Corrosion occurs primarily on the grid and is known as a softening and shedding of 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 key in keeping corrosion in check. To reduce corrosion on long-life batteries, manufacturers keep the specific gravity at a moderate 1.200 when fully charged, compared to 1.265 and greater for high-performing lead acid batteries Read about How to Measure State-of-charge. A lower specific gravity reduces the capacity the battery can hold.
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. Chargers with variable float voltages adjust to the prevailing temperature to help to keep grid corrosion in check. Such chargers are in common use for stationary batteries.
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
The terminals of a battery can also corrode, and this is often visible in the form of white powder. The phenomenon 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.
The term “short” is commonly used to describe a general battery fault when no other definition is available. As the colloquial term “memory” was the cause for all battery ills in the NiCd days, so do battery users often judge non-functioning lead acid batteries simply as being “shorted.” Let’s take a closer look and see what a shorted lead acid battery truly is.
The lead within a battery, especially in deep-cycle units, 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 increases self-discharge. This mechanical action 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 concern.
As the battery sheds its lead to the bottom of the container, a conductive layer forms, and once the contaminated material fills the allotted space in the sediment trap, the now conductive liquid reaches the plates and creates 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 appears normal immediately after a charge and everything seems to function as it should. In essence, the charge has wiped out all evidence of a soft short, except perhaps an elevated temperature on the battery housing. 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. The 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 large 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. This causes a more serious short and is associated with a permanent voltage drop. The shorted cell may have little or no charge and the specific gravity of the electrolyte is close to 1.00. This is mostly a manufacturing defect and cannot be repaired.
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.