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How
to restore and prolong lead acid batteries
The sealed lead acid battery, known as valve regulated lead
acid (VRLA), is designed with a low over-voltage potential.
This is done to prevent water depletion. Consequently, these
systems never get fully charged and some sulfation will develop
over time.
Finding the ideal charge voltage limit 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.
A low voltage protects the battery and allows charging under
a higher temperature but is subject to sulfation on the negative
plate.
Restoring a sulfated battery is difficult and time consuming.
One method that provides reasonably good results is applying
a charge on top of a charge. This is done by fully charging
a battery, then removing it for a 24 to 48 hour rest period
and applying a charge again. This process is repeated several
times and the capacity is checked again with a full discharge.
The lead acid battery is able to accept some overcharge but
too much causes corrosion and loss of electrolyte.
Applying an over-voltage charge of up to 2.50V/cell for one
to two hours can also reverse sulfation. During treatment,
the battery must be kept cool and careful observation is needed.
Prevent venting. Most plastic VRLA batteries vent at 34 kPa
(5 psi). Not only do escaping gases deplete the electrolyte,
they are highly flammable.
Sealed lead acid batteries are also available in cylindrical
form. The Cyclon by Hawker resembles an oversized D sized
cell. If sulfated, applying an elevated charge voltage commonly
reactivates the cell. Initially, the cell voltage may rise
to 5V, absorbing only a small amount of current. In 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 normalizes. When within 2.10V
to 2.40V, the cell starts to accept normal charge. If the
sulfation is advanced, this remedy does not work and the cell
needs replacing.
When applying over-voltage, current limiting must be applied.
Always set the limit to the lowest practical setting on the
power supply and observe the battery voltage and temperature
during charge.
Improving the capacity of an older lead acid battery by cycling
is mostly in vain. Such a battery may simply be worn out and
cycling wears it further down. The lead acid battery is not
affected by memory.
VRLA 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 these batteries. This typically
produces 80 to 90% of the rated capacity. VRLA cells are normally
overrated and manufacturers are aware of this practice.
Simple Guidelines
- Always store lead acid charged. Never let the open cell
voltage drop below 2.10V. Apply a topping charge every six
months or when recommended.
- Avoid repeated deep discharges. Charge more often or use
a larger battery.
- Prevent sulfation and grid corrosion by choosing the correct
charge and float voltages.
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