Does a Car Battery Charge when Idling?

You might know that a car battery charges while driving, but does it also charge when idling?

In other words, can the alternator which charges a car while driving also charge it when idling? An idling car is stationary with its engine running, so it’s in a different state than when driving or moving along. What difference does this make to charging the battery?

A car battery is charged when either driving or idling. An idling car has a lower RPM unless you’re revving continuously, so the time it will take to charge the battery will be much longer, not to mention the negative impact on the engine and environment. Charging a battery by idling is not an ideal way of doing so. If you need to charge it quickly, taking the car for a half-hour spin is better.

FactorsCharging while IdlingCharging while Driving
Engine RPMLowerHigher
Charging EfficiencyLess efficientMore efficient
Charging TimeLongerShorter
Alternator TypeAffects charging efficiencyAffects charging efficiency
Battery SizeAffects charging timeAffects charging time
Battery AgeAffects charging timeAffects charging time
Environmental ImpactNegativeLess negative

Continue reading to learn more about what difference idling makes and how it affects the charging time specifically.

An Idling Car

When a car is idling, the engine still spins at a lower RPM than when accelerating.

It’s not considered good for the engine to let your car idle for long periods. That’s because the engine is not used while idling for any useful purpose. Also, less oil pressure is created while idling, and exhaust fumes are pumped into the atmosphere.

fumes are pumped into the atmosphere by idle car

However, you may be surprised to learn that driving your car is not strictly necessary to charge the battery if that’s all you’re trying to do.

Technically, you can charge your car’s battery when idling. The alternator will continue to convert the mechanical energy (from the running engine) to electrical energy and charge the battery.

The only question is, how efficient is this for charging a car battery compared to charging it while revving or driving? [Note: The RPM is lower.]

How Long to Fully Charge When Idling

How well and quickly a battery charge depends on several factors.

The main factors are the type of alternator, the battery’s size, age, and how you charge it.

Normally, if you drive your car around, an alternator takes 30 minutes and one hour to charge the battery fully. It will take longer if you try to do the same while idling, i.e., with the engine running, without driving the vehicle.

If you leave it idling for a couple of hours, you will probably achieve only 80% charge, so it’s better to take the vehicle for a drive to make the battery charge quickly.

Alternatively, you could step on the gas to increase the engine’s RPM. That would make the battery charge quicker without going anywhere. But it isn’t good for the engine and environment.

So you can see that charging a car’s battery is best done by driving rather than idling for a long time.

The different types of car batteries and their estimated charging times when idling:

Battery TypeEstimated Charging Time When Idling
Lead-Acid Battery4 to 24 hours
AGM Battery2 to 12 hours
Gel Battery6 to 36 hours
Lithium-Ion Battery30 minutes to a few hours

Hybrid Cars & Electric

Hybrid and electric cars have different battery charging methods than traditional gasoline cars.

In a hybrid car, the battery is charged through regenerative braking, which uses the car’s braking system to generate electricity that is then stored in the battery.

The engine in a hybrid car also helps charge the battery when it’s running, but the main charging source is regenerative braking.

In an electric car, the battery is charged by plugging into an external charging station or using the car’s built-in charging system.

There are different types of charging stations, ranging from Level 1, which uses a standard household outlet and can take up to 20 hours to fully charge the battery, to Level 3, which uses high-power DC charging and can charge the battery to 80% in as little as 30 minutes.

It’s important to note that while hybrid and electric cars have different charging systems than traditional cars, the principles of battery charging still apply.

Factors such as battery size, age, and temperature can affect the charging time and efficiency, and it’s important to follow the manufacturer’s recommendations for charging and maintaining the battery to ensure optimal performance and longevity.

Image and Video References

Vehicle Freak:

Vintage Auto Garage

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About Alex Robertson

c3c9d43f1f0d14c4b73cb686f2c81c4e?s=90&d=mm&r=gCertifications: B.M.E.
Education: University Of Denver - Mechanical Engineering
Lives In: Denver Colorado

Hi, I’m Alex! I’m a co-founder, content strategist, and writer and a close friend of our co-owner, Sam Orlovsky. I received my Bachelor of Mechanical Engineering (B.M.E.) degree from Denver, where we studied together. My passion for technical and creative writing has led me to help Sam with this project.

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