fall below 10.5 volts, at a discharge rate of 25 amperes. RC is determined with the battery fully-charged at 26.7° C (80° F). This rating estimates how long the battery might last after a charging system failure, under minimum electrical load.
Ampere-Hours
The Ampere-Hours (AH) rating specifies the current (in amperes) that a battery can deliver steadily for twenty hours, with the voltage in the battery not falling below 10.5 volts. This rating is also sometimes referred to as the twenty-hour discharge rating.
BATTERY MOUNTING
The battery is mounted to a molded plastic tray located in the left front corner of the engine compartment. A U-nut is held in a formation on each side of the battery tray. A molded plastic hold-down strap and thermoguard unit fits across the top of the battery and encloses the sides of the battery case. To secure the battery in the tray, a bolt passes through the hold-down strap on each side of the battery, and is threaded into the U-nut on the battery tray.
A battery tray support is secured at the front with two screws to the front closure panel and radiator yoke, and at the rear with one screw to the inner fender splash shield. The battery tray is secured to the support on the inboard side with two screws. On the outboard side, two screws secure the battery tray to the inner fender splash shield. A single screw secures the battery tray at the front to the front closure panel and radiator yoke.
The battery tray also includes two stanchions that are molded into the rear of the tray, which support the forward end of the Power Distribution Center (PDC).
On some models, a hole in the bottom of the battery tray is fitted with a battery temperature sensor. Models without the battery temperature sensor have a plug fitted to this hole. Refer to Starting and Charging/Charging System for more information on the battery temperature sensor.
When installing a battery, be certain that the hold-down fasteners are tightened to the proper specifications. Improper hold-down fastener tightness, whether too loose or too tight, can result in damage to the battery. See the Battery Replacement procedures for the correct hold-down fastener tightness specifications.