THE AUTOMOTIVE BATTERY

What is the purpose of the battery in a car?

• Provide voltage and current for the starter motor

• Provide voltage and current for the ignition system during cranking

• 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

BATTERY OPERATION

• The automotive battery is an electrochemical device that provides a potential difference.

• Automotive batteries are made up of a series of secondary cells that can be recharged.

• The battery does not store electrical energy, it stores chemical energy that is converted to electrical energy as it discharges.

What is needed for an electrochemical reaction to occur?

• Two unlike metals in a conductive and reactive solution (electrolyte)

• A battery cell is made up of spongy lead, lead peroxide and an electrolyte of sulfuric acid and water.

• This kind of battery is called a lead-acid battery.

• The spongy lead (Pb) is the negative material.

• The lead peroxide (PbO2) is the positive material.

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

The Discharge – Charge Process

• As the battery discharges, a chemical reaction occurs which releases electrons from the lead (Pb) and causes the lead peroxide (PbO2) to gain electrons.

• During this process both of the materials are changing to lead sulfate (PbSO4) and the electrolyte’s acid is changing to water (H2O).

• As the battery is recharged the reaction is reversed and the materials and electrolyte return to their original states.

The process of discharging and recharging

is called cycling.

• Discharging Reaction:

– Pb + H2SO4 PbSO4 + H2 + 2 electrons

– PbO2 + H2SO4 + 2 electrons PbSO4 + H2O + O2

• Recharging Reaction:

– PbSO4 + 2 electrons + H2 Pb + H2SO4

– PbSO4 (-2 electrons) + H2O + O2 PbO2 + H2SO4

Voltage

• Each battery cell produces 2.1v during the discharge reaction.

• Cells are connected in series to gain the desired battery voltage.

• 3 cells yield 6.3v

• 6 cells yield 12.6v

• Charging voltage needs to be 1 volt or more above battery voltage to reverse the reaction.

Battery Construction:

• Cells are made of positive and negative plates which are kept from touching by a porous separator material, usually plastic or fiberglass.

• The plates are made on a lead alloy grid which holds the spongy lead or lead peroxide material.

• The positive grids are connected together with a post strap. The negative grids are also connected together. The grids carry the cell current to the battery posts.

• The battery case is divided into cell cavities which hold the electrolyte and the plate groups. The plate group from one cell is connected to another cell’s plate group through a hole in the cell wall.

Battery Varieties

• Cycling Battery –

– Designed to produce low current for long periods of time.

– They are usually very discharged before recharging.

– Plate material is thicker than an automotive battery.

• Starting Battery –– Designed to produce high current for very short periods of

time.

– There is usually only a slight discharge before recharging.

– Plate material is thinner to reduce heat generation.

Battery Types:

• 1. Non-maintenance Free -

– Lead-antimony grid

– Removable vent caps use to inspect electrolyte and add water.

• 2. Low-maintenance -

– Hidden vent caps

– Lead - calcium or Lead - strontium grid which lowers electrical resistance and heat generation. This reduces gassing and electrolyte loss.

Battery Types:

• 3. Maintenance Free -

– No vent caps. The case has small vents at both ends.

– Grid similar to low-maintenance

– Envelope type separators keep loose plate material near plates

Battery Types:

• 4. Hybrid –

– Lead - antimony in positive grid,

– Lead - calcium in negative grid reduces gassing and internal corrosion

– Spun glass separators allow for a more efficient chemical reaction

– These batteries are able to withstand more deep cycling

Battery Types:

• 5. Recombinant –

– No caps or vents

– Gel type electrolyte

– Special envelope type separators allow hydrogen gas to be reabsorbed. No gassing

– Generate 13.2 volts with six cells

Grid Types

• 1. Square Grid –

– The grid frame is a pattern of squares or rectangles.

– Longer current path, high resistance

• 2. Diagonal Grid –

– The vertical grid bars are set at diagonals to shorten the current path and lower resistance

End Lesson One