How Does A Car Battery Work?

(Last Updated On: March 14, 2024)

Every day, we rely on car batteries, yet have you ever paused and thought: What’s the mechanism behind a car battery’s functioning? The fact that it’s actually a chemical reaction producing the energy required to ignite your car might astonish you.

While there are different kinds of car batteries, all of them follow the same basic principles. We’ll explain it all in detail below.

What’s A Battery?

At its core, every battery is a device that converts energy from a chemical reaction into the electricity that powers our various devices. The cells within the battery are what store the chemical energy. They also convert it into electrical energy when prompted.

How Does A Car Battery Work
A Car Battery

Different types of battery use different materials in the cells. They may also use a different process to generate the chemical reaction. This creates batteries that produce energy in different quantities and accounts for differences like rechargeability and variations in life-span.

What Are Car Batteries Made Of?

The exact components vary depending on the type of battery, but all car batteries contain the same basic components:

  • Casing. Made of non-conducive hard rubber, the exterior coating of the battery protects the components and prevents leaks. The top of the casing holds the terminals, vents, and filter well.
  • Metal plates. Traditionally these are alternating plates of lead and lead oxide. These plates are what store the charge in the battery.
  • Sulfuric acid. The plates are submerged in sulfuric acid or battery acid in layman’s terms. The chemical reaction of the acid with the lead is what provides the charge (hence the name “lead-acid battery”). Some batteries use an electrolyte gel rather than battery acid to facilitate the chemical reaction. This is less likely to spill in addition to being more efficient.

Kinds Of Car Batteries.

There are three main types of battery you’ll see available for cars:

  • SLI batteries. SLI stands for starting, lighting, and ignition. These batteries may also be called lead-acid batteries because of the chemical process used to provide the power (more on that below). These batteries are designed to provide a quick burst of power. This is what turns on your light and radio when you turn your ignition without starting. It also provides the initial power to start the engine.
  • AGM batteries. This is actually a sub-category of SLI batteries, but the different acronym can be confusing so it’s worth mentioning. AGM stands for absorption glass mat, which describes the interior construction of the cells. Like other SLI batteries, these are designed to provide quick bursts of power. They tend to be more efficient than other lead-acid batteries and have a longer overall lifespan.
  • LithiumIon batteries. Found mostly in hybrid and electric cars, lithium-ion (or Li-ion) batteries are more efficient even than AGM batteries. They are also significantly lighter. The main disadvantage is they have a shorter overall lifespan.

You may see terms like “deep cycle” and “wet cell” used in battery discussions. These mostly apply to recreation vehicles, motorcycles, and water or aircraft. When it comes to cars, the vast majority will use one of the three battery styles above.

How Does A Car Battery Work?

That depends on the style of battery. With SLI and AGM batteries, the basic operation is the same. Each cell contains two plates, one each of lead and lead oxide. These plates are known as the anode and the cathode, respectively.

These plates are submerged in a catalyst, whether that’s sulfuric acid or electrolyte gel.  When the catalyst touches the plates, they begin to react. Ions are released by the lead oxide plate, which then react with the lead plate to create lead sulfate and hydrogen. The electrons released in this reaction are what generate the electricity that flows to the terminals.

This chemical reaction also works in reverse. Once your car engine starts, a current flows through it that re-forms the plates from the lead sulfate and hydrogen. This is what allows a car battery to be recharged over and over.

Lithium-ion batteries function on the same basic principles but arrive at the chemical reaction in a different way. They still contain an anode and cathode suspended in electrolyte. In this case, the anode and cathode store lithium instead of being composed of lead. The reaction of the electrolyte with the lithium creates positive lithium ions. These move quickly between the anode and the cathode, creating an electrical current which is sent out as power.


How many volts is a car battery?

Car batteries are labeled as 12 volts, which is slightly inaccurate. Each of the 6 cells in a typical battery actually produces 2.1 volts, for a total voltage of 12.6.

Why do batteries wear out over time?

Some of the plate material is lost in each chemical reaction, even if you’re meticulous about fully recharging the battery. This material accumulates at the bottom of the battery housing and can short out the plates.

The electrolyte balance within the battery can also be disrupted. This normally happens in hotter conditions, which cause the water inside the battery to evaporate.

Can I repair my battery if it dies?

In many cases, yes. Battery maintainers can help to clear away the sediment left behind by the charging process, extending the life of your battery. Many mechanics also offer battery renewal services that can bring even a fully-discharged battery back to life.

Can I use different battery types interchangeably?

That depends on the type. In most cases, you can use AGM batteries as direct drop-ins for lead-acid batteries and vice versa. If you’re unsure whether a battery would work in your vehicle, consult the manual or ask your mechanic.

Lithium-ion batteries are not interchangeable with other car batteries. They will neither fit nor function in a traditional car engine. The opposite is also true: a hybrid or electric car won’t work properly with a traditional SLI battery.

Do I need to add water to a car battery?

Most batteries now have the label of “maintenance-free” and do not need to be re-filled with water. In fact, the caps on these batteries are sealed, and trying to break that seal could damage the battery.

Batteries that are not maintenance-free do need to be periodically re-filled with water to maintain the electrolyte balance. Check the battery’s water level more often in the summer, when evaporation is the biggest concern. Make sure you only use distilled water to refill a battery. Tap water may contain trace metals that can disrupt the chemical process.

Bonus: Best 12V Battery Charger: Top 10 Picks 2021 (Review and Buying Guide)

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