Can a Car Battery Recharge Itself? [Here’s the Truth!]

A dead car battery can be annoying even though you don’t have to pay for a funeral. This is mainly because charging a car battery can be really inconvenient and even daunting if you’ve never used a battery charger or jump cables before.

If you’re in that situation, I can understand why you’re wondering if you can leave the battery alone and have it be charged after a while.

A car battery can recharge itself only when the car is running or idling. Cars have alternators that act as a power source and recharge the car battery once the engine starts. However, if your car is unable to start, the battery won’t recharge without external support such as a battery charger.

In this article, you will learn everything you need to know about recharging your car battery, including what to do if the battery is fully drained or partially empty.

Other things you will discover are the role of the alternator, the time it takes to charge, and whether you can charge the battery in your car without driving it around.

Can a Dead Car Battery Recharge Itself?

A dead car battery cannot recharge itself though if you manage to jumpstart the engine, the car’s alternator will recharge the battery while you drive. But without restarting the vehicle, recharging the car battery requires a third-party power source.

There are two levels of “dead” when it comes to car batteries. The functionally dead car battery is dead in the sense that it doesn’t have enough power to start the engine.

In that case, you can jumpstart the engine and let the alternator recharge the car battery. 

But if the battery has literally no charge or just enough energy that jumpstarting drains it, it is considered “actually dead.” When the car battery is actually dead, you need donor electricity or a car battery charger.

A car battery charger or a tender can not only be used for charging the battery but it can also be used to ensure that your battery doesn’t die when your car has been sitting for too long without use.

This Junior Battery Tender [on Amazon] comes with an option to maintain a full charge on our car battery and your battery’s health even when you don’t use your car that often! It is easy to plug in and safe to use!

Can a Car Battery Charge Itself If You Let It Sit? 

Photo by Adrian Dorobantu from Pexels

You don’t have to drive in order to charge the car battery. As long as your car’s alternator is running, the battery is going to get charged. Since the alternator is engine-contingent, you have to start the engine even if the car is not moving. As you may know, this is called Idling.

Generally speaking, a car battery can be charged while idling because the alternator is operating even if the car is not being driven. But if a car battery is sitting in a parked car with the engine turned off, it cannot be charged by the car.

Idling uses fuel which is converted to electric energy eventually stored in the battery. You should turn off the stereo system, air conditioning, and interior lights, so the electric current from the alternator goes to the battery instead of getting divided to serve the car’s electric load.

Can a Car Battery Recharge Itself While Driving? 

A car battery gets recharged by itself while driving as the alternator feeds electric power to the battery. The source of this electricity is the fuel your engine runs on, and it is responsible for powering the car’s electronics as well.

Depending on the model and make of your car, driving might speed up your alternator’s rotor. Some cars have an electricity-generating mechanism in the wheels.

The rotation of such cars’ rims can generate energy that powers the electronics in the car while the alternator directly powers the battery.

Can a Car Battery Recharge Itself Overnight? 

A car battery cannot recharge itself overnight because it requires a third-party source. Usually, the car’s alternator acts as this source, giving the appearance that the battery is being charged by itself. 

But running the engine overnight to charge the battery would be an inefficient way to charge the battery when you can charge it through a direct power source at home.

Can the Sun Charge the Car Battery? 

The sun cannot charge the car battery. Placing the battery in sunlight can cause overheating and permanently damage it. The reason many people believe that the sun can charge a car battery is that freezing weather can sometimes prevent a battery from operating. 

When such a battery is placed in a warm spot (not direct sunlight), it can start delivering power after a while. This creates the illusion of the car battery getting charged because of the sun when the sun only thaws the frozen electrolyte fluid.

How Exactly Does a Car Battery Charge?

A car battery charges through the alternator, which generators electricity from motion. To understand the process, you need to track the energy. In most vehicles, the car battery provides electric energy that delivers the spark that starts the engine.

All electronics, including the interior lights and the stereo system, are powered by the battery until the engine is started. 

Once the engine is started, the alternator takes the electric load. The gas tank of your car contains chemical energy. This energy turns into kinetic energy as it produces motion.

A common misunderstanding regarding this operation is that the car’s movement charges the battery. 

The car’s wheels do not act like windmills charging the battery, at least not primarily. The rotor in the alternator spins, and its motion produces electricity which feeds the energy demands of the car’s electrical components and also powers the battery.

Since the electric cost of the battery’s power and the alternator’s power is almost the same, it is unwise to use battery power since it takes a long time to fully charge the battery once it is significantly drained.

How Long Does it Take to Charge a Car Battery? 

