Samlex Solar SCC-30AB Mode D'emploi page 16

3
GENERAL INFORMATION: BATTERIEs
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 (2.4 V per cell at 25 ºC / 77 ºF or 14.4 V for a 12 V battery
at 25 ºC / 77 ºF) 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 (performed only for vented /flooded /
wet cell batteries) when the battery is intentionally overcharged (2.5 to 2.6 V per cell
/ 15 to 15.6 V for 12 V 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.
Sealed / VRLA batteries are designed to recombine the Hydrogen and Oxygen back
into water and hence, Sealed / VRLA batteries are not required to be topped up with
distilled water. That is why these batteries are also called maintenance free batteries.
Sealed / VRLA batteries use safety valves to release any excessive gas pressure built
up inside the battery due to malfunction or overheating. If this happens (e.g.,
by overcharging) the valve vents the gas and normalizes the pressure, producing
a characteristic acid smell. Valves can sometimes fail however, if dirt and debris
accumulate, allowing pressure to build up that will result in damage to the battery.
16