|
The charge algorithm for lead acid batteries
differs from nickel-based chemistry in that voltage limiting
rather than current limiting is used. Charge time of a sealed
lead acid (SLA) is 12 to 16 hours. With higher charge
currents and multi-stage charge methods, charge time can be
reduced to 10 hours or less. SLAs cannot be fully charged
as quickly as nickel-based systems.
A multi-stage charger applies constant-current
charge, topping charge and float charge (see Figure 4-3).
During the constant current charge, the battery charges to
70 percent in about five hours; the remaining 30 percent
is completed by the slow topping charge. The topping charge
lasts another five hours and is essential for the well-being
of the battery. This can be compared to a little rest after
a good meal before resuming work. If the battery is not completely
saturated, the SLA will eventually lose its ability to accept
a full charge and the performance of the battery is reduced.
The third stage is the float charge, which compensates for
the self-discharge after the battery has been fully charged.
Figure 4-3: Charge stages of a lead
acid battery.
A multi-stage charger applies constant-current
charge, topping charge and float charge.
Correctly setting the cell-voltage limit is critical.
A typical voltage limit is from 2.30V to 2.45V. If a slow
charge is acceptable, or the room temperature may exceed 30°C
(86°F), the recommended voltage limit is 2.35V/cell. If a
faster charge is required, and the room temperature will remain
below 30°C, 2.40 to 2.45V/cell may be used. Figure 4-4
compares the advantages and disadvantages of the different
voltage settings.
|
|
| |
2.30V
to 2.35V/cell
|
2.40V
to 2.45V/cell
|
|
|
|
Advantage
|
Maximum service life; battery remains
cool during charge; ambient charge temperature may exceed
30°C (86°F).
|
Faster charge times; higher and more
consistent capacity readings; less subject to damage
due to under-charge condition.
|
|
Disadvantage
|
Slow charge time; capacity readings
may be low and inconsistent. If no periodic topping
charge is applied, under-charge conditions (sulfation)
may occur, which can lead to unrecoverable capacity
loss.
|
Battery life may be reduced due to
elevated battery temperature while charging. A hot battery
may fail to reach the cell voltage limit, causing harmful
over charge.
|
|
|
Figure 4-4: Effects of charge
voltage on a plastic SLA battery.
Large VRLA and the cylindrical Hawker
cell may have different requirements.
The charge voltage limit indicated in Figure 4-4
is a momentary voltage peak and the battery cannot dwell on
that level. This voltage crest is only used when applying
a full charge cycle to a battery that has been discharged.
Once fully charged and at operational readiness, a float charge
is applied, which is held constant at a lower voltage level.
The recommended float charge voltage of most low-pressure
lead acid batteries is between 2.25 to 2.30V/cell. A
good compromise is 2.27V.
The optimal float charge voltage shifts with
temperature. A higher temperature demands slightly lower voltages
and a lower temperature demands higher voltages. Chargers
that are exposed to large temperature fluctuations are equipped
with temperature sensors to optimize the float voltage.
Regardless of how well the float voltage may
be compensated, there is always a compromise. The author of
a paper in a battery seminar explained that charging a sealed
lead acid battery using the traditional float charge techniques
is like 'dancing on the head of a pin'. The battery wants
to be fully charged to avoid sulfation on the negative plate,
but does not want to be over-saturated which causes grid corrosion
on the positive plate. In addition to grid corrosion, too
high a float charge contributes to loss of electrolyte.
Differences in the aging of the cells create
another challenge in finding the optimum float charge voltage.
With the development of air pockets within the cells over
time, some batteries exhibit hydrogen evolution from overcharging.
Others undergo oxygen recombination in an almost starved state.
Since the cells are connected in series, controlling the individual
cell voltages during charge is virtually impossible. If the
applied cell voltage is too high or too low for a given cell,
the weaker cell deteriorates further and its condition becomes
more pronounced with time. Companies have developed cell-balancing
devices that correct some of these problems but these devices
can only be applied if access to individual cells is possible.
A ripple voltage imposed on the charge voltage
also causes problems for lead acid batteries, especially the
larger VRLA. The peak of the ripple voltage constitutes an
overcharge, causing hydrogen evolution; the valleys induce
a brief discharge causing a starved state. Electrolyte depletion
may be the result.
Much has been said about pulse charging lead
acid batteries. Although there are obvious benefits of reduced
cell corrosion, manufacturers and service technicians are
not in agreement regarding the benefit of such a charge method.
Some advantages are apparent if pulse charging is applied
correctly, but the results are non-conclusive.
Whereas the voltage settings in Figure 4-4
apply to low-pressure lead acid batteries with a pressure
relief valve setting of about 34 kPa (5 psi), the
cylindrical SLA by Hawker requires higher voltage settings.
These voltage limits should be set according to the manufacturer’s
specifications. Failing to apply the recommended voltage threshold
for these batteries causes a gradual decrease in capacity
due to sulfation. Typically, the Hawker cell has a pressure
relief setting of 345 kPa (50 psi). This allows
some recombination of the gases during charge.
An SLA must be stored in a charged state. A topping
charge should be applied every six months to avoid the voltage
from dropping below 2.10V/cell. The topping charge requirements
may differ with cell manufacturers. Always follow the time
intervals recommended by the manufacturer.
By measuring the open cell voltage while in storage,
an approximate charge-level indication can be obtained. A
voltage of 2.11V, if measured at room temperature, reveals
that the cell has a charge of 50 percent and higher.
If the voltage is at or above this threshold, the battery
is in good condition and only needs a full charge cycle prior
to use. If the voltage drops below 2.10V, several discharge/charge
cycles may be required to bring the battery to full performance.
When measuring the terminal voltage of any cell, the storage
temperature should be observed. A cool battery raises the
voltage slightly and a warm one lowers it.
|
