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4.4 Charging the Lead Acid Battery
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.
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
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.
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.
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.