A lead-acid battery is a rechargeable battery that uses lead plates and sulfuric acid to store and release electrical energy. Invented in 1859, it’s the oldest type of rechargeable battery still in widespread use, powering everything from car engines to backup power systems and golf carts. Its enduring popularity comes down to reliability, low cost, and a nearly perfect recycling rate.
How It Works
Inside a lead-acid battery, two types of lead plates sit submerged in a solution of sulfuric acid and water (the electrolyte). One plate is made of spongy, porous lead. The other is coated with lead oxide. A thin membrane between them prevents the plates from touching while still allowing the acid to flow freely.
When the battery discharges, both plates react with sulfate from the surrounding acid, and both gradually become coated in lead sulfate crystals. This chemical conversion releases electrons, which flow through whatever circuit you’ve connected, powering your starter motor, lights, or equipment. At the same time, the sulfuric acid gets diluted because its sulfate is being consumed, so the electrolyte becomes weaker as the battery drains.
Charging reverses the process. Electrical current strips the lead sulfate off the plates, converting the negative plate back to pure lead and the positive plate back to lead oxide. The sulfate returns to the electrolyte, restoring its concentration. This back-and-forth is why lead-acid batteries can be recharged hundreds of times.
Voltage and State of Charge
Each cell in a lead-acid battery produces roughly 2 volts. A standard car battery has six cells wired together, giving it 12 volts total. You can gauge how much charge remains by measuring the voltage when the battery isn’t connected to anything. A cell reading 2.10 volts at room temperature is about 90% charged. If a cell drops below 2.05 volts during storage, the battery starts to degrade, which is why periodic top-up charging is recommended at least every six months for batteries sitting idle.
Flooded vs. Sealed Batteries
Lead-acid batteries come in two broad construction styles: flooded (also called vented or VLA) and sealed (called VRLA, for valve-regulated lead-acid).
Flooded batteries are the traditional design. The plates sit in liquid sulfuric acid, and you need to periodically add distilled water as the electrolyte evaporates or gets consumed during charging. They’re bulkier and must be kept upright to avoid spills. In return, they tend to last significantly longer, with lifespans reaching up to 20 years in stationary applications like data center backup power.
Sealed VRLA batteries eliminate the liquid maintenance. The electrolyte is immobilized so it can’t slosh or spill, and a pressure valve handles any gas buildup. There are two subtypes. AGM (absorbed glass mat) batteries trap the electrolyte in a sponge-like mat of woven glass fiber. Gel batteries mix the acid with a silicate additive that turns it into a thick, gel-like consistency. Both types are lighter, more portable, and don’t need watering. The tradeoff is a shorter lifespan of roughly 5 to 10 years and more frequent replacements, though upfront costs are lower.
Starting Batteries vs. Deep Cycle Batteries
Beyond construction style, lead-acid batteries are designed for different jobs. The two main categories are SLI (starting, lighting, ignition) and deep cycle.
SLI batteries are what you’ll find under the hood of most cars. They’re built to deliver a massive burst of current for a few seconds to crank the engine, then immediately get recharged by the alternator. Their internal lead plates are thin, which maximizes surface area for that quick energy dump. They shouldn’t be drained below 50% of their total capacity, because deep discharges damage the thin plates and shorten their life.
Deep cycle batteries have thicker lead plates and are engineered to provide a steady, moderate flow of power over several hours. They can handle being drained to 50% to 80% of capacity without significant damage. This makes them the standard choice for golf carts, electric forklifts, marine trolling motors, wheelchair power, and off-grid solar energy storage. Using an SLI battery in a deep-cycle application (or vice versa) will result in premature failure.
Sulfation and Battery Degradation
The most common way lead-acid batteries fail is through a process called sulfation. During normal discharge, lead sulfate crystals form on both plates. In a healthy battery, charging dissolves those crystals completely. But if a battery sits discharged for too long, gets chronically undercharged, or operates in high heat, the lead sulfate crystals harden into a dense, white coating that resists being converted back. This buildup blocks the chemical reactions the battery depends on, reducing capacity and eventually killing the battery entirely.
The practical takeaway: keep lead-acid batteries charged. If you’re storing one, top it off every six months. If you notice a battery that used to hold a charge for hours now drains quickly, sulfation is the likely culprit.
Safety Considerations
Lead-acid batteries carry two main hazards. The first is hydrogen gas. During charging, the chemical reactions produce hydrogen, which is explosive at concentrations between 4.1% and 72% in air. Any space where batteries are being charged needs adequate ventilation to keep hydrogen well below dangerous levels. Batteries should never be charged near open flames, sparks, or cigarettes.
The second hazard is the sulfuric acid electrolyte itself, which causes chemical burns on contact with skin or eyes. Flooded batteries pose the higher risk here because the acid is liquid and can splash. Sealed batteries largely contain this risk, though damaged casings can still leak.
Proper handling also matters because of the lead content. Lead is toxic, but the good news is that lead-acid batteries are the most recycled consumer product in the United States. According to the EPA, 99% of lead-acid batteries are recycled each year. The lead, acid, and plastic casing are all recovered and reused to manufacture new batteries, making this one of the most successful examples of a circular recycling system.
Common Applications
- Automotive: Nearly every gasoline and diesel vehicle uses a lead-acid SLI battery for starting.
- Uninterruptible power supplies (UPS): Sealed VRLA batteries back up servers, hospitals, and telecom equipment during outages.
- Off-grid and solar energy storage: Deep cycle flooded batteries store energy from solar panels for use at night or during cloudy periods.
- Industrial vehicles: Forklifts, floor scrubbers, and airport ground support vehicles run on large deep cycle battery packs.
- Marine and RV: Deep cycle batteries power onboard electronics, trolling motors, and accessories independently from the engine.
Despite growing competition from lithium-ion technology, lead-acid batteries remain dominant in applications where weight isn’t critical, cost matters, and recycling infrastructure is established. Their chemistry is well understood, replacement parts are universally available, and the recycling loop is nearly closed, keeping the environmental footprint lower than their lead content might suggest.