How to charge a car battery after a jump start

How Long to Drive a Car After a Jump Start

Quick Summary

• It takes 4 hours of idling to charge a dead car battery after a jump start.
• It takes at least 30-60 minutes of driving at highway speed to charge a battery after a jump start.
• You also cannot charge a battery after a jump start by driving around the neighborhood.
• Charging your battery with a modern battery charger equipped with a reconditioning setting is the best way to recharge your battery and undo some of the damage from the discharge.

See this post describing how much power a modern car consumes when idling.

Article

After someone helps you jump-start your car, you’ll most likely hear them add this advice: “Just let it idle for a while.” Let me be blunt: that advice is wrong. I’ve tested hundreds of charging systems over the years, and I can tell you that it takes a really long time to recharge a dead battery by letting your car idle.

And if you think driving around the block will do it? No chance.

Let me walk you through what really happens inside your charging system—and exactly how long to drive the car after a jump start to bring the battery back to life.

Why You Can’t Charge a Battery After a Jump Start by Idling

When I test the idle current draw of late-model vehicles, I routinely see them draw 30 to 50 amps. That’s far more than in the old days when the ignition and fuel system drew less power. Today’s alternators not only have to power the power-hungry direct-injection and ignition systems, but also multiple computers and ADAS systems. Even though modern alternators are rated at 150 amps, they only produce 25%–35% of their rated output at idle.

In other words, at idle;

Almost all of the alternator’s output is used by the ignition, fuel pump(s), fuel injectors, ECM, BCM, ABS, and SRS computer, as well as the camera, radar, and lidar for the ADAS systems.

Idling a car for four hours might add a tiny surface charge, but it will not fully charge a battery after a jump start. And the real danger? You can actually overheat your alternator during long idle periods while charging.

See This Warning From Interstate Battery:

“You can charge your car by driving, but only when you drive at highway speeds for a significant period of time. The alternator charges your car’s battery, but some factors may affect the alternator’s ability to charge the battery, including:

The amount of current diverted from the alternator to the battery
How long the current is available (the drive time)
The temperature of the battery
The age of the battery
………..the alternator is not a true car battery charger. It’s running all the onboard electronics. The best way to charge your battery is with a charger. “

Driving After a Jump Start: How Long Is Enough?

If you insist on using the alternator to charge a battery after a jump start, here’s the truth:

You must drive at highway speeds for 30–60 minutes. Here’s why:

Alternators reach their maximum output only at 2,500+ engine RPM. However, today’s CVT transmissions are more efficient and can maintain 60MPH with only 1,700 RPM. So driving at highway speed doesn’t allow the alternator to run at its full charging capabilities.

The Correct Way to Charge a Battery After a Jump Start

I’ve inspected countless alternators that failed prematurely because they were forced to recharge a dead battery. Alternators are designed to maintain a good battery, not revive a dead one.

The right way to charge a battery after a jump start is simple: use a modern smart digital battery charger.

A $60 smart charger can fully recharge the battery and recondition it by running a desulfation procedure during the charge. That restores capacity lost due to sulfation.

A $600 alternator can’t do that

Your alternator is NOT a charger

The alternator’s job is to supply the power needed for all electrical items on the vehicle, plus replenish the battery from the last start up. But the alternator is not a battery recharger so much as it is a battery maintainer. If the alternator has to recharge a discharged battery, it will become overworked, shortening its life. That’s especially true if you try to recharge a dead battery by idling your engine. That puts a tremendous load on the alternator, which causes it to overheat and fail prematurely.

A battery charger costs $60. A new alternator $600 plus labor

© 2012 Rick Muscoplat

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