can you charge a car battery on idle

[AlyssaLynn]

Yes, you can charge a car battery while the engine is idling, but it is an inefficient and slow method that should only be used as a last resort. The vehicle's alternator generates electricity to power the electrical systems and replenish the battery. However, at idle speed (typically 600-800 RPM), the alternator spins much slower than when driving, significantly reducing its charging output. For a moderately discharged battery, idling might take several hours to provide a meaningful charge, whereas driving the car for 20-30 minutes is far more effective.

The primary function of the alternator at idle is to maintain the battery's charge to run essentials like lights and the radio, not to rapidly recharge a deeply drained battery. If the battery is very weak, the energy demanded by the engine control unit and fuel injectors to keep the car running may even exceed what the alternator can produce at low RPM, preventing any net gain in charge.

This method is also impractical from fuel consumption and engine wear perspectives. Modern cars use less fuel while idling than older models, but you're still burning gas for a minimal return. Furthermore, prolonged idling does not allow the engine to reach its optimal operating temperature, leading to incomplete fuel combustion and potential buildup of fuel residues in the oil.

For a faster and more effective charge, driving the car is the best solution. The higher engine RPM (e.g., 1500-2000 RPM during highway driving) allows the alternator to operate at its full potential. Here’s a comparison of estimated charging times under different conditions:

Scenario	Engine RPM	Estimated Charging Time for a 50% Discharged Battery	Key Consideration
Idling in Park	600-800 RPM	2+ hours	Inefficient; may not charge if battery is very weak.
City Driving	1,200-1,500 RPM	45-60 minutes	Good for maintenance charging.
Highway Driving	2,000+ RPM	20-30 minutes	Most effective method while driving.
Using a Dedicated Battery Charger	N/A	4-8 hours (overnight)	Safest and most thorough method for deep charging.

Ultimately, while idling can technically add some charge, it's a weak substitute for driving or using a proper battery charger, especially if the battery is old or has been completely drained.

[VanRiver]

I learned this the hard way. My battery died after I left an interior light on overnight. I jumped it and let the car idle in the driveway for about 45 minutes, thinking it was fine. The next morning, it was dead again. A mechanic friend told me the alternator just doesn't work hard enough at idle to really charge a dead battery. You need to actually drive it to get the RPMs up. Now, if I need to charge it, I take a 20-minute drive on the main road instead of just letting it sit.

[McJayce]

Think of it like trying to fill a swimming pool with a garden hose on a trickle. The alternator is the hose, and the battery is the pool. At idle, you only get a trickle of charge. It's better than nothing if you're stuck, but it's not a real solution. The engine needs to be spinning faster to get a good flow of electricity. For a real charge, you need the water pressure of highway driving. Otherwise, you're just wasting time and gas.