how long to leave car running to charge battery

[DeAlice]

Leaving your car running to charge the battery is an inefficient and generally ineffective method. For a significantly depleted battery, idling the engine for 15-20 minutes might provide just enough charge to start the engine next time, but it will not fully recharge the battery. This is because a car's alternator—the component that generates electricity—is designed to primarily power the vehicle's electronics while maintaining the battery, not to act as a high-output battery charger.

At idle (around 600-800 RPM), the alternator produces its minimum output. For a standard 12-volt car battery with a capacity of, for example, 48 Amp-hours (Ah), idling might only supply a trickle charge of 10-30 amps, which is largely consumed by the car's headlights, climate control, and infotainment system (a phenomenon known as parasitic load). A truly dead battery requires a proper bench charger, which delivers a controlled, multi-stage charge over several hours to restore health without damaging the battery plates.

Charging Scenario	Estimated Time at Idle	Resulting Charge & Notes
Jump-Started Dead Battery	20-30 minutes	Provides minimal "surface charge" sufficient for a single restart. Driving immediately after is crucial.
Weak Battery (Slow crank)	30-60 minutes	May improve starting ability temporarily, but does not address underlying battery age or health issues.
Maintaining Charge (Battery in good health)	Not Recommended	Modern cars have complex electrical systems; idling for long periods is inefficient and can cause fuel dilution and excess emissions.
After a Short Drive with Accessories On	15-20 minutes	Can help recover charge used during a short trip, especially if lights and AC were running.
Using a Dedicated Battery Charger	4-12 hours	The only method for a full, healthy recharge. A 10-amp charger can fully recharge a 50Ah battery in about 5-7 hours.

The most reliable solution is to drive the car. A 30-minute drive at highway speeds (where the alternator spins faster and produces more amps) is far more effective than idling for an hour. If your battery is consistently dead, the problem is likely a failing battery, a faulty alternator, or a parasitic drain that should be diagnosed by a professional.

[SanAmy]

Honestly, you're better off just taking it for a spin around the block. Letting it sit there and idle for ages hardly does anything, especially if you've got the radio or AC on. It's a total waste of gas. If you just got a jump, give it a solid 20-minute drive on a main road instead of letting it run in your driveway. That’ll put a real charge back in it. If it dies again soon after, the battery itself is probably just shot and needs replacing.

[VanMia]

As a DIY guy who's tackled this, here's the deal: idling is a band-aid. The alternator needs RPMs to work well. I hook up a multimeter to the battery terminals. With the engine running, you should see over 14 volts if the charging system is healthy. If it's barely above 12, your alternator might be the problem. A proper battery charger is the real tool for the job. It slowly brings the battery back to 100% without the strain. Idling just puts a shallow charge on top of a deeply drained battery, which can actually shorten its lifespan.

[MarleyLee]

We live in a snowy state, and my neighbor's car battery is always dying in the cold. He thought letting it run for 10 minutes in the morning was enough. It's not. Cold weather makes batteries sluggish, and the draw from the heater and defroster at idle can actually use more power than the alternator puts back. I told him if he's not going to drive it, to use a battery maintainer (a trickle charger). It plugs into a wall outlet and keeps the battery at an optimal charge all winter, which is much cheaper and safer than idling for long periods.

[LeIan]

From a technical standpoint, the math is against idling. A modern car's electrical system, with all its modules and screens, can have a parasitic load of 5-10 amps even when "off." At idle, the alternator may only produce a net gain of 5-15 amps after covering that load. To recharge a 50Ah battery from 50% depth of discharge, you'd need to idle for over two hours—inefficient and harmful to the engine. The engine control unit (ECU) is designed for optimal operation under load while driving, not at idle. The best practice is always a dedicated charger or a sustained drive.