System Operation
SYSTEM OPERATIONThe 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 vehicle's electrical components.
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.
Group Size
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 stamped steel tray located in the left, front corner of the engine compartment. It is secured in the tray by a holddown strap. The holddown strap is secured to the battery tray by two nuts, a stud, and a J-bolt.
The battery tray is spot-welded at the rear to a bracket welded to the dash panel, from underneath to a support welded to the wheelhouse inner panel, and to the left cowl side panel. A hole in the bottom of the tray is fitted with a formed drain tube. The drain tube directs water or electrolyte from a leaking battery to the ground through another hole in the front of the wheelhouse inner panel.
On some models, a second 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 second hole.