| Lead Acid Battery |
| Rechargeable Lead Acid Battery Basics |
A Lead Acid Battery cell is formed by inserting two parallel lead plates ( electrodes), one coated
with lead dioxide (PbO2), in a container with Sulfuric acid (H2So4).
The cell has 2 Volts nominal voltage. Installing six such, series connected, cells in a container
forms the popular 12V Lead Acid Battery.
Chemistry Lead Acid Battery :
Connecting resistive load between the lead electrodes, causes current to flow from the
positive (Pbo2) electrode to the negative (Pb) electrode, thus discharging the battery.
Battery charging is accomplished by connecting the electrodes to a voltage source (charger) with
the proper polarity.
The following chemical reactions take place during the charge - discharge cycle:
During the discharge process, the reactions cause both electrodes to be coated with Lead Sulfate
(PbSO4) crystals. Charging restores the situation.
Electrochemical formula: PbO2 + Pb + 2H2SO4 PbSO4 + 2H2O + 2e¯
More insight is deduced by considering each electrode separately:
At the negative electrode: Pb + SO4 PbSO4 + 2e¯
At positive electrode: PbO2 + SO4 + 4H + 2e¯ PbSO4 + 2H2O
The electrolyte is a mixture of Sulfuric Acid and water. Specific gravity of Electrolyte measures the
content of Sulfuric Acid in the solution. It is typically in the range of 1.200 to 1.300. Whereas
water has Specific Gravity of 1, and pure Sulfuric Acid has Specific Gravity of 1.835. State of
charge and other major battery characteristics are strongly related to the specific gravity of sulfuric
acid.
At charging, electrolysis of water produced oxygen and hydrogen as byproducts . In such process,
the external voltage source induces electrical current in the cell according to Ohms law. The
electrolyte due to its low resistance increases dramatically the current in the cell.
This current dissociates water molecules into (OH) ¯ and H ions.
The reaction at the Positive Electrode: 2H2O O2 + 4H + 4e¯
The reaction at the Negative Electrode: 2H2O + 2e¯ H2 + 2(OH) ¯
Thus, during charging, water is decomposed into its constituents. The oxygen gas,
generated at the positive plate and the hydrogen gas, generated at the negative plate.
Lead Acid Battery Technology:
Flooded batteries and Sealed Lead Acid batteries form the two basic lead acid battery types, they
differ mainly by the way water is restored to the battery.
In Flooded batteries the oxygen and hydrogen gases generated at the plates, are vented to the air
through the top of the battery. Water should therefore be added during maintenance in order to
replace the missing quantity, and sustain the charge storage ability.
In Sealed Lead Acid (SLA) Batteries, generally Valve Regulated Lead Acid (VRLA) Batteries,
pressure relief valves do not allow gases to leave the battery. The oxygen generated at the positive
plate diffuses to the negative plate, where it is recombined with hydrogen to form water. Thus,
water refilling is not required and no maintenance is required.
The valves will however allow gas relief, when internal pressure exceeds allowed value, as may be
the case at battery overcharging.
Lead Acid Battery Applications
Flooded batteries, also called wet batteries, are used for energy storage purpose mainly for
stationery, long duration (several hours) long life systems, for Utilities, Telecom and
Industrial applications.
Sealed lead acid batteries (SLA), often called Maintenance Free Batteries, are favored for standby
application and UPS applications. Their unique features in normal operation, such as maintenance
free, hermetically sealed, non spillable, non gassing and vibration resistance characteristics, makes
them an ideal choice to serve as standard UPS Batteries, as well as in numerous other applications
in additional fields, such as communication, security, transportation, emergency lightning, aviation
and more.
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