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

In the realm of rechargeable batteries, the lead acid type, created by the French physician Gaston Planté in 1859, stands as the pioneer. Despite its long history, this type of battery remains widely utilized today due to its reliability and economical pricing. When it comes to providing high power at a low cost per watt, few alternatives match the efficiency of lead acid batteries. This is why they are favored in automobiles, golf carts, forklifts, marine applications, and as backup power sources through uninterruptible power supplies (UPS).

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The construction of a lead acid battery's grid structure utilizes a lead alloy. Pure lead is too soft to maintain its structural integrity, so other metals such as antimony, calcium, tin, and selenium are generally added to enhance mechanical strength and optimize electrical performance. Therefore, these batteries are often referred to as "lead-antimony" or "lead-calcium".

In particular, the inclusion of antimony and tin improves deep cycling capabilities but results in increased water consumption and a higher need for equalization. Calcium, while reducing self-discharge, can lead to an increase in the size of the positive lead-calcium plate due to grid oxidation when the battery is overcharged. To mitigate these issues, modern lead acid batteries frequently incorporate doping agents such as selenium, cadmium, tin, and arsenic to minimize the quantities of antimony and calcium.

While lead acid batteries are heavy and generally less robust than their nickel- and lithium-based counterparts when it comes to deep cycling, a complete discharge does influence their lifespan. Each cycle of discharge and recharge gradually reduces the battery's capacity. Initially, this decrease is minor as long as the battery remains in good condition, however, the decline becomes more pronounced when performance drops below half of the original capacity. This degradation pattern is common in various battery types.

Lead acid batteries can manage around 200 to 300 cycles depending on the depth of discharge, but their relatively short lifespan is primarily attributable to the corrosion of the positive electrode grid, depletion of active materials, and the expansion of the positive plates. Elevated temperatures and high discharge currents significantly accelerate this aging process (for further insight, refer to BU-804: How to Prolong Lead Acid Batteries).

Charging lead acid batteries is straightforward, but it is crucial to adhere to the appropriate voltage limits. Lowering the voltage limit can protect the battery but results in diminished performance and a build-up of sulfation on the negative plate. Conversely, increasing the voltage enhances performance while risking grid corrosion on the positive plate. Although sulfation can be reversed with timely maintenance, grid corrosion is irreversible (consult BU-403: Charging Lead Acid for more information).

These batteries do not lend themselves well to rapid charging; typically, a full charge can require between 14 to 16 hours. To avoid sulfation, a lead acid battery should be stored in a fully charged state. Insufficient charging leads to sulfation, which detracts from battery effectiveness. Incorporating carbon into the negative electrode can alleviate some of these issues, albeit at the cost of reduced specific energy (see BU-202: New Lead Acid Systems).

Lead acid batteries are neither the longest lasting nor the most prone to memory effect like nickel-based counterparts, but they do exhibit the best charge retention among rechargeable batteries. While nickel-cadmium batteries can lose around 40% of their stored energy within three months, lead acid batteries only self-discharge an equivalent amount over an entire year. Furthermore, these batteries operate exceptionally well in cold climates, outperforming lithium-ion batteries in subzero conditions. According to research from RWTH Aachen University in Germany, the cost of flooded lead acid stands at about $150 per kWh, one of the lowest within battery technologies.

Sealed Lead Acid Batteries

The introduction of sealed or maintenance-free lead acid batteries occurred in the mid-1980s. Engineers criticized the use of "sealed lead acid" as misleading because complete sealing is practically impossible in lead acid batteries. These batteries incorporate valves designed to release gases generated during high-pressure conditions due to charging or rapid discharge. Instead of being submerged in an electrolyte, they utilize a moistened separator to retain the electrolyte, permitting operation in any orientation without risk of leakage.

This sealed format uses less electrolyte than flooded batteries, hence referred to as "acid-starved". A significant benefit of sealed lead acid batteries is their ability to recombine oxygen and hydrogen into water, preventing electrolytic depletion during cycles. This recombination occurs under moderate pressure (0.14 bar or 2 psi), with the valve functioning as a safety vent for excessive gas build-up. Avoiding frequent venting is essential as it can eventually lead to drying out. RWTH Aachen's research notes the cost of valve-regulated lead acid (VRLA) batteries at approximately $260 per kWh.

Among the various sealed lead acid types, gel, also known as valve-regulated lead acid (VRLA), and absorbent glass mat (AGM) are the most prevalent. Gel cells incorporate a silica gel to suspend the electrolyte within a paste. Smaller batteries with capabilities up to 30Ah are often termed sealed lead acid (SLA). Housed in plastic, these batteries serve small UPS systems, emergency lighting, and have widespread utilization in healthcare settings due to their affordability and reliability. Meanwhile, larger VRLA batteries are deployed as power backups for infrastructures like cellular towers, banks, hospitals, and airports.

The AGM design suspends the electrolyte within a specially crafted glass mat, yielding various advantages, such as quicker charging times and instant high-load current support. AGM batteries are apt for mid-range applications, typically holding capacities from 30 to 100Ah; however, they are not well-suited for larger setups like UPS systems. Their standard applications cover starter batteries for motorcycles and micro-hybrid cars, as well as marine and RV systems that benefit from some cycling.

Over time, the capacity of AGM batteries diminishes gradually, whereas gel batteries exhibit a dome-shaped performance curve, sustaining high performance for extended periods before a sudden drop-off at the end of their life span. AGM batteries tend to be pricier than flooded types, but cheaper than gel variants (the latter's costs are excessive for starter-stop functions in vehicles).

