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Automotive Batteries. Safety. Safety. Always wear eye protection. Keep sparks and flames away from battery. Safety. Automotive Batteries. The automobile battery is an electrochemical device that converts chemical energy into electrical energy. Battery Purpose.

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Automotive Batteries

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Automotive Batteries



  • Always wear eye protection.

  • Keep sparks and flames away from battery.


Automotive Batteries

  • The automobile battery is an electrochemical device that converts chemical energy into electrical energy.

Battery Purpose

  • Provide voltage and current for the starter motor

  • Provide voltage and current for the ignition system during cranking

Starter Motor

Alternator (Generator)

Charging system workhorse

Battery Purpose

  • Supplies all electrical power when the charging system is not operating

  • Acts as a voltage stabilizer for the charging system

  • Provides extra current for short periods of time while the charging system is catching up to a new load

Various Battery Terminal Configurations

Battery Ratings

  • In North America, the Society of Automotive Engineers established two ratings for domestic made batteries:

    • cold cranking amps, which indicate the number of amps a battery can deliver at 0° F for 30 seconds and stay above 7.2 volts

    • reserve Capacity, which indicates the number of minutes a fully charged battery at 80° F can deliver 25 amps and stay above 10.5 volts.


  • A typical battery has six cells.

  • The cells are connected in a series configuration.

  • Each cell generates approximately 2.1 volts

  • This equates to each battery having an actual voltage of 12.6 volts.

Battery Construction

  • Each cell contains a series of lead plates.

  • Some lead plates are positive (lead dioxide) in charge and some are negative (lead).

  • The plates are kept from touching by a porous separator material, usually plastic or fiberglass.

Battery Construction

  • A battery produces electricity during the chemical reaction between the lead plates and the electrolyte.

  • The electrolyte is 36% water (H2O) and 64% sulfuric acid (H2SO4) when the battery is fully charged.

Battery Electrolyte

  • Battery electrolyte is measured by its specific gravity.

  • Specific gravity is measured with a hydrometer.

  • The specific gravity of the battery electrolyte correlates to its state of charge. Specific Gravity measurements must be temperature compensated.

  • A fully charged battery will have a specific gravity of 1.270 at 80 degrees Fahrenheit.

Battery Electrolyte

  • Batteries today can have a built in hydrometer to measure the specific gravity or state of charge in the battery. Built in hydrometers show colored dots that are either:

  • Green – 65% or better charged

  • Black – must charged before testing

  • Clear – low electrolyte level, discard battery

Maintenance Free Batteries

  • Most batteries today are said to be maintenance free

  • These batteries do NOT have removable caps on the top.

  • These batteries have an open chamber under the lid

    • Called a liquid/gas separator


  • Check electrolyte specific gravity with a volt meter.

  • 12.6 V = a fully charged battery

    12.4 V = 75% charged

  • 12.2 V = 50% charged

There are many new types of batteries on the market today

  • One of these is the absorbent glass mat battery or AGM battery.

  • This battery contains absorbent mat separators that prevent the electrolyte from leaking

  • This reduces the effects of vibration, which extends battery life.

Battery Operation

  • During the generation of electricity the battery discharges.

  • During discharge the positive and negative plate surfaces become sulfated causing the battery to lose voltage.

As the battery discharges, a chemical reaction occurs

  • The oxygen molecules from the positive plate’s lead dioxide (PbO2) are released into the electrolyte

  • The electrolyte absorbs the oxygen molecules and releases it’s sulfate molecules

  • This causes the positive and negative plates lead (Pb) to gain sulfate molecules (SO4 )

    • both of the plates are changing to lead sulfate (PbSO4)

    • the electrolyte’s acid is changing to water (H2O).

  • As the battery is recharged the reaction is reversed.

Battery Operation

Fully Charged Battery

Discharging Battery

Battery Discharge Conditions

  • Starting a vehicle

  • When the demand of the electrical system exceeds the output of the generator (usually at idle)

  • Using powered accessories while the vehicle is not running.

  • Battery or vehicle storage

  • Normal battery drain (parasitic draw)

    • Clocks

    • Radio station presets

    • Computer memory

Other conditions that can result in a discharged battery include…

  • excessive parasitic drain

  • battery malfunctions

  • starting and charging system malfunctions

Battery Charging

  • When the engine is running at sufficient speed, the generator in the charging system supplies current to the battery. In this condition, the battery is said to be charging.

Battery Charging

  • When a battery is nearly or completely discharged, recharging is required for the battery to provide proper service.

Battery Charging

  • Always charge a battery with the vehicles ignition OFF

  • Always wear safety glasses!

Battery Charging

  • When a charger is first connected to a completely discharged battery, the battery accepts only a small amount of current.

  • As the battery becomes more charged, electrolytes begin to return to sulfuric acid, therefore increasing the charge rate. The charge time for a completely discharged battery may be more than twice as long as that for a half-charged battery.

Battery Charging

  • If you must recharge a battery that’s in the vehicle, make sure that the ignition switch is in the OFF position.

  • If the ignition is not in the OFF position, damage to the vehicles electronic equipment may occur.

  • A battery should be charged at no more than 10% of its CCA rating.

Jump-Start (Negative Ground System)

Storing Batteries

  • When a battery is new the lead and acid will begin to react and it will slowly discharge.

  • Any battery that sits unused for several months will slowly go dead and begin to sulfate.

  • This will ruin the battery..

  • All batteries should be periodically recharged.

  • New batteries, awaiting sale, are given a “trickle charge” (2 - 4A) on a regular schedule

Storing Batteries

Battery Cables

  • Large enough to carry all current demanded

  • Large cable goes directly to the starter

  • Cables for 12-volt systems are usually 4 or 6 gauge

  • Cable clamps are made of steel or lead

Battery Hold-downs

  • Battery must be held in its tray

  • Excessive vibration can harm the battery

  • Holddowns are made of steel or plastic

Reasons for Battery Failure

  • Damaged battery case can leak electrolyte

  • Undercharging can cause sulfation

  • Overcharging can cause buckled/warped plates

  • Vibration can cause more material to be shed from plates

  • Short between the plates can cause a dead cell

  • Age (batteries are good for three to six years)

Hybrid and Other Battery Types

  • Other types of batteries include:

    • Nickel metal hydride batteries

    • Lithium-ion batteries

  • Hybrid battery voltages range 200 – 500 volts

    • This can kill you!

    • Be careful!

    • Do not work on a hybrid until you have been schooled!

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