Sealed Lead Acid
The first sealed, or maintenance-free, lead-acid emerged in the mid-1970s. Engineers argued that the term “sealed lead acid” was a misnomer because no lead acid battery can be totally sealed. To control venting during stressful charge and rapid discharge, valves have been added that release gases if pressure builds up. Rather than submerging the plates in a liquid, the electrolyte is impregnated into a moistened separator, a design that resembles nickel- and lithium-based systems. This enables operating the battery in any physical orientation without leakage.
The sealed battery contains less electrolyte than the flooded type, hence the term “acid-starved.” Perhaps the most significant advantage of sealed lead acid is the ability to combine oxygen and hydrogen to create water and prevent dry out during cycling. The recombination occurs at a moderate pressure of 0.14 bar (2psi). The valve serves as a safety vent if the gas buildup rises. Repeated venting should be avoided as this will lead to an eventual dry-out. According to RWTH, Aachen, Germany (2018), the cost of VRLA is about $260 per kWh.
Several types of sealed lead acid have emerged and the most common are gel, also known as valve-regulated lead-acid (VRLA), and absorbent glass mat (AGM). The gel cell contains a silica type gel that suspends the electrolyte in a paste. Smaller packs with capacities of up to 30Ah are often called SLA (sealed lead acid). Packaged in a plastic container, these batteries are used for small UPS, emergency lighting and wheelchairs. Because of low price, dependable service and low maintenance, the SLA remains the preferred choice for healthcare in hospitals and retirement homes. The larger VRLA is used as a power backup for cellular repeater towers, Internet hubs, banks, hospitals, airports and more.
The AGM suspends the electrolyte in a specially designed glass mat. This offers several advantages to lead-acid systems, including faster charging and instant high load currents on demand. AGM works best as a mid-range battery with capacities of 30 to 100Ah and is less suited for large systems, such as UPS. Typical uses are starter batteries for motorcycles, start-stop function for micro-hybrid cars, as well as marine and RV that need some cycling.
With cycling and age, the capacity of AGM fades gradually; gel, on the other hand, has a dome-shaped performance curve and stays in the high-performance range longer but then drops suddenly towards the end of life. AGM is more expensive than flooded but is cheaper than gel. (Gel would be too expensive for start/stop use in cars.)
Unlike the flooded, the sealed lead acid battery is designed with a low over-voltage potential to prohibit the battery from reaching its gas-generating potential during charge. Excess charging causes gassing, venting, and subsequent water depletion and dry-out. Consequently, gel, and in part also AGM, cannot be charged to their full potential and the charge voltage limit must be set lower than that of a flooded. This also applies to the float charge on a full charge. In respect to charging, the gel and AGM are no direct replacements for the flooded type. If no designated charger is available for AGM with lower voltage settings, disconnect the charger after 24 hours of charge. This prevents gassing due to a float voltage that is set too high.
The optimum operating temperature for a VRLA battery is 25°C (77°F); every 8°C (15°F) rise above this temperature threshold cuts the battery life in half. Lead-acid batteries are rated at a 5-hour (0.2C) and 20-hour (0.05C) discharge rate. The battery performs best when discharged slowly; the capacity readings are substantially higher at a slower discharge than at the 1C-rate. Lead-acid can, however, deliver high pulse currents of several C if done for only a few seconds. This makes the lead-acid well suited as a starter battery, also known as starter-light-ignition (SLI). The high lead content and sulfuric acid make lead-acid environmentally unfriendly.
Lead-acid batteries are commonly classified into three usages: Automotive (starter or SLI), motive power (traction or deep cycle) and stationary (UPS).
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