Even though your car’s battery might seem simple, it’s quite complex and important. Anyone who’s needed a jumpstart understands the critical role the battery plays in getting (and keeping) your car running.
But what is the battery really, and how does it work? The answer isn’t as straightforward as you might think.
How Does a Car Battery Work?
There’s a lot of chemistry going on inside an automotive battery, but don’t worry—we’ll leave out those confusing formulas you used in high school.
Here’s how a traditional, lead-acid battery works, in layman’s terms:
- A 12-volt automotive battery contains six cells connected in series. Partitions separate the cells from one another, and a sulfuric acid/water solution (electrolyte) fills the battery.
- Each cell contains negative (lead) plates and positive (lead dioxide) plates with insulating separators.
- The chemical reaction between the plates and acid solution causes the electrons to flow from the negative plate to the positive plate, creating electrical energy.
- Each cell produces around 2.1 volts through the chemical reaction mentioned above. Because the cells are wired in series, the battery produces approximately 12.6 volts.
- That voltage leaves the battery through the battery terminals. The battery terminals connect to the battery cables, which, in turn, connect to the vehicle.
While supplying electricity to the vehicle, the battery begins to discharge. That means the sulfuric acid starts to move from the electrolyte to the plates.
The discharge process causes the electrolyte to start losing sulfuric acid, so the battery’s output voltage is reduced. For this reason, an alternator is used to recharge the battery when the car is running.
Recharging the battery removes the sulfuric acid from the plates, restoring the electrolyte to a normal-strength solution.
Lead-acid batteries are sometimes referred to as “storage batteries” because they are repeatedly discharged and must be recharged by the alternator.
Car Battery Ratings
Batteries have several ratings, all of which reference the battery’s capacity—the amount of electrical energy that the battery can provide under select conditions.
The capacity primarily depends on the number of plates used inside the battery’s cells.
The different car battery ratings include:
Cold-Cranking Amperes (CCA)
The CCA rating is the most common battery rating method. It indicates the number of amps a 12-volt battery can deliver for 30 seconds at 0 degrees Farenheight without falling below 7.2 volts.
Cranking Amperes (CA)
The CA rating indicates the number of amps a 12-volt battery can deliver for 30 seconds at 32 degrees Farenheight without falling below 7.2 volts. The CA rating is essentially the same thing as another type of rating called the marine cranking amperes.
The ampere-hour rating (an older measurement method) indicates how many amps the battery can deliver over a given period of time.
The reserve capacity is the length of time (measured in minutes) that a 12-volt battery can supply 25 amps before dropping below 10.5 volts.
Another important factor is the group size of the battery. The correct group size ensures the battery will physically fit inside a particular vehicle.
You can find the correct size for your application by looking in the owner’s manual. The group size will also be listed somewhere on the battery.
What Does a Battery Do in Your Car?
The battery in your car serves two primary functions:
- Cranking the engine to get it started.
- Providing electrical energy to the vehicle when the alternator does not provide enough (or any) output.
The battery also acts as a capacitor to smooth out current ripples and protect the vehicle’s sensitive onboard electronics.
Two cables (a negative and a positive) connect the battery to the car. The negative cable connects to a common ground, while the positive cable connects the battery to the starter motor and other necessary points on the vehicle.
Is a Car Battery AC or DC?
Some people wonder whether a car battery produces alternating current (AC) electricity or direct current (DC) electricity. The answer is: automotive batteries always produce DC voltage.
The alternator, on the other hand, produces AC voltage. That’s why the alternator must contain semi-conductors, called diodes, that convert the alternating current to direct current.
What’s the Difference Between AC and DC?
In case you were wondering, the difference between AC and DC is that direct current flows in only one direction, whereas alternating current reverses directions repeatedly. Because of this, AC produces a sinusoidal waveform pattern, and non-pulsating DC produces a straight line pattern.
What are the Different Types of Car Batteries?
So far, we’ve discussed traditional lead-acid batteries. But there are other types of 12-volt automotive batteries as well. Common types include:
Lead-Acid (aka Flooded)
Most 12-volt automotive batteries are of the lead-acid variety. This type of battery is also known as a flooded lead-acid (FLA) battery because it contains a liquid electrolyte.
Absorbed Glass Mat (AGM)
Another design becoming increasingly common is the AGM battery. Instead of using a liquid electrolyte, the acid in an AGM battery is absorbed into the separator.
Typically, this type of battery is used in applications with high charging capacities.
Enhanced Flooded Battery (EFB)
EFB batteries are used in vehicles that feature engine start/stop technology. Basically, an EFB is a type of flooded battery that’s more robust than a traditional lead-acid battery.