Car Battery 101: How Do Car Batteries Work?

Ordinarily, you might take your car battery for granted, but this essential piece of kit is the only power source used to get your vehicle started. If you’ve experienced battery problems before, you’ll know that your car can’t do much without this component. In fact, faulty batteries are one of the most common reasons for the breakdown.

So how do these batteries work, how can you look after yours, and when might you need to get a replacement? Keep reading to find out.

What is a Battery?

A battery is a device that converts chemical energy contained within its active materials directly into electric energy by means of an electrochemical oxidation-reduction (redox) reaction. This type of reaction involves the transfer of electrons from one material to another via an electric circuit.

While the term battery is often used the cell is the actual electrochemical unit used to generate or store electric energy.

In understanding the differences between a cell and a battery, one should think of a battery as one or more of these cells connected in series, parallel, or both, depending on the desired output voltage and capacity.

What is a Car Battery?

An automotive battery or car battery is a rechargeable battery that is used to start a motor vehicle. Its main purpose is to provide an electric current to the electric-powered starting motor, which in turn starts the chemically-powered internal combustion engine that actually propels the vehicle.

Once the engine is running, power for the car’s electrical systems is still supplied by the battery, with the alternator charging the battery as demands increase or decrease.

The majority of these car batteries use a lead-acid chemical reaction. Each of the cells features two plates, one made from lead and the other from lead dioxide. These plates are submerged in sulphuric acid, which acts as a catalyst and triggers a chemical reaction between them.

This reaction produces electrons, which generate electricity. The electricity then flows from the battery to start your car engine. The reaction is reversible, which is why your battery recharges as your engine runs and why you can jumpstart it when it’s flat.

When you switch the ignition on in your vehicle, this sends a signal to the battery telling it to start the chemical reaction. The electricity produced by this reaction supplies power to the starter motor, which then turns the engine over. At the same time, the battery provides power to the spark plugs, which ignite the air and fuel mixture that is compressed in the engine combustion cylinders.

The power provided by the battery is then replaced by the alternator, which is responsible for supplying most of the electrical current to the electrical systems in your car, and for keeping the battery charged.


How Do Car Batteries Work?

Most car batteries rely on a lead-acid chemical reaction to get things moving and grooving. These batteries fall into the “SLI” category. SLI stands for “starting, lighting, and ignition.” This type of battery provides short bursts of energy in order to power your lights, accessories, and engine.

Once the battery jolts the engine to life, power for the car is supplied by the alternator. Most vehicles come with a generic SLI battery from the factory. Learn more about other car battery types.

How do Car Batteries work?

  • A typical SLI battery has six cells. Each cell has two plates or grids: one is made of lead, the other of lead dioxide. Each cell is able to produce about 2 volts of energy. In most car batteries you have six cells, and therefore a 12-volt battery.
  • The plates are submerged in sulphuric acid which triggers a reaction between the two plates. In scientific terms, the acid acts as a catalyst.
  • This acid will trigger a reaction on the lead dioxide plate, causing the plate to produce two things: ions and lead sulfate.
  • The ions produced by the lead dioxide plate react with the adjacent plate to produce hydrogen and lead sulfate.
  • The result is a chemical reaction that produces electrons. The electrons race around the plates and generate electricity. The electricity flows out of the battery terminals to start your engine, turn on your headlights, and play the radio.
  • This chemical reaction is entirely reversible, which is why you can jumpstart your battery and continue to charge it throughout the duration of its life. By applying current to the battery at just the right voltage, lead and lead dioxide will form on the plates and you can reuse your battery, over and over again!

Are There Any Warning Signs That May Indicate My Battery Is On The Fritz?

“If I only knew sooner.” We’ve all been there before. Fortunately, there are various indications and symptoms that your battery may need replacement:

  • Slow engine crank: When you attempt to start the vehicle, the cranking of the engine is sluggish and takes longer than normal to start. You’d best describe it as the “rur rur rur” starting noise sound.
  • Check engine light: The check engine light sometimes appears when your battery power is weak. Strange system indicator lights–such as check engine and low coolant lights–could mean there’s a problem with your battery. (It could also just mean you need more coolant).
  • Low battery fluid level: Car batteries typically have a part of the casing that’s translucent so you can always keep an eye on your battery’s fluid level. You can also inspect it by removing the red and black caps if they are not sealed (most modern car batteries now permanently seal these parts).
  • Bottom line: If the fluid level is below the lead plates (energy conductor) inside, it’s time to have the battery and charging system tested. When fluid levels drop, it’s typically caused by overcharging (heat).
  • The swelling, bloating battery case: If your battery casing looks like it ate a very large meal, this could indicate a battery gone bad. You can blame excessive heat for causing your battery case to swell, decreasing your battery life.
  • Eww, there’s a stinky, rotten egg smell: You may notice a pungent, rotten egg smell (sulfur odor) around the battery. The cause: Battery leaks. Leaking also causes corrosion around the posts (where the + and – cable connections are located.) The gunk may need to be removed or your car may not start.
  • Three years + battery age is considered an old-timer: Your battery can last well beyond three years but, at the very least, have its current condition inspected on a yearly basis when it reaches the three-year mark. Battery life cycles range from three-to-five years depending on the battery. However, driving habits, weather and frequent short trips (under 20 minutes) can drastically shorten the actual life of your car battery.

Why do batteries go flat?

Car batteries should last around five years, or potentially longer, but this does depend on how well they have been looked after. Depending on the types of journeys you make, you might find your vehicle battery starts to show signs of age from around three years.

Effectively, your battery empties itself in one go when you use it to start the engine. It is then gradually recharged by the engine as you drive. If you make a lot of small journeys, the battery may not have the chance to fully recharge before being used again and it’s likely to degrade faster. This could mean you need to replace it more quickly than you would if you regularly made longer trips in your car.

A range of other factors also affects the condition of car batteries, including the outside temperature and how demanding your vehicle’s extra electrical systems are.

Related Article:

How Do I Determine If My Battery Is Too Old?

For one thing, you can check the four- or five-digit date code on the cover of your battery case. The first part of the code is key: look for the letter and digit. A letter is assigned to each month — you know, like A for January, B for February, and so on.

The number that follows nods to the year, as in 9 for 2009 and 1 for 2011. This code tells you when the battery was shipped from the factory to our local wholesale distributor. The additional digits tell where the battery was made.

Car batteries last, on average, three-to-five years. Mind you, there are also weak battery signs to watch for, like a slow engine crank of low fluid level. If your battery case is swollen or bloated, there’s a smelly rotten egg scent coming from the battery, or your check engine light appears, trouble may be beyond the bend. And if it’s over three years old? Consider it time for close monitoring.