chap39 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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OBJECTIVES:
After studying Chapter 39, the reader should
be able to:

•
•
•
•

Describe how a battery works.
List the precautions necessary whenever

working with batteries.
Explain how to safely charge a battery.
Discuss how to perform a battery drain test.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
absorbed glass mat (AGM) battery • ampere-hour (Ah)
rating • antimony
battery electrical drain test • BCI
calcium • cells • cold-cranking amperes (CCA) •
conductance testing • cranking amperes (CA)
deep cycling • dynamic voltage

electrolyte • element
flooded cell battery
Continued
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KEY TERMS:
gassing • gell battery • grids
ignition off draw (IOD) test
jump-starting
lead dioxide (peroxide) • lead sulfate • load test • lowwater-loss battery
maintenance-free battery • marine cranking amperes
(MCA) • meniscus
open circuit battery voltage test

Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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KEY TERMS:
parasitic load test • partitions • pasting • porous lead
radial-grid design • recombinant design battery • reserve capacity
(RC)
sealed lead-acid (SLA) battery • sealed valve regulated (SVR) battery
• sediment chamber • separators • specific gravity • sponge lead
valve regulated lead-acid (VRLA) battery

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Everything electrical in a vehicle receives its current
from the battery.
The battery is one of the most important parts of a
vehicle and is the foundation for the entire electrical
system.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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PURPOSE OF A BATTERY
The primary purpose of an automotive battery is to provide a 
source of electrical power for starting and for electrical demands 
that exceed generator output. 
The battery is a voltage stabilizer because it acts as a reservoir 
where large amounts of current (amperes) can be removed quickly 
during starting and replaced gradually by the generator during 
charging.
The battery must be in good (serviceable) condition before the 
charging system and the cranking system can be tested. It is 
important to test the vehicle battery before further testing of the 
cranking or charging system.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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BATTERY CONSTRUCTION
Most automotive battery cases (container or covers) are constructed 
of polypropylene, a thin (approx. 0.08 inch [.0 millimeters] thick), 
strong, and lightweight plastic. Industrial and truck batteries are 
constructed of a hard, thick, rubber material.
Inside the case are six cells (for 12­volt), each 
with positive and negative plates. Built in the 
bottom are ribs to support the lead­alloy plates 
and provide space for sediment to settle. 
Figure 39–1 Batteries are constructed of
plates grouped into cells, installed in a plastic
case.

This sediment chamber, prevents spent 
material from causing a short circuit between 
the plates at the bottom of the battery. 
Continued

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GRIDS
Each positive and negative plate in a battery is constructed on a 
framework or grid of lead. Lead is soft and must be strengthened 
by adding antimony or calcium for battery use. Grids hold the 
active material and provide electrical pathways for the current 
created in the plate.
Maintenance­Free versus Standard Battery Grids  A normal 
battery uses up to 5% antimony in the plate grids to add strength. 
The more amount of antimony, the more gassing (hydrogen and 
oxygen gases released), and the more water the battery will use.
Maintenance­free batteries use calcium instead of antimony, 

because 0.2% calcium has the same strength as 6% antimony. 
Continued
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Low­maintenance batteries use a low percentage of antimony (about 
2% to 3%) or antimony only in positive plates and calcium for 
negative plates.
The percentages that make up the alloy of the plate grids constitute 
the major difference between standard and maintenance­free 
batteries. Chemical reactions inside each battery are identical 
regardless of type of grid material used.
Radial­Grid Design  Some batteries use a grid design with only 
vertical and horizontal strips. The battery plate creates electricity 

from chemical energy, and this current must flow from where it is 
generated to where it connects to the outside battery post.
The current must move over and up along the grid strips.
Continued
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A grid with a radial­grid design has lower resistance and can 
provide more current more rapidly than can the non­radial­grid 
design used in conventional batteries. 

Figure 39–2 A radial grid from a
battery used in both positive and
negative plates.

The radial spokes act as a superhighway system for the current to 
travel from all areas of the grid to the battery post. 
Continued
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Positive versus Negative Plates  The positive plates have lead 
dioxide (peroxide) placed on the grid frame, called pasting. This 
dark brown active material can react with sulfuric acid of the 
battery. The negative plates are pasted with pure porous lead, or 
sponge lead, and are gray in color.
Separators The positive and the negative plates must be installed 
alternately next to each other without touching. Nonconducting 

separators are used. These separators are porous and have ribs 
facing the positive plate.
Many batteries use envelope­type separators that encase the entire 
plate and help prevent any material that may shed from the plates 
from causing a short circuit between plates at the bottom of the 
battery
Continued
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Cells  Constructed of positive and negative plates, cells have 
insulating separators between each plate. A cell is also called an 
element, and is actually a 2­volt battery, regardless of number of 
positive or negative plates used.

The greater the number of plates used in each cell, the greater the 
amount of current that can be produced. Typical batteries contain 
four positive plates and five negative plates per cell.
A 12­volt battery contains six cells connected in series, producing 12 
volts (6  2 = 12) and has 54 plates (9 plates per cell  6 cells).
If the same battery had five positive plates and six negative, 11 plates 
per cell (5 + 6), or 66 plates (11 plates  6 cells), it would have the 
same voltage, but the amount of current (amps) the
battery could produce would be increased. 
Continued
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Partitions  Cells are separated from other cells by partitions, made 

of the same material as the outside case of the battery. Electrical 
connections between cells are provided by lead connectors that loop 
over the top of the partition and connect the plates of the cells 
together.
Many batteries connect the cells directly through the partition 
connectors, which provides the shortest path for the current and the 
lowest resistance. 
Older­style truck and industrial batteries commonly used connectors 
that extended through the top of the case and over and then down 
through the case to connect the cells.
See Figure 39–3. 
Continued
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Battery capacity is determined 
by the amount of active plate 
material in the battery and the 
area of the plate material 
exposed to the liquid, called 
electrolyte, in the battery.

