Battery Equivalents and Replacements

Car Electrical System Guide: How Does a Car Electrical System Work?

A car electrical system consists of a few general components that are shared among all the car brands, but it can also have a few add-ons that may be very specific.

The car's electrical system ensures proper operation of the car and any malfunction of its components can stop the car. Fortunately, there are few warning signs that allow the user to predict when will the malfunction occur.

Published: November 25, 2021.

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toyota 4x4

Intro to Car Electrical System

Car electrical system of the modern cars may be rather complex, but nonetheless, it is still very similar to the car electrical system from decades ago, at least regarding the most basic components which may include:

- car battery stores energy and release it when required for powering various loads when the main engine is turned Off, including engine starter, lights, fans, and similar.

- engine starter requires plenty of electric energy in a short period of time in order to successfully crank the main engine. Initial currents may go up to, or even more than 1000 Amps, but after just a second or two (or even less), this current quickly drops down to several hundred Amperes.

- alternator is physically connected with the main engine, usually with the transmission/drive belt, and is used to convert mechanical energy into electric energy, which is then used to recharge the battery and to power electric loads while the engine is running.

Other typical loads include various lights, fans, windshield wiper motors, multimedia devices, security systems, and similar.

Additional loads depend on the vehicles and may include various pumps, winches, etc.

Car Battery

The main car battery is still mostly lead-acid starting or dual-purpose battery, with lithium-ion starting batteries slowly emerging on the market, too.

The most common lead-acid starting batteries are wet/flooded batteries, enhanced (maintenance-free) wet/flooded batteries, Absorbent Glass Mat (AGM) batteries, and Gel-cell batteries.

The nominal voltage of the car battery is 12V, although in real life it varies and goes up to 13-15 volts, depending on if the engine is ON, battery charge, electric load, and similar.

The main task of the car's battery is to crank the engine - the battery must be able to provide huge currents for a period of a just few seconds.

Also, when the engine is Off, the main battery must provide energy for systems like security system/alarm, doors lock, lights, radio/multimedia, and similar.

Cars with "Stop & Go" systems require that the battery is able to provide huge currents periodically (whenever the car stops and starts because, for example, traffic lights) and that it must accept charge quickly. For short, if your car has a "Stop & Go" system and You have to replace the battery, be sure to replace the old battery with the new one that supports such use.

Engine Starter

Engine stater is used to spin the main engine and it requires huge currents to do that, regardless if the engine is gas or diesel, although diesel engines require more power due to the higher compression ratios and, up to the point, due to the fuel heaters.

When the ignition key is turned ON, or when the user presses the car's On button, the engine starter is not running yet and in that very short moment, it is practically a short circuit for a battery which then must provide extra current for fraction of a second.

And when the car starter starts to spin, its internal resistance increases, leading to the current drop.

Most passenger cars have a maximum engine starter current below 1000 Amps, which quickly drops down to a few hundred Amps.

When the engine is started, the engine starter is turned off and not used anymore until it has to crank the engine again.


When the engine is running, it is powering the alternator which is mechanically connected to the engine via a drive belt.

car electric system 2

Alternator typically produces AC current which is then converted into the DC current via charge controller that can be based on a simple diode bridge rectifier combined with several capacitors to filter out RF (Radio Frequency) interferences or the charge controller may be an advanced unit with much more features:

- a simple diode bridge rectifier combined with several capacitors actually uses the car battery itself to additionally stabilize the car's voltage.

- advanced battery charger/charge controller combined with the alternator is controlled by a microprocessor, allowing the manufacturer (or user) to accurately control the voltage and the battery charging current. Such charge controllers are not very common yet, but they also may come with the battery temperature sensors, ensuring that the battery is not undercharged during winter and not overcharged during summer, for example.

As starting battery, the car battery must be able to provide energy for the engine starter to start/crank the engine.

But, the car battery also must be able to provide energy for various loads when the engine is not running - such use may discharge the main battery, preventing it from successfully starting the engine.

If the user has larger loads that must be powered while the engine is Off, dual-battery systems are required.

Dual-Battery Systems

Up to now, we have a 12V car electrical system with the car battery, alternator (with its charge controller), and engine starter.

When the user has the need to power larger loads without discharging the main car battery, an additional mostly dual-purpose or deep-cycle battery is added to the car.

This second battery may be used for powering multimedia devices, winches, pumps, fans, appliances, and similar, without the danger of discharging the main battery, as long as the main battery electric circuit and the second battery electric circuit are separated.

The second battery electric circuit may be separated from the main battery electric circuit by a simple power diode or by using a dedicated charge controller which has ground connected to the car chassis, power input is connected to the 12V of the main battery and its power output line is connected to the 12V of the second battery.

Such dedicated charge controllers may support even lithium batteries of 12, 24, or even higher voltages.

Since the second car battery may overload the standard alternator, manufacturers often add larger and stronger alternators for dual-battery car systems.

car electric system 1

Hybrid Cars

Most hybrid cars feature two batteries:

- the first battery is a "standard" starting battery that is used to crank the main engine.

- the second battery is a large lithium battery which is used for powering an electric motor (or electric motors) which are used for propulsion.

