How Does a Car Battery Work?

Power - by Joe Weber - updated on 10/21/2024

Car batteries are something most of us don't think about until they stop working. But have you ever wondered how they actually work? Car batteries are classified as SLI batteries—short for Starting, Lighting, and Ignition.

These rechargeable lead-acid batteries are designed to deliver a burst of power to start your engine, then provide a steady flow to keep your vehicle and its accessories running. Today, we're going to dive into how car batteries are built and the chemical processes that keep your car powered up.

What are the Components of a Car Battery?

It's best to start with its internal design to understand how a car battery works. The industry standard is a 12-volt battery, which you'll find in almost all cars and trucks today. While a few older vehicles still use 6-volt batteries, we'll focus on the 12-volt version for this explanation.

A standard 12-volt lead-acid car battery consists of six individual cells. Each cell holds a series of rectangular grids that contain lead plates. These grids are either positively or negatively charged depending on the materials inside. They're arranged in an alternating sequence of positive and negative grids, separated by a series of insulating separators.

The separators are made from a thin, porous material that allows ionic current to pass between the positive and negative plates while keeping them from short-circuiting. The grids have a lattice of lead wires that channel current to a tab at the top of the grid. This alternating grid setup is known as an element, and each battery cell contains one element.

Each element is placed into one of the battery's individual cells, with the strap of one cell connected to the strap of the next. This series connection links all the cells, combining their charges to create the battery's total voltage. In a standard 12-volt battery, each cell generates about 2.1 volts, resulting in a total output of around 12.6 volts.

The number and thickness of the plates within each grid can vary from one battery to another. Generally, a battery with more plates will have a higher capacity, allowing it to last longer and charge faster, depending on the vehicle's power demands. Batteries with thinner plates tend to have a longer standby life, while those with thicker plates usually have a longer overall lifespan but lower power density (a measure of how quickly a battery can deliver energy).

What is Auto Battery Electrolyte?

Car batteries contain a sulfuric acid solution, known as an electrolyte, which chemically reacts with the materials on the positive and negative grids (we'll get into that shortly). The placement and composition of the electrolyte depend on the type of battery.

• In a flooded lead-acid battery, the electrolyte is found within the cells, with the element submerged in it.
• In an AGM (Absorbed Glass Mat) battery, the electrolyte is suspended in fiberglass separators, and weaved in between the plates.
• Gel batteries, which use a silica gel sulfuric acid (SiO2) as the electrolyte, are another type, though they're rarely used in cars.

How Does a Car Battery Work?

Car batteries store chemical energy that's converted into electrical energy when connected to an external load, like your vehicle. This electricity is generated through a chemical reaction between the material on the plates and the electrolyte. Remember earlier when we talked about the grids being coated with a material that makes the plates either positive or negative? For the positive grids, that material is lead dioxide, a compound of lead and oxygen. The negative grids are coated with a sponge lead paste, made up of hydrogen cations and sulfate anions. The electrolyte contains hydrogen and sulfate ions as well. When the battery terminals are connected to your car, it completes a circuit between the positive and negative grids, triggering a series of chemical reactions. Here's how it works:

• When you turn the ignition, it triggers a forward reaction in all the battery's cells. During this process, the sulfate ions in the electrolyte move toward the negative plates and release their negative charge. This movement of ions is what generates the electrical energy needed to start your vehicle.
• The remaining sulfate combines with the active material on the plates, forming lead sulfate. This process weakens the electrolyte and the lead sulfate that forms on the plates acts as an electrical insulator, reducing the battery's ability to generate power over time.
• The movement of ions in the electrolyte is what generates the current flow. Excess electrons leave the battery through the negative terminal, travel through the car's electrical system via the battery cables, and then return to the positive terminal of the battery, completing the circuit.
• As the battery discharges, the concentration of sulfuric acid in the electrolyte decreases because the acid is consumed during the chemical reactions, resulting in the production of more water. This weakening of the electrolyte reduces the battery's ability to generate power.
• This process continues until the battery's voltage drops so low that it can no longer provide enough electricity to power the vehicle, effectively rendering the battery discharged.
• At this stage, the battery is fully discharged, and the electrolyte's composition becomes more like water, as most of the sulfuric acid has been used up in the chemical reactions.
• At this point, the battery must be recharged.

How Do You Recharge a Car Battery?

Once a battery is discharged, it needs to be recharged to function again. The charging process essentially reverses the chemical reactions that caused the battery to discharge, pushing the sulfate ions back into the electrolyte. Here's how the process works:

• Your vehicle's alternator recharges the battery using a belt-driven system that spins the rotor inside the alternator. As the rotor spins, it gains additional electrons through magnetization. This happens when a copper carbon brush makes contact with an iron clip ring on the rotor shaft.
• The spinning rotor acts as an electromagnet inside the stator coil, generating electricity. This electricity flows in alternating directions, depending on the rotor's movement.
• During this process, the sulfate on both the positive and negative plates is broken down into its original lead and sulfate. The water in the battery splits into hydrogen and oxygen.
• The sulfate from the plates combines with the hydrogen from the water, restoring the sulfuric acid in the electrolyte. At the same time, the oxygen reacts with the lead on the positive plate, forming lead dioxide.
• As a result, the electrolyte is returned to its original sulfuric acid state, replacing the water that was formed during the discharge, and the battery is fully recharged.

Trust the Car Battery Experts at Batteries Plus

Batteries Plus is your go-to destination for car batteries. We carry top-quality brands like Duracell, X2Power, and more. If you're having trouble with your current battery, stop by one of our stores for a free battery test from our in-store experts.

We also offer battery installation for most cars and trucks, and we'll even recycle your old battery for you. Check out our selection of auto essentials, like battery maintainers to keep your battery charged, terminal protection kits to prevent corrosion, and jump starters for those unexpected dead battery moments.

Not sure what's causing your car to struggle when starting? Let us help you figure out whether the problem is with your battery, starter, or alternator. Shopping for a new battery? We can guide you through the differences between flooded and AGM batteries to find the best fit for your vehicle.