Figure 39–3
A cutaway battery showing the connection of
the cells to each other through the partition.

Some water (H2O) escapes 
during charging as a result of 
the gassing produced by the 
chemical reactions. 

Only pure distilled water should 
be added to a battery. If distilled 
water is not available, clean 
drinking water can be used.
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Electrolyte  The electrolyte used in automotive batteries is a 
solution (liquid combination) of 36% sulfuric acid and 64% water. 
This used for both lead­antimony and lead­calcium (maintenance­
free) batteries.
The chemical symbol for this sulfuric acid solution is H2SO4.
H = Symbol for hydrogen
       (subscript 2 means there are two atoms of hydrogen)
S = Symbol for sulfur
O = Symbol for oxygen
       (subscript 4 indicates four atoms of oxygen)
Electrolyte is sold premixed in the proper proportion and is factory 
installed. Additional electrolyte must never be added to a battery 
after the original electrolyte fill. 
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HOW A BATTERY WORKS
A fully charged lead­acid battery has a positive plate of lead 
dioxide (peroxide) and a negative plate of lead surrounded by a 
sulfuric acid solution (electrolyte).
Difference in potential (voltage) between lead peroxide and lead 
in acid is approximately 2.1 volts.
During Discharging  The positive­plate lead dioxide (PbO2) 
combines with the SO4 from the electrolyte and releases its O2 
into the electrolyte, forming H2O. The negative plate also 
combines with the SO4 from the electrolyte and becomes lead 
sulfate (PbSO4). 

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Figure 39–4 Chemical reaction for a lead-acid battery that is fully charged being discharged by
the attached electrical load.

Continued
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The Fully Discharged State  When the battery is fully discharged, 
both the positive and the negative plates are PbSO4 (lead sulfate) 
and the electrolyte has become water (H2O). It is usually not 
possible for a battery to become 100% discharged. However, as the 
battery is being discharged, the plates and electrolyte approach the 
completely dead situation. There is also the danger of freezing when 
a battery is discharged, because the electrolyte is mostly water.
During Charging  The sulfate ion (acid) leaves both the positive 
and the negative plates and returns to the electrolyte, where it 
becomes normal­strength sulfuric acid solution. The positive plate 
returns to lead dioxide (PbO2) and the negative plate is again pure 
lead (Pb).
See Figure 39–5.
Continued
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Figure 39–5 Chemical reaction for a lead-acid battery that is fully discharged being charged by
the attached generator.

Continued
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An Easy Way to Remember Battery Chemical
Reactions
A battery engineer explains to the employees of the plant how a battery
works using these very easy-to-remember statements:

•
•

When the battery is charged, the acid (SO4 ) is in the liquid.
When the battery is discharged, the acid (SO4 ) is on the plates.

When the battery is being discharged, the acid leaves the liquid and is deposited on the plates. When the battery is being charged, the current flows
into the battery, forcing the acid from the plate and back into the liquid
(electrolyte).

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SPECIFIC GRAVITY
Sulfate in the electrolyte is indicated by the electrolyte’s specific 
gravity, the ratio of weight of a given volume of a liquid to the 
weight of an equal volume of water.
The more dense the material (liquid), the higher its specific 
gravity. Pure water is the basis for this measurement and is given a 
specific gravity of 1.000 at 80°F.
Pure sulfuric acid has a specific gravity of 1.835; the correct 
concentration of water and sulfuric acid (called electrolyte—64% 
water, 36% acid) is 1.260 to 1.280 at 80°F.
The higher the battery’s specific gravity, the more fully it is 
charged. See Figure 39–6.
Continued
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Figure 39–6 As the battery becomes discharged, the specific gravity of the battery acid
decreases.

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CHARGE INDICATORS
Some batteries are 
equipped with a built­in 
state­of­charge indicator.
This indicator is simply a 
small ball­type hydrometer 
that is installed in one cell. 

When the ball floats, it appears 
in the hydrometer’s sight glass, 
changing its color. 
Figure 39–7
Typical battery charge indicator. If the specific gravity is low (battery discharged), the ball drops
away from the reflective prism. When the battery is charged enough, the ball floats and reflects
the color of the ball (usually green) back up through the sight glass and the sight glass is dark.
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Figure 39–8 Cutaway of the battery showing the charge indicator. If the electrolyte level drops
below the bottom of the prism, the sight glass shows clear (light). Most battery manufacturers
warn that if the electrolyte level is low on a sealed battery, the battery must be replaced.
Attempting to charge a battery that has a low electrolyte level can cause a buildup of gases and
possibly an explosion.

This hydrometer uses a plastic ball that floats if electrolyte density 
is sufficient (when the battery is about 65% charged).
Because it is only testing 
one cell out of six, and 
because the hydrometer 
ball can easily stick in 
one position, it should 
not be trusted to give 
accurate data about a 
battery’s charge.
Continued
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