Now, there are several different types of hybrid cars regarding the internal design - some models use the main engine only for powering the main generator which is then used for the charging of both batteries, while other models use the main engine for both charging the batteries and for propulsion.

Note: both regular, hybrid and electric cars may also have a backup battery which is used to power the electronics when the main battery (or batteries) are disconnected from the car's electric system, preventing issues with the onboard computer and security systems.

Since large lithium batteries may also accept the charge rather quickly, hybrid cars also support regenerative braking feature - as the car is being slowed down, its kinetic energy is converted into electric energy used to charge the main lithium battery (chemical energy). The regenerative braking feature improves the fuel economy significantly, especially in urban areas where there are many speed changes.

Electric Cars

Electric cars feature a single large lithium battery used for storing the energy used for propulsion.

Also, electric cars may also feature additional small backup batteries for electronics.

As such, electric cars don't have internal combustion engines, not even for electricity production - electric cars must be recharged periodically.

electric car

This provides electric cars with the opportunity to be Eco-friendly vehicles with zero carbon emission, which they are as long as the electricity used for charging them comes from renewable sources like wind or sun.

Note: Hydro-energy is a renewable type of energy, but building dams can lead to various environmental issues...

Personally, lithium-ion batteries are excellent batteries for electric cars that don't require to have a long range on a single battery charge. Since lithium-air batteries are still far from being commercially available, electric cars with lithium-ion batteries will have a range much shorter than the cars with internal combustion engines (regular or hybrid cars). For short - go hybrids! :)

Note: Please, don't get me wrong, electric cars are great, but as soon as You end up in a traffic jam during the summer heat, You will love your hybrid car much more ... Or the petrol/gas-powered car will tow the diesel generator in order to recharge the electric car ...

Most Common Car Electrical Problems

 Most common car electrical problems include:

- engine will not crank: if the engine will not crank, then there is either an issue with the engine starter or with the weak or even dead battery.

If everything else is working properly and the battery voltage is above 12.8 volts, check the engine starter.

If the voltage is below 12 or so volts, the battery is discharged for some reason - in that case, jump-start the engine and recharge the car battery. Also, be sure to find the reason why the battery was discharged in the first place - maybe some device was left running for a long time, or perhaps there are alternator issues.

- dim lights: if the lights are dim, check their connections which may be loose. But, if the lights are dim and the engine will not crank, the battery is discharged. In that case, jump start the car, recharge the battery and find the reason why the battery was discharged. If the battery is too old, consider replacing the battery.

- tripped fuses: if some devices/electric loads are not working, while others are working properly, check the fuses box, maybe there are fuses (or just one fuse) that were activated for some reason - replace them (or it) and if that repeats take the car to the repair shop to find the reason why it is happening.

Note: never replace a fuse with a stronger one.

- loose cables may cause many issues and are often hard to find. Loose cables cause bad connections, that may appear from time to time, or can cause constant problems, similarly to corroded battery terminals, which are much easier to spot.

These are just a few of potentially many problems that can happen with the car electrical system - knowing how the car electrical system works may help the user or mechanic to diagnose the problem.

But, most modern cars are computers with wheels equipped with large monitors displaying various information to the user, often in real-time.

While spotting the issues in such complex systems may be a problem, a large number of sensors monitors various parameters and inform the user of current status and predict the potential issues - for short, as long as there is no message like "Complete Electronic Failure!" or something similar, check the car's status messages and act accordingly.

car pull over

Car Electrical System Frequently Asked Questions (FAQ)

Here are some of the most popular Frequently Asked Questions (FAQ) regarding car electrical systems and their issues.:

What causes electrical problems in cars?

There are many reasons why electrical systems fail, including high temperature, low temperature, vibrations, mechanical impacts, aging of materials and devices, accumulation of dirt, and similar.

Are cars AC or DC current? Are 12V batteries DC or AC?

Cars use Direct Current (DC) electrical systems. Alternate Current (AC) systems are rarely used, mostly for powering various appliances, tools, devices, and gadgets that are operated via AC. AC current can be produced using simple 12V DC-AC inverters.

12V car batteries are Direct Current (DC) electric power sources.

Why do cars use DC current?

AC current is very easy to convert to higher voltages, enabling the energy to be transported to large distances (thousands of kilometers, if required).

Cars use lead-acid batteries which are Direct Current (DC) electric power sources (lithium batteries are also Direct Current (DC) electric power sources).

What are the components of a car electrical system?

The main components of a car electrical system are battery, engine starter, and alternator.

What are the most common car electrical system problems?

The most common car electrical problems include weak/dead batteries, faulty/triggered fuses, dead light bulbs, loose cables, and similar.

Few Final Words

car battery amps mTroubleshooting the car's electrical problems can be a daunting task, especially in complex systems with limited diagnostics.

However, knowing how things work can help in both avoiding and solving the problems.

Personally, always check the status messages of your car and if required, take it to the repair shop before the problem actually stops the car.

Also, if You intend to repair the problem yourself, don't do it unless You are absolutely sure what are You doing. And whatever You do, stay safe!