Experiment-1

Study of lead acid and maintenance free batteries,

Aim: To know the construction, charging and discharging reactions and maintenance of automobile
batteries.
Apparatus: Lead acid batteries in working conditions, Cut model of batteries, Hydrometer and
Multimeter.

Theory:
Battery Function
An automotive battery is an electrochemical device that converts electrical energy into chemical
energy and stores it until needed. When called upon, the battery converts the stored chemical
energy back into electrical energy.

The battery serves four purposes in an automobile:

• It supplies electricity to the accessories when the engine is not running
• It supplies high current to the starter, and system voltage to the ignition system during cranking
• It provides current to the electrical systems when the demand exceeds the output of the
generator
• It acts as a voltage stabilizer in the electrical system.

Automobiles generally use what is classified as a wet cell, lead-acid battery. Batteries produce
current through a
chemical reaction
between the active
materials of the plates
and sulfuric acid in the
electrolyte. Throughout
the life of a battery, it is
either charging or
discharging. When a
battery is supplying
current to accessories or
to the starter, it is said to
be discharging. When
the engine is running at
sufficient speed, the
generator carries the
electrical load and
charges the battery, and both are said to be charging.

A battery is discharging when:

• The engine is not running (parasitic loads or self-discharging)
• The engine is running at a low rpm with conditions of high electrical demand
• There is a fault in the charging system
A battery that is nearly or completely discharged is commonly said to be “dead,” “flat,” or “run
down.” A battery in this condition should be recharged to full capacity to provide proper service.
Although a generator will charge a battery, it is not designed as a “battery charger.” Requiring a
generator to recharge a completely dead battery may cause overheating and damage to the
generator. Unlike “deep cycle” batteries used in some RV and marine applications, an
automotive battery is designed to remain at or near a full state of charge, and not to be
completely discharged.

Battery Construction
A battery is made up of individual cells, electrically connected in series for a cumulative voltage
effect. Each battery cell contains an element made up of positive and negative plates,
separators, and connecting straps. Each plate consists of a stiff mesh grid of a lead alloy,
coated with porous lead on the
negative plates, and lead peroxide or
lead dioxide on the positive plates. A
strap of lead connects the negative
plates to form a group, and another
strap connects the positive plate
group. On each end of the battery,
the straps are extended to form
battery terminals or posts. The
plates are submerged in an
electrolyte solution. Acid fumes and
water vapor are formed and released
during the chemical reactions of
charging. This gassing causes the
loss of electrolyte. Conventional
batteries have removable vent caps,
permitting the electrolyte levels to be
checked and topped off, as well as to
allow chemical testing. “Maintenance
free”
batteries are designed to minimize
gassing.
Between the positive and negative plates are separators, which are constructed to keep the
plates from touching each other and shorting. The separators are porous, to allow electrolyte to
circulate freely and permit the chemical process to take place. Each battery cell is a separate
unit that produces 2.1volts. A “12 volt” automotive battery contains six cells connected in series
for a total of 12.6 volts. Many diesel applications use two 12 volt batteries, connected in parallel,
to provide the high current required to crank a
diesel engine. Batteries connected in this fashion
still supply 12 volts, but have twice the current
capacity of a single battery. Battery cells are
housed in a durable, vented, plastic case, and have
terminals on the top (“top post”) or side
(side terminal). Many aftermarket batteries are
equipped with both types of terminal arrangements.
Negative battery cables are usually grounded to the
engine block. On some applications, a small pigtail
wire also connects the negative terminal to the
vehicle body. The pigtail connects the body ground
to the engine ground, and it must be connected for
the starting and charging system to work properly.
Low Maintenance and Maintenance Free
Batteries
Many batteries are marketed as “maintenance free,” meaning water should not need to be
added during the life of the battery. The plates in these batteries tend to be slightly shorter to
allow them to be submerged deeper in electrolyte. Some maintenance free batteries do not
have removable covers or caps. Others do, to allow for the addition of water in case of
overcharging or severe conditions, and to permit hydrometer testing. These batteries should not
require additional water, but if the electrolyte can be checked, it should be checked
approximately every six months.
Electrolyte and Specific Gravity
Specific gravity is a measure of the density or weight of a fluid, using water as a baseline.
Water has a specific gravity of 1.000, and pure sulfuric acid has a specific gravity of 1.835,
meaning it is 1.835 times heavier than water. Electrolyte contains 64% water and 36% acid,
which gives it a specific gravity of
1.265 to 1.270 in a fully charged
battery (this is often expressed as
“twelve seventy,” etc.). If the
electrolyte is accessible, it can be
checked with a
hydrometer. As a battery is discharged, the electrolyte contains less acid and more water, so a
hydrometer float will not rise as high in the hydrometer barrel, or fewer balls will float. Many
maintenance free batteries have a hydrometer built into one of the cells. We will cover those,
and hydrometer testing, later in the section. For now, keep in mind that the acid is heavier than
water, and a discharged battery has more water in its electrolyte.