In contrast to flooded batteries, sealed lead acid batteries are engineered to prevent reaching gas-generating potentials during charging by maintaining lower over-voltage conditions. Excess charging can lead to gas generation, venting, and subsequent depletion of the liquid, causing dry-out issues. Consequently, gel and, to an extent, AGM batteries cannot be charged to their full potential, which necessitates lower voltage limitations compared to the flooded variety. This charging land limitation is equally relevant for the float charge at full capacity. When no dedicated AGM charger that accommodates lower voltage settings is available, it’s advisable to disconnect the charger after a 24-hour charging period to prevent excessive gas generation due to elevated float voltage settings (see BU-403: Charging Lead Acid).

For an optimal lifespan, a VRLA battery ideally operates at 25°C (77°F); exceeding this temperature by 8°C (15°F) can halve the battery life expectancy (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, functioning best when subjected to slow discharges. A gradual discharge reveals considerably higher capacity readings compared to discharging at the 1C rate. Nevertheless, lead acids can produce high pulse currents of several C, albeit only over brief periods, making them ideal as starter batteries, otherwise referred to as starter-light-ignition (SLI) systems. The dense lead composition combined with sulfuric acid renders lead acid batteries less environmentally friendly.

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

Starter Batteries Explained

Starter batteries are engineered to initiate an engine by providing a momentary, high-power load lasting mere seconds. These batteries boast high current delivery relative to their size but are not designed for deep cycling. They are rated based on Ah or reserve capacity (RC) to represent energy storage capabilities and cold cranking amps (CCA) to signify the current delivered at low temperatures. The Society of Automotive Engineers (SAE) J537 standard outlines a 30-second discharge at -18°C (0°F) at rated CCA without voltage levels dropping below 7.2 volts. RC indicates performance duration, measured in minutes under steady discharge.

Starter batteries achieve low internal resistance by incorporating additional plates which maximize surface area. The plates are characterized by their thinness, and lead is applied in a sponge-like, foamed manner, which further increases the surface area. This plate thickness, crucial for deep-cycle batteries, becomes less significant for starter batteries because the discharge phase is brief, followed by recharging during vehicle operation—emphasizing power over capacity.

Figure 1: Starter battery.
The starter battery has many thin plates in parallel to achieve low resistance with high surface area.
The starter battery does not allow deep cycling. Courtesy of Cadex

Deep-Cycle Battery Defined

Deep-cycle batteries are crafted to deliver steady power for devices such as wheelchairs, golf carts, and forklifts. They are built for maximum capacity and offer a considerable cycle count by incorporating thicker lead plates.

Figure 2: Deep-cycle battery.
The deep-cycle battery has thick plates for improved cycling abilities.
The deep-cycle battery generally allows about 300 cycles. Courtesy of Cadex

It’s important to note that swapping a starter battery with a deep-cycle variant, or vice versa, is not feasible. An inventive user attempting to install a starter battery in a wheelchair instead of the costlier deep-cycle battery will find rapid deterioration due to the thin sponge-like plates becoming compromised from repeated deep cycling.

Combination starter/deep-cycle batteries are available for trucks, buses, and public safety vehicles. However, these units tend to be large and heavy. A simple guideline to remember is: the heavier the battery, the more lead is contained within, typically translating to a longer lifespan. Table 3 shows a comparison of the lifespans of starter and deep-cycle batteries under deep cycling conditions.

Depth of Discharge

Starter Battery

Deep-Cycle Battery

100%

12-15 cycles

150-200 cycles

50%

100-120 cycles

400-500 cycles

30%

130-150 cycles

1,000 and more cycles

Table 3: Cycle performance of starter and deep-cycle batteries.
A discharge of 100% refers to a full discharge; 50% is half and 30% is a moderate discharge with 70% remaining.

Choosing Between Lead Acid and Li-ion for Your Vehicle

Since Cadillac launched the starter motor in the 1910s, lead acid batteries have been a reliable choice. Although Thomas Edison experimented with replacing lead acid batteries with nickel-iron options, lead acid remains favored for its robustness, forgiving nature, and affordability. Today, however, lead acid starter batteries face competition from Li-ion alternatives.

Figure 4 presents a comparison of lead acid and Li-ion characteristics. Both chemistries perform comparably during cold cranking conditions, with lead acid showing slight superiority in watt/kg performance. However, Li-ion excels in cycle life, specific energy in watt-hours/kg, and dynamic charge acceptance. The downsides of Li-ion include higher costs per kWh, intricate recycling processes, and a less favorable safety history compared to lead acid.

Figure 4: Comparison of lead acid and Li-ion as starter battery.
Lead acid maintains a strong lead in starter battery. This is credited to good cold temperature performance, low costs, a solid safety record, and ease of recycling.

Environmental concerns about lead poisoning have led advocates to seek alternatives to lead acid batteries. Europe has effectively prevented NiCd from entering consumer goods, and similar initiatives are emerging for starter batteries. While choices including NiMH and Li-ion exist, their costs are prohibitive, and they struggle with cold temperature performance. With a recycling rate of 99%, the lead acid battery presents minimal environmental hazards and is poised to remain the battery of choice.

Table 5 outlines the pros and cons of widely used lead acid batteries today, excluding new chemistries (see BU-202: New Lead Acid Systems).

Advantages

• Cost-effective and straightforward to produce; low cost per watt-hour
• Minimal self-discharge rate; lowest among rechargeable batteries
• High specific power, capable of delivering significant discharge currents
• Consistent performance at both low and high temperatures

Limitations

• Low specific energy; poor ratio of weight to energy
• Lengthy charging times; a full charge can take 14-16 hours
• Must consistently maintain a charged state to prevent sulfation
• Limited cyclical lifespan; excess deep cycling reduces longevity
• Flooded types necessitate regular watering
• Transportation hazards apply to flooded batteries
• Not environmentally friendly

Table 5: Advantages and limitations of lead acid batteries.
Dry systems boast advantages over flooded types but exhibit lesser durability.