Lead Acid Batteries

Construction
These batteries use an impact and acid resistant case for each
individual cell, which is made from polystyrene compounds. Each
cell case is moulded so that they provide outlets for the terminal
posts so that each cell can be connected with ease to the adjoining
cell, the case also houses a vent valve, which whilst the battery is
being charged allows gases to escape but does not allow the
electrolyte to leak out, the method of connecting the cells varies
dependant on the required use of the battery, and will be discussed
later.
3.12.3.2 Electrolyte
Lead acid batteries consist of cells with an electrolyte made of
sulphuric acid and water mixed to a specific gravity of 1.270 for a
fully charged battery, only distilled water is used in this mix, as the
impurities found in normal tap water will reduce the life and charge
of the cells.
As the cells are discharged and charged, the level of electrolyte will
decrease and so periodically the battery cells will require a “top up”
of distilled water, if this action is required it is to be noted that the
acid is always added to the water, as the reverse procedure is
extremely dangerous, because the water will react violently with the
acid and literally explode when added.
A fully charged cell will have a voltage in excess of 2.5volts after its
charge this will drop to 2.2volts after an hour standing, during
discharge the cell voltage will drop to 2volts and will remain at this
level for the majority of the cell charge life after this time, which is
dependant on the load connected to the cell, the voltage of the cell

will drop to 1.8volts and is considered to be discharged. Because the
voltage remains at a constant 2volts for the vast majority of the time,
this is considered to be the cells nominal voltage value.

The definition of capacity in terms of batteries is the quantity of
electricity that can be taken from a fully charged cell at a specified
discharged rate measured in amps, before the cells nominal voltage
of 2volts drops to a defined level of 1.8volts. Battery capacity is
therefore measured in terms of current and time i.e. ampere-hours,
and is expressed as a percentage against the maximum available
amps/hours for that specific type of cell. The factors, which affect
the battery capacity, are the area and number of plates, strength of
electrolyte and temperature.

/ discharge voltage versus time
It can be seen that during the discharge the voltage of each individual
cell remains constant for a considerable time at approximately 2volts,
however this is only true if the load connected has a small current
draw, If a larger current is drawn from the cell the discharge will
become more linear as the voltage drops more rapidly. What can also
be determined from the graph is that the voltage value cannot be used
to determine the amount of charge in the battery, consider this graph.

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