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10.4 How to Restore and Prolong Sealed Lead Acid Batteries
The sealed version of the lead acid battery is designed with a low over-voltage potential to prevent water depletion. Consequently, the SLA and VRLA systems never get fully charged and some sulfation will develop over time.
Finding the ideal charge voltage limit for the sealed lead acid system 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 and cannot be reversed. A low voltage preserves the electrolyte and allows charging under a wide temperature range, but is subject to sulfation on the negative plate. (In keeping with portability, this book focuses on portable SLA batteries. Due to similarities between the SLA and VRLA systems, references to the VRLA are made where applicable).
Once the SLA battery has lost capacity due to sulfation, regaining its performance is often difficult and time consuming. The metabolism of the SLA battery is slow and cannot be hurried.
A subtle indication on whether an SLA battery can be recovered is reflected in the behavior of its discharge voltage. A fully charged SLA battery that starts its discharge with a high voltage and tapers off gradually can be reactivated more successfully than one on which the voltage drops rapidly when the load is applied.
Reasonably good results in regaining lost capacity 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. This is repeated several times, then the capacity of the battery is checked with a full discharge. The SLA is able to accept some overcharge, however, too long an overcharge could harm the battery due to corrosion and loss of electrolyte.
The effect of sulfation of the plastic SLA can be reversed by applying an over-voltage charge of up to 2.50V/cell for one to two hours. During that time, the battery must be kept cool and careful observation is necessary. Extreme caution is required not to raise the cell pressure to venting point. Most plastic SLA batteries vent at 34 kPa (5 psi). Cell venting causes the membrane on some SLA to rupture permanently. Not only do the escaping gases deplete the electrolyte, they are also highly flammable!
The VRLA uses a cell self-regulating venting system that opens and closes the cells based on cell pressure. Changes in atmospheric pressure contribute to cell venting. Proper ventilation of the battery room is essential to prevent the accumulation of hydrogen gas.
Cylindrical SLA — The cylindrical SLA (made by Hawker) resembles an enlarged D sized cell. After long storage, the Hawker cell can be reactivated relatively easily. If affected by sulfation, the cell voltage under charge may initially raise up to 5V, absorbing only a small amount of current. Within 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 eventually returns to normal. At a voltage between 2.10V and 2.40V, the cell is able to accept a normal charge. To prevent damage, caution must be exercised to limit the charge current.
The Hawker cells are known to regain full performance with the described voltage method, leaving few adverse effects. This, however, does not give credence to store this cell at a very low voltage. It is always best to follow the manufacturer’s recommended specifications.
Improving the capacity of an older SLA by cycling is mostly unsuccessful. Such a battery may simply be worn out. Cycling would just wear down the battery further. Unlike nickel-based batteries, the lead acid battery is not affected by memory.
SLA 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 SLA batteries. This typically produces 80 to 90 percent of the rated capacity. SLA batteries are normally overrated and manufacturers are aware of this.
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. Prevent cell venting.
Important: In case of rupture, leaking electrolyte or any other cause of exposure to the electrolyte, flush with water immediately. If eye exposure occurs, flush with water for 15 minutes and consult a physician immediately.
Simple Guidelines
- Always keep the SLA charged. Never store below 2.10V/cell.
- Avoid repeated deep discharges. Charge more often.
- If repeated deep discharges cannot be avoided, use a larger battery to ease the strain.
- Prevent sulfation and grid corrosion by choosing the correct charge and float voltages.