Does a Car Battery Charge When Idling?


We all know that when you drive your car — the alternator charges the battery. But will the same happen if the car is idling? 

This is a question often asked by those who don’t drive a lot and have their battery go flat frequently, and want to know if leaving the car idling will help charge the car battery. But if you’re just curious if the battery charges while idling:

Yes, assuming everything is in working order, the alternator should charge your car battery while the engine is running, even if it is idling. This is because your alternator is actually powered by the engine, so as long as it is making enough rpm’s, it should charge the battery.

The only catch is that it doesn’t charge very quickly. This is due to the fact that the engine has no load on it when your car is idling. Meaning idling RPMs are much lower than RPMs when driving down the road. Hence the serpentine belt will spin more slowly and produce less electricity to recharge your battery. Later on, we’ll explain why and how it works.

But It could be the case that the electrical components are burning electricity faster then it can charge the battery, like the:

  • AC System
  • Fuel Pumps
  • On-board Computers
  • Electronics attached
  • Etc,


The alternator is not really designed to charge your battery. It is only really designed to maintain its charge, so asking it to charge your battery repeatedly can shorten its life as well. Since it will likely be subjected to sudden voltage spikes, because there isn’t a good “pool” of voltage that a healthy battery provides.

Allowing your battery to go flat repeatedly will significantly reduce its lifespan. Besides, idling isn’t a good idea for a variety of reasons that we’ll go over later.

If you are concerned with your battery going flat, the answer is simple; get a trickle charger (also known as a battery tender!) They are generally cheap, and will minimize the damage done to your car compared to other methods.

And if you can’t or don’t want to, simply driving is a much better option, since that gets things up to operating temperature and gets all the fluids going. Idling for extended periods of time can take a beating on a car.

Related: Does a Car Battery Drain Faster In Cold?

Why Idling Can Charge Your Battery

First, you have to understand that the alternator is mechanically coupled to your engine’s crankshaft. So when the engine is running and producing RPM’s, that rotation from the engine will power the serpentine belt (also called a drive belt,) which will in-turn power the alternator and everything along the belt.

So even when your car is idling, the engine is still producing enough RPM to power the alternator, which in turn charges the battery. You can read more about the car’s charging system here

For example, a stock alternator in a 1997 Camry would produce ~80-90 amps while driving, and around 60 amps at idle (600 engine rpm.) Which is less than when driving, but still sufficient to charge the battery, albeit at a slower rate.

Essentially, the energy for powering the alternator comes from the combustion of gasoline. So all of the energy used to power your car’s electronics, actually comes from gasoline.

Why Idling Is Not A Great Idea

Aside from the previously mentioned reasons, such as potential alternator and battery damage, many modern car include sophisticated battery management systems designed to help extend battery life. As a result, your car’s ability to charge at low RPMs may be limited.

Besides, cars are not designed to idle for long periods of time. As when the car is idling, the engine wont be operating at it’s peak temperature, meaning fuel wont combust completely, leaving residue and increased exhaust particles, which will begin to carbon-up the engine, resulting in decreased performance, increased emissions, and eventually, although unlikely, component failure (i.e. catalytic convertor).

Also, condensation forms in the exhaust, and the car will be unable to blow this buildup of water out of the tailpipe without the high airflow provided by driving. This will eventually cause the exhaust system to rot and potentially fail prematurely.

The exhaust from your car also pollutes the air even more than usual when Idling, as it produces more exhaust particles than any other activity, which is bad for the environment.

How Long Should You Idle To Charge The Battery?

If you don’t care about the impact of idling, it can take several hours to charge your battery by idling. Considering that an idling engine produces low rpms, combined with the fact that there are probably other electronics that are draining the battery, it will charge much, much slower, than what it would usually take.

And at that point, it’s not practical at all, and is much better to take your car for a spin, or get a battery tender, instead of just leaving it idling.

How Long Should You Drive To Charge The Battery?

A good 20-30 minute drive should be sufficient to charge the car battery more than halfway. But if the car will be sitting for extended periods of time, It’s advisable to invest in a trickle charger. They are available for around $25 and will always keep your batteries charged.

Read: The Truth About Seafoam: Negative Effects

Keep Your Battery Healthy!

Above all, whether you’re idling or not, make sure your battery is in good shape to begin with. You don’t want to repeatedly discharge it to the point of damaging it, and if you’re noticing that the battery isn’t lasting as long as it used to, it’s possible that it already happened.

Related: How Long Can a Car Battery Sit Unused?

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