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These types of batteries confine the
electrolyte, but have a vent or valve to allow gases to
escape if internal pressure exceeds a certain threshold.
During charging, a lead-acid battery generates oxygen gas at
the positive electrode.
Sealed lead-acid batteries are designed so
that the oxygen generated during charging is captured and
recombined in the battery. This is called an oxygen
recombination cycle and works well as long as the charge
rate is not too high. Too high of a rate of charge may
result in case rupture, thermal runaway, or internal
mechanical damage.
The valve-regulated battery is the most
common type of sealed battery. It was developed for
stationary and telecommunication battery applications. These
types of sealed batteries have a spring-controlled valve
that vents gases at a predetermined pressure. Typical
pressure thresholds are from 2 to 5 psig, depending on the
battery design. Although the term "valve
regulated" is often used synonymously to describe
sealed lead-acid batteries, not all sealed batteries are
valve-regulated. Some battery designs employ replaceable
vent plugs or other mechanisms to relieve excess pressure.
Sealed batteries were developed to reduce the maintenance
required for batteries in active service. Since electrolyte
levels are preserved by trapping and recombining off-gasses,
there should not be any need to add distilled water over the
life of the battery. These batteries are often misnamed
"maintenance free." In fact, all maintenance
practices applicable to unsealed type batteries are
applicable to sealed type batteries. The only exception is
that electrolyte levels cannot, and should not need to be,
maintained.
Sealed type batteries are often avoided for
backup power source applications for several reasons. One
reason is that the state of charge of sealed type batteries
cannot be ascertained by the usual specific gravity
measurement. Reliable alternative methods to measure the
state of charge for sealed type batteries are under
development. A second reason is their sensitivity to high
temperatures.
Typical Cylindrical Sealed Lead Acid
Battery
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