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Invented by the French physician Gaston Planté in 1859, lead
acid was the first rechargeable battery for commercial use.
Today, the flooded lead acid battery is used in automobiles,
forklifts and large uninterruptible power supply (UPS) systems.
During the mid 1970s, researchers developed a maintenance-free
lead acid battery, which could operate in any position. The
liquid electrolyte was transformed into moistened separators
and the enclosure was sealed. Safety valves were added to
allow venting of gas during charge and discharge.
 Driven by diverse applications, two designations of batteries
emerged. They are the sealed lead acid (SLA), also known under
the brand name of Gelcell, and the valve regulated lead acid
(VRLA). Technically, both batteries are the same. No scientific
definition exists as to when an SLA becomes a VRLA. (Engineers
may argue that the word ‘sealed lead acid’ is a misnomer because
no lead acid battery can be totally sealed. In essence, all
are valve regulated.)
The SLA has a typical capacity range of 0.2Ah to 30Ah and
powers portable and wheeled applications. Typical uses are
personal UPS units for PC backup, small emergency lighting
units, ventilators for health care patients and wheelchairs.
Because of low cost, dependable service and minimal maintenance
requirements, the SLA battery is the preferred choice for
biomedical and health care instruments in hospitals and retirement
homes.
The VRLA battery is generally used for stationary applications.
Their capacities range from 30Ah to several thousand Ah and
are found in larger UPS systems for power backup. Typical
uses are mobile phone repeaters, cable distribution centers,
Internet hubs and utilities, as well as power backup for banks,
hospitals, airports and military installations.
Unlike the flooded lead acid battery, both the SLA and VRLA
are designed with a low over-voltage potential to prohibit
the battery from reaching its gas-generating potential during
charge. Excess charging would cause gassing and water depletion.
Consequently, the SLA and VRLA can never be charged to their
full potential.
Among modern rechargeable batteries, the lead acid battery
family has the lowest energy density. For the purpose of analysis,
we use the term ‘sealed lead acid’ to describe the lead acid
batteries for portable use and ‘valve regulated lead acid’
for stationary applications. Because of our focus on portable
batteries, we focus mainly on the SLA.
 The
SLA is not subject to memory. Leaving the battery on float
charge for a prolonged time does not cause damage. The battery’s
charge retention is best among rechargeable batteries. Whereas
the NiCd self-discharges approximately 40 percent of
its stored energy in three months, the SLA self-discharges
the same amount in one year. The SLA is relatively inexpensive
to purchase but the operational costs can be more expensive
than the NiCd if full cycles are required on a repetitive
basis.
The SLA does not lend itself to fast charging — typical charge
times are 8 to 16 hours. The SLA must always
be stored in a charged state. Leaving the battery in a discharged
condition causes sulfation, a condition that makes the battery
difficult, if not impossible, to recharge.
Unlike the NiCd, the SLA does not like deep cycling. A full
discharge causes extra strain and each discharge/charge cycle
robs the battery of a small amount of capacity. This loss
is very small while the battery is in good operating condition,
but becomes more acute once the performance drops below 80 percent
of its nominal capacity. This wear-down characteristic also
applies to other battery chemistries in varying degrees. To
prevent the battery from being stressed through repetitive
deep discharge, a larger SLA battery is recommended.
Depending on the depth of discharge and operating temperature,
the SLA provides 200 to 300 discharge/charge cycles.
The primary reason for its relatively short cycle life is
grid corrosion of the positive electrode, depletion of the
active material and expansion of the positive plates. These
changes are most prevalent at higher operating temperatures.
Applying charge/discharge cycles does not prevent or reverse
the trend.
There are some methods that improve the performance and prolong
the life of the SLA. The optimum operating temperature for
a VRLA battery is 25°C (77°F). As a rule of thumb, every 8°C
(15°F) rise in temperature will cut the battery life in half.
VRLA that would last for 10 years at 25°C would only
be good for 5 years if operated at 33°C (95°F). The same
battery would endure a little more than one year at a temperature
of 42°C (107°F).
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Advantages
and Limitations of Lead Acid Batteries
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Advantages
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Inexpensive and simple to manufacture — in terms
of cost per watt hours, the SLA is the least expensive.
Mature, reliable and well-understood technology —
when used correctly, the SLA is durable and provides
dependable service.
Low self-discharge —the self-discharge rate is
among the lowest in rechargeable batterysystems.
Low maintenance requirements — no memory; no electrolyte
to fill.
Capable of high discharge rates.
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Limitations
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Cannot be stored in a discharged condition.
Low energy density — poor weight-to-energy density
limits use to stationary and wheeled applications.
Allows only a limited number of full discharge cycles
— well suited for standby applications that require
only occasional deep discharges.
Environmentally unfriendly — the electrolyte and
the lead content can cause environmental damage.
Transportation restrictions on flooded lead acid —
there are environmental concerns regarding spillage
in case of an accident.
Thermal runaway can occur with improper charging.
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Figure 2-4:
Advantages and limitations of lead acid batteries.
The SLA has a relatively low energy density compared with
other rechargeable batteries, making it unsuitable for handheld
devices that demand compact size. In addition, performance
at low temperatures is greatly reduced.
The SLA is rated at a 5-hour discharge or 0.2C. Some batteries
are even rated at a slow 20 hour discharge. Longer discharge
times produce higher capacity readings. The SLA performs well
on high pulse currents. During these pulses, discharge rates
well in excess of 1C can be drawn.
In terms of disposal, the SLA is less harmful than the NiCd
battery but the high lead content makes the SLA environmentally
unfriendly. Ninety percent of lead acid-based batteries are
being recycled.
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