You might have had this thought: “The battery starts the car. Alternator charges the battery. If the battery’s dead, I just have to drive long enough for the alternator to recharge it.”
Sounds perfectly logical. Too bad it's dead wrong.
How long does it take to charge a car battery from driving?
About four to eight hours at highway speeds is what it takes to actually charge a car battery. However, it will never reach 100 percent while you’re driving.
If you look it up, you might see “Drive 30 minutes after you jump a car to recharge the battery.” Versions of this myth have been passed down for generations. They might have even been somewhat true back to the 1950s car culture.
Is it still true today?
Interstate Batteries auto electronics expert, Jeff Barron, knows the real answer. Interstate lab manager and decades-long expert on all things batteries and cars, Barron manages quality at Interstate and hosts ProClinic® seminars for automotive shop owners and technicians across the country.
His answer might shock you.
“You would need to drive for a long time to charge a car battery. Your alternator is not a battery charger,” he said. “There is no way your battery will get to 100 percent while you’re driving.”
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Barron affirmed that the myth is half-true. Yes, you can charge a car battery from driving. Yes, the alternator does charge the battery — if you’re driving at highway speeds. Otherwise, the alternator is busy. All the onboard electronics, from your AC to the little lights on your roll-up windows, run on the alternator. So do the computers managing your transmission, the sensors tracking oxygen intake and hundreds of other control modules.
That’s why the engine needs to be at least 1,000 rpm before it feeds your car battery an amp or two.
Plus, charging a car batterytakes time.
Actual battery chargers take 10-24 hours to charge a car battery. That’s the fast, smart chargers. If you’re using a trickle charger, you could be charging your battery for three or more days. Charging slowly protects the battery. Charge too fast, and you can overheat and kill your car battery. Your car’s electrical system manages the charging voltage and amperage that your car battery needs.
Barron has one more important reminder: If you just jumped the car, you’re facing a battery too depleted to start the engine or a deeper engine issue. If the problem is the battery itself, no amount of driving and charging will bring it back.
“It’s already half gone. If a battery is so low that it can’t start the car, then it’s already got permanent damage,” Barron said. “There’s a chance you got to it just as it ran down, but if it’s been weak overnight, that battery can’t hold as much charge as it used to. You should go looking for a new battery.”
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However, the electrical system in your car can’t charge your car faster than a plug-in battery charger.
At least you are generating a charge. Driving to charge your car battery works better than idling the engine.
Can I charge a car battery while idling?
It could take days to charge a car battery by idling. If you just jumped a car, the battery is either too depleted to start the engine — or something else is wrong and idling won’t help anyway.
Leaving your car parked will not charge your battery. The engine’s rpm reading is low, and the alternator is running all the electronics. At highway speeds, your engine’s running hard enough for the alternator to send spare amps to the battery.
But when parked, the engine doesn’t have spare amps.
No, you can’t charge a car battery while idling. At best, it’ll charge the battery a few amps, but not nearly the hundreds of amps it took to start the engine. After a few days of starting and then idling the engine, you’ll steadily deplete the battery instead of charge it.
How far do you have to drive to charge a car battery?
You just jumped your car. You figure you need to charge the battery. So, you’re hitting the road. Your engine needs to be running at highway speeds for the alternator to start seriously charging the battery.
How far do you have to drive to charge your battery?
The short answer: Far.
Plug-in battery chargers estimate 10-24 hours to fully charge a car battery. Let’s assume your car battery is 50 percent charged. (Which is probably why your car wouldn’t start.)
It can take about eight hours of highway driving to charge a car battery to almost full.
Eight hours at 65 mph is 520 miles.
That’s roughly the distance from New York City to Columbus, Ohio.
That eight-hour road trip will charge your car battery to 75 percent or 80 percent. That might be as far as your alternator can take it. You see, as you charge a battery, it takes more and more voltage to raise the charge. However, such high voltages will also wreck your onboard computers. So, the alternator stops just short of giving your battery the full charge.
Need to change your battery? Let’s test it first.
Visit any place Interstate® is sold for a fast, accurate battery test to see how much longer you can trust it. We can help you change your battery before it dies.
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How do I charge my car battery faster while driving?
To charge your car battery faster while driving, conserve onboard electricity and keep your engine rpm constant.
The eight-hour estimate also assumes you’re traveling at a constant 65 mph. Hit a highway speed to charge your battery. Then keep the rpm constant. Road turns and hills can divert the engine’s power. So will shifting gears. Pick a straight path and hold it as long as you can at the same speed. A 10-minute slowdown from traffic will add 10 minutes to how long it’ll take to charge your battery.
Conserving onboard electricity means turning off any optional electronics:
- Drive without air conditioning or heating.
- Only turn off your headlights if it’s safe.
- Turn the brightness down on any touch screens to conserve power.
- Unplug your phone charger.
- Turn off the radio.
- You can also turn down the brightness of your dashboard lights.
Saving enough amps here and there will redirect power to charging your car battery while you drive. Keep in mind, dozens of hidden electronics also draw your alternator’s attention.
In the end, driving for hours isn’t the most efficient nor cost-effective way to charge your car battery.
Instead, visit a repair shop. Ask for a battery test. The analysis you get will show more than how much charge your battery has in it at the moment. You’ll uncover how many years (or weeks) you have left in your battery. Then you can work with a pro to replace your car battery before it dies again.
Also, try to kill the 30-minute myth. Science doesn’t support it. Whoever told you that driving for 30 minutes will charge your car battery only shared half the story.
Take it from the battery experts.
Charge or change your car battery? Find out for sure.
After you jump a car, there’s a chance the battery is still good. To be sure before you change it, visit Interstate All Battery Center® store or your nearest location where Interstate® is sold. A battery test will give you the analysis you need.
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