How long to leave a car running to charge the battery? — explained

If you are taking a lot of short drives with your car, you may eventually end up with a low-charge battery. Still, as long as it’s not completely discharged, it’s possible to charge the battery by using a portable charger or jumpstarting the car.

What most drivers don’t realize is that you can increase the charge of your battery by simply running your engine. It’s all about letting the engine run for some time, so the question is – how long to leave a car running to charge the battery? You should leave the car running for 20 to 30 minutes to charge the battery, but it depends on a lot of factors as we’ll discuss.

Why does the car battery lose its charge?

To understand how you can fill up the charge of your battery by simply running the engine, you first need to understand why it loses charge in the first place. There are various reasons behind a draining battery, and if the charge gets too low, you might receive a battery discharge warning on the dashboard.

Car battery cells degrade over time, so that’s why you have to completely replace the battery if it dies. However, there are plenty of reasons why your battery might lose its charge without getting completely flat. If you use your car’s electronic features without running the engine, your battery’s charge only gets lower.

You might use the interior lights or the radio both significantly affect the battery charge if the engine isn’t running. Also, keeping your headlights on even for 10 minutes without starting the engine wastes a lot of charges.

Over time, battery connections and terminals also get corroded, and even extremely cold or hot weather affects the battery’s charge. Ultimately, your battery loses its charge if you only take short drives for a while. Since you can charge the battery by letting your engine run for 20 to 30 minutes, taking frequent short rides will deplete the battery.

Charging the battery via running engine

If your car’s battery loses charge, you can restore it by running your car for at least 20 to 30 minutes. The thing is – your battery builds up its charge once the engine is running. The alternator provides electric charge to the battery by using mechanical energy from the engine.

The alternator is powered by a serpentine belt and that’s how the rotational force gets transferred to the alternator. However, it takes some time for the alternator to charge the battery. That’s why you won’t be able to charge it if you only run the engine for a short period.

Constantly driving your car only for short drives depletes the battery charge and the alternator won’t have enough time to charge it up while the engine is running. So, if you haven’t driven your car for some time, it’s important to let the engine run for at least 20 minutes to be able to start it again the next time.

Running the car idle

As long as your engine is running, the serpentine belt is driving the alternator which powers the battery and electrical features of your car. However, running the car idle won’t be as efficient as driving your car for 20 to 30 minutes.

Once you start your engine, that cranking alone takes some charge, and it takes more time to recover this charge level by idling when compared to driving your car. Idling your car for at least 30 minutes will charge your car’s battery a bit, but it might take hours to charge it up in a way you would when driving it.

The best way to test your battery charge is by using a voltmeter after letting your car idle for some time. If the reading shows an electric charge of 12 volts, your battery has restored the charge completely. If it reads a lower charge, you’ll need to drive a bit longer to restore the charge.

What affects the battery recharge time?

While you’ll usually have to leave your car running for 20 to 30 minutes to restore the battery charge, the time depends on a lot of factors. If your battery gets completely dead, it’s not possible to charge it just by running your car.

Also, if the charge is too low, it might only be possible to jumpstart the car or use a portable charger to restore the charge enough so you can start your car in the first place. Here are some of the main factors that affect the time needed to charge the battery by running your car

• The condition of your alternator
• Age of the battery
• Battery type

The weather conditions could also affect the time needed to charge the battery. If it’s too cold, you’ll need to let the car run longer than 30 minutes. Likewise, extreme heat might affect the time and you might even be able to restore a higher charge to the battery by running it for less than 30 minutes.

Condition of the alternator

Running the car to charge your battery only works if your alternator is in optimal condition. If you have a bad alternator, the engine might be running without charging the battery. At this point, a low battery charge combined with a poor alternator might make it impossible to even start your car.

Also, you might notice a battery warning on the dashboard, and see your headlights dim. However, it’s possible to jumpstart a car with a bad alternator, although it’s just a temporary solution. If your alternator is bad, you’ll have to replace it and check your battery’s charge afterward, since it could all just be a consequence of the alternator.

Age of the battery

If your battery is old, it might be the reason why it doesn’t keep the charge for long. So, running your engine to charge the battery won’t do much difference in this case. Most car batteries last between 3 and 5 years, so you should check the age of your battery and measure its capacity with a voltmeter.

Older batteries tend to take much more time to charge by simply using the power of the alternator and running the car.

Battery type

A lead-acid battery takes a long time to charge as it has a high capacity. So, you also need to consider the capacity of your battery to determine the time needed to charge it. Overall, it’s best to rev your car since this causes the alternator to work harder and charge the battery faster.

Letting the engine run idle will ultimately be less efficient since the alternator won’t be working as hard as when driving and revving your car.

Our take

The question of “how long to leave a car running to charge the battery” has been present among drivers for quite some time. After all, it’s the best way to restore some portion of the battery charge if you don’t have jumper cables or a portable charger.

As long as your battery isn’t completely depleted, and the alternator is working, you should be able to generate enough charge by simply driving your car, at least until you recharge the battery properly.