Are sealed lead-acid batteries safe to use indoors?

Are sealed lead-acid batteries safe to use indoors?

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Sealed Lead-Acid (SLA) batteries, often referred to as valve-regulated lead-acid (VRLA) batteries, do indeed have a vent. Thus, the term "sealed" can be somewhat misleading. Every uninterruptible power supply (UPS) I've come across contains one or more SLAs, which confirms their general safety for indoor use. Here’s a snippet from a white paper by APC:

Valve regulated lead acid (VRLA) batteries [...] do not require special battery rooms and are suitable for use in an office environment. Air changes designed for human occupancy normally exceed the requirements for VRLA [...] ventilation. Vented (flooded) batteries, which release hydrogen gas continuously, require a dedicated battery room with ventilation separate from the rest of the building.

The difference in gas output is substantial for VRLA batteries:

VRLA batteries are considered to be “sealed” because they normally do not allow for the addition or loss of liquid. A vented battery can give off sixty times more gas than a VRLA battery in normal use.

The recombination of hydrogen and oxygen into water within a "sealed" or VRLA battery minimizes gas emissions. Gas only escapes when internal pressure surpasses the pressure valve's rating. This pressurization is why under severe misuse or abuse, these batteries can visibly bulge. The plastic container is designed to withstand such conditions.

Smaller SLAs, like the ones most households may use, typically employ a gel as an electrolyte suspender, safeguarding against spills, even when cracked. Larger SLAs utilize a glass mat instead, as gel can be more costly.

BU-201: How does the Lead Acid Battery Work?

Invented by French physician Gaston Planté in 1859, lead-acid was the first rechargeable battery for commercial use. Despite its age, lead-acid remains popular today for its dependability and cost-effectiveness, offering a low cost-per-watt ratio. Few batteries deliver bulk power as cheaply as lead-acid, making it ideal for automobiles, golf carts, forklifts, marine applications, and uninterruptible power supplies (UPS).

The grid structure of a lead-acid battery comprises a lead alloy. Pure lead is too soft to support itself, so small amounts of other metals like antimony, calcium, tin, and selenium are added to enhance mechanical strength and electrical properties. These batteries often go by the names "lead-antimony" or "lead-calcium."

Adding antimony and tin improves deep cycling but also increases water consumption and the necessity to equalize. Conversely, calcium minimizes self-discharge, although the positive lead-calcium plate tends to grow due to grid oxidation when overcharged. Modern lead-acid batteries incorporate doping agents like selenium, cadmium, tin, and arsenic to lower antimony and calcium content.

Lead-acid batteries are heavy and less durable than nickel- and lithium-based systems when deep-cycled. Each full discharge inflicts strain, gradually diminishing battery capacity. While the loss is minimal when the battery is in good condition, it accelerates as performance drops to half the nominal capacity. This wear-down applies to all batteries but varies in degrees.

Depending on the depth of discharge, lead-acid batteries for deep-cycle applications offer 200 to 300 discharge/charge cycles. The short cycle life owes to grid corrosion on the positive electrode, depletion of active material, and positive plate expansion. Elevated operating temperatures and high discharge currents further hasten aging. (See BU-804: How to Prolong Lead Acid Batteries).

Charging a lead-acid battery is straightforward, but adhering to voltage limits is crucial. A low voltage limit preserves the battery, although it leads to sulfation buildup on the negative plate. A high voltage limit enhances performance but causes grid corrosion on the positive plate. While sulfation can be reversed if addressed in time, corrosion is irreversible. (See BU-403: Charging Lead Acid).

Lead-acid batteries are not conducive to fast charging. Achieving full charge typically requires 14-16 hours. The battery must always be stored fully charged to prevent sulfation, which hinders performance. Adding carbon to the negative electrode mitigates this issue but reduces specific energy. (See BU-202: New Lead Acid Systems).

While lead-acid batteries offer moderate lifespans and lack the memory effect seen in nickel-based systems, their charge retention ranks highest among rechargeable batteries. For instance, NiCd batteries lose approximately 40 percent of their stored energy in three months, whereas lead-acid batteries self-discharge the same amount in a year. Lead-acid performs well in cold temperatures and outshines lithium-ion under subzero conditions. According to RWTH, Aachen, Germany (2018), flooded lead-acid costs about $150 per kWh, making it one of the most economical battery options.

Sealed Lead Acid

The first sealed or maintenance-free lead-acid batteries appeared in the mid-1970s. "Sealed lead-acid" is somewhat of a misnomer since no lead-acid battery is entirely sealed. To manage venting during stressful charging and rapid discharge, valves are incorporated to release gases if pressure builds up. Unlike traditional flooded batteries, sealed designs saturate the electrolyte into a moistened separator, akin to nickel- and lithium-based systems. This design prevents leakage and permits operation in various orientations.

