Can a Car Battery Be Charged at Idle Speed When It's Dead?

[VanHarper]

When a car battery is dead, it can be charged at idle speed. The functions of a car battery include: 1. Providing power to the starting system, ignition system, and electronic fuel injection system when starting the engine; 2. Supplying power to the car's electrical equipment when the engine is stopped or at low idle speed; 3. Assisting in powering electrical equipment when the generator's output is insufficient; 4. Mitigating voltage surges in the electrical system to protect the car's electronic devices; 5. Storing excess electricity generated by the alternator. The maintenance methods for a car battery are: 1. Clean the exterior of the battery with a damp cloth; 2. Wipe off any dirt on the terminals and posts that could cause leakage; 3. Open the battery water cap and check if the water level is at the normal position; 4. Verify whether the battery is charging properly.

[Anthony]

When the battery is dead, idling can indeed recharge it, but the efficiency is really low. Last time I asked about this at the repair shop, the mechanic said the alternator runs at a low speed during idle, delivering only about 10 amps of charging power—roughly one-third of normal driving conditions. To fully recharge a depleted battery, you'd need to idle for at least three to four hours, wasting fuel and risking engine carbon buildup. Plus, many modern cars have intelligent power management systems that deliberately reduce charging intensity at idle to save fuel. If your battery is drained, it's better to jump-start it first, then drive at highway speeds for half an hour or use a dedicated charger to top it up. This approach is much healthier for the battery.

[VonTucker]

A veteran driver with twenty years of experience tells you that idling to charge the battery is just a drop in the bucket. Last week, my nephew's car had a dead battery. After starting, it idled for forty minutes, but still couldn't start the next morning. The principle is simple: when the engine runs at 800 rpm, the alternator speed is less than 2000 rpm, and the power generated is simply not enough to maintain the vehicle's electrical needs and charge the battery simultaneously. Especially when the air conditioning or audio system is on, the current may even reverse and drain the battery. A reliable method is to turn off all electrical devices after starting, let the car idle at around 2500 rpm for ten minutes before hitting the road. This way, you protect the battery without overheating the catalytic converter.

[Will]

Every mechanic knows that idling to charge the battery is the most damaging to the car. We've seen too many cases where customers, trying to save trouble, charge their batteries at idle speed, only to end up with sulfated and ruined batteries. The alternator belt lacks sufficient tension at idle, making it prone to slipping, which causes the charging current to fluctuate wildly. Especially for start-stop batteries, they require a high current to activate, which idle speed simply can't provide. Nowadays, diagnostic tools can easily show that the charging voltage at idle is only 13 volts—you need to rev the engine to 1500 RPM to stabilize it at 14 volts. If you really need an emergency charge, it's better to lift the car, put it in D gear, and press the brake to maintain the RPM at 1500. Twenty minutes of this is equivalent to an hour of idling.

[DiGianna]

During automotive engineering studies, we conducted tests showing poor idle charging efficiency has physical causes. A 12V battery normally requires 14V for proper charging, but at idle speed, the alternator's insufficient rotation results in rectified DC voltage barely exceeding 13V. Worse still, modern vehicles' numerous electronic devices consume about 3A current through the body control module at idle, leaving only a trickle charge for the battery. Experimental data reveals that a completely drained 60Ah battery takes 7 hours to fully charge at idle, whereas it only requires 90 minutes at 2000 RPM. So don't fret over fuel costs - revving the engine after startup is the practical solution.