How long does it take to charge a car battery at idle after a jump start?
3 Answers
It takes 10 to 15 hours to charge a dead car battery at idle. Here are specific methods to determine whether charging at idle is possible: 1. Voltage assessment: First, the voltage is absolutely sufficient. According to national standards, the voltage requirement for a 12-volt system alternator is constant at 14.5V±0.25V, meaning the alternator output voltage remains unchanged whether at idle or at maximum RPM. This clearly indicates sufficient voltage for charging. 2. Current assessment: The ideal charging current for a standard lead-acid battery is 1/10 of its capacity. Taking the most common 65AH battery as an example, its ideal charging current is 6.5 amps. From the national standards for alternator output current requirements, even the lowest-power alternator has a minimum output current of 16A, far exceeding the charging requirements.
I've also experienced a dead battery before. After getting a jump start, I let the engine idle to recharge, but that process is painfully slow. Usually, it takes about 30 minutes to an hour of idling for the battery to recover slightly. However, if the battery is old or of poor quality, it might take even longer; a fresh battery might only need around 40 minutes. I recommend not just idling—driving the car is more efficient. Once the engine revs up, the alternator outputs more power, and about 20 minutes of driving should be enough for an emergency charge. If your battery keeps dying like this, it's probably nearing the end of its life—time to replace it or check the alternator.
From years of driving experience, idling to charge the battery is just a time-consuming task. The current is only around 10 amps, so after jump-starting, you need at least an hour to be safe. It's best to drive around for a while; revving up the engine within ten minutes significantly improves charging efficiency. Idling not only charges slowly but also wastes fuel. If the battery is severely depleted, it might take over two hours to fully charge. One more thing to note: after starting, check the alternator voltage—it should normally be between 13.8-14.4 volts. If it's lower, there's an issue with the charging system.
