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Plastic SLA batteries arriving from vendors with
less than 2.10V per cell are rejected by some buyers who inspect
the battery during quality control. Low voltage suggests that
the battery may have a soft short, a defect that cannot be
corrected with cycling. Although cycling may increase the
capacity of these batteries, the extra cycles compromise the
service life of the battery. Furthermore, the time and equipment
required to make the battery fully functional adds to operational
costs.
The Hawker cell can be stored at voltages as
low as 1.81V. However, when reactivating the cells, a higher
than normal charge voltage may be required to convert the
large sulfite crystals back to good active material.
Caution: When
charging a lead acid battery with over-voltage, current limiting
must be applied once the battery starts to draw full current.
Always set the current limit to the lowest practical setting
and observe the battery voltage and temperature during the
procedure. If the battery does not accept a normal charge
after 24 hours under elevated voltage, a return to normal
condition is unlikely.
The price of the Hawker cell is slightly higher
than that of the plastic equivalent, but lower than the NiCd.
Also known as the ‘Cyclone’, this cell is wound
similar to a cylindrical NiCd. This construction improves
the cell’s stability and provides higher discharge currents
when compared to the flat plate SLA. Because of its relatively
low self-discharge, Hawker cells are well suited for defibrillators
that are used on standby mode.
Lead acid batteries are preferred for UPS systems.
During prolonged float charge, a periodic topping charge,
also known as an ‘equalizing charge’, is recommended
to fully charge the plates and prevent sulfation. An equalizing
charge raises the battery voltage for several hours to
a voltage level above that specified by the manufacturer.
Loss of electrolyte through elevated temperature may occur
if the equalizing charge is not administered correctly. Because
no liquid can be added to the SLA and VRLA systems, a reduction
of the electrolyte will cause irreversible damage. Manufacturers
and service personnel are often divided on the benefit of
the equalizing charge.
Some exercise, or brief periodic discharge, is
believed to prolong battery life of lead acid systems. If
applied once a month as part of an exercising program, the
depth of discharge should only be about 10 percent of
its total capacity. A full discharge as part of regular maintenance
is not recommended because each deep discharge cycle robs
service life from the battery.
More experiments are needed to verify the benefit
of exercising lead acid batteries. Again, manufacturers and
service technicians express different views on how preventive
maintenance should be carried out. Some experts prefer a topping
charge while others recommend scheduled discharges. No scientific
data is available on the benefit of frequent shallow discharges
as opposed to fewer deep discharges or discharge pulses.
Disconnecting the float charge while the VRLA
is on standby is another method of prolonging battery life.
From time-to-time, a topping charge is applied to replenish
the energy lost through self-discharge. This is said to lower
cell corrosion and prolong battery life. In essence, the battery
is kept as if it was in storage. This only works for applications
that do not draw a load current during standby. In many applications,
the battery acts as an energy buffer and needs to be under
continuous charge.
Important: In
case of rupture, leaking electrolyte or any other cause of
exposure to the electrolyte, flush with water immediately.
If eye exposure occurs, flush with water for 15 minutes
and consult a physician immediately.
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