The charge algorithm of the SLA differs from the NiCd and NiMH in that voltage-limit rather than current-limit is used. The charge time of the SLA is a slow 16 hours. With higher charge currents and multistage charge methods, the charge time can be reduced to 10 hours and less. In no way can the SLA be charged as quickly as the NiCd.

A multistage charger applies three charge stages consisting of constant-current charge, topping charge and float charge (see figure below). During the constant current charge, the battery charges to 70% in about five hours; the remaining 30% is completed by the topping charge. The slow topping charge lasting another five hours is essential for the well-being of the battery and can be compared to a little rest after a good meal. If deprived, the SLA eventually loses the ability to accept a full charge and the performance of the battery is reduced. The third stage is the float charge which in part compensates for the self-discharge after the battery has been fully charged.

Charge Stages of an SLA Battery

During the constant current charge, the SLA battery is charged at a high current, limited by the setting of the charger. Once the voltage limit is reached, the current starts to drop gradually, governed by the need for the battery to attain full saturation. Full charge is reached when the current drops to a preset threshold value or settles at a low steady state plateau.

The proper setting of the cell-voltage limit is critical and relates to charge times and ambient charge temperatures. A typical voltage limit range for Stage 2 is from 2.30V to 2.45V. If a slow charge is acceptable or the room temperature exceeds 30C (86F), the recommended voltage limit is 2.35V/cell. If a faster charge is required and the room temperature remains below 30C, 2.40V to 2.45V/cell may be used. The table below 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 degrees C (86 degrees 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 below and inconsistent. If no periodic topping charge is applied, under charge condition ( sulfation ) may occur 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.

Recommended Charge Voltage Limit on an SLA Battery
Whereas the settings in the table above are valid for the widely used plastic SLA designs with a pressure relieve valve setting of 5 psi, the cylindrical SLA by Hawker (formerly Gates) allows higher voltage settings. Failing to apply the recommended voltage threshold for these batteries will cause a gradual decrease in capacity due to sulfation. Typically, the Hawker cell has a pressure relief setting of 50 psi, allowing some recombination of the gases during charge.

The recommended float charge voltage for the plastic SLA is 2.25 volts per cell (higher for the Hawker cell). Too high a float charge voltage should be avoided because it accelerates grid corrosion. During float charge, the charge current is minimal.

An SLA must always be stored in a charged state. A topping charge should be applied every six months (or other time intervals as recommended by the manufacturer) to avoid the voltage from dropping below 2.10 volts per cell.

An approximate charge-level indication can be obtained by measuring the open terminal voltage of the cell while in storage. A voltage of 2.11V reveals that the cell has a charge of 50% 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.

Plastic SLA batteries that arrive from the vendor with less than 2.10V per cell are commonly rejected when examined by the buyers Incoming Inspection Departments. Although the capacity of these batteries can be increased by cycling, it is felt that the extra cycles needed compromise the service life of the battery. Furthermore, the time and equipment required to get the battery fully functional adds to the operational costs. Low voltage also suggests that the battery may contain soft shorts, a defect that cannot be corrected with cycling.

The Hawker cell c

an be stored at voltages as low as 1.81V. However, when reactivating the cells, a higher than normal charge voltage may be required to convert the large sulfite crystals back into good active material (Restoring SLA Batteries).

When measuring the terminal voltage of any cell, the storage temperature should be observed. A cool battery raises the voltage and a warm one lowers it slightly. It is best to store the SLA in cool and dry place.

The SLA is ideal for standby operation. During prolonged float charge, a periodic topping charge is recommended to fully charge the plates and prevent sulfation. Depending on manufacturer, the frequency of topping charge varies between three to twelve months.

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.

Important: In case of rupture, leaking electrolyte or any other cause of skin or eye exposure to the electrolyte, immediately flush with water. If eye exposure, flush with water for 15 minutes and consult physician.

Restoring SLA Batteries

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