Samlex Solar SCC-30AB Owner's Manual page 10

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GENERAL INFORMATION: BATTERIEs
• Lead-Calcium batteries have lower self-discharge rates and therefore, will draw less current
while kept in storage
Electrochemical Reactions during Charging and Discharging
of Lead Acid Battery
Electrical power in the Lead Acid Battery is produced by reversible electrochemical
reactions as follows:
During discharging, the Sulphuric Acid in the electrolyte reacts with the Positive and
Negative plates resulting in conversion of the active materials in the two plates to
Lead Sulphate. At the same time, electrons are released that create the electrical
current that flows to the load. The concentration of Sulphuric Acid in the electrolyte is
reduced as the battery gets discharged (the electrolyte becomes pure water when the
battery is fully discharged).
During charging, reverse electrochemical reactions take place. Under the influence
of the charging voltage fed to the battery by the external battery charger / charge
controller, electrons are fed back to the battery and the Lead Sulphate at the Positive
and Negative Plates is converted back to Lead Dioxide at the Positive Plate and Lead
at the Negative Plate and the concentration of Sulphuric Acid is restored (will revert to
33.5% v/v when the battery is fully charged).
Gassing Due to Excessive Overcharging
During charging, the battery is required to be charged in a controlled manner in the
final Absorption Stage (2.4V per cell at 25ºC / 77ºF or 14.4V for a 12V battery at 25ºC
/ 77ºF) that restores the last 20% to 30% of the capacity. On completion of this stage
of charging, the Lead Sulphate at the Positive and Negative Plates is fully converted
back to Lead Dioxide at the Positive Plate and Lead at the Negative Plate. Any further
charging at this voltage or higher than this voltage results in electrolysis of water in
the electrolyte to Hydrogen and Oxygen and this undesirable condition contributes to
waste of energy. This process is known as "gassing". Gassing is also produced during
the timed Equalization Stage (normally performed only for vented /flooded / wet
cell batteries) when the battery is intentionally overcharged (2.5 to 2.6V per cell / 15
to 15.6V for 12V batteries and 30 to 31.2V for 24V batteries) so that weaker cells are
brought up to the full charge too (equalized).
Non-sealed / vented / flooded / wet cell batteries have open vents to release Hydrogen
and Oxygen produced during gassing. The above un-intentional electrolysis of water
during overcharging results in loss of water and reduces the level of the electrolyte in
this type of batteries. When the level of the electrolyte is reduced, the upper portion
of the plates in the cells will not be immersed in the electrolyte and will result in loss
of battery capacity. Hence, these types of batteries are required to be topped up with
distilled water periodically to ensure that the plates in the cells are fully immersed
in the electrolyte. Some non-sealed / vented / flooded / wet cell batteries come with
catalytic caps to recombine any emitted Hydrogen and Oxygen.
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