How Long Should Idle Charging Last?

[ArcherMarie]

If the car has been parked for 10 days without moving, idling for 10 minutes is sufficient for charging. However, if it has been parked for more than 20 minutes, at least 20 minutes of charging is required. More relevant information is as follows: 1. Idling: The car can be charged while idling in place. If the usual purpose is just to help replenish the battery, idling is sufficient. When the car is completely out of power, charging at 1500 rpm for 20 minutes can provide enough power for the starter to start the engine once. Charging at 1500 rpm for 30 minutes can provide enough power for a start the next day. If charging at 1500 rpm for more than 60 minutes, the battery can be almost fully charged. 2. Note: When the car is not used for a long time while away, it is advisable to unplug the power cord to prevent severe battery discharge. If you forget, you can ask family or friends to help start the car for charging. It is also a good idea to keep a set of jumper cables in the car for emergencies.

[LeMatthew]

Recently, my husband often idles his ride-hailing car to recharge the battery. I asked a mechanic and found out this method is actually not ideal. When the battery is low, idling for half an hour can start the engine, but it takes at least an hour of driving to fully charge it. During city traffic jams, idling to recharge is extremely inefficient—it’s better to take a weekly highway drive. The worst part is that prolonged idling harms the car and wastes fuel, while carbon monoxide levels near the exhaust pipe skyrocket. Doing this in a garage can be deadly—two years ago, someone in our neighborhood was rushed to the ER due to poisoning from this. If you really need a longer charge, find a ventilated open space and keep a carbon monoxide detector in the car for safety. Now, we always keep a portable charger in our car. If all else fails, calling roadside assistance is much safer than taking risks.

[LaBrady]

I've modified my car's audio system, which consumes a lot of power, and I've specifically researched idling charging. At 2000 RPM, the alternator can charge 5-8Ah per hour into the 12V battery. A 65Ah battery would take at least 8 hours to go from depleted to full—meaning idling all night! Not to mention the fuel cost could buy a new battery, and the catalytic converter might even clog. Last time I tested with a voltmeter, after idling for 20 minutes, the voltage only rose by 0.5V. It only stops being effective when the voltage reaches 12.6V. Practical advice: For mild depletion, idle for 30 minutes; for severe depletion, drive to a repair shop and use a dedicated charger. Be extra careful with modified circuits—a friend of mine fried his amplifier while idling to charge due to unstable current damaging the equipment.

[SanSadie]

Last time my battery died and I called for assistance, the technician said that cars with start-stop function are most afraid of idling charging. For regular cars, idling for 15 minutes can replenish the starting power, but cars with AGM batteries can't charge the deep-level power at all. My Mercedes-Benz was tested, and to maintain 70% battery level for the start-stop battery, it needs to drive over 50 kilometers weekly. Short-term remedy: Turn off the start-stop button when using the air conditioner to let the alternator charge at full capacity; long-term suggestion is to install a voltage monitor and take immediate action when it drops below 11.8V. A 4S shop technician secretly told me that for cars with frequent battery depletion, it's more cost-effective to directly replace it with a regular battery, just that the start-stop function will be disabled.