In a conventional lead-acid battery, the grid plate is cast from an alloy of lead and up to 5-12% antimony. (Some manufacturers use arsenic.) Adding the antimony to the lead (to be sure, when a substances, mostly metal, is added to another metal, an alloy is formed) strengthens the soft lead, improves adhesion of active mass and protects the lead against corrosion. They also serve a purpose in the manufacturing process; they help achieve mechanical strength, handling and casting during production and maintain the rigidity and structural integrity of the casted plates, thus improving the quality and yields during the manufacturing process.
However, while the antimony helps stabilize the active material of the positive electrode improving the cycle life of the battery, it migrates to the negative plate where it is precipitated and impacts the charge voltage output of the battery as well as self-discharge and increased water loss from the battery.
As a consequence, most battery manufacturers tried to minimize or even eliminate the antimony addition especially in batteries for stationary applications where smaller demands are made in respect to cycle service. The two responses are lead-calcium and lead-selenium alloy batteries.
Lead-Calcium Batteries
Calcium batteries replaces antimony in the plates of the battery to give it an improved resistance to corrosion, no excessive gassing, less water consumption over the life of the battery in addition to lowering the battery's self-discharge characteristics. Silver is another additive used by some manufacturers,; it enable the battery to be more resilient to high temperatures. Tin and aluminium are also being added by manufacturers to the calcium to stabilize it.
They are usually sealed and maintenance-free. They require a higher charge voltage than conventional batteries. When used in a deep-cycle situation, a charger designed for calcium batteries or one that has a calcium charging mode has to be used to get the maximum life out of the battery.
Lead-Selenium Batteries
In the case where selenium is added to the alloy compositions, it helps to stabilize the antimony, and leads to a more hardened lead with a finer, denser grain structure. This grain refinement creates a lead alloy that is more corrosion-resistant in addition to reducing inter-granular corrosion.
The lead alloy selenium alloy combines the advantages of both the calcium and lead-antimony alloys while reducing the disadvantages. Lead-selenium batteries have the following benefits: good grid density, conductivity and tensile strength; reduced water losses; super cycling and deep discharge performance.
Disadvantages
One of the major features of lead-alloy batteries is that they come in transparent jars. The growth of the positive plate due to the precipitation of corrosive lead compound at the grain lead boundaries can lead to the failure of the jar casing and the seals of its terminals.
Batteries using these two alloys also exhibit out-gassing during normal charge/discharge conditions primarily in the form of oxygen and hydrogen. Also, under extreme voltage conditions or excessive charge, acid vapour can be generated. This is a major concern that requires adequate ventilation precautions during their installation to avoid the potential issue of hazardous gas evolution and explosion. However, being valve-regulated, a vent is provided to allow hydrogen to escape and prevent a dangerous build up of pressure inside the battery.
General Features
They come in transparent or translucent jars that allow the visual inspection of the plates.
These types of lead-alloy batteries come in 2V and have to be connected in series to produce higher voltages.
They are designed to stand in a vertical orientation in order to conserve floor space.
They are also maintenance-free. Serviceable batteries have removable caps so the electrolyte can be checked regularly. Maintenance-free batteries are sealed for life and the water level does not need to be maintained.
They also have a built in hydrometer giving some indication of the charge in one cell.
They are designed to have an operational life of about 10 years.
Most importantly, they are Front Access Terminal type batteries. As the name 'front access' directly implies, all the terminals are placed where they can be reached easily for both during installation and maintenance. They are built to be connected in a row using just a small solid interconnecting bar, rather than varying lengths of cable or the long solid bar connectors required for top terminal batteries.
Lead-alloy batteries are not necessarily superior to the conventional lead-acid types. But they are designed for very harsh operating conditions and are far better suited for use in stationary storage situations and switchgear applications like in the telecomm industry, in railway signals, in emergency systems or in industrial and manufacturing lines, and in petrochemical and processing plants.
SolarKobo helps its clients in Lagos and throughout Nigeria make the best possible choice of batteries and other power storage and switchgear systems that fits their budget and meets their power needs.