How long does idling take to fully charge the car battery?

[VanThomas]

Idling the car for 20 minutes can fully charge the battery. Here is some information about car idling: 1. The purpose of car idling: Idling is an operating condition of a car, where the engine runs in neutral gear. The engine speed during idling is called idle speed. The idle speed can be adjusted by changing the throttle size. Idling means the engine is "producing power without doing work." When the engine is running, if the accelerator pedal is completely released, the engine is in an idling state. 2. Reasons for unstable idling: Direct causes refer to mechanical parts being dirty, worn, or incorrectly installed, leading to changes in cylinder power, resulting in an imbalance of power among cylinders and causing unstable idling. Indirect causes refer to abnormalities in the engine's electronic control system, leading to poor combustion of the air-fuel mixture, making it difficult to balance the power among cylinders and causing unstable idling.

[VonGenesis]

I've experienced the hassle of charging a dead car battery by idling. That time, after returning from a business trip, I found my car wouldn't start, and a friend suggested trying to idle it. At idle, the engine speed is usually only around 700-800 RPM, and the alternator output current is pitifully small, making charging the battery as slow as a snail's pace. From my experience, for a standard 12V car battery in a discharged state, relying solely on idling might only charge about 5% per hour, meaning it would take 20 hours or more to fully charge. There are too many influencing factors: the battery's capacity, whether it's new or old, the alternator's performance, and the temperature all affect the charging speed. It's slightly more efficient in hot weather, but overall, it's still too slow. Worse, long-term idling wastes fuel, increases emissions, and causes carbon buildup. So I learned my lesson and now, in such situations, I simply drive out for a half-hour highway run—it charges faster and is more convenient. Alternatively, keeping a charger at home for topping up is more worry-free and safer. In short, don't count on idling as an emergency solution; at best, it can temporarily maintain the charge.

[DiRiley]

Charging the battery at idle is theoretically possible but highly inefficient. At idle speed, the engine runs at low RPM, resulting in insufficient output current from the alternator. While driving at 60 km/h, the alternator spins rapidly and can deliver 20-30 amps, but at idle, it may only produce around 5-10 amps. For a 50 Ah battery, charging at idle in a depleted state might replenish just a few percent per hour, potentially requiring over ten hours to reach full charge. This process varies significantly: new batteries charge faster than old ones, and passenger car alternators are typically smaller than those in larger vehicles. More practically, idle charging isn't recommended. It wastes fuel, increases emissions, and prolonged idling—especially in winter—can cause carbon buildup and component damage. For faster charging, I suggest driving for half an hour or using a smart charger plugged into a household outlet for a few hours—both quicker and more eco-friendly solutions. Prevention is better than cure: regularly checking battery health eliminates the need to rely on this method.

[Rodrigues]

Idle charging is time-consuming and inefficient. When the engine is idling, the alternator's power output is weak, usually only capable of slow charging the battery. For a typical depleted battery, it takes at least several hours to fully charge via idling, depending on the battery size and age. New car batteries might charge slightly faster, but older ones are slower. For instance, a large battery may only gain 3% to 5% charge per hour at idle, possibly taking an entire day to reach full capacity. This method isn't worth long-term use as it easily wastes fuel and causes carbon buildup. I recommend driving at highway speeds for about ten minutes for faster and more efficient charging.