How to restore and prolong lead-acid batteries
The sealed lead-acid battery is designed with a low over-voltage potential to prohibit the battery from reaching its gas-generating state during charge. This prevents water depletion of the sealed system. Consequently, these batteries will never get fully charged and some sulfation will develop over time.
Finding the ideal charge voltage threshold is critical and any level is a compromise. A voltage limit above 2.40 volts per cell produces good battery performance but shortens the service life due to grid corrosion on the positive plate. The corrosion is permanent. A voltage below the 2.40V/cell threshold strains the battery less but the capacity is low and sulfation sets in over time on the negative plate.
Driven by diverse applications, two sealed lead-acid types have emerged. They are the sealed lead acid (SLA), and the valve regulated lead acid (VRLA). Technically, both batteries are the same. Engineers may argue that the word 'sealed lead acid' is a misnomer because no lead acid battery can be totally sealed.
The SLA has a typical capacity range of 0.2Ah to 30Ah and powers personal UPS units, local emergency lighting and wheelchairs. The VRLA battery is used for large stationary applications for power backup. We are looking at methods to restore and prolong these two battery systems separately.
The sealed lead-acid (SLA)
SLA batteries with mild sulfation can be restored but the work is time consuming and the results are mixed. Reasonably good results 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 process is repeated several times and the capacity is checked with a final full discharge and recharge.
Another method of improving performance is by applying an equalizing charge, in which the charge voltage threshold is increased by about 100mV, typically from 2.40V to 2.50V. This procedure should last no longer than one to two hours and must be carried out at moderate room temperature. A careless equalize charge could cause the cells to heat up and induce venting due to excessive pressure. Observe the battery during the service.
Sealed lead-acid batteries are commonly rated at a 20-hour discharge. Even at such a slow rate, a capacity of 100% is difficult to achieve. For practical reasons, most battery analyzers use a 5-hour discharge when servicing these batteries. This produces 80% to 90% of the rated capacity. SLA batteries are normally overrated and manufacturers are aware of this practice.
Cycling an SLA on a battery analyzer may provide capacity readings that decrease with each additional cycle. A battery may start off at a marginal 88%, then go to 86%, 84% and 83%. This phenomenon can be corrected by increasing the charge voltage threshold from 2.40V to 2.45V and perhaps even 2.50V. Always consider the manufacturer's recommended settings. Cyclone batteries require slightly higher voltage settings than the plastic version.
Avoid setting the charge voltage threshold too high. In an extreme case, the limiting voltage may never be reached, especially when charging at elevated temperatures. The battery continues charging at full current and the pack gets hot. Heat lowers the battery voltage and works against a further voltage raise. If no temperature sensing is available to terminate the charge, a thermal runaway can be the result.
The recovery rate of SLA batteries is a low 15%. Other than reverse sulfation, there is little one can do to improve SLA. Because the SLA has a relatively short cycle life, many fail due to wear-out.
Valve regulated lead-acid (VRLA)
The charge voltage setting on VRLA is generally lower than SLA. Heat is a killer of VRLA. Many stationary batteries are kept in shelters with no air conditioning. Every 8°C (15°F) rise in temperature cuts the battery life in half. A VRLA battery, which would last for 10 years at 25°C (77°F), will only be good for 5 years if operated at 33°C (95°F). Once damaged by heat, no remedy exists to improve capacity.
The cell voltages of a VRLA battery must be harmonized as close as possible. Applying an equalizing charge every 6 months brings all cells to similar voltage levels. This is done by increasing the cell voltage to 2.50V/cell for about 2 hours. During the service, the battery must be kept cool and careful observation is needed. Limit cell venting. Most VRLA vent at 0.3 Bar (5 psi). Not only does escaping hydrogen deplete the electrolyte, it is highly flammable.
Water permeation, or loss of electrolyte, is a concern with sealed lead acid batteries. Adding water may help to restore capacity but a long-term fix is uncertain. The battery becomes unreliable and requires high maintenance.
- Always store lead-acid in a charged condition. Never allow the open cell voltage to drop much below 2.10V. Apply a topping charge every six months or when recommended.
- Avoid repeated deep discharges. Charge more often. Use a larger battery to reduce the depth of discharge.
- Prevent sulfation and grid corrosion by choosing the correct charge and float voltages. If possible, allow a fully saturated charge of 14h.
- To reverse sulfation, raise the charge voltage above 2.40V/cell for a few hours.
- Avoid operating lead-acid at elevated ambient temperatures.
Note: Wheelchair batteries don't last as long as golf cart batteries because of sulfation. The theory goes that a golf cart battery gets a full 14 hours charge whereas a wheelchair only gets 7 hours while the user sleeps.