Lead Acid Batteries: How They Work, Types and Care

8 min read

THE OLDEST BATTERY

Why Lead Acid Batteries Still Start Your Car

Car battery secured in an engine bay with covered terminals. Illustrative image

There's one in almost every car you've ever ridden in, in the boats at the marina, and in plenty of off-grid cabins.

It's a lead acid battery, and a French scientist named Gaston Planté made the first one in 1859. Most technology from the 1850s is in a museum, but this one is still in your driveway.

How Lead Acid Batteries Work

Dry plate and separator samples beside a closed battery. Illustrative image

It's exactly what it sounds like: lead and acid, packed in a plastic box.

The positive plate is lead dioxide and the negative plate is plain lead. They sit in a liquid that scientists call the "electrolyte". That's just a fancy word for the stuff that lets electrical charge move from one plate to the other, and the electrolyte is where all the action happens.

When you discharge it, a chemical reaction kicks in. The acid reacts with both plates, they become lead sulfate, and electrons flow out through the wires to your lights. It's like a playground slide where the electrons only get to ride if they go through your lamp.

Charging is the same process running backwards. A charger has to supply current to reverse it, and the plates turn back into lead and lead dioxide, ready to store energy for the next trip.

What Kind Of Acid Is In A Car Battery?

Three replacement batteries on a workshop shelf. Illustrative image

Sulfuric acid, mixed into a strong solution. It's the same in cars, boats and off-grid banks.

The materials are cheap and common, and a lead acid battery can provide the huge burst of power required to crank an engine. Every auto parts store on the planet sells one.

The best part? In the US, about 99 percent of the lead from old ones gets recycled into new batteries.

Types Of Lead Acid Batteries

Three battery designs arranged on a wooden workbench. Illustrative image

The types of lead batteries you'll meet in the shop all use the same chemistry. What changes is how the acid is held in place.

Flooded (Wet-Cell)

Flooded battery with six filler caps on a containment tray. Illustrative image

The classic. Unscrew the caps and you can see the liquid inside. It's the cheapest kind, but it needs a top-up now and then, and if you tip it over you've got acid on the floor.

AGM (Absorbent Glass Mat)

Closed battery secured by a strap in a boat compartment. Illustrative image

Here the acid is soaked into a fiberglass mat, like juice in a paper towel. Nothing sloshes, so it can lie on its side and take the bumps on a boat.

Gel (Valve Regulated)

Sealed battery on a shelf beside a ventilation grille. Illustrative image

The acid is turned into a thick jelly. AGM and Gel are both called "VRLA" batteries. They're sealed, so there's nothing to top up.

Common Applications

Closed backup power cabinets in an equipment room. Illustrative image

You'll find them in the following jobs: starting vehicles and boats, backup power cabinets that keep hospitals running in a blackout, cell towers, and budget off-grid solar storage. For the big picture of storage at home, here's how home batteries work.

The Trade-Offs Against A Lithium Battery

Heavy battery resting on a wheeled workshop trolley. Illustrative image

Lead is dense. For the same energy, lead-acid weighs about three times as much as a lithium-ion battery. That's fine in a basement, however it's a big problem in anything you carry.

Heat is the other enemy. Above about 25°C (77°F), every extra 8°C (15°F) cuts a sealed battery's life in half.

How Long Does A Lead Acid Battery Last?

Battery beside a water bottle, goggles and gloves. Illustrative image

A deep-cycle one lasts about 400 to 500 cycles if you only drain it halfway, but only 150 to 200 if you drain it fully every time. How you treat it matters a lot.

Don't let it sit empty. Left in a low state of charge for weeks or months, the lead sulfate hardens into crystals. Scientists call this "sulfation". That lost capacity usually never comes back, which is why a car left dead for months often needs a new lead acid battery.
Stay above half. Lead-acid off-grid banks are often sized to never go below 50 percent. The battery calculator helps you size yours.
Top up the open kind. Check the acid level regularly. If it's low, add distilled water, never tap.

A good charge controller brings it back to fully charged from your panels without overdoing it.

When To Pick Lead-Acid

Solar-powered timber cabin beside a closed ventilated shed. Illustrative image

Pick it for a car, when your off-grid budget is tight, or somewhere remote where any mechanic in town can help.

When To Skip It

Portable power station beside a tent and backpack. Illustrative image

Skip it if you'll cycle it every day for years, if weight matters in small vehicles, or if you want zero upkeep. A lithium bank with a good BMS just sits there and works.

Safety Notes

Protective goggles and gloves beside a battery on a tray. Illustrative image

It's important to respect the acid, because it eats skin, clothes and concrete. Wear gloves and goggles around an open lead acid battery, and keep baking soda nearby to neutralize a spill.

Charging makes hydrogen gas, which can explode. Charge in a well-ventilated space, keep the vent caps on, and keep sparks, flames and cigarettes away.

Old ones go to the recycler, never the trash. The lead is too valuable to waste and too toxic to dump.

What's Next For Lead-Acid

Prototype battery and material samples on a laboratory bench. Illustrative image

Labs keep working on it. Lead-carbon designs add carbon so it charges and discharges better under tough conditions. Want something newer? Look at saltwater batteries.

Sources

Hard Words

Term	What it means
Lead-acid battery	A cheap, heavy battery with lead plates sitting in acid.
Electrode	The positive or negative plate: lead dioxide or plain lead.
Sulfuric acid	The strong acid that lets charge move between the plates.
Flooded (wet-cell)	The open-top type with liquid acid you can see.
AGM (Absorbent Glass Mat)	Sealed, with the acid soaked into a fiberglass mat.
Gel battery	Sealed, with the acid turned into a jelly.
VRLA	Valve-regulated lead-acid: sealed and maintenance-free.
Sulfation	Hard crystals on the plates from sitting empty too long.
Deep-cycle	Built to be drained low and recharged many times.
Cycle life	How many drain-and-recharge rounds before it wears out.
Distilled water	The pure, mineral-free kind used for topping up.

Common Questions

Why is a battery design from 1859 still in my car?

It's cheap, sold everywhere, and very good at the huge burst of power a car needs.

What's the difference between flooded, AGM, and Gel?

The open kind has liquid acid you top up. AGM and Gel hold the acid in a mat or a jelly, so they're sealed.

Why are lead-acid batteries so heavy?

Lead is dense, so for the same energy they weigh about three times as much as lithium-ion.

What kills a lead-acid battery fastest?

Sitting empty. Hard sulfation builds up, and recharging usually doesn't undo it.

How deep can I drain it?

Stay above 50 percent. Full drains every time cut its life to half or less.

Are lead-acid batteries recyclable?

Yes. In the US about 99 percent of the lead is recycled into new batteries.