What Charges The Battery In A Car?

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Although not a big component in a car, the battery has an essential role. You basically cannot start the car on your own without it. It is a vital component, that is also providing power to all the essential modules that store information when the automobile is stopped. As this being said, the battery needs also some help to keep it charged after starting the engine.

So what charges the battery in a car? The alternator is the car component which is responsible for charging the battery. The alternator provides enough power supply to all the other electrical components of the vehicle and keeps the battery fully charged.

What is the car battery and what function has?

An automotive car battery is used to power the engine starter. This engine starter cranks the motor on his own until it initialize it and the engine car run on its own.

The design of the batteries is in such a way to deliver maximum current for a short period. After we start the engine, all the power feeding is assured by the alternator.

Another role of the battery is to act as a stabilizer if voltage transients occur. Essentially, if there are voltage spikes, it will not interfere with the electronic components.

They introduced the first car batteries in the 1920s when the engine starters appeared. Firstly, the used voltage was 6V and then in the 1950s was regulated to 12V. The automotive industry was going towards 42V in the 1990s, but the evolution of motors, wiring and digital controls phased out the voltage switch.

The main parameters of a car battery is the physical dimension, current per hour (Amp hour), crank amperage (used to start the engine) and the RCM (reserve capacity minutes) – ability to sustain a minimum load for a certain period.

So now that we start the engine, it’s the alternator’s turn to deliver power to the vehicle.

How does an alternator charges a battery?

Let’s try and explain this in more simples words. Usually the alternator supplies between 13.5 – 14.5 volts DC (direct current) with the help of a voltage regulator.

The alternator is a motor that produces alternating current (AC) when the load is active. If you don’t know what alternating current is, I will leave you a link here. In short, alternating current is switching polarity back and forth as the rotor is turning inside the stator of the motor.

Alternating current (AC) means that the flow is equally positive and negative in a period. This current is not suitable for the car’s battery that uses direct current (DC).

Direct current (DC) is a current that does not alternate in a period and has a constant flow.

To achieve this, we use a diode bridge. The diode bridge rectifies the current and makes it positive on a load for each alternating phase. I will leave a video below so it’s easy to understand it.

The alternator is also a very important part in a vehicle because it provides a power supply to electrical components and charges the car battery. The car cannot function much time, only with the energy provided by the battery. As a result, this aspect is highlighted by an LED on the dashboard if the alternator stopped functioning.

How long it takes to charge the car battery?

It usually depends on the battery capacity and the charger used. I’ve created this table below with different battery capacities in (Amp hour) and different chargers used. You can find these specifications on the battery itself and on the device you use.

Battery capacity (Amp Hour) Device charging capability (Amp) Time to fully charge (Hours)
60 4 15
55 1 55
45 4 11.25
75 40 1.875
70 20 3.5
44 12 3.6

The idea is simple. You need to check the car battery capacity (Amp Hour) and you need to divide it by the device charging capability (Amp). Then you get the number of hours in which the battery will be fully charged.

As shown above, if we consider the alternator as the device, usually for a small car the current output is 60Amp at maximum rpm. Considering that, we can talk half (30Amp) at 2000 rpm. So it’s enough current to charge the battery, but usually, these values are regulated to increase the battery lifetime and to keep it within the limits.

Do car batteries charge when idling?

As seen above we can also make some estimation for this question. When the car it’s idling, it means that the engine is already ON and the alternator is running. Given this, let’s assume that the engine is idling at 900 rpm. For this reason, the output current of the alternator is around 15Amp.

As an example for a 45Amp car battery, it can be fully charged in approximately 3 hours. The answer is yes, the car battery charges when the car is idling and the engine is running constantly.

It must be remembered that in case the battery is old or is on a wear-out phase, even if it is fully charged the chemical components are not good enough anymore to keep the voltage stable for a long period. In fact, a car battery that cannot maintain the constant voltage, when it is not used for a longer time, it requires some attention.

Car charging system diagram

Here is a nice picture made by Sun auto service about the charging system in the newer car models:

car charging system

As you can see, there are a lot of components covered by us above. By using more electronic modules and reducing the size of the battery on newer car models, there is a need for an external power regulator to control the input and output voltage.


Although the answer to what charges a car battery is short (the alternator), we tried to cover a variety of topics that are related to the car battery charging. Most components of the charging system are very important and play a vital role in the good functionality of your automobile.

It’s essential to drive your car periodically so that your battery does not die because of not receiving enough voltage.

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