Section 1.3 The Sealed Lead Acid (SLA) Battery, states that the SLA is designed with a low over-voltage potential to prevent water depletion. Consequently, the SLA system never gets fully charged and sulfation may occur over time.
Once the 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 comparison can be made to a child in a famine-stricken country that can only digest a small amount of food but is able to increase the intake gradually.
A subtle indication of whether an SLA battery can be recovered or not is reflected in the behavior of its discharge voltage. A fully charged SLA battery which starts its discharge with a normal voltage and tapers off gradually has a better success rate of being reactivated than a battery that begins with a low terminal voltage and drops rapidly soon after a 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. The procedure is repeated several times after which the capacity of the battery is checked. Unlike other battery chemistries, overcharge does not harm the SLA, provided the terminal voltage is properly according to the section on charging the SLA battery .
Another method in reversing the effect of sulfation of a plastic SLA is by applying an over-voltage charge of up to 2.50 volts per cell for one to two hours. During that time, the battery must be kept cool and careful observation is essential. Extreme caution is required not to allow the cell pressure to rise too high (most plastic SLA batteries vent at 5 psi). Not only does cell venting cause damage to the battery, the escaping gases are highly flammable!
If the SLA does not take a charge current, the battery may be affected by sulfation, a condition that occurs if the SLA has been stored in a partially discharged state. Early stages of sulfation can sometimes be reversed by applying a charge voltage that is twice that of the terminal voltage.
The cylindrical SLA by Hawker can be reactivated quite easily after storage. If affected by sulfation, the cell voltage may initially be as high as 5 volts, forcing a small amount of current into the cell. Within about two hours, this small charging current converts the large sulfate crystals back into active material. As this occurs, the cell impedance decreases and eventually returns to normal condition. At a voltage between 2.10V and 2.40V, the cell is able to accept a full charge. To prevent damage, caution must be exercised to limit the charge current once the cell starts to accept charge. The Hawker SLA cells are known to regain full performance with the voltage method, leaving little or no adverse effects.
Improving the capacity of an older SLA by cycling is mostly unsuccessful. Such a battery may simply be worn out. Further cycling would just wear the battery down further. Unlike nickel-based batteries, the SLA 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% is difficult to reach. For practical reasons, most battery analyzers use a 5 hour discharge, a rate that produces typically only 80% to 90% of the rated capacity. Manufacturers are known to overrate their SLA batteries. With the popularity of battery analyzers, some manufacturers have lowered their ratings to be more in line with reality.
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.