What Are Lead-Acid Batteries?

What Are Lead-Acid Batteries? The lead–acid battery is the oldest practical rechargeable battery, with a history dating back to the mid-19th century. Here's how they work, and what they are used for today.

Written by 
Andrew Hagan
Edited by 
Victoria Chen-Englert
Updated on 
March 20, 2024
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📖 This post is part of our Batteries 101 series 📖

1. Quick Intro: What Are Lead-Acid Batteries?

The lead–acid battery is the oldest practical rechargeable battery, with a history dating back to the mid-19th century. This battery type played a crucial role in the development of early electrical power systems and remains widely used today on account of its reliability, low cost, and ability to deliver high surge currents.

2. History of Lead–Acid Batteries

The first lead-acid battery was invented by French physicist Gaston Planté in 1859, featuring a simple yet effective design with two lead sheets separated by rubber strips and immersed in sulfuric acid. This innovation marked the creation of the first viable secondary battery that could be recharged, paving the way for advancements in electrical storage technologies. Planté's initial design was later improved upon by Camille Alphonse Faure in 1881, who coated lead grids with a paste of lead oxides to enhance battery capacity. Subsequent commercialization efforts by pioneers like Henri Tudor led to the widespread adoption of lead-acid batteries in applications such as electric lighting systems and early car batteries.

An early Planté battery cell composed of two lead plates rolled into a spiral. Image source: Popular Science Monthly Volume 38, public domain, via Wikimedia Commons

3. How Lead–Acid Batteries Work

Basic Battery Design

Like all batteries, lead–acid batteries use electrochemical reactions to convert chemical energy into electrical energy. Modern lead–acid batteries for automotive applications typically contain six cells that together provide a total voltage of 12.6 V when fully charged.

Each of these cells contains the following components:

  • A positive electrode (cathode) primarily composed of lead dioxide
  • A negative electrode (anode) made of spongy lead
  • An electrolyte solution of sulfuric acid and water, in which both electrodes are immersed

The two lead-based electrodes also usually contain small amounts of other elements, including calcium, tin, antimony, and/or selenium, which improve their mechanical and performance characteristics such as rigidity and lifespan. Separators, nowadays usually based on plastic, are inserted between the plates to prevent physical contact that would otherwise lead to a short circuit while allowing ions to pass through, and the entire assembly is enclosed in a plastic casing.

Electrochemical Reactions

Here's a simplified description of the electrochemical reactions occurring during discharging and charging:

  • Discharging: During discharge, the spongy lead of the anode reacts with sulfuric acid to form lead sulfate, positively charged hydrogen ions, and negatively charged electrons, while the lead dioxide of the cathode reacts with sulfuric acid, hydrogen ions, and electrons to produce lead sulfate and water. These two reactions drive a flow of electrons from the anode to the cathode through the external circuit, providing electrical power.
  • Charging: Applying an external electrical current to the battery reverses these reactions, converting the lead sulfate at the cathode back into lead dioxide, the lead sulfate at the anode back into spongy lead, and the mostly aqueous electrolyte back into sulfuric acid, thus replenishing the battery's capacity.

4. Types of Lead–Acid Batteries

Starting/Cranking Batteries vs. Deep-Cycle Batteries

Nowadays, there are two main types of lead–acid batteries, which vary in terms of both design and intended applications:

  • Starting/cranking batteries: These utilize a large number of thin plates to supply the brief surge of high electric current needed to start an engine.
  • Deep-cycle batteries: These are designed to provide a lower current over longer periods by using a smaller number of thicker plates for sustained energy delivery.
A modern lead–acid car battery (sealed wet-cell type). Image source: Jack.randle on Wikimedia Commons, distributed under a CC BY-SA 4.0 license

Flooded Batteries -- Must be Topped Up with Water

Moreover, technological advances in recent decades have resulted in additional designs of lead–acid batteries. Technically, the basic format described above using free-flowing aqueous sulfuric acid as the electrolyte is known as a flooded or wet-cell lead–acid battery. While common and affordable, these batteries must be installed vertically and periodically topped up with water.

"Maintenance-Free" Gel and Absorbent Glass Mat (AGM) Batteries

These issues prompted the development of "maintenance-free" and "sealed" batteries. Although both of these descriptions are something of a misnomer (all batteries require some degree of maintenance, and complete sealing would not be safe on account of the gases generated during overcharging), these batteries do not require regular top-ups and can be installed in other orientations besides vertical.

A better term is valve-regulated lead–acid battery, where relief valves allow for excess pressure to be safely released and the sulfuric acid electrolyte is either made into a gel by mixing with silica or incorporated into absorbent mats woven from glass fiber. These are referred to as gel batteries and absorbent glass mat (AGM) batteries, respectively. These designs provide superior performance to wet-cell batteries, albeit with the compromise of higher cost.

5. Applications of Lead–Acid Batteries

Primary Use Cases

Despite being developed over 150 years ago, lead–acid batteries still see a variety of applications. Some primary use cases are as follows:

  • Automotive: Lead–acid starting/cranking batteries are extensively used in cars owing to their ability to deliver the high surge current necessary to start an internal combustion engine. Deep-cycle batteries are also employed in some small vehicles such as golf carts and motorized wheelchairs, although lithium-ion batteries are increasingly common.
  • Backup power supplies: The reliability and cost-effectiveness of lead–acid batteries make them suitable for use in uninterruptible power supplies (UPS) for hospitals, cellphone towers, and data centers.
  • Energy storage: Lead–acid batteries are still used for stationary energy storage in renewable energy systems, such as solar and wind power installations, although newer technologies such as sodium-ion batteries are beginning to compete in this space.
  • Industrial and marine: Lead–acid batteries power electric forklifts and industrial machinery, as well as providing electricity for boats and marine equipment.

Environmental Impacts and Recycling

Despite the widespread use of lead–acid batteries, potential environmental impacts include the toxicity of lead and the corrosive nature of sulfuric acid. However, recycling initiatives and regulations are in place to mitigate these. In actual fact, lead–acid batteries are highly recyclable, thus offsetting at least some of the concerns regarding the hazards of their constituent materials.

The spent lead and plastic components can be smelted and reused, while the sulfuric acid can be either neutralized and released or reclaimed. According to Battery Council International, the recycling rate for lead batteries in the United States is 99%, far exceeding that of other battery types and other products in general.

Future Alternatives

With ongoing advances in battery technology, alternatives such as lithium- and sodium-ion batteries are becoming more prevalent in many applications on account of their higher energy densities and longer lifespans. However, at least for now, lead–acid batteries remain a critical component of our electrical systems owing to their proven reliability and cost-effectiveness for certain applications.

6. Conclusion

From their invention in the 19th century to their prevalent use today, lead–acid batteries have been a cornerstone of energy storage. Their simple yet effective operation and recyclability have ensured their place in multiple applications. Although newer technologies may offer higher efficiency or energy density, the reliability, cost-effectiveness, and well-established manufacturing and recycling infrastructure make lead–acid batteries a vital component of modern energy solutions.

As we advance into an era of more sustainable energy consumption, the role of lead–acid batteries, particularly in vehicles, renewable energy storage, and backup power, is likely to evolve. Nonetheless, the legacy of lead–acid batteries as a pioneering and enduring energy solution is secure, demonstrating their fundamental role in powering our past, present, and perhaps — for the time being at least — our future.

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