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How to Restore and Prolong Sealed Lead Acid Batteries
The sealed version of the lead acid battery is
designed with a low over-voltage potential to prevent water
depletion. Consequently, the SLA and VRLA systems never get
fully charged and some sulfation will develop over time.
Finding the ideal charge voltage limit for the
sealed lead acid system is critical. Any voltage level is
a compromise. A high voltage limit produces good battery performance,
but shortens the service life due to grid corrosion on the
positive plate. The corrosion is permanent and cannot be reversed.
A low voltage preserves the electrolyte and allows charging
under a wide temperature range, but is subject to sulfation
on the negative plate. (In keeping with portability, this
book focuses on portable SLA batteries. Due to similarities
between the SLA and VRLA systems, references to the VRLA are
made where applicable).
Once the SLA battery has lost capacity due to
sulfation, regaining its performance is often difficult and
time consuming. The metabolism of the SLA battery is slow
and cannot be hurried.
A subtle indication on whether an SLA battery
can be recovered is reflected in the behavior of its discharge
voltage. A fully charged SLA battery that starts its discharge
with a high voltage and tapers off gradually can be reactivated
more successfully than one on which the voltage drops rapidly
when the load is applied.
Reasonably good results in regaining lost capacity
are achieved by applying a charge on top of a charge. This
is done by fully charging an SLA battery, then removing it
for a 24 to 48 hour rest period and applying a charge
again. This is repeated several times, then the capacity of
the battery is checked with a full discharge. The SLA is able
to accept some overcharge, however, too long an overcharge
could harm the battery due to corrosion and loss of electrolyte.
The effect of sulfation of the plastic SLA can
be reversed by applying an over-voltage charge of up to 2.50V/cell
for one to two hours. During that time, the battery must
be kept cool and careful observation is necessary. Extreme
caution is required not to raise the cell pressure to venting
point. Most plastic SLA batteries vent at 34 kPa (5 psi).
Cell venting causes the membrane on some SLA to rupture permanently.
Not only do the escaping gases deplete the electrolyte, they
are also highly flammable!
The VRLA uses a cell self-regulating venting
system that opens and closes the cells based on cell pressure.
Changes in atmospheric pressure contribute to cell venting.
Proper ventilation of the battery room is essential to prevent
the accumulation of hydrogen gas.
Cylindrical SLA — The cylindrical SLA
(made by Hawker) resembles an enlarged D sized cell. After
long storage, the Hawker cell can be reactivated relatively
easily. If affected by sulfation, the cell voltage under charge
may initially raise up to 5V, absorbing only a small amount
of current. Within about two hours, the small charging
current converts the large sulfate crystals back into active
material. The internal cell resistance decreases and the charge
voltage eventually returns to normal. At a voltage between
2.10V and 2.40V, the cell is able to accept a normal charge.
To prevent damage, caution must be exercised to limit the
charge current.
The Hawker cells are known to regain full performance
with the described voltage method, leaving few adverse effects.
This, however, does not give credence to store this cell at
a very low voltage. It is always best to follow the manufacturer’s
recommended specifications.
Improving the capacity of an older SLA by cycling
is mostly unsuccessful. Such a battery may simply be worn
out. Cycling would just wear down the battery further. Unlike
nickel-based batteries, the lead acid battery is not affected
by memory.
SLA batteries are commonly rated at a 20-hour
discharge. Even at such a slow rate, a capacity of 100 percent
is difficult to obtain. For practical reasons, most battery
analyzers use a 5-hour discharge when servicing SLA batteries.
This typically produces 80 to 90 percent of the rated
capacity. SLA batteries are normally overrated and manufacturers
are aware of this.
Caution: When charging an SLA
with over-voltage, current limiting must be applied to protect
the battery. Always set the current limit to the lowest practical
setting and observe the battery voltage and temperature during
charge. Prevent cell venting.
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.
Simple Guidelines
- Always keep the SLA charged. Never store below 2.10V/cell.
- Avoid repeated deep discharges. Charge more often.
- If repeated deep discharges cannot be avoided, use a larger
battery to ease the strain.
- Prevent sulfation and grid corrosion by choosing the correct
charge and float voltages.
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