Fact Checked

What is a Car Battery?

Joseph W.
Joseph W.

A car battery is a physical reserve of energy within the car system. The battery is usually charged by an alternator — a device that converts the mechanical energy from the engine into electrical energy. This energy is then used to prompt reactions within the electrolyte of the battery, which is most commonly composed of lead oxide plates submerged in a mixture of sulfuric acid and water. When a battery is in use, the sulfuric acid reacts with the lead oxide plates, to form lead sulfate. Recharging the battery involves reversing this reaction, which allows it to store energy chemically.

The battery in a car is used to power the vehicle during ignition and provide energy for the lighting system. In gasoline engines, it is also used to ignite the fuel, so this type of engine cannot work without a battery. Diesel engines may work without batteries, if the initial electrical energy is provided at ignition. The battery is also used to provide traction energy for electric vehicles.

Car battery.
Car battery.

There are several types of car batteries available. The most common is the flooded type, which is basically an arrangement of lead plates immersed in sulfuric acid in a pseudo-open reservoir. This type requires regular topping up with distilled water to compensate for the water lost as hydrogen and oxygen gases during the electrolysis process. A slight modification of this results in a different type of battery known as a sealed battery, which is basically a flooded battery which is sealed so that no water can be lost.

Car battery in situ.
Car battery in situ.

A valve regulated lead acid (VRLA) battery is also sealed. One type has safety valves that allow for the safe escape of hydrogen and oxygen gases generated during charging. There is another type of VRLA battery that doesn't have safety valves and instead has an additional catalyst surface on which the two gases recombine to form water again.

The glass mat kind of car battery is radically different from the VRLA, sealed, and flooded types of batteries. Its composites are boron silicate glass mats saturated with electrolyte between the plates. By using the recombinant technology, they don’t lose hydrogen or oxygen, and they also store charge for a much longer period, losing only 1% to 3% monthly. They are usually two to three times more expensive than the normal flooded batteries, however. Another option is silica gel batteries, but due to their slow recharge rates, they are not common in cars.

A car battery is usually charged by an alternator.
A car battery is usually charged by an alternator.

Two other electrolytes are used less commonly in automotive batteries. One is the nickel metal hydride (NiMH), which packs a relatively higher energy within a unit of space. This is also referred to as high energy density. NiMH batteries can be easily recycled, since they do not contain toxic metals. Lithium-ion (Li-ion) batteries also has a high energy density. They also self discharge the least over a period of time.

Car batteries power a vehicle's lighting system.
Car batteries power a vehicle's lighting system.

When storing sulfate batteries, it is important a person to ensure that they are always charged, preferably to the maximum. Discharged sulfate batteries tend to degrade by sulfating, which involves the formation of large lead-sulfate crystals in the batteries. This hinders the normal reverse reaction that defines recharging. The batteries should be stored in a cool place, as warm temperatures result in a higher rate of discharge and corrosion at the contact plates.

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Discussion Comments


@jmc88 - Good questions. I think the easiest answer I can tell you is to go to your local automotive store and have them check it. Nearly every store will do this for free, since it is pretty easy if you have the right equipment. I've had it done at Autozone, but that's just because it's the closest to my house. Advance, NAPA, or even Wal-Mart will run the tests for free. They will check your battery, alternator, and starter, and let you know if any of them are coming up with bad readings.

As for your problem, it does sound like the alternator. It is responsible for recharging the battery after it has been in use. The battery itself is 12 volts and the alternator gives the electrical system 2 more volts during operation, which is why it normally reads 14. If the voltmeter goes down, it's probably because the truck isn't getting the 2 extra volts from the alternator. I would have it checked ASAP as a bad alternator can fail quickly and leave you stranded.


When you start to have electrical problems with your car, how do you tell whether the problem lies with the battery or the alternator? For the last couple weeks, the voltmeter on the dashboard of my truck has been going back and forth between 14 volts (where it usually is) and 12 volts. Whenever it drops down to 12, the battery light usually comes on.

The truck seems to still drive okay, though, and I have never had a problem with it starting. This makes me think that it could be the alternator, but I'm not sure. What exactly is the purpose of the alternator, and how long will it last if it is going bad? Are there any easy tests to determine whether my battery is still good or if something else might be causing the problem?


@Izzy78 - I'm not an expert on the electric cars, but I believe I have read that most of them now use lithium batteries. Like the article mentions, they usually hold a charge for the longest. If I am not mistaken, though, their ability to hold a maximum charge can diminish over time, so I would guess that car companies are looking for other alternatives. That might be where the nickel-based batteries come into play.


@orangey03 - Good advice. Salvage yards are a great place to find replacement alternators. Like you mentioned, they are very expensive to buy brand new, but as long as the used ones are in good condition they should work for several years. Even if you have to buy two used ones, it's usually a lot cheaper than buying one new one.

I didn't realize that there were so many different kinds of batteries in cars, though. I was just familiar with the normal flooded and sealed types found in most cars. I wasn't thinking about all the new types of batteries that have come about for the electric cars. What types do they use? I know they usually have an array of several batteries that are all linked together.


@shell4life – The same thing happened to my husband. It turned out that he had a bad alternator instead of a bad battery.

The battery just wasn't getting charged properly, because the alternator was malfunctioning. He got a new alternator, and that fixed everything.

Alternators are not cheap, though. We found out that a brand new one was going to cost hundreds of dollars, so we went to my friend's junkyard and found one that matched for just forty bucks.


I went to buy a car battery last week, because my car seemed to be dying. It would click awhile when I turned the key, but the engine would not get started, and the lights inside the vehicle all came on, but they dimmed down quickly.

The new battery worked for about a day, but then, everything started happening again. Did I just buy a bad battery, or is something else going on?


I have been stranded at work a couple of times because of a dead car battery. The first time, I had no idea what was wrong.

I tried to crank my car, and nothing happened. It didn't even try to rev up or anything.

My dad came to my rescue with a voltage meter, and he determined that my battery had died. Luckily, I worked in the city, so there were plenty of places around that sold car batteries.


What type of the battery do I need for a Plymouth Voyager '99 van?

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      By: Avantgarde
      Car battery.
    • Car battery in situ.
      By: zest_marina
      Car battery in situ.
    • A car battery is usually charged by an alternator.
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      A car battery is usually charged by an alternator.
    • Car batteries power a vehicle's lighting system.
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      Car batteries power a vehicle's lighting system.
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