Skip to Content

How Does a Car Battery Work?

Batteries Plus Blog - Power - by Bryan Veldboom - updated on 10/25/2021

Car batteries are one of those things most of us take for granted. But how do they actually work? Car batteries fall into the designation of SLI batteries, which stands for Starting, Lighting and Ignition. SLI batteries are rechargeable lead-acid batteries designed for applications that require lots of power upfront to start a vehicle’s engine, followed by slower, more consistent delivery to keep the vehicle and its accessories running. Today, we’ll be taking an in-depth look at how car batteries are constructed, as well as the chemical processes they use to power your car.

What are the Components of a Car Battery?

To understand how a car battery works, we have to start with their internal design. A 12 volt battery is the industry standard. Most cars and trucks will have one of these under the hood. There are some vehicles that run on 6 volt batteries, although for our purposes here, we’ll be using a 12 volt battery as our example.

A typical 12 volt lead acid auto battery contains 6 separate battery cells. Each cell contains a series of rectangular grids, housing a number of lead plates. Due to the material within it, each grid is either positively or negatively charged. These are arranged in a sequence of alternating positive and negative grids, divided by a series of separators.

The separators are made of a thin, porous material that allows ionic current to flow between the positive and negative plates, while preventing them from short circuiting. The grids themselves contain a lattice of lead wires that are used to channel current to a tab located at the top of the grid. This series of alternating grids is called an element. Each cell of the battery will contain a single element.

What is an Auto Battery Electrolyte?

The battery also contains a sulphuric acid solution called an electrolyte which reacts chemically with the material on the positively and negatively charged grids (more on that below). The location and make-up of the electrolyte will depend on the type of battery you have. In a flooded lead acid battery, the electrolyte lives in the battery’s cells with the element submerged within. In an AGM (short for Absorbed Glass Mat) battery, the electrolyte is suspended in the separators. In a gel battery, the electrolyte is a silica gel sulfuric acid (SiO2), although gel batteries are rarely used in automobiles.

Each of the battery’s elements is placed into one of the battery’s individual cells with the strap of one cell connected to that of the adjacent cell. This setup connects all the cells in a series, allowing each cell’s charge to combine into the battery’s total voltage. In a typical 12 volt battery, each cell produces roughly 2.1 volts of electricity, allowing the battery to produce approximately 12.6 volts of electricity.

The number of plates each grid contains and how thick they are will vary from battery to battery. A greater number of plates typically give the battery a greater capacity, allowing it to run longer and charge faster, although this depends on the power demand of the vehicle it is used in. A battery with thinner plates will have a longer standby life, whereas one with thicker plates will have a longer overall life, but less power density.

How Does a Car Battery Work?

Car batteries contain stored chemical energy that is converted into electrical energy when it is attached to an external load, such as your car. Electricity is produced by the chemical reaction between the material on the plates and the electrolyte. Remember when we mentioned that the grids had a material added to them which made the plates inside positive or negative? For positive grids, this material is a compound of lead and oxygen called lead dioxide. The negative grids have a sponge lead paste applied, which is a compound of hydrogen cations and the sulfate anion. The electrolyte itself contains hydrogen and sulfate ions. When the battery terminals are connected to your car, a circuit is completed between the positive and negative grids, causing a series of chemical reactions. Here’s how the process works:

  • When you turn on your ignition, it causes all the cells to have a forward reaction in which the sulfate ions in the electrolyte move to the negative plates and give up their negative charge.
  • The remaining sulfate combines with the active material on the plates to form lead sulfate. This reduces the strength of the electrolyte and the sulfate on the plates acts as an electrical insulator.
  • The movement of the ions in the electrolyte are what create current flow. The excess electrons flow out of the battery’s negative terminal and transmit throughout the car via the battery cables, then travel back to the positive side of the battery.
  • At the positive battery terminal, the electrons rush back in and are accepted by the positive plates.
  • As the battery begins to discharge, the acid concentration of the electrolyte will get weaker as sulfuric acid is consumed through these reactions, creating additional water.
  • This will continue until the battery’s voltage drops so long that it can no longer deliver electricity.
  • When it reaches this point, the battery is discharged and the electrolyte more closely resembles water.

How Do You Recharge a Car Battery?

Once a battery is discharged, it needs to be recharged in order to function again. The charging process is essentially the reverse of the chemical process that caused the battery to discharge in the first place, driving the sulfate ions back into the acid. This is how it works:

  • Your alternator is responsible for recharging the battery using a belt-driven system that turns the rotor in the alternator cables.
  • The rotor gains additional electrons through the magnetization of little particles. This is done when a copper carbon scratches an iron clip ring on the beam.
  • The turning of the electromagnet inside the stator coil produces the electricity inside them. The flow of electrons that produce electricity is alternating, meaning it flows forward or backward, depending on the direction of the rotor.
  • As this occurs, the sulfate in both the positive and negative plates are split back into their original lead and sulfate.
  • The water is divided into hydrogen and oxygen.
  • The sulfate in the plates combines with the hydrogen in the water and is restored to sulfuric acid.
  • Simultaneously the oxygen with water combines chemically with the lead of the positive plate to form lead dioxide.
  • The electrolyte is restored to its original state as sulfuric acid is formed, replacing the water in the electrolyte.

Trust the Car Battery Experts at Batteries Plus

Batteries Plus is your car battery headquarters. We sell top-quality car batteries from brands like Duracell, X2Power and more. If you’re experiencing issues with your current battery, our in-store experts can test it for you for free. We also offer free battery installation on most cars and trucks and will even recycle your old car battery. Be sure to shop our selection of additional auto essentials, including battery maintainers to keep your battery charged, terminal protection kits to help keep your terminals free of corrosion and jump starters for jumping an engine with a dead battery.

Having trouble starting your car? Find out whether the battery, starter or alternator is at fault. Shopping for a new car battery? Learn the differences between flooded and AGM batteries.

Need a new car or truck battery?

Shop Car & Truck Batteries