The sealed battery design uses less electrolyte than flooded types, hence the term "acid-starved." Perhaps the most significant advantage is its ability to recombine oxygen and hydrogen into water, ensuring the battery doesn't dry out during cycling. Recombination occurs at a moderate pressure of 0.14 bar (2 psi), with the valve acting as a safety vent for gas buildup. Frequent venting should be avoided to prevent dry-out. As of 2018, RWTH Aachen estimates VRLA costs at about $260 per kWh.

Several types of sealed lead-acid batteries exist, with gel and absorbent glass mat (AGM) being the most common. Gel cells use a silica-based gel to suspend the electrolyte, forming a paste. Smaller units up to 30Ah are often labeled as SLA (sealed lead-acid) batteries, enclosed in plastic, and utilized in small UPS systems, emergency lighting, and wheelchairs. Due to low cost, reliable service, and low maintenance, SLA batteries are widely used in healthcare settings like hospitals and nursing homes. Larger VRLA batteries provide backup power for cell towers, internet hubs, banks, hospitals, airports, and other critical infrastructure.

AGM batteries suspend the electrolyte in a specially designed glass mat, offering several advantages, including faster charging and the ability to deliver instant high-load currents. AGM batteries are best suited for mid-range applications, typically ranging from 30 to 100Ah, and are less ideal for large systems like UPS. Common uses include starter batteries for motorcycles, start-stop functions in micro-hybrid cars, and marine and RV applications requiring some cycling.

AGM batteries experience gradual capacity fade with age and use, while gel batteries exhibit a more stable performance curve, maintaining high performance levels longer before a sudden drop at the end of life. AGM is pricier than flooded batteries but cheaper than gel, making gel less practical for applications like start/stop systems in vehicles.

Unlike flooded batteries, sealed lead-acid batteries are designed with low over-voltage potential to prevent gas generation during charging. Excessive charging leads to gassing, venting, water depletion, and dry-out. Consequently, gel and AGM batteries cannot be charged to their full potential, necessitating lower charge voltage limits than flooded batteries. This also applies to float charges. If a specific charger for AGM with lower voltage settings is unavailable, it's wise to disconnect the charger after 24 hours to avoid gassing due to a high float voltage. (See BU-403: Charging Lead Acid).

To learn more, visit our website for Features Of Sealed Lead Acid Batteries.

The optimal operating temperature for a VRLA battery is 25°C (77°F). Every 8°C (15°F) increase above this threshold halves the battery’s lifespan. (See BU-806a: How Heat and Loading affect Battery Life). Lead-acid batteries are rated at 5-hour (0.2C) and 20-hour (0.05C) discharge rates, performing best with slower discharges. Despite low discharge efficiency, lead-acid batteries can emit high pulse currents for a few seconds, making them well-suited for starter-light-ignition (SLI) applications. However, the high lead and sulfuric acid content makes lead-acid environmentally unfriendly.

Lead-acid batteries fall into three primary categories: automotive (starter or SLI), motive power (traction or deep cycle), and stationary (UPS).

Starter Batteries

Starter batteries are designed to crank engines with momentary high-power loads. They deliver high current for their size but aren't built for deep cycling. These batteries are rated in Ah or RS (reserve capacity) to indicate their energy storage capability. Cold Cranking Amps (CCA) denotes the current a battery can deliver at cold temperatures. SAE J537 specifies 30 seconds of discharge at −18°C (0°F) at the rated CCA amperage without the voltage dropping below 7.2 volts. RC reflects runtime in minutes at a steady discharge of 25 amperes. These batteries achieve low internal resistance with thin plates and sponge-like lead to maximize surface area.

Deep-cycle Battery

Deep-cycle batteries provide continuous power for devices like wheelchairs, golf carts, and forklifts. Designed for maximum capacity and cycle count, they feature thick lead plates to handle deep discharge cycles. However, full discharges induce stress, and cycle counts correlate with depth-of-discharge (DoD). These batteries are rated in Ah or minutes of runtime, typically for 5-hour and 20-hour discharges.

Starter batteries should not be used in place of deep-cycle batteries. Despite potential cost-saving, starter batteries would quickly deteriorate with repeated deep cycling. Combination starter/deep-cycle batteries are available for trucks, buses, and military vehicles but are larger and heavier. Generally, the heavier the battery, the more lead it contains and the longer it will last.

Lead Acid or Li-ion in your Car?

Since Cadillac introduced the starter motor in 1912, lead-acid batteries have been the go-to choice for vehicles. Thomas Edison's attempt to replace lead-acid with nickel-iron (NiFe) failed due to lead-acid’s rugged, forgiving nature and low cost. Today, Li-ion batteries challenge lead-acid for vehicle use.

Lead-acid and Li-ion perform similarly in cold cranking. Lead-acid excels in specific power, while Li-ion offers better cycle life, specific energy, and dynamic charge acceptance. However, Li-ion's high cost, complex recycling, and safety concerns make it less appealing compared to lead-acid.

If you have further questions, connect with us to discuss What is an AGM battery?. Our knowledgeable sales team is ready to assist you in identifying the best options for your needs.