Maybe you’ve thought this before: “The battery starts the car. The alternator charges the battery. If the battery is dead, I will just have to drive long enough for the alternator to charge it.”
Makes sense, right? Too bad it is dead wrong.
How long does it take to charge a car battery from driving?
It takes about 4 to 8 hours at highway speeds to charge a car battery. That being said, you will absolutely never charge to 100 percent while you are driving.
If you look for “drive 30 minutes after you jump a car to charge the battery,” you will find it everywhere. Creations of this myth have been handed down from generation to generation. Perhaps they were even somewhat true then, when we had cars from the 1950s.
Is it still true today?
Interstate Batteries auto electronics expert Jeff Barron knows the real answer. Barron is the Interstate lab manager and has decades of experience with batteries and cars. He manages the quality operation at Interstate and speaks at ProClinic® seminars for automotive shop owners and technicians around the country.
His answer may surprise you.
“To charge a car battery, you would have to drive for a long time. Your alternator is not a battery charger,” he said. “There is no way your battery will get to 100 percent while you’re driving.”
Barron agreed that the myth is halfway true. It is true you can charge a car battery from driving and the alternator is doing the charging — if you’re driving with highway speeds. Below highway speeds, alternator is busy too.
All of the onboard electricity, from your air conditioning to the little lights on your roll-up windows, runs by the alternator. So does the computer that runs the transmission, the oxygen intake sensor, and hundreds of control modules.
That’s why Barron said you need to be above 1,000 rpms for the engine to feed your car battery an amp or two.
Also, the amount of time needed to charge a car battery is significant.
Regular battery chargers take 10-24 hours to charge a car battery, and that’s the quick, smart chargers. If you are charging using a trickle charger, you can be charging your battery for 3 or more days!
By charging the battery more slowly, you are protecting it. Charge too quickly and you can kill your car battery by overheating. Your car’s electrical system manages the charging voltage and amperage the battery requires.
Now, Barron has an important caveat: If you just jumped the car, you have a battery that is too depleted to start, or another issue unrelated to the battery. If the battery is bad, no amount of driving/charging will help.
“It’s already half gone. If a battery is so low that it can’t start your car, when it is only partially flat, it has permanently damaged itself,” Barron said. “Yes, there is a chance you did catch it as it just saw its death, but in the case where the car was weak overnight, the battery cannot take the same charge it used to. You should call the battery guys.”
To put everything in perspective, your car’s electrical system cannot charge the car battery faster than a plug-in charger.
At least you are producing a charge. Driving your car to charge a car battery is better than idling your car while doing nothing!
Can I charge a car battery while idling?
Good luck charging a car battery by idling — it could take days. If you just jumped a car, the battery is so low that it could not supply enough power to start the engine. Or, there was some other problem with the vehicle, and idling does you no good at all.
The simple action of parking and leaving the vehicle will not charge the battery. Idling at low rpm leaves no spare amps available for the battery to receive. The alternator is too busy running the electrical demands of the vehicle. The only time your engine is running hard enough to generate spare amps to charge the battery is at highway speeds.
But, at a stoplight or parked, you are not making spare amps.
And the bottom line is: no, you can’t charge a car battery while idling. A running engine may charge the battery a couple few amps at best, but it won’t come close to the hundreds of amps it took to crank the car and start the engine. After a several days of starting the vehicle and idling the engine, you will be steadily depleting the battery and not gaining anything.
How far do you have to drive to charge a car battery?
You just jumped your car with jumper cables, and your assumption is “I need to charge my car battery,” and you jump in the car and hit the road. Remember, AFTER you jump-start the vehicle, you need to get the engine running at highway speeds to have a real chance at charging the battery.
Let’s try to answer a very common question: How far do you have to drive to charge your car battery?
The short answer is: far.
As a quick reference for plug-in battery chargers, it takes 10-24 hours to charge a car battery. Same if we have an uncharged car battery that is 50 percent charged (and the reason it wouldn’t start).
A quick hypothetical scenario: With a 100 percent dead battery that we jumped, we drive the vehicle and continue to run the engine for several hours. It can take around 8 hours of highway driving to fully charge a car battery.
8 hours at 65 miles per hour = 520 miles.
It is roughly the distance from New York City to Columbus, Ohio.
If (big if) a car battery was charged and can take any charge at a near 100 percent rate of engineering, then you will simply at 75 percent or 80 percent battery charge at the end of that drive.
Because, as the battery charges, it requires more and more voltage. But, at such a high voltage, an alternator’s internal voltage regulation can simply pass max load voltage — which could destroy sensitive internal electronics or the onboard computers.
So, your alternator stops short of charging your battery.
How do I charge my car battery faster while driving?
To charge your car battery faster by driving, reduce onboard electricity consumption, and keep your rpm steady.
The eight-hour estimate assumes you are maintaining a constant 65-mph speed on the highway. Then, while driving as fast as you can, hold the RPMs constant. Changes in speed due to turns and hills will take away from what your engine was able to provide, as will the gear that you’re in.
Pick one straightaway and hold it for as long as possible without changing speed. If you are slowed down by ten minutes from traffic, you add ten that it will take to charge your battery.
To manage the electrical use from other functions of the vehicle:
- Drive without heating or air conditioning.
- Drive with all lights off except if necessary for safety reasons.
- Turn down the brightness of any touch screens to save power.
- Unplug a phone charger.
- Turn the radio off.
- You can also turn the brightness of your dashboard light down.
If you save some amps here and there, you can then redirect that power for charging your car battery while driving. Be aware that there are a huge number of hidden electronics that will also demand your alternator’s attention.
However, if you’re still planning to drive for hours to charge your battery, reconsider. It isn’t the most efficient or cheapest way to charge a car battery.
Please visit a repair shop. Request a battery test. The analysis you get will tell you much more than how much charge you have, but also, how many years (or weeks) you can expect from your battery. Then you can work on replacing your car battery with the help of a professional before it dies again.
And please also try to disavow the 30-minute myth. There is no science supporting it. Anyone who has ever told you driving your car for only 30 minutes will charge your car battery has only told you half of the immediate story. What do the battery experts have to say??