Can a car battery be charged at idle speed?
2 Answers
It is possible to charge a car battery at idle speed. The details are as follows: Idling refers to a working condition where the engine runs without load, the gear is in neutral, consuming fuel while the vehicle remains stationary. As long as the engine is running, it drives the alternator to work simultaneously. For household cars at idle speed, the alternator provides a voltage of 14V and a current of around 40A. Therefore, a car can indeed charge its battery while idling. At idle speed, although the RPM is low, it still drives the magneto to work, thus outputting a stable DC voltage. Since the battery's power capacity is not that large, the charging speed will be slightly slower. Modern cars use AC generation, and the battery's power is mainly used to start the car, which consumes a significant amount. However, once the engine is running, the battery's power is no longer consumed.
As a car enthusiast, I know that when the engine is idling, the alternator can still function, but the charging efficiency is much lower compared to when driving. At low engine speeds, the power generation is minimal, and charging a depleted battery might take one or two hours, progressing very slowly—especially in cold winter temperatures. In the past, when my car battery was aging, I tried idling overnight to barely get it started, but it was far less effective than driving for a few minutes. Additionally, idling wastes a lot of fuel, leads to severe engine carbon buildup, and increases maintenance costs. Battery lifespan may also suffer due to slow or overcharging. While technically possible, it's not ideal; I recommend using it briefly in emergencies. For long-term battery depletion, driving to charge or replacing the battery is better. By the way, check the voltmeter—during idling, it usually reads around 13 volts, enough to maintain but not fully charge the battery.