Chemical Reactions while Discharging and Charging

In a fully charged battery, the active materials in the positive and negative plates are distinctly
different in chemical composition, and the electrolyte has a high acid content. Positive plates
contain a compound of lead and oxygen (PbO2), while negative plates contain lead (Pb). The
electrolyte is composed of water (H2O) and sulfuric acid (H2SO4). Sulfuric acid is a compound
of hydrogen, sulfur and oxygen. As a battery begins to discharge, the composition of the plates
becomes more similar, and the water content of the electrolyte increases. Lead sulfate (PbSO4)
is formed on both the positive and negative plates, trapping the oxygen and sulfur, and leaving
water molecules behind (left side of illustration). The voltage potential of a battery is dependent
on the dissimilarity of the active materials in the positive and negative plates. As the lead sulfate
content in the plates increases, the voltage and available current decreases. This process is

reversed to charge the battery. Current applied to the battery causes the lead sulfate residing on
the plates to release its oxygen into the electrolyte. This release increases the acid content of
the electrolyte, and returns the plates to their original compositions (right side of illustration).
The basic electrochemical reaction equation in a lead acid battery can be written as:

Discharge
During the discharge portion of the reaction, lead dioxide (PbO2) is converted into lead sulfate
(PbSO4) at the positive plate. At the negative plate sponge lead (Pb) is converted to lead sulfate
(PbSO4). This causes the sulfuric acid (2H2SO4) in the electrolyte to be consumed.

Charge
During the recharge phase of the reaction, the cycle is reversed. The lead sulfate (PbSO4) and
water are electrochemically converted to lead (Pb), lead dioxide (PbO4) and sulfuric acid
(2H2SO4) by an external electrical charging source.

Hydrometer Testing
Hydrometers are used to measure the specific gravity of electrolyte. A hydrometer with a single
float and a numerically graduated scale is recommended. This type of
hydrometer usually has a built-in thermometer to make necessary temperature
corrections. Smaller hydrometers that use
multiple, colored balls are generally not reliable.
Hydrometer floats and barrels are made of thin glass and are fragile.
Handle with care!
This chart shows the charge level and voltage for specific gravity readings taken
with a hydrometer.

Testing Battery: If the starter motor does not run, the battery may be
discharged or is faulty. Before carrying out testing of the starting system, test
the battery at no load condition. Follow the steps to test the 12 Volt batteries
with digital multimeter on no load condition (Fig ).
1. Turn Ignition key off.
2. Turn on headlights for 10 seconds to dissipate battery surface charge.
3. Insert black test lead into COM test lead jack.
4. Insert red test lead into test Voltage lead jack.
5. Disconnect positive (+) battery cable.
6. Connect red test lead to positive (+) terminal of battery.
7. Connect black test lead to negative (-) terminal of battery.
8. Turn multimeter rotary switch to 20V DC range.
9. View reading on display.
10.Test Results.
Compare display reading in Step 9 with table below.
Voltage Percentage battery charged
12.60 V or greater 100 %
12.45 V 75%
12.30 V 50 %
12.15 V 25 %

If battery is not 100% charged, then charge it before doing Battery testing
further electrical system tests.

Battery Safety
There are important safety concerns to keep in mind when working on or around automotive
batteries. Batteries can explode, and have enough power to arc weld. Always respect the
power of a battery,even a “dead” battery. The sulfuric acid in electrolyte is extremely
corrosive, and can cause severe chemical burns to the skin and eyes. It will also damage
painted surfaces and many other materials, including clothing. Always wear approved safety
glasses when working around batteries and the use of rubber gloves is recommended when
working with electrolyte.
You should know the locations of fire extinguishers and the first aid kit. First aid kits should
contain a bottle of sterile, acid-neutralizing eyewash. Larger facilities often have an emergency
shower and eyewash station located in the battery storage and service area.
Batteries release explosive hydrogen and oxygen gasses. A battery can explode with a sound
like a shotgun discharging, rupturing the case and spraying acid in all directions. Avoid creating
sparks around a battery. The following guidelines will help to reduce the chance of arcing or
sparks:
• The ground terminal of a battery should always be disconnected first and reconnected last.
• Connect battery chargers to a battery before plugging in the charger.
• When jump-starting a vehicle, follow the proper procedure.
Do not connect the jumper cable to the negative battery terminal of the vehicle you are jump-
starting.
• Do not attempt to charge, jump-start, or load test a battery with a broken or loose post, a
cracked case, or one in which the electrolyte is frozen.

Accidental shorting of the positive battery terminal or any system voltage source to
ground with a tool or metal object can cause severe burns.
Metal jewelry can be heated to its melting point in seconds. Even a brief short of this nature can
damage the PCM and other electronic components.
Never hammer on a battery terminal or cable end, or attempt to remove a cable by prying.
To avoid damage to the battery or terminals, and possible personal injury, use a clamp
spreading tool if the clamp doesn’t seat at the bottom of the post, and use a cable clamp puller
to remove stubborn clamps. Avoid contact with the white, flaky or powdery corrosion that builds
up around battery terminals and trays. This substance is sulfate and/or sulfide; it is corrosive
and can cause chemical burns.

Conclusion:
1. The students learnt about the lead acid battery construction and how it works.
2. The students learnt about the sealed maintenance free battery construction and its
working.
3. The students learnt about the chemical reactions happening during charging and
discharging in the battery.
4. The students learnt about testing of the battery with hydrometer and multimeter.
5. Safety precautions to be followed while working with batteries.