How to restore and prolong lead-acid batteries
(BU35)
The sealed lead-acid battery is designed with a low over-voltage
potential to prohibit the battery from reaching its gas-generating
state during charge. This prevents water depletion of the sealed
system. Consequently, these batteries will never get fully charged
and some sulfation will develop over time.
Finding the ideal charge voltage threshold is critical and any level
is a compromise. A voltage limit above 2.40 volts per cell produces
good battery performance but shortens the service life due to grid
corrosion on the positive plate. The corrosion is permanent. A voltage
below the 2.40V/cell threshold strains the battery less but the
capacity is low and sulfation sets in over time on the negative
plate.
Driven by diverse applications, two sealed lead-acid types have
emerged. They are the sealed lead acid (SLA), and the valve regulated
lead acid (VRLA). Technically, both batteries are the same. Engineers
may argue that the word 'sealed lead acid' is a misnomer because
no lead acid battery can be totally sealed.
The SLA has a typical capacity range of 0.2Ah to 30Ah and powers
personal UPS units, local emergency lighting and wheelchairs. The
VRLA battery is used for large stationary applications for power
backup. We are looking at methods to restore and prolong these two
battery systems separately.
The sealed lead-acid
(SLA)
SLA batteries with mild sulfation can be restored but the work
is time consuming and the results are mixed. Reasonably good results 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 The process is repeated several times and the capacity is checked with a
final full discharge and recharge.
Another method of improving performance
is by applying an equalizing charge, in which the charge voltage threshold is
increased by about 100mV, typically from 2.40V to 2.50V. This procedure should
last no longer than one to two hours and must be carried out at moderate room
temperature. A careless equalize charge could cause the cells to heat up and induce
venting due to excessive pressure. Observe the battery during the service.
The cylindrical SLA, made by Hawker and sold under the Cyclone name, requires
slightly higher voltages to reverse sulfation. An adjustable power supply works
best for the service. Set the current limit to the lowest practical setting and
observe the battery voltage and temperature during charge. 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 cell starts to
clamp the voltage. At around 2.30V, the cell accepts charge. If the sulfation
is advanced, this remedy does not work and the cell needs replacing.
Sealed lead-acid batteries are commonly rated at a 20-hour discharge. Even at
such a slow rate, a capacity of 100% is difficult to achieve. For practical reasons,
most battery analyzers use a 5-hour discharge when servicing these batteries.
This produces 80% to 90% of the rated capacity. SLA batteries are normally overrated
and manufacturers are aware of this practice.
Cycling an SLA on a battery
analyzer may provide capacity readings that decrease with each additional cycle.
A battery may start off at a marginal 88%, then go to 86%, 84% and 83%. This phenomenon
can be corrected by increasing the charge voltage threshold from 2.40V to 2.45V
and perhaps even 2.50V. Always consider the manufacturer's recommended settings.
Cyclone batteries require slightly higher voltage settings than the plastic version.
Avoid setting the charge voltage threshold too high. In an extreme case,
the limiting voltage may never be reached, especially when charging at elevated
temperatures. The battery continues charging at full current and the pack gets
hot. Heat lowers the battery voltage and works against a further voltage raise.
If no temperature sensing is available to terminate the charge, a thermal runaway
can be the result.
The recovery rate of SLA batteries is a low 15%. Other
than reverse sulfation, there is little one can do to improve SLA. Because the
SLA has a relatively short cycle life, many fail due to wear-out.
Valve regulated
lead-acid (VRLA)
The charge voltage setting on VRLA is generally lower than SLA.
Heat is a killer of VRLA. Many stationary batteries are kept in
shelters with no air conditioning. Every 8°C (15°F) rise
in temperature cuts the battery life in half. A VRLA battery, which
would last for 10 years at 25°C (77°F), will only be good
for 5 years if operated at 33°C (95°F). Once damaged by
heat, no remedy exists to improve capacity.
The cell voltages of a VRLA battery must be harmonized as close
as possible. Applying an equalizing charge every 6 months brings
all cells to similar voltage levels. This is done by increasing
the cell voltage to 2.50V/cell for about 2 hours. During the service,
the battery must be kept cool and careful observation is needed.
Limit cell venting. Most VRLA vent at 0.3 Bar (5 psi). Not only
does escaping hydrogen deplete the electrolyte, it is highly flammable.
Water permeation, or loss of electrolyte, is a concern with sealed
lead acid batteries. Adding water may help to restore capacity but
a long-term fix is uncertain. The battery becomes unreliable and
requires high maintenance.
Simple Guidelines
- Always store lead-acid in a charged condition. Never allow
the open cell voltage to drop much below 2.10V. Apply a topping
charge every six months or when recommended.
- Avoid repeated deep discharges. Charge more often. Use a
larger battery to reduce the depth of discharge.
- Prevent sulfation and grid corrosion by choosing the correct
charge and float voltages. If possible, allow a fully saturated
charge of 14h.
- To reverse sulfation, raise the charge voltage above 2.40V/cell
for a few hours.
- Avoid operating lead-acid at elevated ambient temperatures.
Note: Wheelchair batteries don't last as long as golf cart batteries
because of sulfation. The theory goes that a golf cart battery
gets a full 14 hours charge whereas a wheelchair only gets 7
hours while the user sleeps.
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Created: April 2003, Last edited: February
2007
