Illustration 1: The original size of positive plate versus end of life size of positive plate
Original size
of the positive plate
End of life* size
of the original positive plate
End of life is defined as the
point where the battery can
deliver only 80% of its rated
capacity
Sulfation: Lead sulfate is created at both the negative and positive plates during discharge. If the battery is left discharged, or is undercharged for long durations this accumulation builds up as a high resistance coating. Sulfation in a flooded battery can sometimes be reversed by charging the battery at voltage in excess of the manufacturer’s normally specified maximum value. Consult your battery’s manufacturer if you wish to attempt this. Sulfation reversal in absorbed glass mat / valve regulated lead acid (AGM/VRLA) batteries is not practical because the required sulfation reversal voltage is high enough to cause outgassing through the battery’s pressure relief valves, leading to battery failure due to electrolyte loss. Therefore, sulfated AGM/VRLA batteries must be replaced. Sulfation can be reduced by insuring the battery is recharged promptly after use, and by insuring that the battery is charged at a high enough voltage to reverse regular sulfation.
Ni-Cd batteries do not suffer from damage due to sulfation.
Loss of electrolyte: Both flooded and AGM/VRLA batteries are subject to electrolyte loss. Hydrogen and oxygen evolve as a byproduct of charging. Gradual loss of electrolyte from flooded batteries is easily managed by periodic maintenance that includes addition of deionized water. In contrast, electrolyte lost from AGM/VRLA batteries cannot be replaced. The only protection against an AGM/VRLA battery drying out is insuring that it is not charged at too high a voltage.
As with lead-acid batteries, Ni-Cd batteries lose electrolyte during charging and must be periodically topped up with deionized water to maintain proper electrolyte level.
Flooded lead-acid battery versus AGM/VRLA lead-acid battery
The plates of most SLI batteries are immersed in liquid electrolyte that is held inside the battery by gravity. This arrangement is referred to as either a “flooded” or “vented” battery because plates are normally submerged. Oxygen and hydrogen gases liberated from the battery during recharge escape to the atmosphere through the battery vent caps. Some loss of these gases during charging is normal, and is the reason deionized water must be periodically added to a flooded battery. Overcharging the battery greatly accelerates gas generation and water loss. Permanent battery damage occurs when electrolyte level is too low, allowing active material to dry out.
East Penn Manufacturing maintenance regime. These will all be discussed below.
Some lead-acid batteries immobilize the
liquid electrolyte using a technology called absorbed glass mat (AGM). Inside the AGM battery electrolyte is absorbed by a porous fiberglass separator material wrapped around the battery’s plates. This technology offers two potential advantages: (1) there is little free electrolyte available to leak if the battery is damaged, and (2) hydrogen and oxygen recombine inside the battery back into water, meaning that under ideal conditions water does not have to be added. The vent caps of AGM batteries are thus permanently fixed, meaning there is no possibility to replenish water. The AGM battery is sometimes referred to as a “maintenance-free” battery.
Illustration 2
1000x magnification of absorbed glass
mat material in AGM battery4
Cell caps on AGM batteries look like vent caps on flooded batteries but are very different in function. Each cell of an AGM must be closed to the atmosphere so that oxygen and hydrogen created during charge may recombine back into water, rather than escape the battery. The cell cap of an AGM battery is a one-way pressure relief valve set at about 5 pounds per square inch. When correctly charged the pressure relief valve does not operate and gas stays within the battery jar. When overcharged, though, gas is generated faster than it can recombine back into water. The pressure relief valve(s) then open until cell pressure drops to about 3 psi. The gas is lost for good and there is no way to replenish the correct proportions of oxygen and hydrogen. AGM batteries thus dry out each time they are overcharged. When sufficiently dry they will fail completely.
Lead-acid batteries with pressure relief valves are referred to generically as “VRLA”, for Valve Regulated Lead Acid. Because the vent caps are not removable, battery state of charge cannot be assessed by measuring electrolyte specific gravity.
Illustration 3: Temperature-compensated charging voltage in a lead-acid starting battery
While all automotive voltage regulators vary their output with temperature, many battery chargers are not temperature compensated. This means that such chargers either over- or undercharge batteries, increasing risk of genset start failure. Because genset chargers are on 24/7 instead of a few hundred hours per year, the consequence of incorrect charging is roughly 18 times worse for a genset battery than a vehicle battery. It is up to the user or specifying engineer to demand temperature compensated charging in order to assure this important feature is provided.
The examples below demonstrate the problem of using a non-temperature compensated battery charger:
- A lead-acid battery operated at a normal summer temperature of 50 degrees C10 (122F) but charged at the manufacturer’s recommended 2.25 volts/cell at room temperature would suffer a reduction in life of more than 70%11 because it is overcharged.
Battery Application Handbook for Cyclon® & Genesis® Sealed-Lead Products.
While the shape of the curve is common to all lead-acid batteries, values on the curve may vary from supplier to supplier. It is essential that users insure that float and boost charging voltages are set to the values recommended by their battery’s manufacturer.
Battery voltage data: Caterpillar Inc. Chart: Stored Energy Systems LLC (SENS). NOTE 1: This curve represents only one battery type and is not ideal for all batteries. NOTE 2: The relatively high voltage below 0 degrees C may not be suited to all batteries or genset electrical systems. NOTE 3: To enable compliance with battery makers’ different temperature compensation slopes, the battery charger’s temperature compensation slope should be adjustable.
- During winter in most parts of North America the same lead-acid battery could be exposed to 0 degrees C (32F) temperature or colder. If charging voltages were not increased above the room temperature value of 2.25 volts/cell to about 2.35 volts/cell, the battery would be undercharged. Undercharging would degrade the battery’s already poor cold temperature performance and increase risk of irreversible damage due to sulfation.