Section 3 Engine Systems
204
A similar chemical reaction takes place at the
negative plate, where the pure lead (Pb) combines
with the sulfuric acid electrolyte (H
2
SO
4
) to form
water (H
2
O) and lead sulfate (PbSO
4
). However,
when this reaction is complete, there are two
leftover electrons, which keep the plate negative
despite the fl ow of electrons through the circuit to
the positive plate.
As the process continues, more and more of
the electrolyte is changed to water and the plates
become more sulfated. Gradually, the chemical
reaction becomes unable to keep up with the fl ow
of electrons through the circuit. The voltage con-
tinues to drop as the charges on the plates equalize
until the battery becomes fully discharged.
When a battery is recharged, a controlled direct
current is passed through the battery in the reverse
direction from normal operation. This causes a
reversal in the chemical action and restores the
plates and electrolyte to their original conditions.
Recycling R Re c cy li g Batteriesia e
Today o o o da y’ s s s lea ad -acid a b ba tteries typically con nt ain a large
perce e e e c en t t ta ge e of f recyc y cl ed materials. This is because
lead-ac e e ad d c ci d batteries b t s are recycled and some m of their
materia m ma te a al s can ca be re r eu sed. As a result, les ss new material
needs e e e ed t t to go g o into n a a new n battery. Automot ti ve shops
collect o o o l ec a an d d send e de d ea d batteries to off-si it e recycling
facilit a a ci ti es. s s Used Us d batt t te ries can be stored for f up to a year
befor e e e o o re being b recycl r c le d. It is best to store e used lead-
acid c c c d d batter b a ri es inside, d away from the drainage system
and n n n d a an y y y sourc o ce of f i ig nition. Cracked batteries t should
be e e e co c c on t t ta ine n ed , and d a an y spills from a battery t crack
should h h h ou be c cl eaned a d up as quickly as possible.s
R R l B
Green Tech
number of negative and positive plates in each
cell. The greater the number of plates per cell, the
higher the ampere-hour rating (capacity to provide
current for a specifi c length of time) of the battery.
The positive plates have a lead oxide covering. The
negative plates have a porous or spongy surface.
Battery Voltage
In a charged battery, the lead oxide covering
of the positive plate reacts with the electrolyte to
generate a positive charge, and the pure lead of
the negative plates reacts with the electrolyte to
create a negative charge. This creates a potential
difference (voltage) between the plates. All plates
of a like charge are electrically connected, causing
accumulative charges to be present at the positive
and negative battery terminals.
Each cell of a battery in good condition con-
tributes approximately 1.95V to 2.08V to the total
charge of the battery. Six fully charged cells will
produce at least 12V. If they do not, the battery
must be recharged or replaced.
The Discharging/Charging Cycle
The charge in a battery remains relatively stable
until a circuit is completed between the battery
terminals. When this happens, current fl ows through
the circuit and a chemical reaction takes place
between plates and electrolyte as the battery attempts
to equalize the potential difference between the
plates.
The lead oxide (PbO
2
) on the positive plates
combines with the sulfuric acid (H
2
SO
4
) in the
electrolyte to create lead sulfate (PbSO
4
) and water
(H
2
O). In this reaction, each molecule of lead oxide
must supply two electrons. This surrendering of
electrons helps the plate stay positively charged,
despite the infl ux of electrons through the circuit
connecting the battery terminals.
Copyright Goodheart-Willcox Co., Inc.