Grid Corrosion-The loss of contact between plates and some of its paste material. This is a naturally occurring result of batteries on serve but can also happen pre-maturely. The lead plates found inside the batteries are consumable materials which means that they will corrode over time. When grid corrosion occurs, this film of corrosion insulates part of the plates from the outer paste material, eventually reducing the overall capacity of the battery.
Positive Plate Growth - Over time, material deposits on the positive plates causing them to physically expand and grow larger. In a VRLA battery, this collection of material can actually bulge the battery enough to crack and break the case over time. It may also lift near the positive battery terminal causing seal leakage at the post. As the positive plate grows, more active material or paste loses contact with the plate because those cast-in “waffle pockets” also stretch out.
Discharge Cycles - As the battery experiences continuous or cyclical discharge cycles over time, the active paste changes its molecular character. Basically, the paste turns to mush, further reducing its contact with the plates and the useful life of the battery is essentially used up. Batteries are engineered for a specific cycle life ranging from a 200 to 300, all the way to several thousand and the number varies by the Depth of Discharge (DOD). If a product is capable of 1200 cycles at an 80% DOD, it is typical they would sustain several hundred more at a 50% DOD.
Environmental Temperature - All of these aging conditions will accelerate along predictable lines as the environmental temperature of the battery increases. Lead-acid batteries thrive at 77 degrees Fahrenheit (25 Celsius). As the temperature rises above that, aging increases linearly. A 15 degree Fahrenheit increase over 77 F, which is 92 F, ages a battery by half-life.
Accordingly, a battery designed to last twenty years will last only ten years in a 92F environment. It’s important to note that each 15-degree (F) increase is a half-life so a 107F environment reduces battery life to ¼ and so on. Many lead-acid VRLA Batteries are designed for a ten-year life. When used in outdoor cabinets in hot climates such as Phoenix, AZ, they generally last 12 to 18 months on average. A glance at the average operating temperature in one of the battery cells will give a good idea of temperature at the site. For de-rating purposes, approximate or data log the average temperature over the battery life. A few hot weeks per year won’t seriously accelerate a battery’s aging process but it certainly plays a role in longevity.
Also, cold temperatures have an equally beneficial effect with one trade off. Cool temperatures reduce the internal discharge of a battery reducing float current consumption This means that the battery requires less power to fully charge. This is because the lower float current consumption has the added effect of lowering any accumulation of heat generated by trying to keep the battery at a full state of charge when operating at a higher temperature.
There are four common causes for battery failure: grid corrosion, positive plate growth, discharge cycle, and environmental temperature. The below sections talk about each cause of failure in more detail.