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How to restore and prolong lead acid batteries The sealed lead acid battery, known as valve regulated lead acid (VRLA), is designed with a low over-voltage potential. This is done to prevent water depletion. Consequently, these systems never get fully charged and some sulfation will develop over time.
Finding the ideal charge voltage limit 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. A low voltage protects the battery and allows charging under a higher temperature but is subject to sulfation on the negative plate.
Restoring a sulfated battery is difficult and time consuming. One method that provides reasonably good results is applying a charge on top of a charge. This is done by fully charging a battery, then removing it for a 24 to 48 hour rest period and applying a charge again. This process is repeated several times and the capacity is checked again with a full discharge. The lead acid battery is able to accept some overcharge but too much causes corrosion and loss of electrolyte.
Applying an over-voltage charge of up to 2.50V/cell for one to two hours can also reverse sulfation. During treatment, the battery must be kept cool and careful observation is needed. Prevent venting. Most plastic VRLA batteries vent at 34 kPa (5 psi). Not only do escaping gases deplete the electrolyte, they are highly flammable.
Sealed lead acid batteries are also available in cylindrical form. The Cyclon by Hawker resembles an oversized D sized cell. If sulfated, applying an elevated charge voltage commonly reactivates the cell. Initially, the cell voltage may rise to 5V, absorbing only a small amount of current. In 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 normalizes. When within 2.10V to 2.40V, the cell starts to accept normal charge. If the sulfation is advanced, this remedy does not work and the cell needs replacing.
When applying over-voltage, current limiting must be applied. Always set the limit to the lowest practical setting on the power supply and observe the battery voltage and temperature during charge.
Improving the capacity of an older lead acid battery by cycling is mostly in vain. Such a battery may simply be worn out and cycling wears it further down. The lead acid battery is not affected by memory.
VRLA 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 these batteries. This typically produces 80 to 90% of the rated capacity. VRLA cells are normally overrated and manufacturers are aware of this practice.
Simple Guidelines
- Always store lead acid charged. Never let the open cell voltage drop below 2.10V. Apply a topping charge every six months or when recommended.
- Avoid repeated deep discharges. Charge more often or use a larger battery.
- Prevent sulfation and grid corrosion by choosing the correct charge and float voltages.
