How Does A Car Battery Work?

Understanding how function of a car battery is the way you can know how to prevent battery problems.

You know that a car battery is the reason why your car starts in the morning, but do you know how a car battery works? As long as it is doing its function of starting your car, I’m sure your battery has seldom, or maybe not even once has it impinged on your consciousness.

Don’t feel guilty, most car owners are like you, totally oblivious to the workings of a battery until one day when the car refuses to start. I wrote this article so that a battery will no longer be a puzzle to you and the others like you. When your battery dies, because it will surely die, you would know why..

How does a car battery work? Most car batteries are lead-acid batteries made up of a series of cells with two plates – one made of lead, and the other of lead dioxide – immersed in a sulfuric solution called the electrolyte.

When you start the car and current is drawn from a battery, the electrolyte in each cell undergoes an electrolysis. It is a chemical decomposition that produces hydrogen ions (positively charged) and oxygen ions (negatively charged) which are transmitters of electric charges.

These charges induce the hydrogen ions to seek the negative plates and the oxygen ions to go after the positive plates. At the same time, the electrolyte breaks up into hydrogen ions.

When the different ions get into contact with the electrodes to which they are attracted, they release their electrical charges. Then, a chemical reaction occurs with the plate’s active material causing electricity to be released by the battery.

This turns the electrolyte into water and both plates into lead sulphate. As a consequence, the electrolyte is diluted and its pressure drops.

This chemical reaction is reversed when the battery is charged. Lead dioxide and lead develop once more on the plates and the acid returns to the electrolyte so that you can use the battery again and again.

What Is A Car Battery?

Now, you know how a battery works, but do you know what kind of battery is in your car?

The battery in your car is most likely a lead-acid rechargeable battery that supplies electric current to your car. Its raison d’etre is to start the engine of your car. Generally, starting the engine discharges no more than 3% of the capacity of the battery. Because of this, car batteries are devised to deliver a powerful burst of electricity in a split second.

Once the engine has started, the alternator takes over in powering the electrical system of the car. In addition to starting the engine, the battery is also called upon to supply extra power when the electrical requirements of the numerous electronic devices in the car is too much for the alternator.

That is why your battery is also referred to as an SLI battery which stands for Starting, Lighting and Ignition. SLI batteries are not meant for deep discharging. A total discharge can decrease the lifespan of an SLI battery.

The latest versions of SLI lead-acid batteries have six cells arranged in a series to theoretically provide 2.1 volts each for a 12-volt system for smaller vehicles and 12 cells providing 24 volts for heavy-equipment and large trucks. A 12-volt battery is fully charged at 12.4 volts or even higher and discharged at 12.39 volts or lower.

Voltage appertains to the potential amount of electricity that the battery holds. The smallest amount of decrease in the voltage of a battery matters a lot on the battery’s performance.

How Is A Car Battery Designed?

All cells of lead-acid batteries are identical in design. The cells contain two vertical plates, one negative and the other positive. These plates are made from a lead alloy. When the battery is fully charged the positive plate is coated with lead dioxide and the negative plate with a spongy lead.

The plates are separated from each other by a separator which is an insulating material and immersed in an acid solution made up of water and sulfuric acid known as the electrolyte. The maximum level of an electrolyte is a half inch above the top of the plates.

As mentioned earlier, the cells in a battery are connected in a series – the positive plates are connected together and so are the negative plates via an external connection called the terminals.

The terminals are also the battery’s connection to the car’s electrical system which is an earth-return circuit. That’s why one of the terminals (negative) is connected to the body and is said to be earthed.

The capacity of a battery is measured in ampere-hour or Ah and is dependent on the lead plates’ total area. A typical car battery has a 40Ah capacity which means that it can supply 4-amps of current for an uninterrupted 10 hrs. (40Ah / 4 amps = 10hrs).

Charging A Battery

Your battery discharges (around 3% of its capacity) when you start the engine, but the charge is replenished by the alternator as you drive. In the first three years of its life, you are not supposed to experience any problem.

But if for some reasons such as having left a parking light on overnight, the battery goes flat or dies completely, you can always recharge it using an external power source.

The best way to charge a dead SLI lead-acid battery is with the use of a trickle charger, although the fastest way is to jump-start it. Slow charging is preferable to jumping since it will not only restore the battery to a fully charged state, but this also allows the battery to hold the charge longer. Frequent jumping is also discouraged since it will tend to damage the alternator when done often.

The charging process is the reverse of the discharging process. When you recharge the battery, the electricity flows from the external source to the battery in the opposite direction.

It enters the positive terminal instead of going out from it which is what happens when it is being discharged. During charging, the plates revert to its original condition – the negative plate to lead and the positive plate to lead dioxide. The acid also returns to the electrolyte.

The power delivered by a battery, as well as the chemical activity inside it, are affected by the temperature. The starting capability of a battery is lesser in the cold climate when the engine oil is thicker and more viscous and requiring more power to crank an engine.

That is why you more often will have a weak battery in winter rather than during the summer months.

SLI batteries should not be fully discharged for two reasons.

● First, fully discharging a battery will cause sulfation, the accumulation of lead sulfate crystals on the plates. Sulfation is a major cause of the early death of a battery. Since lead sulfate crystals are large, a heavy build-up of these on the plates could result to cracks on the grid and the falling off of the lead oxide and lead coating.
● Second, a sulfated battery is difficult to charge since converting the lead sulfate back to its original state becomes more and more laborious.

A cell’s voltage drops rapidly from its normal value of 2.2 volts to slightly more than 2 volts when being discharged. It stays fixed at this value up to the time the battery is discharged fully. Once it reaches this stage, the reduction in the voltage accelerates and the build-up of sulfation becomes substantial, unless it is quickly charged.

Charging and discharging a battery the correct way is a guarantee that you will always get the most out of your battery.

Battery Life

In general, batteries are give a lifespan of from three to four years. But, with proper care and maintenance, some batteries extend their useful life to much longer than that. Some batteries also die an early death, even before it has reached its maximum life expectancy.

There are factors that influence the life of your battery other than maintenance and care. The weather is a major determinant of how long your battery will last, and so are driving habits and vehicle type. Signs of a failing battery are quite obvious.

It usually starts with the battery’s inability to hold a charge or even to take a charge. When you wake up one cold morning and find your car refusing to start, in all probability, the battery is the culprit.

However, you can prolong the life of your battery by adhering to the following suggestions:

1. Be diligent in regularly checking the condition of your battery. Top off the electrolyte solution religiously and keep it always clean and dry. Remove any sign of corrosion the moment you see them.

2. Avoid frequent short drives. Normally, an alternator will take at least eight hours of straight driving to fully charge your battery. Short trips result to undercharging and are responsible for shortening the life of your battery.

Undercharging can cause the buildup of crystalline deposits on the battery’s negative terminal that prevent the battery from taking a charge. The process is called sulfation and is bad news for your battery.

3. Avoid parking in the open during winter time. Cold weather is one of the worst enemies of batteries. Park your car inside the garage to keep the battery warm to make it easy to start in the morning.

4. If you have to store your battery for a period of time, store it indoors and remove the battery from the car or just simply disconnect the negative terminal. Long period of unuse can drain the battery with what is known as a self-discharge or even by a parasitic drain.

5. Use only distilled water when topping off a battery.


After this article you no longer have the right to ask “How does a car battery work?’.
We have covered all the issues that pertains to car batteries. Knowledge is power, and having a working knowledge of what a car battery is and its function, how it works, and how to prolong its life is power indeed.


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