Utility applications, either power generation or substation, demand batteries that can deliver full performance, even after 20 years of full-float or service operation. Cell construction, plate design and specific gravity of the acid electrolyte each significantly impact the overall operating life of a battery. In addition, proper maintenance procedures must be observed or even the finest battery design will suffer shortened life.
Utility batteries are typically used in full-float operation. Under these conditions, the battery, charger and equipment are all connected in parallel, and the charger supplies dc power to the equipment and holds (floats) the battery at the same voltage level to maintain full charge.
The typical utility battery discharge profile includes a high initial current drain to set breakers, followed by several hours of moderate current to sustain key operating systems, concluding with several minutes of high current drain to reset breakers. Batteries are sized according to the power required by the particular discharge profile of the specific installation and are expected to deliver this performance throughout their intended service life. As a result, the reliable flooded lead-acid battery is widely used in the industry.
The cell is the basic building block of a battery. Each lead acid “wet cell” operates at a nominal voltage of two volts and has a rated capacity and current-carrying capability roughly proportional to its physical size. Figure 1 illustrates a typical flooded cell. All cells have the same basic parts: a case (jar) and cover; the element (alternating positive and negative plates separated by layers of material called the separator); acid electrolyte; and terminal posts that extend through the cover for electrical connections. Although the specific design of these parts can affect battery performance over time, plate thickness and acid specific gravity are the two most critical features in ensuring long float life and long-term performance.
Plate thickness. The most common element design in flooded utility batteries is the flat plate style. Both the positive and negative plates of the element are made of active material pasted on grids cast from either lead-calcium or lead-antimony material. Two different cells may have the same rated capacity, but could have vastly different internal plate designs. One may use fewer, thicker plates, while the other may use a greater number of thinner plates. Although both have the same rated capacity, the cell with the thinner plates contains less active material and achieves its rating by discharging at a higher voltage. Since it is common for some of the pasted active material to shed from the plates over time, the amount of active material available for discharge after years of float service is critical. Thus, a cell designed with thicker plates will contain more active material and is better suited to withstand abusive conditions and long-life service. It is critical that utility batteries contain positive plates with a minimum thickness of 0.25 inches and negative plates with a minimum thickness of 0.20 inches in order to achieve optimum long-life performance.
Acid specific gravity. During years of float service, it is normal for the positive plates of a cell to grow. Positive plate growth is the normal failure mode of a flooded lead-acid battery. This plate growth is the result of the aggressive nature of acid electrolyte attacking grain boundaries of the grid. A higher acid content, indicated by a higher specific gravity number, can increase the rate of this chemical reaction. The typical utility battery has a specific gravity of 1.215; however, some flooded batteries specify an acid gravity as high as 1.250. Although this higher acid content will enhance rate capability and increase overall capacity, it can have a detrimental effect on float life by accelerating plate growth. To ensure long-life under float conditions, it is critical that specific gravity of a utility battery not exceed 1.220.
Grid material. Elevated temperature and frequent cycling of the battery exacerbate the inter-granular corrosion reaction discussed above, significantly decreasing service life. Its effects can, however, be reduced by carefully controlling the casting process or by adding small amounts of a grain refiner to the grid material. Commonly used grid refiners are tin, arsenic, cadmium and selenium. Each has roughly the same effect on optimizing the grain structure of a cast lead-antimony grid. For example, “selenium battery” is a commonly used term. However, this is not a radical new concept in battery design, but rather a standard lead-acid battery with a small amount of selenium added to the lead-antimony grid as a grain refiner. In general, cells using lead-antimony grids (whether or not they contain selenium) are fairly maintenance intensive, since they require more frequent water addition than cells that use lead-calcium grids. For most utility applications, lead-calcium is the preferred grid material.
Plate support and sediment space. The overall cell design of a long-life battery must take into account plate growth that will occur over years of float service. The ideal cell design supports positive plates from the top, allowing them to hang above a void space at the bottom of the jar. Plate growth is then directed downward into the void space. If positive plates rest directly on the bottom of the jar, plate growth will be directed upward, eventually stressing the post and cover, leading to cracks and acid leakage. As mentioned previously, it is normal for some shedding of active material to occur over the life of a cell. This material will fall to the bottom of the cell jar and collect as sediment. Insufficient void space below the plates could lead to a short-circuit condition as this sediment collects over time. It is imperative that the cell design allows at least one-half-inch void space between the bottom edge of the positive plates and the cell jar bottom to accommodate both plate growth and sediment collection.
Post seal assembly. The terminal post seal assembly should be free to move or slide against the cover, sealing the inside of the cell to prevent electrolyte leakage using high compression O-rings as illustrated in Figure 2. A simpler design, which bonds the cover directly to the post, is susceptible to compromise under mechanical stresses brought about by plate growth, shipping and handling, or a seismic event.
Regardless of the battery design, proper maintenance procedures are critical to achieving maximum performance and service life. Exposure to heat will significantly decrease the life of a battery, and use of improper float voltage can have a similar effect. Improper electrolyte level maintenance can have catastrophic results. To avoid these problems and ensure optimum battery life, the Institute of Electrical and Electronics Engineers (IEEE) recommends the maintenance schedule shown in Table 1.
Although specified for valve-regulated batteries, IEEE does not specify cell impedance measurements for flooded batteries. Nevertheless, it is good practice to take these readings annually to note any irregular increases in internal resistance before a critical load is applied to the battery.
Yuasa Inc. is manufacturer of advanced batteries and related products for utilities, switchgear, UPS, telecommunications and other backup power applications. Additional information is available at www.yuasainc.com.