can you leave a car running to charge battery

[MacViolet]

No, idling your car is an inefficient way to charge a battery and can actually be harmful if done regularly. While the alternator does charge the battery when the engine is running, it operates at a much lower output at idle speed (typically around 600-800 RPM). At this low RPM, the alternator produces just enough power to run essential systems like the fuel pump and ignition, leaving very little surplus for charging a significantly depleted battery. For a meaningful charge, the engine needs to be running at a higher RPM, which is best achieved by driving the car for at least 20-30 minutes.

The primary role of the alternator is to maintain the battery's charge, not to recharge a dead one from scratch. Attempting to charge a completely dead battery by idling can put excessive strain on the alternator, potentially leading to premature failure—a costly repair. Furthermore, prolonged idling is wasteful, contributing to unnecessary fuel consumption and engine wear without the benefit of bringing the vehicle up to optimal operating temperature.

This method should only be considered a temporary fix in an emergency, such as reviving a battery with just enough charge to start the engine after a minor drain (like leaving an interior light on). For a healthy battery that is simply low from infrequent use, a short drive is vastly superior to idling.

Scenario	Idling Effectiveness	Recommended Action
Jump-Started Dead Battery	Very Poor	Drive for 30+ minutes to recharge safely.
Battery Drained by Left-On Light	Limited/Short-Term	Idle for 10-15 mins, then drive to fully charge.
Maintaining Charge on Healthy Battery	Sufficient for Maintenance	Regular driving is better; idling is unnecessary.
Frequent Need to Recharge	Ineffective & Damaging	Indicates a failing battery or parasitic drain; professional diagnosis required.

[StEmersyn]

It's a common misconception, but you're better off taking a short drive. Idling the engine just doesn't spin the alternator fast enough to put a real charge back into the battery. You'll burn gas and add wear to your engine for very little gain. If the battery is really dead, idling might not even help at all. Save yourself the time and just drive around the block a few times.

[MadisonFitz]

From an efficiency standpoint, idling to charge a battery is a poor practice. Modern alternators are designed to provide sufficient charge while driving. At idle, the energy produced is prioritized for immediate engine operations. This method is inefficient, increases fuel costs, and contributes to unnecessary emissions. For the battery's long-term health and overall vehicle efficiency, connecting a dedicated battery charger or simply driving the vehicle is the correct approach.

[VonBentley]

Think of it like this: your alternator is a water pump, and your battery is a bucket. At idle, the pump is only dripping, barely keeping up with small spills. To actually fill the bucket, you need a steady stream, which you only get by driving. Letting the car sit and run is like hoping those drips will fill the bucket—it'll take forever and probably won't work if the bucket is nearly empty. A quick drive is the surefire way to get the job done right.

[SilasLynn]

As a car guy who's dealt with this, idling is a weak solution. The alternator's output is directly tied to engine RPM. At a standstill, you're not generating enough amps for a solid charge, especially on a modern car with all its electronics drawing power. You might get enough juice to start the car next time, but you won't fully replenish the battery. This can mask a deeper issue like a dying battery. A proper drive or a trickle charger is the only way to ensure a complete and healthy charge.