PK-001 How does the Lead Acid Battery Work?

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 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 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 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 the 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).

Starter Batteries

The starter battery is designed to crank an engine with a momentary high-power load lasting a second or so. For its size, the battery is able to deliver high current but it cannot be deep-cycled. Starter batteries are rated with Ah or RS (reserve capacity) to indicate energy storage capability, as well as CCA (cold cranking amps) to signify the current a battery can deliver at cold temperature. SAE J537 specifies 30 seconds of discharge at –18°C (0°F) at the rated CCA ampere without the battery voltage dropping below 7.2 volts. RC reflects the runtime in minutes at a steady discharge of 25. (SAE stands for Society of Automotive Engineers.)

Starter batteries have a very low internal resistance that is achieved by adding extra plates for maximum surface area (Figure 1). The plates are thin and the lead is applied in a sponge-like form that has the appearance of fine foam, expanding the surface area further. Plate thickness, which is important for a deep-cycle battery is less important because the discharge is short and the battery is recharged while driving; the emphasis is on power rather than capacity.

Deep-cycle Battery

The deep-cycle battery is built to provide continuous power for wheelchairs, golf cars, forklifts and more. This battery is built for maximum capacity and a reasonably high cycle count. This is achieved by making the lead plates thick (Figure 2). Although the battery is designed for cycling, full discharges still induce stress and the cycle count relates to the depth-of-discharge (DoD). Deep-cycle batteries are marked in Ah or minutes of runtime. The capacity is typically rated as a 5-hour and 20-hour discharge.

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A starter battery cannot be swapped with a deep-cycle battery or vice versa. While an inventive senior may be tempted to install a starter battery instead of the more expensive deep-cycle on his wheelchair to save money, the starter battery would not last because the thin sponge-like plates would quickly dissolve with repeated deep cycling.

There are combination starter/deep-cycle batteries available for trucks, buses, public safety and military vehicles, but these units are big and heavy. As a simple guideline, the heavier the battery is, the more lead it contains, and the longer it will last.?Table 3?compares the typical life of starter and deep-cycle batteries when deep cycled.

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Lead Acid or Li-ion in your Car?

Ever since Cadillac introduced the starter motor in 1912, lead acid batteries served well as battery of choice. Thomas Edison tried to replace lead acid with nickel-iron (NiFe), but lead acid prevailed because of its rugged and forgiving nature, as well as low cost. Now the lead acid serving as starter battery in vehicles is being challenged by Li-ion.

Figure 4?illustrates the characteristics of lead acid and Li-ion. Both chemistries perform similarly in cold cranking. Lead acid is slightly better in W/kg, but Li-ion delivers large improvements in cycle life, better specific energy in Wh/kg and good dynamic charge acceptance. Where Li-ion falls short is high cost per kWh, complex recycling and less stellar safety record than lead acid.

Lead is toxic and environmentalists would like to replace the lead acid battery with an alternative chemistry. Europe succeeded in keeping NiCd out of consumer products, and similar efforts are being made with the starter battery. The choices are NiMH and Li-ion, but the price is too high and low temperature performance is poor. With a 99 percent?recycling rate, the lead acid battery poses little environmental hazard and will likely continue to be the battery of choice.

Table 5?lists advantages and limitations of common lead acid batteries in use today. The table does not include the new lead acid chemistries.

source from Battery University

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