Automotive Batteries: Summary

  • An automotive battery is an electrochemical device that provides for and stores electrical energy.
  • Electrical energy is produced in the battery by the chemical reaction that occurs between two dissimilar plates that are immersed in an electrolyte solution.
  • An automotive battery has the following important functions:
    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.

    Automotive Batteries

  • Electrical loads that are still placed on the battery when the ignition switch is in the OFF position are called key-off or parasitic loads.
  • The amount of electrical energy that a battery is capable of producing depends on the size, weight, and active area of the plates and the specific gravity of the electrolyte solution.
  • The conventional battery is constructed of seven basic components:
    1. Positive plates.
    2. Negative plates.
    3. Separators.
    4. Case.
    5. Plate straps.
    6. Electrolyte.
    7. Terminals.
  • Electrolyte solution used in automotive batteries consists of 64% water and 36% sulfuric acid by weight.
  • The electrolyte solution causes the chemical actions to take place between the lead dioxide of the positive plates and the sponge lead of the negative plates. The electrolyte is also the carrier that moves electric current between the positive and negative plates through the separators.
  • The automotive battery has a fully charged specific gravity of 1.265 corrected to 80°F.
  • Grid growth is a condition where the grid grows little metallic fingers that extend through the separators and short out the plates.
  • Deep cycling is discharging the battery almost completely before recharging it.
  • In a conventional battery, the positive plate is covered with lead peroxide and the negative plate is covered with sponge lead.
  • In maintenance-free batteries, the cell plates contain calcium, cadmium, or strontium to reduce gassing and self-discharge.
  • The grid construction of the hybrid battery consists of approximately 2.75% antimony alloy on the positive plates and a calcium alloy on the negative plates.
  • The recombination battery uses separators that hold a gel-type material in place of liquid electrolyte.
  • Absorbed glass mat (AGM) batteries hold their electrolyte in a moistened fiberglass matting that is sandwiched between the battery’s high-purity lead plates. Separation of the plates is done by acid-permeated vitreous separators that act as sponges to absorb acid.
  • Within a valve-regulated lead-acid (VRLA), the oxygen produced on the positive plates is absorbed by the negative plate, causing a decrease in the amount of hydrogen produced at the negative plate and combining it with the oxygen to produce water that is returned to the electrolyte.
  • Methods that are being used and developed for the electrical architecture of the 42-volt system include a single 42-volt system or a dual-voltage system.
  • The dual voltage system may use a dual generator system where one generator operates at 42 volts, while the other operates at 14 volts.
  • A dual stator, dual voltage system produces dual voltage from a single alternator that has two output voltages.
  • A DC/DC converter is configured to provide a 14 V output from the 42-volt input. The 14 V output can be used to supply electrical energy to those components that do not require 42 volts.
  • Cell construction of the NiCad battery is the cathode (positive) electrode is made of fiber mesh covered with nickel hydroxide, while the anode (negative) electrode is a fiber mesh that is covered with cadmium. The electrolyte is aqueous potassium hydroxide (KOH).
  • During discharge, ions travel from the anode, through the KOH, and on to the cathode. During charging, the opposite occurs.
  • The cathode electrode of the NiMH battery is a fiber mesh that contains nickel hydroxide. The anode electrode is made of hydrogen-absorbing metal alloys. The cathode and anode electrodes are separated by a sheet of fine fibers saturated with an aqueous and alkaline electrolyte-KOH.
  • Under load, the cell discharges and the hydrogen moves from the anode to the cathode electrode. Since the electrolyte only supports the ion movement from one electrode to the other, it has no active role in the chemical reaction, and the electrolyte level does not change.
  • A 300-volt NiMH battery is constructor of 240 cells that produce 1.2 volts each. The cells are made into a module, with each module having 6 cells. The modules are connected in series to create the total voltage.
  • A service disconnect in the HV battery is used to disable the HV system if repairs or service to any part of the system is required. This service connector provides two functions that are used to separate the HV battery pack into two separate batteries, with approximately 150 volts each.
  • Contactors are heavy-duty relays that are connected to the positive and negative sides of the HV battery.
  • The contactors are normally open and require a 12-volt supply to keep them closed.
  • Ultra-capacitors are capacitors constructed to have a large electrode surface area and a very small distance between the electrodes.
  • Ultra-capacitors are used in many present day hybrid vehicles and in some experimental fuel cell electric vehicles because of their ability to quickly discharge high voltages and then be quickly recharged.
  • Hybrids that use regenerative braking, a starter/generator with the stop/start feature, and the 42-volt system will use ultra-capacitors to restart the engine.
  • The three most common types of battery terminals are:
    1. Post or top terminals: Used on most automotive batteries. The positive post will belarger than the negative post to prevent connecting the battery in reverse polarity.
    2. 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.
  • The most common methods of battery rating are cold cranking, cranking amps, reserve capacity, and ampere-hour.