How Long Does It Take to Fully Charge the Battery While Idling?
3 Answers
Car idling charging time depends on the remaining battery power. If the battery is completely dead, it may take around 20 hours. Below are the influencing factors of charging and the hazards of idling charging. Factors affecting charging: Vehicle battery charging is influenced by many factors, such as whether high-power equipment is being used in the car, whether the air conditioning is turned on, or whether USB is supplying power. These factors will all increase the charging time. Hazards of idling charging: Idling charging consumes more fuel and produces severe emissions pollution, while also causing carbon deposits. Therefore, it is recommended to use a battery charger whenever possible.
I often encounter friends asking similar questions. Generally speaking, for a regular gasoline car's 12-volt lead-acid battery, idling to charge is quite inefficient. At idle speed, the engine runs at low RPM, and the alternator output current may only be around 30 to 50 amps. If your battery is deeply discharged (e.g., below 12 volts), a full charge might take two to three hours or even longer, depending on the battery capacity and vehicle condition. I've seen some car owners trying to charge while warming up their cars in winter, only to find the voltage barely increased after half an hour—far less effective than driving for a few minutes, which charges much faster. Doing this long-term also increases fuel consumption and the risk of carbon buildup. It's advisable to regularly check battery health and maintain charge through medium to short-distance driving. If you're in urgent need of power, it's best to use an external charger—it saves time and hassle.
Speaking of this, I have some personal experience, especially with hybrid models like Toyota's hybrid system. Idle charging is much more efficient because the motor and battery design are optimized. The generator can provide strong output even at idle, and it usually takes only 15 to 30 minutes to recharge the 12-volt battery, depending on how much charge is left. I've driven my friend's hybrid car a few times, and if the battery is in good condition, idling for an afternoon can fully charge it. However, don't overlook the actual operating conditions of the vehicle—high temperatures or running the AC at full blast can affect the charging speed. Also, older lead-acid batteries might require multiple stops for charging, so it's more reliable to keep the vehicle running normally whenever possible.
