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Separators are used between the positive and
negative plates of a lead acid battery to prevent short circuit through
physical contact, Dendrites (‘treeing’) most and shredded
active material. Separators cause some obstructions for the flow of ions
i.e. electricity between the electrodes. Separators therefore must have the
following characteristics:
- They must be
porous—high porosity gives a high rate of flow of ions.
- Pore size must be
small enough to restrict the flow of colloid particles but not
restrict the ions.
- They must be as
thin as possible.
- Electrical
resistance must be very high.
- They are a little
larger than the plates to prevent material shorting the plates.
To balance these criteria, the choice of
separator shifted from wood to rubber to glass mat to cellulose based
separators to sintered PVC separator to micro porous PVC/polyethylene separator.
An effective separator must meet a number of mechanical properties.
Permeability, porosity, pore size distribution, specific surface area,
mechanical design and strength, Electrical resistance, ionic conductivity,
and chemical compatibility with the electrolyte. In service the separator
must have good resistance to acid and oxidation.
In the battery service condition the
following reaction can be shown:
·
PbO2 + 2H+ + SO4-2 = PbSO4 + H2O + ½ O2
·
PbO2 + (oxidizable separator material) + H2SO4 = PbSO4 +
(oxidized material)
Moreover, the battery service temperature
can be as high as 70 to 80 degrees Celsius. The separator must be capable
of resisting thermal degradation as far as possible.
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