How Long Should the Car Run After a Jump Start?
3 Answers
After jump-starting the car, it needs to run for more than half an hour to ensure the battery is sufficiently charged. Signs that the car battery is fully charged: It needs to be judged based on the battery, terminal voltage, and electrolyte. A large number of bubbles appear inside the battery, indicating a boiling phenomenon. The terminal voltage rises to the maximum value and no longer increases within two hours. The electrolyte rises to the highest level and no longer increases within two hours. Method for jump-starting a car: Drive the working car next to the dead car, with the front ends facing each other. Open the hoods of both cars and take out the jumper cables. Locate the batteries of both vehicles, attach the negative clamps to the negative terminals of both car batteries, and attach the positive clamps to the positive terminals of both car batteries. Start the working car, charge for a few minutes, and then start the dead car.
After jump-starting a car, it's generally safe to let it run for 15 to 30 minutes. I once experienced a situation where the car stalled immediately after jump-starting and couldn't be restarted. Later, a mechanic explained that jump-starting is only a temporary solution, and the alternator only begins properly charging the battery once the engine is running. You need to wait until the charging stabilizes to ensure enough power for the next ignition. The exact time depends on the battery capacity and how depleted it was. For regular sedans, charging is most effective at around 1,500 RPM. In winter, when temperatures are low, it's safer to charge for at least 20 minutes or more, while in summer, a slightly shorter duration may suffice. If you frequently take short trips with constant start-stop cycles, the battery is more prone to draining again. In such cases, it's advisable to let the engine run a bit longer. For older cars, it's best to let them run for over 30 minutes, avoiding high-power devices like air conditioning during this time.
After jump-starting the car, I usually let it run for about 20 minutes before turning it off. Last time when the battery was completely dead, the mechanic checked it while explaining that shutting it down immediately might still leave it unable to start. This is because it takes approximately 20 minutes to fully charge one cell of a lead-acid battery, and the alternator only begins working after the engine starts—with the charging rate depending on the RPM. I typically keep the engine steady at around 1500 RPM, slightly higher than idle speed for better efficiency. In summer, the duration can be slightly reduced to 15 minutes, but if the car is over five years old or has recently experienced a dead battery, be sure to charge it longer. Meanwhile, watch the dashboard for any charging fault warnings—headlight brightness is also a simple indicator.