Blue Silver Black Car Engine – pixabay

It takes anywhere between 30 minutes to 24 hours to charge a car battery based on the method of charging and the degree to which the battery is drained. There are two ways to charge the car battery. The primary method is to charge by driving, and the alternative is to use a charger.

Driving Duration to Charge a Car Battery

If your car battery has lost less than 50% of its power, driving is a practical way to bring it to 100%. It’s mainly because the car will start without a problem, and as long as you keep the electric load low, the battery will be back in 15 minutes on your regular commute.

You will not need to take time and drive extra miles just to charge the car battery. To recharge a car battery, you need to drive around 30 minutes on the highway or 50 miles.

This is feasible for most American adults who drive to work. If you drive 15 minutes one way and take the same time returning, your car battery will be charged. But if the car battery is fully drained, getting it to full charge requires a lot of driving, which is not feasible.

You not only need to drive for hours but must do so in a single session because turning off the engine is risky as there is no knowing if the car will start back up.

Unless a multi-hour drive makes sense in your routine, you will be driving more than you usually do, which is more expensive than using a charger. That said, you’ll likely save time because the duration to charge a car battery while driving is significantly shorter.

Duration to Charge a Car Battery Using a Charger

Using a charger, you can charge the car battery in 2 to 4 hours. If the charger is low amp, it can take up to 24 hours to fully charge your car battery. The table below best indicates the charging times of different amp chargers.

Charger AmpDuration to fully chargeDuration to functional charge
2 Amp Charger48 hours24 hours
4 Amp Charger24 hours12 hours
10 Amp Charger6 hours3 hours
20 Amp Charger4 hours2 hours
40 Amp Charger1 hour30 minutes

Please note that the most common car battery charger is the 2 Amp variety, which means you’ll need to charge the battery for the whole day for it to be functional. Alternatively, you can take the battery to a mechanic. 

Automotive mechanics usually have higher Amp chargers and can charge the battery in a few hours. The problem with this approach is getting to a mechanic when you cannot drive.

Hybrid approaches are more practical because you can factor in your routine drive, the initial energy required, and the time it takes to charge a battery. 

The ideal hybrid charging approach would weave together charging the battery with a charger and driving to charge the battery to full capacity. Depending upon whether you have a higher Amp charger, you might also include taking the car to a mechanic.

Charging the car battery with a charger    • You can do other things while the battery is being charged
    • It can be done even when the battery is actually drained
    • Takes too long
    • Requires a little more expertise
Charging the car battery by idling    • You can do other things (within reason) because you’re not focused on driving.
    • It takes less time than using a charger
    • Needs a higher level of supervision
    • It does not work if you cannot start/jumpstart the car
Charging the car battery by driving    • It is the quickest way to charge a dying car battery.
    • It can usually be done without breaking your daily routine.
    • Requires jumpstarting/starting the engine
    • It is not feasible when the battery is nearly empty, and you don’t usually drive a lot

What to Do If Your Car Battery is Dead? 

If your car battery is dead, but the alternator is working, you can try to jumpstart the car and drive with minimal electric load to get the battery charged. In case you’re at home or near a mechanic’s, you can use a charger initially.

But since car chargers can take a while before fully charging a battery, it is okay to charge the battery to around 30%. The rest can be safely charged with the car’s alternator.

Please note that if your car battery dies mid-drive, there could be issues with the alternator. That requires a separate set of steps, for which you should read my post on what to do if your car battery dies while driving.

In most cases, when you’re dealing with a dead car battery, you need to charge the dead battery at home. Some people prefer to take the car to the workshop. There is no universal answer as to whether the latter is bad.

But if your car’s alternator is working, there is no reason why you should not charge the battery at home.

How to Charge a Dead Car Battery at Home?

To charge a dead car battery at home, you must use a charger or jumpstart and idle the car. If using a charger, you must get the charger out in an open, clear space near an electric outlet. Then, you should place the battery on a raised, non-conductive surface and connect the appropriate clamps to it.

Turn on the electric supply and let the charger work for at least one hour. Depending on the amp rating of the charger, it can take anywhere between 4 hours to 48 hours to fully charge the car battery.

It is best to charge the battery to the point where you can start the car and then use the car’s alternator to charge the battery. 

Alternator ratings start at 60 amps and go up to 200 amps. In contrast, a car battery charger usually generates 2 amps, but advanced chargers can have a 40-amp rating.

In other words, the slowest car alternator charges the battery faster than the fastest battery chargers on the market. That said, not all cars’ alternators charge while idling.

So in the rare instance that your alternator simply powers the electronic components of your car when idling and does not charge the battery, you might not have any option except to rely on the battery charger unless you want to go on a 30-minute highway drive.

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