Find out what the user can do to reduce battery corrosion and shedding.
Corrosion occurs primarily on the grid and is known as a softening and shedding of the lead off the plates. This reaction cannot be avoided because the electrodes in a lead acid environment are always reactive. Lead shedding is a natural phenomenon that can only be slowed down and not eliminated. A battery that reaches the end of life through this failure mode has met or exceeded the anticipated life span. Limiting the depth of discharge, reducing the cycle count, operating at a moderate temperature and controlling overcharge are preventive actions the battery user can embrace to keep corrosion in check.
To reduce corrosion on long-life batteries, manufacturers keep the specific gravity at a moderate 1.200 level when fully charged, compared to 1.265 and greater for high-performing lead acid batteries. (See BU-903: How to measure State-of-charge) A lower specific gravity decreases the specific battery energy.
Applying prolonged overcharge is another contributor to grid corrosion. This is especially damaging to sealed lead acid systems. While the flooded lead acid has some resiliency to overcharge, sealed units must operate at a correct float charge. (See BU-403: Charging Lead Acid)
Chargers with variable float voltages adjust the charge voltage to the prevailing temperature. Reduce the float charge if the ambient temperature is higher than 29°C (85°F) and increase it when cold. Most chargers for stationary batteries feature temperature control, but this not common in vehicles. A fully charged starter battery is kept at 14.40V (2.40V/cell) while driving and this can lead to overcharge. As a laptop battery that is plugged into the main has a relatively short life being, so will also long cruises hasten corrosion by keeping the starter battery at full voltage when a lower voltage float charge would be better.
To attain maximum surface area, the lead on a starter battery is applied in a sponge-like form. With time and use, chunks of lead fall off and reduce the performance. Figure 1 illustrates the innards of a corroded lead acid battery.
Figure 1: Innards of a corroded lead acid battery
Grid corrosion is unavoidable because the electrodes in a lead acid environment are always reactive. Lead shedding is a natural phenomenon that can only be slowed and not eliminated.
The terminals of a battery can also corrode, and this is often visible in the form of white powder. This is a result of oxidation between two different metals connecting the poles. Terminal corrosion can eventually lead to an open electrical connection. Changing the connecting terminals to lead, the same material as the battery pole of a starter battery, will solve most corrosion problems.
Besides shedding, electrical short is another failure mode, especially with starter batteries on trucks. The term “short” is openly used to describe a battery fault when no other definition is available. The lead within a battery is mechanically active and when a battery discharges, the lead sulfate causes the plates to expand. This movement reverses during charge and the plates contract.
The cells allow for some expansion but over time the growth of large sulfite crystals can result in a soft short that manifest itself as elevated self-discharge, a mechanical action that also causes shedding of the lead material. On a starter battery, the shedding is manageable because the lead plates are thin and the battery does not go through a deep discharge. On a deep-cycle battery, on the other hand, shedding is a larger concern.
As the battery sheds its lead to the bottom of the container, a conductive layer forms that will gradually fill the allotted space in the sediment trap. In time, the now conductive liquid may reach the plates, creating a shorting effect. The term “short” is a misnomer and elevated self-discharge or a soft short would be a better term to describe the condition.
“Soft shorts” are difficult to detect because the battery functions normally immediately after charge and everything seems fine. In essence, a charge wipes out all evidence of a soft short condition, except perhaps an elevated temperature that can be noticed when touching the battery housing. However, once rested for 6–12 hours, the battery begins to show anomalies such as a lower open-circuit voltage and reduced specific gravity. The measured capacity will also be low because self-discharge has consumed some of the stored energy. According to the 2010 BCI Failure Modes Study, shorted batteries accounted for 18 percent of battery failures, a drop from 31 percent five years earlier. Improved manufacturing methods may account for this reduction.
Another form of soft short is mossing. This occurs when the separators and plates are slightly misaligned as a result of poor manufacturing practices. This causes parts of the plates to become naked. Such exposure promotes the formation of conductive crystal moss around the edges, which leads to elevated self-discharge.
Lead drop is another cause of short in which chunks of lead break loose from the welded bars connecting the plates. Unlike a “soft” short that develops with wear-and-tear, a lead drop often occurs early in battery life due to a manufacturing defect. This induces a more serious short that is associated with a permanent voltage drop and could lead to a thermal runaway.
The most radical and serious form of short is a mechanical failure in which the suspended plates become loose and touch each other. This results in a sudden high discharge current that can lead to excessive heat buildup and thermal runaway. Sloppy manufacturing as well as excessive shock and vibration are the most common contributors to this failure.
Last updated 4/22/2015
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