THE AUTOMOTIVE BATTERY
Dec 23, 2015
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