How Long To Leave A Car Running To Charge Its Battery?

(Last Updated On: July 19, 2023)

As you must have guessed, there are various ways to recharge a car’s battery. And so, in this article, we will consider one such method: leaving a car running to charge the battery. How exactly does this work, and how long to leave a car running to charge the battery?

If you are here to get an idea of the time frame, leave the car running for approx. 15 – 30 minutes will put enough charge in the battery to turn the vehicle off and restart it again. Please bear in mind that every car has a different alternator and charging system output, so no two vehicles charge at the same rate. The other thing to consider is if the battery has a problem like it no longer has any fluid (distilled water), it can no longer hold a charge and will lose its charge as quickly as you put it back in.

How Does A Car Battery Charge? 

It’s no news that a car uses tons of power to ignite and run other accessories in the vehicle; due to the car’s energy consumption, the car battery can quickly run low. So, cars use a rechargeable battery and a self-charging system, otherwise known as the alternator.

A car battery comprises lead plates in its core that are submerged in a mix of sulfuric acid and distilled water. Each lead plate connects to a metal rod covered by a metal nub at the top of the battery. These nubs/terminals are what the battery cables connect to.

When a car starts, an initial jolt of electric current brings a chain reaction within the battery that produces electricity. This current is transferred to the starter to get the motor running. This process is continuous, so the vehicle requires an alternator to regulate and input charge back into the battery.

An alternator is like a car’s generator and is one of the critical components that make up the electrical charging system of a vehicle. The alternator is a part of the car’s engine made up of a magnetic coil and rod. Its function is to convert the mechanical energy of a vehicle into electrical energy. Powered by a belt, the electrical energy passes through to the battery, charging it. It uses an electromagnet to keep the charging system fully functional and the battery in power. The battery, in turn, can bring to life other electrical elements of the engine, including the starter, with which the vehicle can start

How will Leaving The Car Idling Charge The Battery? 

Technically, leaving your car idle can help charge the battery. The magic is in the alternator, the generator of the vehicle. As earlier highlighted, the alternator manages the car’s battery, which keeps electricity in supply to continue giving power to the battery. This is only possible if the car’s engine is running, as this will keep the alternator pulley rotating, producing more electricity and charging the battery.

How Long To Leave A Car Running To Charge Its Battery

Considering a flat battery, the time given for the alternator to recharge the battery depends on a couple of factors—the charging output of the alternator and the car’s battery’s ability to accept a charge. I.e., if the battery is at the end of its life, it may not longer hold a charge or accept it.

To fully charge a car’s battery, driving long distances is expected as the car will need to be kept in motion for a long time. However, keeping the engine at idle for about 15 – 30 minutes should get the battery powered up if it is serviceable. Then the alternator can fully charge the battery when you hit the road.  

If driving the vehicle to charge the battery, one thing to be mindful of is putting more back in than you take it out. To explain, if it’s a cold dark day, having the headlights on, the heater at full blast, and the radio on, you are using a considerable amount of electricity. So, it will take a long drive to charge the battery fully. Obviously, don’t drive around with the lights off when it’s dark outside!

Does Revving The Engine Charge The Battery Faster?

Revving an engine makes the crankshaft of a car turn faster, and practically, the faster the crankshaft of the car turns, the quicker it turns the belt running the alternator, which produces more electricity when it runs faster. However, revving an engine to charge a car battery is only safe for about 30 seconds; further, usage may cause damage to the battery or other components of the car.

How To Know If The Battery Is Being Charged

There are several ways to tell if your battery is being charged. The most efficient way is to make use of a voltmeter. But if you do not have one, there are other methods you can use to know this. Firstly, check to see if the charging indicator is on if the car has one. If this is the case, then your battery is possibly charging.

Another way would be to turn on your headlights; if they appear dim but brighten up when you rev the engine, the battery will likely be charging. But the most reliable way you can be confident your battery is charging is to use a voltmeter. With a voltmeter connected to the battery and the vehicle idling, a good alternator should charge between 13.4 and 14.4 volts.

How Long Will A battery Last Without Being Charged

The lifespan of a car battery without being charged is not a lengthy one. A new car battery can last about two weeks or more before needing a charge. And if not charged, it can endure for up to two or three months before hitting the flat line.

As soon as you turn off your car engine, your car battery stops charging. The car battery can remain in this state for a few weeks or months before it loses its charging capacity. So, a recommendable way to maximize your car’s battery life will be to have it plugged into a trickle charger if being stored for an extended period. Failing that be prepared to keep a battery jump pack handy.

To summarise

Leaving your car running is the best way to ensure your car battery is adequately charged. The longer the car is on, the better chance of you achieving a fully charged car battery. Around 30 minutes of idling, the car alternator will have put in enough charge (providing no other faults) to safely switch off the engine and back on a short while later.

My name is Tom although my friends call me Tommy. Messing around with cars and bikes has always been a hobby of mine even from a young age. So I made it my day job 17 years ago. I am a fully qualified mechanic as you would expect. I've worked in all different areas of the motor trade, valeting, panel beating, engine repairs, I'm sure you get the idea. I enjoy sharing my wealth of knowledge and experience with others, which is the reason I spend a lot of time here writing for this website.

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